Knife blade materials

This page collects work comparing various knives to summarize the information gathered on blade materials and as well provide additional reference links on material properties.

Composition, name and heat treating information : main

Composition tables/information :

Tool steel name cross reference information :

Heat treating :

Maker commentary :

Materials information :

Steels discussed :

Steel Carbon Manganese Chromium Nickel Vanadium Copper Molybdenum Silicon HRC 1084 0.84 0.75 — — — — — — 45-66 1095 0.9-1.03 0.3-0.5 — — — — — — 45-66 52100 0.98-1.10 0.25-0.45 1.3-1.6 — — — — — 58-62 50100-B 0.93 0.43 0.60 0.03 0.21 0.16 0.24 — 56-61

The specifications for 52100-B is the composition of an individual blade.

Nominal composition of 1084 :

Carbon : 0.84%

Manganese : 0.75%

Silicon : 0.22%

Note 1084 is a carbon steel which has a specific definition :

Carbon steels are regarded as steels containing not more than 0·5% manganese and 0·5% silicon, all other steels being regarded as alloy steels - American Society for Metals 1964

The first thing that is obvious is that this is a high carbon steel. Looking at the diagram to the right, 1084 has much more carbon than is necessary even for full hardness in steels. The extra carbon which isn't dissolved in the martensite is used to form cememtite, or iron carbide, which increases the wear resistance over a steel such as 1060 which could obtain similar hardness.

The other thing which is obvious is that it only has a small amount of alloying elements which increase hardenability and so it will require a very fast quench, either water or a very fast oil.

Note in the isothermal transformation diagrams to the right how the small amount of Chromium and the much larger amount of Manganese in O1 (just under 1.5%) makes a severe impact on the hardenability.

The O1 shows a severe weakening or suppression of the diffusion based transformations (pearlite) during the cooling and thus can take a much slower quench. This reduces issues of warping and quench cracking which is why slower quenching steels can be favored by makers/manufacturers.

Note that in addition to Carbon and Manganese, 1084 will typically contain Silicon, Phorphorus and Sulphur. The last two are impurities and are present in all steels, Silicon will in general as well as it is commonly used as a deoxidizer and will be present in the original ore.

Silicon does serve as an alloying element in steels as it dissolves in ferrite and strengthens it. It also enhances corrosion resistance, commonly used in austenetic stainless steels. It is also very significant in regards to its effect on carbide formation as it suppresses it both during the quench and during tempering. However in order for these effects of silicon to be significant, they require more substantial amounts than what is typically found in 1084.

In regards to the carbon content of 1084, it is very close to the eutectoid point for the iron/carbon system whihc is ~0.77%. This is a fairly technical point but has some implications for the knife maker.

As noted in the diagram on the right, a steel close to the Eutectoid point will, upon heating, transform from from ferrite+pearlite to pure austenite. Steels which have a lower carbon content will transform to austenite but will still contain ferrite and steels which have a higher carbon content will contain cementite instead of the ferrite.

Again, this is a technical point, but it is of interest to those hardening the steel.

In regards to the actual properties of 1084, it will have physical properties similar to other carbon and low alloy steels. Often times much argument will be made about small changes in steels which imply large differences but the metallurgical data doesn't reflect such extravagant claims. For example, it is true that in general 1095 will have a slightly higher amount of cementite than 1084 which will give it a higher wear resistance, but how much is likely not to be significant. In some detail, from research looking at the wear on steels in grinding ore, here are the relative wear rates of 1040 carbon steel vs 1090 carbon steel : 1

0.040 C : 140 WF

0.090 C : 130 WF

Note that this is comparing a very large difference in carbon content, from 0.4 to 0.9%, much larger than say 1084 vs 1095 and yet the wear factors of the tooling change by less than 10%.

If this seems odd then realize that the volume of cementite (iron carbide) in these pure carbon steels is relatively small. For example the picture on the right is of DIN 12206 which is a 1.5% carbon steel. Even in this much higher carbon content there is still only approximately 5% of carbide by volume of the steel.

As the carbon content drops so does the volume of carbide. In steels such as 1084 or 1095, the carbide volume will only be a couple of percent. Comparing something like 1084 vs 1095 then in regards to wear resistance is talking about a very small fraction of a percent difference in carbide volume. If the other aspects of the steels are similar, which are strongly influenced by the initial steel quality and the heat treatment, the performance of 1084 vs 1095 is likely to be near identical.

What is the significant difference then in 1084 vs the lower carbon content steels such as 1045 or similar alloys such as 4340? The critical aspect is the toughness and in particular the ductile to brittle transition. This is important in steels as it is the temperature at which steels will fail in a brittle mode vs a ductile one. This means that they will crack with little or no deformation.

In high carbon steels, the ductile to brittle failure point is actually at high temperatures, above room temperature which means that when fully hardened they tend to break when over stressed with little deformation and crack cleanly. It also means that the amount of energy they absorb is severely reduced. However as noted in the figure in the right, the low to mid carbon steels have a ductile to brittle temperature which is below zero which means they are more suitable for impacts and harsh work in colder weather.

Knives used in 1084 :

The bush knife from Takach forge was a solid example of 1084. It could easily do extended wood work and suffer no significant fracture. It was hard enough to also resist rolling and tended to blunt by slow wear. As this is a low carbide steel with only a small volume of cementite and no alloy carbide it ground very easily even on natural stones. As it has a decent working hardness and quality hardening from the maker, it also sharpens easily to a fine finish and has no significant issues with burr formation.

Nominal composition of 1095 :

Carbon : 0.9 - 1.03%

Manganese : 0.3 - 0.5%

Sulfur : < 0.05%

Phosphorous : < 0.04%

1095 is a very high carbon steel steel. The carbon content allows it not only to reach maximum hardness in martensite, it has excess carbon to form a significant volume of cementite. This gives it a significant advantage in abrasive wear resistance over the medium carbon steels in which all of the carbon typically is utilized in hardening the martensite.

As 1095 has a very restricted alloy content, it has a very low hardenability as it does not contain significant amounts of elements to increase hardenability :

Molybdenum

Chromium

and only has a small amount of Mangenese. Note in order to get a full oil hardening steel the Mangenese content has to be much higher as seen in O1 for example at 1.0 - 1.4% .

However as knives are generally made from very thin sections of steels then the shallow hardening isn't a critical concern and fast quenching oils are commonly used to harden 1095.

In regards to properties, as 1095 also has no alloy content which effects tempering :

Chromium

Molybdenum

Tungsten

Vanadium

It softens continuously as the tempering temperature is raised and there is a smooth loss of hardness and strength.

Note that 1095 is subject to TME (tempered martensite embrittlement) with tempering temperatures in the neighborhood of 500F even though it doesn't consistently show up on charpy/izod impact tests. However it does show up on torsional impact/ductility measurements which has to do with the axiality dependence of TME due to cementite precipitation.

Knives personally used in 1095 :

The Ontario machetes had suffered gross damage losing large pieces out of the edge on hard woods. The Ontario Spec Plus knives, also had problems; one blade was too brittle, another had an unhardened tang, and one was even ground the wrong way, the edge was thicker than the spine. The Ontario RTAK also took severe damage clipping off branch stubs .

The Livesay RCM also deformed during limbing though not as severe as the Ontario's. The custom RTAK from Livesay did not have such a problem and the performance was solid for a wood cutting tool.

The TOPS Steel Eagle also had several problem in regards to brittle fracture. The teeth cracked when used as wire breakers and the tip and edge cracked when digging an arrow head out of a piece of wood ref. It also had reduced slicing edge retention compared to a Battle Mistress in INFI on carpet and wood.

The narrow Mora had problems with brittle tip fracture on light work and in general the edge retention was low on hemp and carpet.

The custom paring knife was heat treated to maximize torsional toughness and give a very high torsional strength and ductility and compressional strength. The edge had a very thin profile and a very acute grind yet was still able to cut woods, bone and even sod with no significant damage. The edge holding in general was also very high on both hemp and carpet.

Summary : 1095 is a plain carbon steel which offers a very high working hardness, moderate wear resistance, and very high apex stability. It can also be tempered for high toughness by drawing above the TME point around 500F. Unfortunately it is often used in very inexpensive and low quality knives and therefore it often has very low performance and issues with durability.

Nominal composition of 52100 :

Carbon : 0.98 - 1.1

Chromium : 1.3 - 1.6

Manganese : 0.25 - 0.45

Silicon : 0.15-0.3

52100 is a high carbon steel, the carbon content is well beyond what is necessary to reach maximum hardness in martensite and thus it will have a small amount of excess carbide both in the forms of cementite (iron carbide) and chromium carbide. This will give it an advantage in wear based properties over the mid carbon steels which lack the abundance of carbon to both fully harden the martensite and have a surplus to form cementite. This difference is pretty dramatic on standard materials testing, for example using ASTM G65 (200/300 mesh sand/silica) 52100 has almost a 2.5:1 advantage over 1040 :

-5% NaCl quench, 200C temper - 1040 : weight loss = 0.17 grams -5% NaCl quench, 200C temper - 52100 : weight loss = 0.07 grams -Abrasive wear characteristics of carbon and low alloy steels for better performance of farm implements, M. Kumar and R. C. Gupta

The advantage, though not as drastic holds for the high carbon steels due to the small amount of chromium carbides in 52100 :

At a hardness of HRC = 61, 52100 steel has a better cutting performance than 1086 steel and both are generally better than Damascus steel. -Wear tests of steel knife blades - John D. Verhoeven a,*, Alfred H. Pendray b, Howard F. Clark

Note in the above Verhoeven is referring to a higher CATRA score, which is a measure of edge retention cutting abrasive paper, when he notes "cutting performance". The same work also shows AEB-L to have slightly improved performance again in the same check for the same reason, a slightly higher volume of chromium carbides.

Aside from wear resistance though carbides, the Manganese and Chromium in 52100 also significantly increase the hardenability over a plain carbon steel such as 1084 or 1095. Both of these elements do this in slightly different ways.

The chromium in the steel increases the hardenability by reducing the high temperature diffusion reactions. As the blade cools, the Chromium slows the formation of pearlite. The manganese addition has a similar effect, it also reduces the formation of pearlite, but it does by expanding the austenite phase to being stable (existing) lower temperatures. This ideally prevents the ferrite from forming until ideally the temperature has been reduced by the quench to the point martensite starts to form.

Note how 52100 compares well to O1 in regards to hardening response as it has increased Chromium but reduced Magnesium and the balance gives a similar suppression of diffusion reactions.

As 52100 is a common industrial bearing steel there is a wealth of data on how it performs as such compared to other bearing steels which show among other things : 52100,S90,440C,REX20

at the same hardness, 52100 has higher compression strength than 440C and S90V

1550F soak produces a higher strength, at the same hardness than a 1650 F soak

And the heat treatment of it has been studied in detail to determine for example how to minimize the retained austenite and maximize the hardness :

The experiments conducted show that austenitizing and tempering temperatures have the most influence on the retained austenite and the hardness in the heat treatment of 52100 steel. The austenitizing and tempering temperatures of 827C and 177C, respectively, gave the lowest austenite and highest hardness values for both the second and final Taguchi analyses, indicating that no further refinement of the experiment is necessary. -Iterative taguchi analysis: optimizing the austenite content and hardness in 52100 steel P.W. Mason and P.S. Prevéy, Lambda Research

Note that critical in the heat treatment of 52100 is to adjust the soak time and temperature so that enough carbon gets dissolved in the austenite to achieve full hardness, but just that amount. As noted in the above, the hardness tends to maximize at approximately 0.6% of carbon dissolved in the austenite and raising above this tends to increase the retained austenite, promote plate (vs lathe) martensite and reduce wear resistance by dissolve more of the chromium carbide.

52100 has also been extensively studied in regards to the properties of bainite and duplex bainite+martensite structures :

Bainite + martensite duplex microstructures can be produced in AISI 52100 steel via combination of austempering and quenching processes. Bainite + martensite duplex microstructure yields in high levels of hardness (55-64 HRc) and enhanced impact toughness (24-54 Joule). -Effect of austempering temperatures on microstructure and mechanical properties of a bearing steel Volkan Kilicli, Ph.D.1 and Mucahit Kaplan, M.Sc.

In that work it is clearly shown how optomizing the microstructure allows an increase in toughness of almost three fold with a small loss of hardness from 61 to 59 HRC by adjusting the austempering time at temperature from 15 to 30 minutes. However while standard bainite does have increased toughness over martensite, the opposite is true in regards to wear resistance :

Austempered samples have highest impact strength, the least being martempered samples. The impact strength increased with soaking time in austempered samples up to certain level. 20% improvement is observed with austempering process.(iii) Annealed samples have the highest wear, while martempered samples have the least wear. Approximately 50-60% wear resistance is increased with martempering process. -Effect of Austempering and Martempering on the Properties of AISI 52100 Steel P. Vamsi Krishna, R. R. Srikant Mustafa, Iqbal and N. Sriram

Now 52100 can't be discussed in regards to knives without talking about the grain structure. To start, there is much confusion in the industry where both the austenite grain boundaries and the primary carbides are both referred to as grain size which is misleading. It is very necessary to separate them as they have completely different effects in steels.

It is known that a reduced grain size strengthens steels and can do so significant as calculated by the Hall-Petch equation which shows an increase in yield strength (in ferrite) which increases as the inverse of the square root of the grain size. However a strong concern as a result of the reduction in grain size is a corresponding reduction in hardenability.

Martensitic transformation in an ultrafine-grained (UFG) steel was investigated. The UFG steel was a plain carbon steel containing 1.0 wt.% C and prepared by thermomechanical treatment. The grain size was approximately 1 µm. The sample was reheated at 1023 K for 5 min and then cooled in water to conduct a martensitic transformation. In addition to martensite, lamellar pearlite was observed in microstructure. When the heating time was prolonged to 7min, after quenching process, matensite was only observed. The results indicate that when heating time is shorter, the grain size is small (2.5 µm) and lamellar pearlite can be formed as quickly as martensitic transformation in quenching process. When the heating time is prolonged, grains grow (4 µm) and no lamellar pearlite is observed. This phenomenon is greatly related the increasing resistance for martensitic transformation and accelerated diffusion of carbon atoms due to the refinement of grains in the steel -Ultrafine-grain Effect on Martensitic Transformation in a Hypereutectoid Steel - Fuliang Lian, Yongning Liu, Hongji Liu, Junjie Sun, Xuejiao Sun

In short, a 1% C steel (1095) had ultra-fine grain (15 ASTM) and when it was water quenched it formed a significant amount of pearlite due to the very fine grain. In order to get full martensite with a water quench, the grain had to be coarsened to increase hardenability. Beyond 12-13 ASTM then water isn't a sufficient quenchant for 1% carbon steels. This fact has become known to some knifemakers who have had to resort to accelerated quenches in order to preserve full hardness when the grain size was reduced through thermal cycling, using water on 52100 for example or even brine or "super quench".

Knives personally used in 52100 :

The Blackjack small was used for a lot of work, in regards to edge retention it was behind S30V and 10V in slicing cardboard with various edge finishes South Fork Review.

Ray Kirk's test blades were made from 1084, L6, 52100 and D2 at various hardness levels, which were used unmarked so the steel types were unknown until after the work was completed to examine sharpness and edge retention on used mats, as well as ease of sharpening and durability by cutting bone and concrete. At high sharpening angles (20 degrees) there was no difference in initial sharpness. In edge retention on the used mats the D2 blade consistently outlasted the 52100 knife on the mat cutting which out cut the other two steels. On the bone cutting the L6 blade fared the worst due to the low hardness. On the concrete cutting the 52100 blade had the best balance of strength and durability to minimize damage. While there was a large difference in machinability, the blades which sharpened the fastest tended to be just the ones which suffered the least damage or wear in use, so either D2 or 52100 depending on what was cut.

Ray's ABS bowie was used for a lot of wood work and rope cutting and found to have lower edge retention than a straight handled Battle Mistress (INFI) on cutting used poly, however the edge angles was lower on Ray's knife and it still cut significantly better even when the edge was degraded. The bowie was also subjected to both light and hard impacts off of a concrete block and it did well taking only minimal edge damage inspite of its very acute edge profile, and was much more durable than a tactical knife from Strider in a much thicker edge profile.

The MEUK in 52100 was several blades on used carpet and it had significantly better edge retention than a Swiss Army knife, a custom in LM1 and another 52100 blade by Ed Fowler though was behind another custom in CPM-10V. The MEUK was also behind S30V and 10V several high alloy steels slicing carpet. One of the main drawbacks to 52100 is the low corrosion resistance, and this was evident with the MEUK which would developed a patina quickly in the kitchen on acid foods.

Ed Fowler's Pronghorn was used for a lot of work include cutting hemp where at best it matched the performance of an Opinel. On used carpet it did poorly, being outperformed by a Swiss Army knife. On lateral loads it did very poorly showing little flexibility and taking a set immediately and being very easy to bend, acting essentially like unhardned steel. However this blade was hardness tested and found to have a very low hardness, less than 40 HRC.

Summary : 52100 tended to show what would be expected from the materials properties, that it is behind higher carbide steels such as S30V on abrasive cutting, but it has superior grindability, toughness, and ease of sharpening.

50100-B / 0170-6C / Carbon V: main

This is one of the more common carbon steels in the cutlery industry, however it is rarely called by that name. Cold Steel calls it Carbon V, Camillus calls it 0170-6C, and Case calls it "chrome vanadium", W7 is tool steel with a similar composition.

Knives personally used in 0170-6C / Carbon V :

The Twistmaster was used for a lot of hemp rope cutting and with a fine ceramic finish at 22 degrees per side it had only a fraction of the edge retention of a D2 custom from Mel Sorg at 62 HRC. With the edge sharpened to a fine diamond finish at the same angle, it was identical to the Becker CU/7 once the edge had been reprofiled to a similar level of cutting ability by adjusting the angle which makes sense as they are the same steel at the same hardness. It was however significantly behind higher and harder alloy steels such as VG-10 and D2.

The Machax was used for a lot of wood work compared to multiple large blades, the edge retention was much lower than the Busse Battle Mistress. The Machax took edge damage in the form of chips and dents cutting hard woods with the stock profile. With the edge angle reduced the damage was much reduced but the edge retention was much lower than on the Battle Mistress as the edge would lose slicing aggression much faster. The Machax was also given a soak in salt water and as expected the blade formed surface rust readily but didn't tend to pit.

The Patrol Machete was used on a variety of light vegetation where it did well, however it took gross damage on light limbing. A replacement suffered extensive edge damage on the same work and then cracked easily with a few light slap on a log. While the edge was too thin which can be used to argue for the rippling, there was gross fracture under far too low a level of stress.

The Becker CU/7, similar to the Twistmaster, also had a large disadvantage compared to the Sorg Custom, this time the comparisons was at a very rough finish, left by a 100 grit aluminum oxide belt. The Becker was not even in the same class. With a much reduced edge profile the CU/7 was subjected to very hard impacts, the edge fractured readily, but there was no gross damage up into the primary grind as seen for example with the WB from Strider.

The Combat Bowie was used for some bone chopping compared to the Camp Tramp in SR101 and had a slight advantage after a short round of bone cutting with both blades at similar angles and finishes. However the much higher chopping ability of the Becker would give it an advantage and in general this type of work needs to be repeated to insure that it isn't just bone variance or a bad swing. However it would be reasonable to conclude the edge durability is at least similar in class.

Overview : This steel is one of the most common production grade cutlery steels, called different names by different manufacturers. It is basically a low alloy enhancement of 1095 designed to give deeper hardening, refine the grain and as well provide slight increases to wear resistance and corrosion resistance. The performance seen in production knives was significantly varied which is common with the more inexpensive blades.

Tool steels

Steels discussed :

Steel Carbon Manganese Chromium Nickel Vanadium Molybdenum Tungsten Cobalt Nitrogen HRC L6 0.65-0.75 0.25-0.80 0.6-1.2 1.25-2.0 0.2-0.3 0.5 — — — 45-62 O1 0.85-1.0 1.0-1.4 0.4-0.6 0.30 0.30 — 0.5 — — 55-64 A-2 0.95-1.05 1.0 4.75-5.50 0.30 0.15-0.50 0.9-1.4 — — — 58-60 D-2 1.4-1.6 0.60 11.0-13.0 0.30 1.10 0.70-1.20 — — — 58-62 M-2 0.95-1.05 0.15-0.4 3.75-4.5 0.30 2.25-2.75 4.75-6.50 5.00-6.75 — — 62-66 M-4 1.3 0.3 4.0 — 4.0 4.5 5.5 — — 58-68 Calmax 0.6 0.8 4.5 — 0.2 0.5 — — — 58-60 INFI 0.5 — 8.5 0.74 0.36 1.3 — 0.95 0.11 58-60 CPM 3V 0.80 — 7.5 — 2.75 1.30 — — — 58-62 CPM 10V 2.45 0.5 5.25 — 9.75 1.30 — — — 58-64 CPM 15V 3.4 0.5 5.25 — 14.5 1.3 — — — 58-65

Nominal composition of L6 :

Carbon : 0.75

Chromium : 0.9

Manganese : 0.75

Molybdenum : 0.35

Nickel : 1.75

Note L6 refers to a family of steels, not a specific steel :

The high-carbon low-alloy tool steels, represented by AISI L6, are designed to provide oil-hardening capabilities and higher toughness and resistance to tempering than available from plain carbon steel steels. Carbon levels are above 0.65 percent to achieve the required hardness and wear resistance. hardeningabiliy is obtained by sing at leat 1.5 percent nickel and 0.75 percent chromium, sometimes supplemented by molybdenum and vanadium. -Nickel Alloy Steels Data Book

15N20 is a very similar high nickel steel, often used in bawnsaw blades. Nominal composition of 15N20 :

Carbon : 0.75

Manganese : 0.4

Nickel : 2.0

Nickel has many uses in steel :

increases toughness (strong influence on cleavage fracture)

moves the ductile to brittle transition temperature to low temperatures

increases strength

increases corrosion resistance

It does this by a number of ways as it has a number of strong influences on the phases present in steel and their nature. For one, as noted in the diagram on the right it is a very strong austenite stabilizer. As the carbide forming elements (chromium, molybdenum, etc.) are ferrite stabilizers, Nickel is often added to ensure ferrite isn't present in the steel.

The effect of the ductile to brittle transition temperature is so strong that nickel is often used for steels which retain toughness even in extreme cold as in cryogenic temperatures. This however requires significantly more Nickel than is in L6.

It also effects carbide formation, specifically suppressing it :

An increase of nickel content in the investigated structural steels causes a decrease of epsilon carbide concentration in their microstructure after tempering. -The effect of carbide precipitate morphology on fracture toughness in low-tempered steels containing Ni, Krawczyk J, Bala P, Pacyna J.

and also reduces the Martensite start temperature :

The investigations show that the Ms, As and Af temperatures decrease with increasing nickel and manganese contents. -Influence of Manganese and Nickel on the a´ Martensite Transformation Temperatures of High Alloyed Cr-Mn-Ni Steels Andreas Jahn, Alexander Kovalev, Andreas Weiß and Piotr R. Scheller

which promotes a finer martensite structure.

Note the graph on the right which shows the extreme increase in toughness between L6 and a carbon steel of the same carbon content in terms of charpy impact toughness. L6 is tougher at a higher hardness than the plain carbon steels. This is why in general alloy steels are used in severely demanding applications.

In regards to hardenability, as noted in the isothermals in the right, L6 has a strongly suppressed diffusion based reaction compared to not only W1 but also O1. This is due to the fact that it has three alloying elements which retard these reactions :

Mangenese

Chromium

Molybdenum

Note the effect of Molybdenum is so strong on the pearlite that it splits the curve completely and pulls the pearlite away from the bainite formation as it suppresses bainite to a greater extent than it retards pearlite. In any case, for the knife maker this means L6 can use a less severe oil (slower) than O1 and thus has again lower risk of quench cracking and warping.

In regards to retained austenite, while nickel is a austenite stabilizer as is carbon, there are only a minimal amount of high carbide formers and other elements which suppress the martensite formation temperatures and thus the amount of retained austenite in L6 steels is very close to the same amount in pure carbon steels of the same carbon amount.

Note the sudden drop in retained austenite in the L series tool steels which decreases to almost zero at a tempering temperature of 300C. This is because in the temperature range of 200 to 300C there is a decomposition of retained austenite in all steels which don't have this reaction suppressed. Silicon for example will suppress it and L6 doesn't have a signficant amount of Silicon.

This temperature range is a fairly interesting one in regards to the effect on toughness in general because the reaction of L6 to tempering in this range is different depending on how the toughness is measured.

The interesting thing is that this minimum toughness peak show in the torsional data is well understood as tempered martensite embrittlement :

The embrittlement is concurrent with the replacement of epsilon carbide by interlath cementite during tempering, and the mechanical instability of interlath films of austenite (as a consequence of this carbide precipitation) during subsequent loading. -Mechanism of Tempered Martensite Embrittlment in Low Alloy Steels, Horn and Ritchie

It also shows up in charpy-v notch tests and measurements of strain fracture toughness (see the above paper) but doesn't show up in unnotched charpy/izod. This may be due to its sensitivity on grain orientation, transverse tests would clarify that concern.

Knives personally used in L6 type steels :

Ray Kirk's test blades were made from 1084, L6, 52100 and D2 at various hardness levels, which were used unmarked so the steel types were unknown until after the work was completed. At high sharpening angles (20 degrees) there was no difference in initial sharpness. In edge retention on the used mats the D2 blade consistently outlasted the 52100 knife on the mat cutting which out cut the 1084 blade which outclassed the much softer L6 blade showing the heavy influence of hardness. On the bone cutting the L6 blade fared the worst again due to the low hardness. On the concrete cutting the 52100 blade had the best balance of strength and durability to minimize damage, the L6 blade again suffered due to lack of compression resistance. While there was a large difference in machinability, the blades which sharpened the fastest tended to be just the ones which suffered the least damage or wear in use, so either D2 or 52100 depending on what was cut. Note in this comparison there was an issue with the L6 blade, there was a problem with the hardening. It was included just as a check on the consistency of the testing.

The Running Dog Traditional Tanto is made from 15n20, a swedish bandsaw steel similar to L6. It was compared to a D2 custom from Mel Sorg (62 HRC, full cryo) which showed superior edge retention on hard woods . The corrosion resistance was quite low, it would rust in minutes when exposed to food acids .The Tanto was also compared to a Sub Sniper in ATS-34 cutting wood and had significantly lower edge retention, but the much greater machinability allowed similar sharpening times.

Summary : L6 is a very tough, mid-carbon steel which offers a very high toughness at a high hardness. The above blades could not readily examine these attributes as the L6 test blade by Kirk was severely hampered by lack of hardness and the Running Dog Tanto was not examined significantly as it was just on loan.

The following specifications cover O1 Tool Steels :

ASTM A681

DIN 1.2510

SAE J437

SAE J438

UNS T31501

Nominal composition of O1 :

Carbon 0.85 - 1

Chromium 0.4 - 0.6

Manganese 1 - 1.4

Silicon 0.5 max

Phosphorus 0.03 max

Sulphur 0.03 max

Tungsten 0.4 - 0.6

Vanadium 0.3 max

As a high carbon, low alloy tool steel, it has a very find distribution of very fine carbides, mainly cementite.

As a bit of detail on the composition, as noted in the image on the right, it only requires a maximum of 0.6% carbon to produce maximum hardness, above that there is very little increase and there are issues with retained austenite and formation of plate (vs lathe) martensite. Why then does O1 have such a high level of carbon?

The extra carbon will form carbides which are much harder than the steel (martensite) such as cementite, and more importantly since O1 has a small but significant amount of alloy carbide formers in Tungsten, Vanadium and Chromium, all of which will tie up carbon in the formation of carbides. The carbon needs to be increased above 0.6% to ensure that a free amount of carbon is left to go in solution in the martensite to enhance the hardness.

In regards to the carbide formers, the tungsten and vanadium are mainly there as they will not dissolve in the austenite and thus they pin the austenite grains as they form and thus keep the grain very small which increases the strength and toughness of the steel. These very small and very hard carbides are much harder than the martensite and the cementite 1 and thus will contribute to the low stress abrasive wear resistance over a pure carbon steel such as 1095. However to really make this significant then a lot more Tungsten is needed, several percent such as steel in the cold work grades such as F2. 2

The other significant alloy influence in O1 is the manganese which is very beneficial to steels in many respects (it is a deoxidizer) and chromium, and to a lesser extent the silicon. These elements all increase the hardenability, or ease of forming martensite. This is why O1 can oil harden but 1095 for example needs a much faster quench and is usually water hardened. For the knife user, this makes little effect, but for the knife maker, not having to deal with the extreme quench of water and the risk of cracking can be of benefit.

The chromium in the steel increases the hardenability by reducing the high temperature diffusion reactions. This means as the blade cools, the chromium stops pearlite from forming The manganese addition has a similar effect, it also reduces the formation of pearlite, but it does so in another way. manganese expanding the austenite phase to being stable (existing) lower temperatures. This ideally prevents the ferrite from forming until ideally the temperature has been reduced by the quench to the point martensite starts to form. These alloy additions to O1 over the plain carbon steels and the effect they have on the reduction of the pearlite and thus the increase in the hardenability can be readily seen in the TTT curves to the right.

O1, like most of the high carbon or high alloy steels can benefit from an extended quench where the steel is taken to below room temperature. The main reasons for this are the :

increased transformation of austenite to martensite

formation of very fine eta carbides

The combination of these two effects can produce an increased hardness of 2-3 HRC points and an increase in low stress wear resistance by a factor of 2. 2a .

What does all of this mean as to how the steel performs? As always, it is of benefit to look at some comments/feedback on steels from the woodworking industry as it is a very common chisel and plane steel. In general O1 is considered an entry level steel for such materials when compared to White, Blue and HSS chisels using M2 or similar steels. All of these materials will have a significantly higher carbide content, but still retain a very fine, well distributed carbide network as noted in the image on the right. 3 The apex stability in M2 and similar steels is therefore still high enough they tend to blunt by slow wear and can maintain a high sharpness.

The PM-V11 and the White Steel really do deliver. The gap between them and the A2 and O1/HCS is very large. -Derek - Four Chisel Steels Compared: PM-V11, A2, White Steel, O1/HCS

Now a frequent point of contention in such materials is O1 vs A2 and in general the argument is A2 will make a stronger and more wear resistant edge but it is harder to grind :

A2 is a great steel that offers a real improvement in edge retention. O1, on the other hand, is still preferred by many for its relative ease of sharpening and its ability to get sharper. -Ron Hock

However the difference in these materials in terms of abrasive wear resistance is actually quite small and they both have the same working hardness ranges and maximum obtainable hardness. In practice then what is often seen in terms of one out performing the other is dependent more on which manufacture made which chisel in a particular steel and the random stresses on it in a particular use. Brent Beach for example compared a large range of planes in various steels and while the HSS blades in M2 did consistently offer superior performance the performance of A1 vs O2 was just a random spread around each other :

Lee Valley A2 , 6 Lie Nielsen A2 , 6 Lee Valley block plane A2 , 12 Lie Nielsen #62 O1 , 18 Hock O1 , 9 Knight O1 , 6 -Brent Beach, plane blade testing

While the data show a weak increase in performance of A2 over O1, the numbers listed (which are the wear bevel sizes) differ in the random spread much larger than the difference between them so there is no statistical significance. Beach was also doing a very controlled comparison, in normal work it is even more unlikely a consistent performance increase would be seen unless careful observations were made over a very long time period.

Maker commentary:

Knives personally used in O1 :

In regards to the Uddo in O1, it was used for extended slicing comparisons on 1/2" hemp and cardboard and performed well, similar to other steels in its class in regards to generally blunting by slow wear and resisting chipping and significant deformation. In order to have the performance significantly exceeded in regards to edge retention it was necessary to step up to steels such as Elmax and M4, or use a steel similar to this one, but hardened differently such as customs in 1095 which are at maximum hardness, 66/67 HRC. Of course while the edge retention slicing abrasive materials is higher in those examples, they trade off grindability and toughness to obtain the higher strength and wear resistance.

On harder work, cutting carpet. an example of the kind of tradeoff in terms of toughness and how it influences performance can be seen. used carpet. The O1 blade from Uddo shows its versatility here compared to various steels as it could easily do extended slicing without any significant damage and just blunted by slow wear. The fact that it takes no visible damage in such work has a significant effect on the ease, or speed of resharpening. The top performance in edge retention in that comparison was seen in S30V. However the O1 blade was significantly easier to grind and when the two are combined to represent a kind of edge retention - efficiency measurement then the O1 is ahead of the S30V. Now of course if the knife is power sharpened or very coarse / high end stones are used to grind it, then this kind of measurement is moot as grinding speed can be rapidly reduced in such methods.

The same kind of benefit was seen to an even greater extent when the knife was used to cut up some sods alongside a few other folders. It was among the fastest to sharpen as it again took very little damage and the steel has a high grindability.

In regards to sharpening, O1 in general gets high praise for ease of sharpening, not only as noted by the woodworkers as noted previously but by knife users as well. It is one of the easier to sharpen steels possible as it has :

high grindability

high apex stability

As this blade is hardened for high durability, then it compromises a little on the edge retention in light use for performance in light use as noted in the above. However This means that in general the chip resistance is fairly high (compared to steels such as D2, TS-34, 10V) and in heavier use it can excel where such steels would chip. In general due to the combination of toughness and strength the edge tends to blunt by slow wear as noted in the image on the right which also increased the ease of sharpening by reducing the necessary grinding as seen in the carpet cutting previously.

The TUSK from McClung, handled low stress cutting well and the hard edge stayed crisp a long time cutting wood, ropes and other soft media. However it chipped badly cutting light sheet metal and suffered gross damage readily on edge torques, a replacement fractured in the same manner, the maker claimed abuse. The behavior may be explained by the above torsional graph which shows strong embrittlement regions for O1 in both torsional impact and strength with a low ductility.

A pATAK in O1 from McClung also suffered gross damage during a review. A Howling Rat was personally used to do the same work with no damage. Mike Turber also compared a ATAK in O1 from McClung to several other knives and it also suffered edge chipping cutting woods. This, as is common with critisms of McClung's knives caused significant controversy as McClung claimed the knife was a fake inspite of the knife being bought from an offical dealer of McClung's knives and other customers reporting the same behavior on multiple knives from McClung. Will Kwan also noticed problems with prying in woods as well as light chopping into metals scan down through this cashed link of a deleted thread to see Kwan's commentary.

The Randall #1 in O1 didn't have problems with chipping however due to the low hardness (55/56 HRC) the edge retention was in general lower than slightly harder Randall #5 in 440B stainless. The 440B Randall also has of course a much greater corrosion resistance, the O1 blade will patina visibly while cutting acidic foods. When compared to harder and more wear resistant alloys such as S30V, a production folder out cut the O1 Randall by about 3:1 on cardboard. The O1 Randall machined very easily as it was soft enough to file, the low hardness does mean it could be problematic to get a crisp edge on v-rod rigs.

As with all steels, heat treating is critical. The custom O1 blade at 63.5 HRC was compared to a Sebenza in S30V on cardboard with both sharpened to very low edge profiles, the performance was reversed from seen on the Randall and the O1 blade had much better edge retention. It also did very well slicing used carpet. It formed a patina very fast when exposed to food acids.

Summary : O1 is a general purpose tool steel known for moderate wear resistance and toughness and low corrosion resistance. It makes a very nice light utility knife. Its performance in larger knives was not as impressive but may be due more to choices in heat treating rather than intrinsic properties of the steel.

A2 is an air hardening cold work die steel. The significant amounts of chromium and molybdenum makes it more dimensionally stable than O1. They also require much higher austenization temperatures to dissolve than the water/oil hardening steels and thus A2 is typically austenized at 1750/1800 F. The heating is usually done in stages to minimize thermal gradients in the steel and reduce the hold time at the austenization temperature to prevent grain growth.

The dissolved alloy elements and high carbon content will cause significant percentage of retained austensite if the quench is halted at room temperature and result in a loss of 2.5-3 HRC points. With oil + cold A2 can can harden up to 64/65 HRC and will resist significant softening up to 1000 F (57/59 HRC). A2 has moderate wear resistance (A2 is 6, O1 is a 4 and D2 is an 8 : ref) and good impact toughness. The grain fracture size with standard industy heat treatment is 8.5. Some specification and performance data sheets on A2 from various manufacturers :

Materials data :

Maker commentary :

Knives personally used in A2 :

The Mission MPK took edge damage chopping while the Recon Scout from Cold Steel in Carbon V did not, which may simply be an issue of hardness. The MPK also had much lower edge retention on hemp with a DMT 600 finish compared to a Becker CU/7 which is also made from the same steel as the Recon Scout. The Mission was problematic in regards to durability and cracked in half under a light impact from a framing hammer in an attempt to cut a piece of tension bar (mild steel), no progress was made on the bar cut. Much higher durability has been seen with other blades such as the TAC-11 and Howling Rat).

The Project I, similar to the MPK also showed damage readily just on wood chopping.

Summary : A2 is an air hardening tool steel known for a solid combination of wear resistance and toughness. The personally used knives were not impressive in regard to durability or edge retention however this is more likely an issue with choice of heat treatment. One of the reasons could be large amounts of retained austenite which will transform to untempered martensite over time, expecially with heavy work which leaves the steel very brittle.

D2 is a cold work die steel with a much higher alloy content than A2, specifically the chromium and carbon percentages are both increased to generate a large volume of chromium carbide. D2 is typically austenized at just slightly higher temperatures, 1850 F, which like A2 is usually done in stages. It has very high wear resistance due to the carbide content which also lowers machinablity and grindability. The corrosion resistance is high for a tool steel, significantly more than A2 and it resists forming a patina strongly. D2 however doesn't have the corrosion resistance of martensitic stainless steels as most of the chromium in D2 is in the form of primary carbides due to the high carbon content and low austenizing temperatures.

D2 has a coarse carbide structures, the primary chromium carbides which can be up to 50 microns in length, the fracture grain size is 7.5. It is commonly used in industry for punches, dies and various types of knives. It has significant retained austenite retained after quenching to room temperature which can be reduced by cold treatments. It has a very wide temper range from 300F for maximum hardness (64 HRC) and wear resistance, up to 950F (58/60 HRC) for toughness. The high temperature tempers will transform retained austenite to martensite in the cooling to room temperature after the tempering, as the ausentite is conditioned by carbide precipitation during the temper which raised the Ms point. Generally multiple tempers should be used to temper the freshly transformed martensite.

D2 can also be soaked much hotter, up to 2050 F, which forces much more of the alloy content into the austenite which lowers the Ms point and produces a lower as quenched hardness. However there is now a much greater secondary hardening responce which can increase the hardness well above the as quenched hardness due to secondary carbide precipitation and the transformation of the retained austenite to martensite. Some specification and performance data sheets on D2 from various manufacturers :

Other reference information :

Knives personally used in D2 :

The Deerhunter in D2 was compared to identical blades in AUS-8 and VG-10 stainless steel. In edge retention on hemp rope the D2 blade could cut double the amount of the VG-10 blade before achieving a similar state of significant blunting, and the VG-10 knife 50% more than the AUS-8A. However when the influence of corrosion was added by soaking the blades in lemon juice, the D2 blade was far behind the two stainless steels which were similar in edge retention on the hemp. The blade were also used for hard work, batoning, cutting bone and metal and impacted into concrete. The VG-10 blade consistently showed the lowest durability and the D2 the highest.

Ray Kirk's test blades were made from 1084, L6, 52100 and D2 at various hardness levels, which were used unmarked so the steel types were unknown until after the work was completed, to examine sharpness, edge retention on used mats, ease of sharpening and durability by cutting concrete. At high sharpening angles (20 degrees) there was no difference in initial sharpness. In edge retention the D2 blade consistently outlasted the 52100 knife on the mat cutting which out cut the other two steels. On the concrete cutting the 52100 blade had the best balance of strength and durability to minimize damage.

The Dozier Agent was compared to the Safari Skinner on cardboard to check it compared to another D2 blade and no significant difference was observed in edge retention in both push/pull sharpness : ref. The K2 from Dozier was compared to a small Sebenza in S30V from Chris Reeve knives on slicing cardboard and there was no significant difference in slicing edge retention between the two : ref. The K2 was also used to slice used carpet and compared to numerous other knives where it did well in general, though was outperformed by several very hard tool steels : ref. The Agent was also compared to a S30V Paramiltary from Spyderco cutting used carpet, and the edge retention was similar when the material was dry, however with cutting performed in the rain the D2 blade fell behind showing the influence of the lower corrosion resistance of D2 vs S30V : ref 1, 2. In regards to heavy impact, the Dozier Agent was subject to a variety of impacts off of hard objects and was readily outperformed by tougher steels like SR101 : ref.

Two Spyderco Folders in S30V were compared to the Heafner bowie in D2 for edge retention on used carpet and the same behavior was noted in regards to rusting effecting edge retention : ref. In that case the D2 blade was further behind as the rain was heavier. The Heafner bowie also showed one of the drawbacks for D2 in large blades as when it accidently hit a rock when clearing some grasses the edge chipped readily : ref. However Swamp Rat knives have demonstrated highly levels of impact toughness with their D2 : ref.

The custom from Mel Sog was used to cut a lot of hemp with various edge finishes and profiles showing the influence of both and how D2 makes an excellent rope slices with a thin edge and x-coarse finish : ref.

The Uluchet was used for a lot of wood cutting and in general performed well, it has a fairly robust edge profile which allows it to resist damage from woods and even bone. It was also able to be used as a baton impact tool with no problems.

There was little done with the Cuda MAXX as it was bought mainly to check the ability of the knife to take "flicking", intertial wrist openings.

Summary : D2 is a tool steel known for high wear resistance through its very heavy chromium carbide content and high obtainable hardness. In general it makes a nice steel for fine cutting blades, at moderate sharpening angles, and especially for coarse finishes. The corrosion resistance is high for a tool steel, though it tends to pit readily in salt water soaks, and the resistance to impact is also low. There is quite a bit of variation in regards to durability, Swamp Rat D2 and Dozier's D2 were seen to be extremely different.

Nominal composition of Calmax :

Carbon : 0.6

Silicon : 0.35

Mangenese : 0.8

Chromium : 4.5

Vanadium : 0.2

Molybdenum : 0.5

A very versatile steel with adequate wear resistance and very high toughness. It is very suitable for low to medium production volume tooling for blanking thick production materials or in general when the tooling is exposed to high stresses. -Uddeholm Pocket Book

Note Calmax falls in the elemental range of the Uddeholm patent EP0388415B1 which was for a family of high toughness steels for heavy duty stamping.

In regards to composition, with a small amount of Molybdenum, Calmax would be expected to have a secondary hardening response but not a strong one. The moderate amount of austenite stabilizers (nickel, manganese, or cobalt/copper) and low amount of carbon in solution (due to the carbide formers chromium, vanadium and molybdenum) would produce small amounts of retained austenite. These are indeed the properties as shown in the graphs on the right.

With the moderate amount of Mangense and large amount of Chromium, this is a deep hardening, air hardening steel though oil would be a preferred quench for knives (to minimize diffusion reactions and precipitations of carbide from the austenite). It would resist a patina much more so than carbon steels due to the chromium in solution, but would not be stainless as it would not have enough chromium to passivate and would pit if left exposed to corrosive environments.

The mid amount of carbon would ensure lathe matensite and promote very high toughness which would only be exceeded by the very high shock steels with even less carbon and more nickel and possibly silicon to remove TME and allow tempering in the 500F range.

The main attraction of Calmax as a tool steel is the growing understanding that workhorse steels like D2, which have been long used in the tooling industry due to high :

compression strength

abrasive wear resistance

can suffer premature failure due to chipping and generate fracture. This loss can come from direct impact failure or long term fatigue.

Calmax is far tougher than D2 and has a similar level of compressive strength and thus can equally resist deformation. In cases then when D2 tools are failing by fracture then Calmax is a solid choice over steels both due to the high impact toughness and standing fatigue.

For knives this means that it is an option when heavy impacts are frequent part of the use and the edge may come in contact with very hard objects and the knife has to be able to strongly resist fracture. The air hardening ability also makes it attractive for makers / manufacturers who can not handle the oil hardening requirement of other tough steels such as L6.

The main attraction however for larger knives isn't simply the toughness but the fact that it combines that toughness with a very high grindability. This means that not only does it strongly resist fracture, it is also very easy to restore to sharpness after any deformation / damage. For example compared to 3V :

Carbon : 0.6

Chromium : 4.5

Vanadium : 0.2

Molybdenum : 0.5

Silicon : 0.35

Mangenese : 0.8 Calmax Carbon : 0.8

Chromium : 7.5

Vanadium : 2.75

Molybdenum : 1.3 3V

3V has a significantly higher carbide volume through the greater amount of mainly Vanadium, this also reduces the free carbon so 3V has a significantly lower hardening response than Calmax. 3V also has a very high toughness and strongly resists fracture, but the large amount of vanadium carbide means it is more difficult to grind and takes more time or more expensive abrasives to restore. Now to be clear, there is always a balance, if Calmax is wearing too fast then 3V might be a superior choice.

However there is more to steel than impact toughness, there are other issues such as :

wear resistance

fatigue strength

compressive strength

As Calmax is a mid carbon, low carbide steel, while it ranks high in toughness and grindability, it will have lower fatigue strength than similar steels such as Caldie which are ESR grades and thus are cleaner. It also will have lower strength than high carbon and high carbide steels.

Note to the right that Calmax for example has a much lower bend strength and compression strength than not only D2 but even a simple 1% carbon steel.

Knives used in Calmax :

Voyager : Kyley Harris, cKc knives

The Voyager is a large blade, used mainly for chopping wood and scrub brush. Calmax worked very well in that knife, it was hard enough to resist deformation well, would tend to fail by plastic deformation when over loaded and hitting harder objects such as metals and/or rocks/dirt in the bark of trees. The high grindability due to the low carbide volume also made it very simple to grind/sharpen, even on natural stones.

Summary : Calmax is a tool steel designed to air harden, have high toughness and solid fatigue. It works well in larger knives due to the combination of high hardness to resist deformation, toughness and fatigue strength to resist chipping/fracture and high grindability for ease of maintenance and sharpening.

INFI is a propriety steel used by Busse Combat through hardened to 58/60 HRC.

The performance of INFI in the blades of Busse Combat has been demonstrated live by jerry Busse at live and public demonstrations at knife shows, as well as in videos and pictures, these include thousands of push cuts on full once inch hemp rope without sharpening, cuts though a hanging bundle of 10 strands of inch hemp, multiple 2x4's chopped with the knife still shaving, and very heavy prying loads and bends to a very high degrees without breaking on a fully hardened blade.

Knives personally used in INFI :

The straight handled Battle Mistress easily outlasting a TOPS knife in edge retention on both carpet and wood.

The battle Mistress E was used for very heavy work to check durability and the performance was very high, resisting hammer impacts, cuts into nails, concrete and even rock with minimal damage.

The badger Attack 3 was used mainly as a heavy utility knife and with a custom modified edge profile worked extremely well as a heavy wood craft blade.

Summary : INFI is a tool steel known for overall solid performance with a excellent balance of corrosion resistance, toughness and edge retention. It makes a superb large blade as well as smaller blades which need to handle tougher work.

M2 is a high speed steel (HSS) which means it retains its hardness at the high temperatures induced from cutting at high speeds. High speed steels achieve this "hot hardness" through the use of alloy elements such as W, Mo and V to form secondary carbides during tempering. They require very high austeniting temperatures (2250F-2350F) to dissolve the alloy carbides. M2 is air hardening up to 66/67 HRC with oil quench and cold treatements and has very high wear resistance and low impact toughness, this is the hardness in hacksaw blades. The fracture grain size for HSS is 9 to 9.5. Some specification and performance data sheets on M2 from various manufacturers :

other materials data :

Maker perspective :

Note that the tempering temperature of HSS strongly effects the corrosion resistance because chromium rich carbides both form at certain temperatures and dissolve at others :

[...] 575 °C < t < 700 °C, redissolution of M4C3 and part of M23C6 carbides -In¯uence of heat treatment on the corrosion of high speed steel

Knives personally used in M2 :

The mini-AFCK was compared mainly to a AUS-8A stainless steel blade from spyderco and found to have better edge retention in cutting cardboard, plastics, and insulation. It also sharpened easily to a very fine edge.

Summary : M2 is a HSS tool steel which high obtainable hardness and wear resistance with low impact toughness. It has a very fine grain structure and makes an excellent low impact cutting knife. The corrosion resistance is lower than stainless cutlery steels but high in general for a tool steel.

M4 is a high carbon, high speed steel. In the knife industry there was, and still is to a reduced extent, some promotion of it as having high toughness, however such claims are often problematic due to the lack of reference points. The materials data, even in the most positive case as presented by the manufacturer show it to have lower toughness compared to tool steels such as A2 and is similar to high carbide stainless steels such as S30V. This is not surprising as it is a high carbon, and thus high wear modification of M2.

As noted in the image to the right, it has a high carbide volume, with well distributed small carbides (on the order of 2-3 micron). Given an optimized hardening (for strength and wear resistance) it would be expected to hold a fine sharpness for an extended period of time, have little issues with carbide tear out and aside from gross impact concerns (chopping), the main issues would likely be with difficulty of grinding.

In a little detail on toughness as that was, and to some extent still it, one of the main promotional points of the steel, it was often promoted as being tough because of use as in the BladeSports competition blades. However it has to be realized that :

those knives are not hardened for maximum hardness/wear resistance

those knives have low long term durability

The latter point was a well kept secret for a number of years. The M4 blades when first used would actually suffer extreme brittle failure. To prevent this the blades were under hardened by lowering the soak temperature. However even with such underhardening the blades still have a very short lifetime :

Blade sports competitors push the limits and some of these very thin blades work harden and fracture or crack after a year or two on competition, and are replaced. Personally I used 52100 clad with 15N 20 for several years, and the knife is still undamaged. - Ed Schempp

In regards to the effect of tempering and soak temperature on toughness :

Blade performance was examined by hardness, 3-Point bend, impact, and CATRA (edge retention) testing. The results show that the austenitizing temperature is a significant factor that affects all mechanical properties tested. The max load in 3-Point bend test increases with the carbides fraction that can be maximized by controlling austenitizing temperature. Both austenitizing temperature and tempering temperature have significant effects on the hardness. As for the impact performance, the impact toughness increases with the carbides density. Additionally, we can achieve comparatively high impact toughness in low austenitizing temperature without decreasing hardness through lowering the tempering teperature, because tempering temperature has no significant effect on impact toughness. The edge retention of CPM-M4 steel relates to its hardness. Harder materials can provide a better edge retention for knife blade. - Heat treatment effects on CPM-M4 tool steel performance as edged blade material Lian, Sidi

As M4 is a very well used steel in industry there is a wealth of data on it including on such topics as the use of salt for heat treating.

One of the benefits of conductive heating in salts is the use of low austenization temperatures. High speed tools heat treated in salts also require much shorter holding times at temperature. -Hardening high speed steels : metallurgical benefits of salt - Gregory W. Dexter

The difference in austenite grain can increase the traditional fine grain of such HSS (ASTM 9-10) to the ultra fine grain size of 11-12 which increases the strength and toughness at the same hardness .

Beyond material data, an interesting point in regards to M4 as a cutting steel can be seen in the wood working industry where the standard knife demos on hemp/cardboard are replaced by wood working :

This was a test of 5 steels used in chopping dovetails in hardwood. There were relatively good performances from 3V and M4, and superlative performances from Koyamaichi laminated white steel. -Derek

The M4 did much better than the 10V which likely had issues with edge stability, and as well over a standard O1 chisel which would suffer from a lower hardness. Note that wear resistance was of little benefit in such a comparison as the chisel edges all blunted from wear and deformation. However chopping isn't the only use for chisels and the performance in paring was not identical :

Taken overall, 10V struggled to hold an edge over the duration of the test. I strongly suspect that this was due difficulties in creating a good edge at the outset. This highlights the major drawback in using this steel, and it is doubtful that the average woodworker would see any advantage here. I cannot recommend 10V as a steel for woodworking blades. In the initial stages the laminated WS appeared to take the sharpest edge, better than any other blade in the test, but it did not hold this as well as the M4 and 3V blades. The latter made up for this by holding a good edge longest. -Derek

This then showed the critical idea that in regards to edge retention, the stopping point is critical as the steels which had early best performance were not the same as those which had the best performance later on. However as with all such comparisons, conclusions should be left tentative without multiple runs to ensure the results are consistent.

As for corrosion resistance, M4 has a small amount of chromium which is dissolved in the steel in the soak to prevent diffusional phases in the quench and enhance the secondary hardening response. The same chromium however also makes it significantly more corrosion resistant over simple steels such as 1095 and L6. How corrosion resistant is it? It is strong enough that it is generally considered difficult to force a patina on it compare to those basic steels. It is however not a stainless steel and there are frequent comments that the knives can in fact come with corrosion on them when bought as noted in the video on the right.

As an interesting point of view :

I forced a medium patina on the knife when it was new with boiling vinegar to help prevent rust and pitting. Opinions vary on the value of a patina. I cannot definitively say that it helps prevent rust but it's just something I do that seems to work for me. I have used the knife every day for the last 6 weeks as a fish cleaning tool. Typically I will come in and spend 15-45 minutes at the cleaning table depending on the number of fish I have. During this time, the knife blade is covered with blood, fish goo and residual saltwater from the kayak and fish. The only effect I have seen on the steel is a gradual and even darkening of the patina. There has been zero rust, pitting or red/orange residue. It should be noted that I always rinse the knife within a half hour of finishing. I usually dry it too but sometimes I just leave it out wet and have seen no ill effects from this. For the first few weeks I would spray it with a shot of wd40 after cleaning it but recently I have stopped bothering because it doesn't seem to be necessary. As far as ocean use its a little less capable. I took it on the kayak a couple of times. I have a small mesh enclosed cavity in the center where I kept the knife in its sheath. The cavity constantly has about a half inch of saltwater in it so the knife was basically bathing in ocean water for the entire 5 hours. When I got in there was light orange swirls on both side of the blade. No pitting at all but the rust process had definitely begun. I was able to remove all signs of corrosion in 30 seconds of light rubbing with one of those green and yellow kitchen pads. -Surfingringo

Note that the tempering temperature of HSS strongly effects the corrosion resistance because chromium rich carbides both form at certain temperatures and dissolve at others :

[...] 575 °C < t < 700 °C, redissolution of M4C3 and part of M23C6 carbides -In¯uence of heat treatment on the corrosion of high speed steel

That temperature range produces a maximum corrosion resistance peak.

Knives used in M4 :

The Air was used for stock cutting on cardboard and had very high performance. It was difficult to grind, and required specialized stones to sharpen effectively. However with those stones it would easily take a very high sharpness with very little burr formation.

The 710 axis was used alongside the Air and was found to be similar in regards to grinding, sharpening and edge retention.

Summary : M4 is hard to grind, easy to sharpen (minimal burr formation), but requires specialized hones. It has enough chromium to strongly resist a patina, but will readily corrode if exposed to salt water and/or acids. It is very abrasive resistant, does very well in cutting ropes and cardboard and in general works well at lower angles than coarse steels such as D2 and ATS-34.

Nominal composition of 3V :

Carbon : 0.8

Chromium : 7.5

Vanadium : 2.75

Molybdenum : 1.3

A hot-worked, fully dense, wear resistant, vanadium-rich, powder metallurgy cold work tool steel article having improved impact toughness. This is achieved by controlling the amount, composition and size of the primary carbides and by insuring that substantially all the primary carbides remaining after hardening and tempering are MC-type vanadium-rich carbides. The article is produced by hot isostatic compacting of nitrogen atomized powder particles. -Patent

This steel for Crucible marked a turning point in their development as they discovered something about carbide volume and nature which was critical to maximizing toughness at a given wear resistance :

The notable improvement in toughness obtained with the articles of the invention is based on the findings that the impact toughness of powder metallurgy cold work tool steels at a given hardness decreases as the total amount of primary carbide increases, essentially independent of carbide type, and that by controlling composition and processing so that substantially all the primary carbides present are MC-type vanadium-rich carbides, the amount of primary carbide needed to achieve a given level of wear resistance can be minimized. -Patent

It has also been discovered that in comparison to conventional ingot-cast tool steels with compositions similar to those of the articles of the invention, that production of the articles by hot isostatic compaction of nitrogen atomized, prealloyed powder particles produces a significant change in the composition as well as in the size and distribution of the primary carbides. The former effect is a hereto unknown benefit of powder metallurgical processing for cold work tool steels, and is highly important in the articles of the invention because it maximizes the formation of primary MC-type vandium-rich carbides and largely eliminates the formation of softer M7C3 carbides, which in addition to MC-type carbides are present in greater amounts in ingot-cast tool steels of similar composition. -Patent

CPM-3V then is able to provide a very high level of toughness at a high wear resistance through approximately 5% MC type carbides which are vanadium rich.

An interesting piece of work on 3V, 10V and 15V :

Abrasive wear tests demonstrated that the ion-nitriding treatment and the addition of vanadium were very effective in enhancing hardness and abrasive wear resistance of HIP steels such as CPM-3V,10V, and 15V, as compared to the same steels that were only quenched and tempered. Besides, the formation of a thinner compound layer of gamma-Fe4N and VN resulted in optimum wear properties -Study of the wear behavior of ion nitrided steels with different vanadium contents, R.M. Muñoz Riofano, L.C. Casteletti, P.A.P. Nascente

Note that a very coarse and very hard silicon carbide abrasive was ground against the steels (pin on disk, 500 mesh silicon carbide) which is why the initial performance of the 3V, 10V and 15V steels are so close together. Silicon carbide is comparable in hardness to vanadium carbide hence it can produce rapid wear even on high vanadium carbide steels.

Knives used in 3V :

The Ed Schott camp knife was used for a variety of work and the edge held up well, especially considering it was ground at nine degrees per side. It even held up to several dozen full hits from a framing hammer without gross fracture however it did have problems with a weakness through the tip.

The Extreme Judgement showed poor performance in regards to edge retention and durability, taking damage readily in terms of chipping on woods even though the edge was 0.027-0.028" thick and ground at 15-16 degrees per side. In retrospect, this likely was caused by heat damage to the steel in the initial power grinding or similar likely transient damage and not indicative of the performance of the steel in general.

Summary : On paper CPM-3V looks to combine high toughness and wear resistance, however the two knives used did not exhibit such performance.

Nominal composition of CPM-10V :

Carbon: 2.45

Manganese : 0.5%

Silicon: 0.9%,

Chromium: 5.25%

Vanadium : 9.75%

Molybdenum : 1.3%

Design description :

A hot-worked, fully dense, wear resistant, vanadium-rich, powder metallurgy cold work tool steel article having improved impact toughness. This is achieved by controlling the amount, composition and size of the primary carbides and by insuring that substantially all the primary carbides remaining after hardening and tempering are MC-type vanadium-rich carbides. The article is produced by hot isostatic compacting of nitrogen atomized powder particles. -Patent

10V is the most popular of the extremely high vanadium steels developed by Crucible for cold work, resisting very high wear abrasive demands. This family of steels includes up to 20% vanadium steels, however aside from 10V, the others are not very popular and rarely used in knives.

The very high vanadium carbide volume of 10V produces very high adhesive and abrasive wear resistance at the cost of a low grindability, apex stability and toughness. As a point of comparison vs A2 :

half the toughness

twice the wear resistance

Knives personally used in CPM-10V :

The utility hunter in CPM-10V was used for a lot of cutting on various materials from cardboard, woods, plastics, hemp and used carpet. It consistently did well and outperformed steels such as D2 and ATS-34 for slicing edge retention. It did not form a patina, showing a corrosion resistance higher than steels like L6 and O1 which would be expected based on the much higher chromium content.

Summary : CPM-10V is a very high wear steel and provides high wear resistance and hardness, producing high slicing edge retention when cutting to a low sharpness.

CPM-15V is an extremely high carbon, high vanadium cold work tool steel which offers improved wear resistance over 10V.

Nominal composition of CPM-15V :

Carbon: 3.5

Manganese : 0.5%

Silicon: 0.9%,

Chromium: 5.25%

Vanadium : 17.5%

Molybdenum : 1.3%

Design description :

A hot-worked, fully dense, wear resistant, vanadium-rich, powder metallurgy cold work tool steel article having improved impact toughness. This is achieved by controlling the amount, composition and size of the primary carbides and by insuring that substantially all the primary carbides remaining after hardening and tempering are MC-type vanadium-rich carbides. The article is produced by hot isostatic compacting of nitrogen atomized powder particles. -Patent

CPM-15V offers even more wear resistance than 10V through a much larger MC carbide volume. However this increased wear resistances comes at the cost of decreased :

impact toughness

grindability

apex stability

Knives personally used in CPM-15V :

The Roger Dole folder was on loan so it was not used for extensive comparisons. It was not found to have an advantage in push cutting edge retention compared to D2 or ATS-34. However it did have better edge retention slicing carboard than an ATS-34 custom.

Summary : CPM-15V is an extremely high wear cold work steel. The abrasive wear resistance is improved approximately 50% over 10V however the toughness and grindability are severely reduced. The difficulty of working/fabrication is so high that few makers/manufacturers will use it.

Steels discussed :

Note Phosphorus is normally considered an impurity in steels and limited to under 0.04% in most cutlery stainless, similar tolerances for sulfur, there are also small amounts of copper as well. The composition of ZDP-189 is also of some debate.

H1 is a stainless steel which through a high nickel content 6-8% allows age hardening as opposed to the method of soak/quench/temp commonly used for most cutlery stainless steels. The main advantage of precipitation hardening steels in general is that they can be supplied to a manufacturer in an condition of optimal machinability and then the only heat treatment required is an extended very low temperature soak. This grade has the near immunity to corrosion which has been verified by independent curious individuals. Some users have reported corrosion but it seems to be an issue with contamination. Spyderco's H1 blades have tested in hardness at 58 on the spine for both plain and SpyderEdge versions and 65 HRC on the edge of the plain edge version and 68 HRC on the edge of SpyderEdge model which has been proposed to be due to work hardening Some publically posted materials data on H1 :

Knives personally used in H1 :

A plain edge Pacific Salt in H1 was compared to a S30V and D2 blade slicing used carpet and by adjusting the grit finish of the H1 blade to a more optimal level (rougher) the performance was competitive, showing the importance of edge finish However at the same edge grit finish, the H1 knife had approximately 50% of the sharpness of the S30V blade after 254 slices through used carpet. Comparing the Pacific Salt to a Byrd Meadowlark on cardboad there was no significant difference in slicing edge retention The Pacific Salt was also compared to a Meadowlark in 8C13CrMoV and small Sebenza (S30V) with acute edge profiles slicing up plywood, and the H1 steel held its own to the 8C13CrMoV and both were far ahead of the Sebenza.

The Alantic Salt, which has a SpyderEdge profile, used carpet : out cut a plain edged S30V blade slicing used carpet The Alantic Salt was also used for some very heavy cutting, slicing up a steel belted tire and readily outperformed several plain edged knives in various hard and high wear steels .

Frank k (Bladeforums handle) also compared H1 to VG-10 on cardboard and found a large difference between the plain edged blades, multiples times more material cut with the VG-10 knife for both plain edged blades to have the same level of blunting. However the serrated edges cut for so long it was not possible to determine the superority of either

Summary : H1 is a precipitation hardening stainless steel, extremely resistant to corrosion, fairly tough and ductile for a stainless steel and will tend to deform plastically rather than break when over stressed. The edge holding in cutting abrasive materials like cardboard and used carpet will be low compared to high wear stainless steels like VG-10 and especially S30V however it holds it own with many of the softer and less wear resistant stainless cutlery grades and has much better corrosion resistance.

The following specifications cover 420 stainless steels :

S42000

420S37

1.4021

X20Cr13

2303

SUS420J1

Nominal composition of 420 :

Carbon: 0.15

Silicon: 1.0%,

Chromium: 14%

Similar steels include 420J2 Azom :

Carbon: 0.15-0.36%

Silicon: 1.0%,

Manganese: 1.0%,

Chromium: 14%

and 420HC : Stal.com :

Carbon: 0.44%

Chromium: 13%

Care has to be taken in general from inferring properties of these steels because the 420 label can be applied to steels with a carbon content as high as 0.38 % Stal.com and some of the alloys such as Molybdenum and Nickel are not always present under the 420 label.

In regards to the alloy composition, the main alloying elements are the carbon content and the chromium content. It is a little complicated to discuss as there is Molybdenum in some 420 steels and not in others and this effects the carbon and chromium dynamic as noted in the phase diagrams on the right.

The addition of Molybdenum :

increases the carbide volume

decreases the free chromium

expands the M23C6 carbide range at the expense of the M7C3 phase

This is why in general it isn't a common alloying element in razor blade steels which are attempting to have the smallest carbides (M7C3) to maximize the apex stability to allow the edge retention at a very high sharpness and low edge angle. Now Molybdenum of course has it uses, in particular in stainless steels it increases :

increases pitting and crevice corrosion

increases secondary hardening

The pitting resistance is much more improved with Molybdenum (and Nitrogen) than Chromium as shown in the Pitting Resistance Equivalent (PRE) number

PRE = Cr + 3.3 Mo + 16 N Thompson Creek Metals

In regards to hardness, a critical property of steels for knives (and other uses), first considering that the 420 label applies to a large range of steels, Cincinnati Tool Steel which can include up to 0.3 to 0.4% carbon and a range of chromium, carbon in solution is going to vary significantly dependently on exactly what 420 is being considered. Secondly, how it is hardened is critical as well.

As just a general reference, considering the diagram to the right and realizing that the alloy in 420 will increase the hardness slightly beyond the carbon content of the martensite, then the working hardness of this steel will typically be :

54 to 60 HRC

Where the highest hardness comes from the steel which has the highest carbon, lowest chromium and minimal (or no) molybdenum and it hardened from a very hot soak, has a fast quench and an extended quench and has a low temper. Now in general, as 420 class steels often get used on very basic cutlery, they often have the simplest of hardening :

air vs oil quench

room temperature vs cold treatements

This means in a lot of 420 knives the hardness will tend to be towards the lower end of the working range and this explains the common perceptions that such steels are soft and weak.

Being more specific, have a look at the isothermal graphs on the right which show the properties of the steel when austenized at :

1000 C (1832 F)

1100 C (2012 F)

Note the very different expected difference in Carbon and Chromium in solution at the two different temperatures. Taking the 420J2 referenced by Azom for example which can have as high as 0.36% Carbon and 14% Chromium, the martensite would be expected to have :

0.28% C / 12.5% Chromium, with a 1000 C austenization

However it is below the saturation line at 1100 C which would mean all of the Carbon and Chromium would be insolution at that point hence austenization temperatures at likely to only be as high as 1050 C. Note however the very large difference in martensite hardness which can result from the extra carbon insolution and the increase in corrosion resistance as the chromium is in solution as well.

The raw data produced on hardening reflect the values predicted from the phase diagrams as shown in the graph to the right :

The peak hardness of the 420A steel is reached at just above the austenitizing temperature of 1050 °C. -Corrosion and microstructural characterization of martensitic stainless steels submitted to industrial thermal processes for use in surgical tools

Consider the as quenched hardness > 60 HRC for the 420 steel which is in stark contrast to how it is typically used in cutlery where it is typically much softer/weaker. This is a reflection of not the inherent properties of the steel but simply that it is used on inexpensive knives which are not well hardened. Note the same work shows that the corrosion resistance of the 420 steel is signficantly higher than the 440C which would again be inferred fro the phase diagrams which predict a much higher level of Chromium in solution.

In regards to material properties, 420 stainless is a very commonly used steel in industry for many applications and thus it is not difficult to find materials data which compares it for example to other valve steels Properties of High Strength Steels :

1095 : 53 HRC

420 : 51 HRC

716 : 54 HRC (0.37%C, 13.5% Cr, 1% Mo)

301 : 43 HRC

17-7 PH : 46 HRC

In short :

the tensile strength is very similar between 1095, 420 and SS 716

the two martensite stainless steels are tougher than 1095