HOUSTON — Ryan Gassett had already put in a full day, moving heavy boxes and furniture for $15 an hour, when his introductory welding class began at 10 p.m. By the time he arrived at Lone Star College north of Houston, the highway toll collectors at the exit for the school had closed for the night and the campus janitors were mopping bathrooms.

The graveyard-shift course was not his first choice, Mr. Gassett, 19, explained, but “there were no other openings.” So he took what he could get.

In recent decades, welding — like other blue-collar trades that once provided high-school graduates with a reliable route to the middle class — seemed to have about as promising a future as rotary phones. But many of these once-faltering occupations are finding new life in Texas and the Gulf Coast region, where an industrial revival built around the energy boom continues to spawn petrochemical plants and miles of new pipeline despite the plunge in crude oil prices.