Method for heat treating structural members

A method for induction heat treating and quenching of a structural member of carbon steel to a uniform martensitic structure, said structural member having a web and at least one flange substantially perpendicular thereto consisting of

• induction preheating the structural member to a temperature in the range of 900.degree. to 1400.degree. F. by means of at least one coil utilizing the frequency in the range of 180 to 3000 cycles;
• continuting in a separate induction heating stage to heat the structural member to the austenitizing temperature in the range of 1450.degree. to 1800.degree. F. by the use of at least one coil having a higher frequency than that of the prior step;
• liquid quenching the structural member with a multiplicity of nozzles immediately, said nozzles being disposed at an angle in the direction of the advance of the structural member and the pressure regulated from the bottom of the quench zone liquid quench within the range of 10 to 20 pounds per square inch, from the top of the quench zone liquid quench in the range of 1.5 to 6 pounds per square inch and from the sides of the quench zone liquid quench in the range of 5 to 25 pounds per square inch of the structural member to minimize distortion thereof;
• concurrently restraining the member by its web in a series of segmented restraining rolls placed on the bottom and the top thereof so as to present slots through which the liquid may be sprayed to quench the structural member until the structural member has reached a temperature below 1000.degree. F. to minimize localized bow distortion;
• induction heating the structural member to a temperatering temperature in the range of 750.degree. to 1250.degree. F. by the use of a coil having a frequency in the range of 180 to 3000 cycles;
• and
• subsequently restraining the structural member while cooling to avoid camber distortion by passing the structural member through web restraining and tapered roll flange forming rolls, followed by tapered vertical restraint rolls, and finally flange surface engaging rolls to minimize cambering distortion.