Patent ID: 8518195

Claim:
A method of heat treating a low alloy carbon steel composition workpiece to improve its ductility for shaping into an article of manufacture and to provide improved strength in the shaped article, the low alloy carbon steel workpiece having a carbon content and, initially, a ferritic microstructure; the method comprising: heating the steel composition workpiece to a first temperature above its A 3 temperature until the microstructure of the steel composition workpiece is transformed into grains of austenite; cooling the steel composition workpiece to a second temperature below its A 3 temperature for a cooling period to commence precipitation of ferrite at the grain boundaries of the grains of austenite, the second temperature and the duration of the cooling period being determined to retain a major portion of the austenite grains in the microstructure; reheating the steel composition workpiece to a temperature above its A 3 temperature for a re-heating period to re-form new austenite grains from precipitated ferrite at austenite grain boundaries that were not transformed to ferrite; repeating the cooling of the steel composition workpiece below its A 3 temperature and the reheating above its A 3 temperature to obtain a predetermined microstructure of changed and reformed austenite grains; and then, when the steel composition workpiece is at a temperature, either above or below its A 3 temperature; quenching the steel composition workpiece to a quench temperature below its M s temperature and above its M f temperature to commence formation of martensite from the changed and reformed austenite, and to obtain a proportion of retained austenite in the microstructure of the workpiece; and, thereafter quenching the steel composition workpiece to an ambient temperature to prepare the steel composition workpiece for a forming operation.