A full worked heat-treatment process for an AISI 4140 gearbox shaft — steel grade selection for hardenability, a TTT-diagram-based oil quench schedule, a tempering calculation to a 32–38 HRC target, and a hardness verification plan.
The project brief: an industrial gearbox manufacturer needs a complete heat-treatment process specified for a 1.25 in diameter output shaft that must reach 32–38 HRC through-hardened, with a tempered-martensite microstructure that balances strength against the toughness the shaft needs under repeated torque reversals. Every decision in the process — steel grade, austenitizing schedule, quench medium, temper temperature, and cooling method — depends on the one before it.
This module walks the whole process the way a working metallurgical engineer would: why AISI 4140's hardenability (not raw hardness potential) makes an oil quench sufficient, how the TTT diagram's pearlite/bainite nose governs whether that quench actually works, a worked tempering calculation that lands hardness inside the target band with margin, and a verification plan built on Jominy testing, core hardness, and metallography rather than a single surface reading. The full worked numbers, complete process parameter table, and finished design reasoning are part of the unlocked module below.