I have a question on vacuum hardening 440C stainless steel. Just combing the literature, I see references to 1700°F (925°C) and 1900°F (1040°C) austenitizing as well as low tempering 350°F (175°C) and 900°F (315°C) for secondary hardening. There are also references to the need to freeze for retained austenite (RA). The old heat treat I worked at did not do much if any 440C, so I have no real hands-on experience to draw upon. How would you process it?
The parts are essentially an integral bearing race, and I think we need 60 HRC (minimum). Parts are small, so large sections are not an issue.
Standard vacuum processes used in the bearing industry for 440C typically yields hardness values in the 58-62 HRC range. This involves preheating at 1440°F (780°C) for 45-60 minutes and then ramping to 1940°F (1060°C), soak for 45-60 minutes and gas (nitrogen) quench – basically backfilling with nitrogen to -5 inches Hg and fan cooling. This is followed by a deep freeze at -120°F (-84°C) for two hours minimum (to help minimize retained austenite), followed by a temper at 350°F (175°C) for 2-3 hours. For section thicknesses over 0.25 inches (6.4 mm), an oil quench should be used.
AMS 2759/5 calls out an austenitizing temperature of 1925°F (1050°C) with gas or oil quench allowed. They call out various tempering temperatures with corresponding hardness ranges. AMS 2759/5 also requires a double temper. Note: retained austenite levels (typically <7% requirement) and dimensional stability requirements (typically <0.00010 in./in.) are not normally achieved with a single temper.
The result is an average hardness in the 59-61 HRC. Gas quenching helps with distortion (on thin-walled rings).