Question:
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.

Answer:
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).