Can 440C stainless be hardened?
Yes. 440C austenitizes at 1010-1065 °C, air or oil quenches, and tempers to 58-60 HRC. Add a cryogenic step for stability and accept that corrosion resistance drops with hardness.
440C (UNS S44004) is the hardest of the standard stainless steels: 0.95-1.20% carbon and 16-18% chromium, a martensitic grade that hardens through like a tool steel. Annealed it sits around 95-100 HRB (about 20 HRC equivalent) and machines reasonably. Hardening is austenitize at 1010-1065 °C (1850-1950 °F), hold 30-60 minutes, then quench in still air, positive-pressure vacuum, or warm oil for heavy sections. As-quenched hardness is 60-62 HRC. Tempering at 150-200 °C (300-400 °F) for 2 hours, twice, gives the working range of 58-60 HRC used for bearings, knives, valve seats and pump parts.
Two additions matter. First, 440C retains a lot of austenite after quenching because of its high carbon and chromium, which makes the part unstable dimensionally and softer than it could be. A cryogenic soak at -75 to -195 °C (-100 to -320 °F) between quench and temper converts most of it; specify it for bearings, gauges and anything with a tight tolerance. Second, tempering above about 425 °C (800 °F) drops hardness quickly and, in the 425-565 °C range, cuts corrosion resistance and toughness (temper embrittlement), so 440C is used either at low temper (58-60 HRC) or, less often, at 600 °C and above for about 40-45 HRC with better toughness.
Corrosion resistance is the trade. Hardened 440C is stainless in the sense that it resists fresh water, mild atmosphere and many cleaning agents, but at 58-60 HRC the carbides tie up chromium and it is well below 304 in chloride resistance and will pit in seawater. Vacuum or protective-atmosphere heat treatment keeps the surface bright; an air furnace leaves scale that must be removed and the surface passivated afterwards. Distortion is modest with an air quench but real: allow 0.05-0.10% size change and grind after hardening. Machining is done annealed, with grinding or hard turning (CBN) after heat treatment for final dimensions.
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What to specify
- Material: 440C per ASTM A276 (bar) or AMS 5630, supplied annealed unless the part is to be made from pre-hardened stock
- Heat treat to 58-60 HRC (or another stated range) per AMS 2759/5, with austenitizing at 1010-1065 °C, quench, cryogenic treatment and double temper
- Cryogenic treatment (-75 °C or colder, 1-2 hours minimum) for bearings, gauges and tight-tolerance parts
- Vacuum or protective-atmosphere heat treatment to keep the surface bright, or state that scale removal and passivation follow
- Which dimensions are finished after hardening (grind allowance of 0.1-0.25 mm / 0.004-0.010" on diameters), and which are as-machined
- Hardness test location on a ground flat, and a certification with the furnace record
- Passivate after finishing per ASTM A967 using a citric or inhibited nitric bath; plain nitric passivation can etch 440C
Pitfalls
- Tempering in the 425-565 °C (800-1050 °F) range: hardness falls, toughness falls and corrosion resistance falls all at once
- Skipping cryogenic treatment on a precision bearing race or gauge, then watching it grow and soften over months as retained austenite transforms
- Expecting 304-level corrosion resistance at 60 HRC; hardened 440C pits in seawater and chloride cleaners
- Finishing to size before hardening; allow for size change and grind afterward
- Air-furnace hardening without protection, leaving heavy scale and decarburization on the surface
- Grinding burns on hardened 440C that re-temper the surface locally; specify a light finish grind with plenty of coolant
Frequently asked questions
- How hard can 440C stainless get?
- 60-62 HRC as quenched, and 58-60 HRC after a low temper at 150-200 °C (300-400 °F). That is the working hardness for bearings, knives and valve parts. Tempering higher softens it but only improves toughness above about 600 °C.
- Does 440C need cryogenic treatment?
- For anything precision, yes. 440C retains significant austenite after quenching; a cold soak between quench and temper converts it, adds a point or two of hardness and stops the part from changing size in service.
- Does hardening 440C reduce its corrosion resistance?
- Yes. Chromium tied up in carbides is not available for the passive film, so hardened 440C is less corrosion resistant than annealed 440C and much less than 304. It is fine in fresh water and mild atmosphere, not in seawater.
