440A Stainless Steel
Type 440A stainless steel is the lowest-carbon member of the 440 family (0.60–0.75% C, 16–18% Cr), achieving a maximum hardness of 54–56 HRC while retaining the highest corrosion resistance of the three 440-series grades. The lower carbon content compared to 440B and 440C results in somewhat fewer carbides in the hardened microstructure — reducing wear resistance slightly but significantly improving corrosion resistance in moderately aggressive environments.
440A is used where the combination of high hardness, good corrosion resistance, and moderate wear resistance is required — primarily in cutlery, medical instruments, and certain bearing applications. AMS 5631 covers bar and forging stock for aerospace and precision applications where the 440A composition provides a better corrosion resistance-to-hardness balance than 440C.
Product Forms & Size Range of 440A Stainless Steel
440A SS bar and forging stock per AMS 5631 is available in round bar from 0.125″ to 6.000″ and as billet for die forgings, annealed (hardness ≤26 HRC). Bar is the predominant product form as 440A is machined to final geometry and then hardened after machining to target hardness.
HEAT TREATMENT OF 440A STAINLESS STEEL
440A SS is austenitized at 1850–1950°F (1010–1066°C), oil quenched, and tempered at 300–500°F (149–260°C) for maximum hardness of 54–56 HRC. A temper at 600°F reduces hardness to approximately 50–52 HRC with improved toughness and slightly improved corrosion resistance. Cryogenic treatment between quench and temper is recommended for bearing and precision dimensional applications. Annealing at 1550–1650°F with slow furnace cool restores the soft machinable condition.
440C Stainless Steel
Highest-carbon 440-series grade (AMS 5618/5630) — maximum hardness (58–62 HRC) for bearing races and precision cutting tools where wear resistance takes priority.
420 Stainless Steel
Lower-carbon martensitic SS (AMS 5506/5621) — good hardness with better toughness and weldability than 440-series grades.
Certified Material for Critical Service
440A stainless steel is supplied to AMS 5631 (bar and forging stock). Mill certifications document carbon content (0.60–0.75%), full heat chemistry, and annealed mechanical properties. Heat treatment to required hardness is performed after final machining. AMS 5632 covers the free-machining 440F variant with sulfur additions for improved machinability.