SUS440A Stainless Steel: Martensitic Grade Balancing Hardness and Toughness

What SUS440A Is

SUS440A is the lowest-carbon member of the 440 martensitic stainless steel family (AISI 440A / UNS S44002). With 0.60-0.75% carbon and 16.00-18.00% chromium, it sits between the high-hardness 440C grade and the lower-carbon 420 series. That mid position is deliberate: the chromium content provides a useful level of corrosion resistance and supports hardening response, while the relatively low carbon level leaves enough toughness that the quenched and tempered steel does not chip as readily as the higher-carbon members of the family. In the annealed condition the steel is soft enough to be machined and ground into finished tooling; a hardening and tempering treatment then raises it to 55-57 HRC, which is the practical working hardness for most cutlery and light-duty wear parts.

Chemical Composition and Equivalent Grades

Element (wt%)	SUS440A
C	0.60-0.75
Si	1.00 max
Mn	1.00 max
P	0.040 max
S	0.030 max
Cr	16.00-18.00
Mo	0.75 max
Fe	Balance

Equivalent designations in common use are AISI 440A, UNS S44002, GB/T 7Cr17 and the European grade 1.4109 (X65Cr14). Free-cutting ferritic grades such as 1.4104 (X14CrMoS17) are frequently mis-quoted as the European equivalent of 440A; they are a different, lower-carbon family and should not be used as a substitute where a hardenable martensitic grade is required.

Mechanical Properties After Heat Treatment

Because 440A is specified by hardness rather than by strength class, published tensile figures must always be read together with the tempering condition. In the annealed state the steel is relatively soft, with typical tensile strength around 760 MPa, yield strength near 450 MPa and elongation of roughly 20%. Once quenched and tempered to 55-57 HRC, tensile strength rises into the region of 1900-2000 MPa and yield strength approaches 1600-1700 MPa, with ductility falling to a few percent. That combination is exactly what a knife blade, shear edge or bearing component needs, but it also means the component should be designed so that service stresses stay well below the material limits and impacts are distributed rather than concentrated.

Property	Annealed	Quenched and tempered
Hardness	About 22 HRC	55-57 HRC
Tensile strength	About 760 MPa	About 1900-2000 MPa
Yield strength	About 450 MPa	About 1600-1700 MPa
Elongation	About 20%	A few percent
Impact behaviour	Good, readily formed	Limited; edge chipping risk rises with hardness
Typical service window	Machining and forming stage	Cutting edges, wear surfaces, light impact duty

Heat Treatment Practice

The established route is austenitising at 1010-1050 C followed by an oil quench, then tempering at 180-220 C with an air cool. That low-temperature temper produces the 55-57 HRC working hardness and gives the best balance between wear resistance and edge stability. Where higher toughness is more important than maximum hardness, tempering at 300-350 C brings hardness down to about 48-52 HRC in exchange for noticeably better resistance to chipping. Tempering in the intermediate range around 400-550 C should be avoided altogether, because tempering of this martensitic family in that window can reduce toughness sharply. Parts should be protected from decarburisation during austenitising, and hardness should be checked after the temper rather than after the quench.

Applications and Fabrication Notes

Cutting tools: high-grade kitchen knives, scissors, utility blades, shear and punch edges

Wear components: bearing races and balls for low-speed, medium-load duty, pump and valve trim, valve cores for medium-pressure systems

Precision parts: mould cores for general wear duty, textile machine needles, measuring tools and gauges

Fabrication: machine and grind in the annealed state, harden and temper to final hardness, then finish by grinding and polishing

Finishing: a dry polishing sequence with progressively finer abrasive grit followed by fine polishing paste gives a uniform surface and a mirror finish is achievable

Passivation: after grinding and polishing, a passivation treatment restores the chromium oxide film and removes embedded foreign particles from the surface

Welding: not recommended for load-bearing joints on a 0.6-0.75% carbon steel; the martensitic weld zone is hard and crack sensitive

Frequently Asked Questions

Q: What is the best heat treatment for SUS440A?
Austenitise at 1010-1050 C, oil quench, then temper at 180-220 C and air cool. This route reliably delivers 55-57 HRC and the best combination of wear resistance and edge stability. Tempering at 300-350 C trades hardness down to about 48-52 HRC for greater toughness.

Q: How does SUS440A differ from SUS420J2?
SUS440A carries more carbon and chromium, so it hardens to a higher working hardness with better wear resistance and slightly better corrosion resistance. SUS420J2 becomes cost-effective for general wear parts where maximum edge retention is not required.

Q: Is SUS440A suitable for food-contact cutting tools?
Yes, when the tool is properly hardened and then passivated so that a dense, stable oxide film covers the surface. Avoid prolonged soaking in acidic or strongly chlorinated detergents, which attack that film and can lead to pitting.

Q: What should be avoided during tempering?
Tempering in the region of 400-550 C. Martensitic stainless steels of this type can lose toughness in that window, so the practical choice is either a low-temperature temper for hardness or a 300-350 C temper for toughness.

Q: How should SUS440A be polished?
Use a dry sequence of progressively finer abrasive belts, then fine polishing paste. Keep pressure controlled to avoid local overheating, and always clean and passivate the surface after polishing.

Q: Can SUS440A components be welded?
Welding is generally avoided on finished tooling because of the high carbon content and the hard, crack-sensitive martensite that forms in the heat-affected zone. Where a joint is unavoidable, preheating and post-weld heat treatment are required.

Q: How does service life compare with other 440 grades?
In moderate wear duty the grade lasts considerably longer than a 420-class steel. In heavy, high-load wear duty the higher-carbon 440B and 440C grades retain an edge longer, and the part should be moved to those grades instead.