420 and 440 are both martensitic stainless grades, which means both harden through heat treatment rather than staying soft like 304 or 316. Past that similarity, the two grades diverge on one variable — carbon content — and that variable decides hardness ceiling, wear resistance, and corrosion resistance all at once.
Composition Is Where the Difference Starts
420 runs 12 to 14 percent chromium and 0.15 to 0.4 percent carbon. The 440 series runs higher chromium, 16 to 18 percent, and carries substantially more carbon: 440A at 0.60 to 0.75 percent, 440B at 0.75 to 0.95 percent, and 440C at 0.95 to 1.20 percent, as supplied in bar stock.
That carbon jump is the whole story. More carbon means more room for hard carbides to form during heat treatment, which raises the achievable hardness ceiling. It also means more chromium ends up locked into those carbides instead of staying in solution to protect the surface from corrosion — the tradeoff runs in both directions at once.
Hardness Ceiling: 440C Wins, But Not by as Much as the Marketing Suggests
Properly heat-treated 420 typically hardens into the low-to-mid 50s on the Rockwell C scale, with some sources citing a ceiling as high as 55-56 HRC depending on the exact heat treatment and chemistry within the grade's range. That's already a respectable hardness for a lot of applications.
The 440 series goes further because of its higher carbon: 440A typically tops out around 55 HRC, 440B reaches into the upper 50s, and 440C — the highest-carbon member of the family — can reach 58 to 60 HRC, occasionally slightly higher under optimized heat treatment. 440C's edge over 420 is real, but it's a few points of hardness, not a different class of material.
Corrosion Resistance Runs the Opposite Direction
This is the part that's easy to miss if you're only comparing hardness numbers. Higher carbon content in the 440 series ties up more of the alloy's chromium in carbides, which means less free chromium is available to form the passive oxide layer that gives stainless steel its corrosion resistance. 420, with its lower carbon and correspondingly higher free-chromium fraction, holds up better against rust and general corrosion than 440 does — a direct consequence of the same chemistry that gives 440 its hardness advantage.
Neither grade is a marine-duty stainless. Both are martensitic grades chosen for hardness and edge retention, not for the corrosion resistance that 304 or 316 austenitic grades provide. But between the two, 420 is the more corrosion-tolerant choice, and that's worth weighing against 440C's hardness edge depending on the application.
Manufacturing Process Also Affects the Real-World Result
Heat treatment isn't the only lever on final performance. A 2025 peer-reviewed study on spray-formed 440C found that the spray-forming process produces a finer, more evenly distributed carbide structure than conventional casting, which translated into improved wear resistance in the finished material. The takeaway isn't that spray forming is required — most 440C in circulation is conventionally processed and performs well — but that carbide distribution, not just carbon content on a spec sheet, plays a real role in how a given batch of material actually wears in service.
Which Grade Fits Which Job
420 is the more forgiving choice when the part sees some moisture or mild corrosive exposure alongside a need for moderate hardness — cutlery, surgical instruments, and mold components that don't need 440C's hardness ceiling all commonly use 420.
440C earns its place when wear resistance and edge retention matter more than corrosion resistance — bearing races, high-end knife blades, and precision instrument components where a harder surface directly extends service life, and the operating environment doesn't put heavy corrosive demand on the part.
Profab Machine machines martensitic stainless components in both 420 and 440 grades to the hardness and finish specifications a given application calls for. If you're not sure which grade fits your part's actual service conditions, send the drawing and application details for a direct review.
