Martensitic grades are supplied hard or annealed, in bar, strip and sheet; the condition on the order decides what the machinist can do with it.
What Makes a Stainless Grade Martensitic
Three things separate this family from the rest of stainless: it carries enough carbon to respond to heat treatment, it has a body-centred tetragonal lattice after quenching, and it is magnetic. The chromium is there for corrosion resistance, but the carbon is what the designer is buying. Without it the alloy would be soft and formable; with it, the alloy can be hardened, tempered and ground to a working surface.
The practical consequence is that a martensitic part is specified in two states, not one. It arrives annealed and machinable, and it is hardened and tempered afterwards — either at the mill, so the stock comes in the condition the drawing asks for, or in a heat-treat shop after machining. A drawing that names the grade but not the condition leaves the shop guessing, and the two guesses machine very differently.
Surface finish is part of corrosion control on this family: a polished face sheds chlorides that a rough ground face holds.
The Grades in Common Supply
The family is quoted from a short list. What changes across it is the carbon level, the chromium level, and whether nickel or molybdenum is present to buy back some corrosion resistance.
410 — the general-purpose grade
Around 11.5 to 13.5% chromium with carbon capped near 0.15%. It hardens to a useful working hardness, machines well in the annealed condition and is the cheapest way into the family. Its corrosion resistance is the weakest of the group, so it belongs indoors, in oil, or behind a finish — not in washdown or salt air. Fasteners, shafts, pump parts, valve trim and cutlery that is not expected to survive a dishwasher all appear in 410.
420 — the cutlery and tool grade
More carbon, typically 0.15 to 0.40%, with chromium at 12 to 14%. That combination takes it to roughly 50 to 55 HRC after hardening and tempering, which is the range where blades, surgical scissors, mould inserts and wear plates live. It is harder to weld than 410 and less forgiving of a missed temper.
431 — the tough, higher-chromium option
Chromium rises to 15 to 17% with 1.25 to 2.5% nickel, which buys back corrosion resistance and toughness at the cost of some achievable hardness. Shafts that see both stress and moisture, propeller hardware and aircraft fittings are the classic uses. It is the grade to reach for when 410 would rust and 17-4 PH would be over-specified.
440 series — the high-carbon end
440A, 440B and 440C step the carbon from about 0.60% to 1.20% at 16 to 18% chromium, and the hardness follows: 440C reaches roughly 58 to 60 HRC. This is bearing-race, blade, valve-seat and wear-part territory. The trade is toughness and corrosion resistance, both of which fall as carbon rises, and the grade is normally machined annealed and then hardened because a hardened 440C part is ground, not cut.
Chemistry and Mechanical Values
Typical ranges for the grades above, quoted to the ASTM product specifications. Values are for guidance: the purchase order should name the standard and the condition, and the mill certificate should be checked against both.
| Grade | C % | Cr % | Ni % | Mo % | Typical hardness after hardening |
|---|---|---|---|---|---|
| 410 | ≤ 0.15 | 11.5–13.5 | — | — | ~40 HRC maximum |
| 420 | 0.15–0.40 | 12.0–14.0 | — | — | ~50–55 HRC |
| 431 | ≤ 0.20 | 15.0–17.0 | 1.25–2.50 | — | ~45–50 HRC |
| 440C | 0.95–1.20 | 16.0–18.0 | — | ≤ 0.75 | ~58–60 HRC |
Mechanically, the annealed condition is soft enough to machine and the hardened condition is strong enough to replace a low-alloy steel in a wear application:
| Grade | Condition | Tensile strength | Yield strength | Elongation |
|---|---|---|---|---|
| 410 | Annealed | ~480–520 MPa | ~275 MPa | ~20% |
| 410 | Hardened and tempered | ~650–850 MPa | ~450–600 MPa | ~15% |
| 420 | Hardened and tempered | ~700–950 MPa | ~550–750 MPa | ~12% |
| 431 | Hardened and tempered | ~900–1,050 MPa | ~700–800 MPa | ~12% |
Hardening and Tempering
Hardening is three steps and the temper is the fourth: austenitise, quench, then temper to the hardness the part actually needs. Heating to around 1,000 to 1,060°C dissolves the carbides and puts the alloy into its austenitic form; quenching — oil or air depending on section and grade — traps the carbon in a tetragonal lattice and leaves the part hard and brittle. Tempering is what makes it usable: it relieves the internal stress and trades hardness for toughness along a curve the designer should choose deliberately.
Two shop-floor facts follow from that. First, the part moves. Distortion and dimensional change during quench are normal, so a feature held to a tight tolerance is normally ground after hardening rather than finished before it. Second, the annealed state is the machinable state: 440C in the hardened condition is ground, EDM'd or polished, not turned.
Where a drawing needs hardness and a tight tolerance together, the sequence is usually: rough machine annealed, harden and temper, then finish grind. That sequence belongs on the process sheet before the first cut, not after a part comes back oversize.
What It Does Well, and Where It Stops
Strength and hardness. This is the family's reason to exist. A 440C part at 58 HRC resists abrasion that would write off a 304 part in a week, and a 420 blade holds an edge that no austenitic grade can.
Corrosion resistance. Moderate, and lower than the austenitic grades. The chromium is partly consumed by carbides, and the structure is less uniform, so pitting starts sooner in chlorides. Free-machining variants are worse again, because sulphur added for machinability becomes a nucleation site. Passivation after grinding, and a rinse regime where salt accumulates, are not optional extras on this family.
Magnetism. Strongly ferromagnetic, unlike 304. Useful for workholding and sensors, awkward anywhere a non-magnetic grade is assumed.
Weldability. The weak point. The heat-affected zone hardens and can crack, so welded assemblies need preheat, matching filler and a post-weld temper, and pressure-retaining work is normally outside the family's scope. Design the wear part as a separate piece wherever possible.
Where the Family Gets Used
The pattern across industries is the same: a surface that has to resist wear or hold an edge, in an environment that is damp rather than immersed.
- Automotive and off-road: exhaust components in the ferritic grades, but shafts, wear plates and valve parts in the martensitic ones.
- Aerospace: actuator and structural fittings where 431's strength-to-corrosion balance fits; hardened bushings and pins.
- Medical and cutlery: scissors, forceps, needles and blades in 420 and 440, ground and passivated after hardening.
- Food processing: slicer blades, dicing grids, mixer shafts and grinder plates, where a hard edge plus a cleanable finish matter more than maximum corrosion resistance.
- Architecture and consumer goods: cladding, fixings and decorative hardware in 410 and 431, usually with a brushed or polished finish and a maintenance schedule.
Choosing It, or Choosing Something Else
Pick a martensitic grade when the part needs hardness, wear resistance or an edge, and the environment is mild. Pick an austenitic grade when corrosion is the governing failure and the part does not need to be hard. Pick a duplex or a precipitation-hardening grade when both matter and the budget allows it.
Three checks settle most cases. Is the part ground after hardening? If not, the tolerance may be unobtainable. Is it welded? If so, the family is probably the wrong choice. Does it see chlorides? If so, move up to 431 or out of the family entirely.
We supply the family as round bar, sheet and strip, annealed or in a hardened condition, with the mill certificate and the heat number on the paperwork. Where the part is machined here, the machining route covers the rough and finish operations and we can quote the heat treatment with it. If the grade you need is not on the materials page, send the specification and we will confirm what the mills can supply.
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