The Stainless Grades That Can Be Hardened Like Steel

September 21, 2026

Most stainless conversations stay inside the austenitic grades. The martensitic family is the other half of the picture, and it is where you go when a part has to be hard rather than merely strong.

What makes them different

Austenitic grades like 304 and 316L cannot be hardened by heat treatment. They only get harder by being deformed, which is the work hardening problem that makes them slow to cut.

Martensitic grades have enough carbon and low enough nickel to transform when quenched, exactly like a carbon steel. Heat, quench, temper, and the part is hard all the way through. The price is chromium. To keep the carbon available for hardening these grades run leaner on chromium than the austenitic family, so corrosion resistance drops.

They are also magnetic in every condition, which rules them out where a non magnetic part is specified.

The four you will meet

Grade	Carbon	Typical hardened hardness	PREN	What it is for
410	0.15 percent max	35 to 45 HRC	around 12	General purpose, valve parts, fasteners
416	0.15 percent max, sulphur added	35 to 42 HRC	around 13	The free-machining one, shafts and studs
420	0.15 to 0.40 percent	48 to 52 HRC	around 13	Cutlery, moulds, surgical instruments
440C	0.95 to 1.20 percent	58 to 60 HRC	around 17	Bearings, knife edges, wear parts

These are published typical values. Actual hardness depends on the heat treatment the shop or the heat treater runs, and it should be reported.

Note the direction. Carbon rises down the table and so does achievable hardness. Corrosion resistance stays poor throughout, and 440C is the odd one out only because its higher chromium partly offsets its carbon.

Machinability, and the catch

In the annealed state these cut reasonably well. 416 in particular is one of the easier stainless grades in the shop, with a published machinability rating near 85 against free-machining carbon steel B1112 at 100. That is above 303 at 78 and far above 316L at 36.

The catch is that the annealed state is not the delivered state. A 440C bearing race is useless at annealed hardness. So the real sequence is:

1. Machine in the annealed condition, leaving grinding stock on critical surfaces
2. Harden and temper
3. Grind or hard turn the critical features to size

Step 3 is where the money goes and it is the step buyers leave off the drawing. A part quoted as milling and delivered needing cylindrical grinding on two diameters is a requote, not a rounding error. Say which features are finished after hardening and what stock to leave.

There is also distortion. Quenching moves parts, more on thin or asymmetric sections. A long slender shaft in 420 may need straightening between hardening and grinding. Mention the geometry in the RFQ and a shop that runs these grades will plan for it.

Corrosion, honestly

This family is stainless in the sense that it resists rust better than carbon steel. It is not stainless in the sense that 316L is.

• 410 and 416 will rust in a damp environment if they are not kept oiled or coated. 416 is worse because of the sulphide stringers, exactly as 303 is worse than 304.
• 420 in the hardened and tempered condition holds up in mild environments and in body contact for short periods. Surgical instruments in 420 are cleaned and dried between uses for a reason.
• 440C is the best of the four and still well below 316L.

Passivation helps and it is worth specifying. Note that nitric passivation can attack high carbon and free-machining grades, so citric is usually the right call here. The citric against nitric article covers why.

None of these belong in sea water, in continuous chloride service or in a wash down bay. If a part needs both hardness and real corrosion resistance, the answer is usually 17-4 PH at lower hardness, or a hard coating on an austenitic part.

What belongs on the drawing

1. Grade and the hardness you need, as a range, with the test method and where on the part it is measured.
2. Which features are ground after hardening, and how much stock to leave.
3. Whether straightening is acceptable if the part distorts.
4. Passivation, with the standard, and a note to use citric on 416 and 440C.
5. Any coating, and whether a hydrogen bake is needed after plating.

The short version

Pick a martensitic grade when hardness is the requirement and the part stays dry. 416 when it mostly needs to be cheap to cut, 420 for edges, 440C for wear and bearing surfaces. Then put the post hardening operations on the print, because that is the half of the job that decides the price.

The grade table shows these against the austenitic and precipitation hardening grades, and the buyer guide sets out the four families side by side.

Frequently asked questions

Figures in this guide are published specification limits or published typical values for the alloy, not measurements of any part. Machinability is rated against free-machining carbon steel B1112 at 100 and PREN is chromium plus 3.3 times molybdenum plus 16 times nitrogen. The conventions are set out in thereference section and the editorial standards.

Keep reading

• When to Machine 17-4 PH and When to Age It
• Duplex 2205 and the Parts That Actually Need It