Hardness is the number a designer writes when they want a part to resist wear, and the number a machinist watches when a thread keeps tearing. On stainless steel it behaves differently from carbon steel: most of the grades used for formed and turned parts cannot be hardened by heat treatment at all.
How hard is stainless steel?
Annealed austenitic stainless - 304 and 316 - reads roughly 70–95 HRB (about 150–220 HV), and it cannot be hardened by heat treatment; it hardens only by cold work, so a drawn or rolled part is harder than the sheet it came from. Ferritic 430 and free-machining 430F sit in a similar band, around 75–95 HRB. Martensitic grades - 410, 420, 440C - can be hardened by heat treatment, from about 40 HRC for 420 up to 58–60 HRC for 440C, which is why they are used for wear parts rather than for deep drawn housings. The scale matters as much as the number: HRB is used for soft material, HRC for hardened material, and HV or Vickers for thin sheet and case-depth work where a Rockwell indenter is too coarse.
Typical hardness by grade
| Grade | Family | Typical hardness (annealed) | Hardenable by heat treatment? |
|---|---|---|---|
| 304 / 304L | Austenitic | ≈ 70–95 HRB (150–220 HV) | No - cold work only |
| 316 / 316L | Austenitic with Mo | ≈ 70–95 HRB | No - cold work only |
| 430 | Ferritic | ≈ 75–95 HRB | No |
| 430F | Free-machining ferritic | ≈ 75–95 HRB | No |
| 410 | Martensitic | ≈ 80 HRB annealed; 35–45 HRC hardened | Yes |
| 420 | Martensitic | ≈ 40–50 HRC hardened | Yes |
| 440C | Martensitic, high carbon | ≈ 58–60 HRC hardened | Yes |
Values are the usual published ranges and depend on product form, thickness and condition. For anything that matters, the hardness belongs on the mill certificate or on the heat treatment specification rather than in a general note on the drawing.
Why a drawn 304 part is harder than the sheet
Austenitic stainless work-hardens faster than mild steel. Every forming operation raises the hardness of the material that was formed, so a deep drawn housing can be noticeably harder in the wall than in the flange, and a rolled thread is harder than the bar it was rolled from. That is useful - it is why cold-worked stainless makes a decent spring or a wear-resistant thread - and it is also why the same grade that formed cleanly on the first draw can tear on the third, and why machinability drops in steps as the part is worked.
Which scale to specify
- HRB - soft annealed stainless, turned fittings, sheet. The right scale for most of the parts on a sensor assembly.
- HRC - hardened martensitic grades and any hardened wear surface.
- HV (Vickers) - thin sheet, drawn walls and case depth, where a Rockwell indenter would punch through the material or read the anvil underneath.
- Micro-hardness (HV0.1 or similar) - for a wall that is 0.8 mm thick and has a hardness gradient through the bend or the drawn wall.
- Tensile strength instead of hardness - for a formed part, tensile properties and elongation usually predict behaviour better than hardness, and the mill certificate carries both.
When hardness actually matters on a part
Threaded fittings want a hardness that holds a thread without galling, which is why we run free-machining 430F for the oxygen sensor hex nuts and austenitic 304 where the assembly cannot tolerate a magnetic part. Wear parts - a seat, a valve face, a pin - want a martensitic grade with a stated HRC so the wear surface can be verified. Drawn housings rarely need a hardness callout at all; they need elongation, a wall that holds its shape and a surface that does not score the moulding around it. Putting an HRC requirement on a 304 housing that cannot be hardened is one of the more common specification errors we see, and it delays the quotation while everyone works out what the number was meant to describe.
Key point
On 304 and 316 the hardness comes from cold work, not from heat treatment; on 410, 420 and 440C it comes from the furnace. Specify the scale, the test location and the condition, and use tensile and elongation instead of hardness where the part is formed rather than worn.
Frequently asked questions
What is the hardness of 304 stainless steel?
Annealed 304 is typically 70-95 HRB (about 150-220 HV). It cannot be hardened by heat treatment; it hardens by cold work, so sheet and drawn parts read differently.
Is stainless steel harder than mild steel?
Not in the annealed state - a mild steel and an annealed 304 are in a similar hardness band. Stainless steel work-hardens far faster, so a drawn, rolled or thread-rolled part ends up harder than the equivalent mild steel part.
Can 316 stainless steel be hardened?
No. 316 is austenitic and non-hardenable by heat treatment. Cold working raises the hardness, and stress relief will not raise it further; if you need a hardenable stainless, the martensitic grades 410, 420 or 440C are the route.
Why does my stainless thread tear during tapping?
Because the grade that was specified work-hardens quickly. Free-machining 430F machines and threads much more cleanly, which is why sensor hex nuts are usually specified in 430F rather than 304 where the assembly allows it.