SUS304 (AISI 304 / X5CrNi18-10) — why it cannot be hardened by heat treatment
What actually happens when you heat treat SUS304, why strength in a 304 fastener comes from cold work and ISO 3506 classes, and what we can and cannot do with it.
Grade data
| JIS designation | SUS304 |
|---|---|
| Closest equivalents | AISI 304 / DIN X5CrNi18-10 |
| Carbon (%) | ≤ 0.08 |
| Steel family | Stainless steel |
| Heat-treatment route | Not hardenable by heat treatment |
| Typical surface hardness (HRC) | Not applicable |
| Typical hardness specified in service (HRC) | Not applicable |
| Hardenable by heat treatment | No |
Hardness values are typical industry ranges for the small sections we process, and depend on the part, the section size, the quench and the governing specification. They are not a guarantee and do not replace your drawing or standard.
Reviewed:
What it is
SUS304 is an austenitic stainless steel and the default corrosion-resistant fastener material. Its structure is austenite at room temperature and it stays austenitic through any normal thermal cycle, which is exactly why it cannot be hardened by quenching — there is no transformation to martensite to exploit.
How it is heat treated
There is no hardening route. The heat treatment that exists for this grade is solution annealing, and it makes the material softer, not harder — it dissolves carbides and restores corrosion resistance. Strength in a 304 fastener comes from cold work during forming, which is what the ISO 3506 A2-70 and A2-80 classes describe. Quenching a 304 bolt from red heat does nothing useful.
Plating and hydrogen embrittlement
Zinc plating is not normally applied to stainless and generally should not be — the correct treatment is passivation, which removes free iron and restores the chromium oxide film. Austenitic 304 in the normal condition is outside the population ISO 4042 baking is aimed at, although heavily cold-worked stainless containing strain-induced martensite is not automatically immune.
Not normally indicated
ISO 4042 calls for embrittlement-relief baking on electroplated fasteners at property class 10.9 and above, or at roughly 390 HV (≈ 40 HRC) and above. Many customer specifications set a lower trigger — the drawing decides, not this page.
Hydrogen de-embrittlement bake plannerTypical ISO 898-1 property classes
When to choose it
Choose it when corrosion resistance is the requirement and moderate strength is acceptable — general stainless fasteners for wet, food, marine-adjacent and outdoor service.
When not to choose it
Do not choose it when the drawing needs hardness, wear resistance or high tensile strength; no heat treatment will give you those in this grade. Choose a hardenable stainless such as SUS420J2, or a coated alloy steel, and decide deliberately between corrosion resistance and strength.
Frequently asked questions
Can SUS304 be hardened?+
Not by heat treatment. It can be work-hardened by cold forming, which is where the strength of an A2-70 or A2-80 fastener comes from, but there is no quench-and-temper route. Anyone offering to harden a 304 bolt is describing a different material.
Why did our 304 parts become magnetic after forming?+
Cold work can transform some of the austenite into strain-induced martensite, which is magnetic and harder. It is normal in heavily formed 304 and it is not evidence of the wrong material — but it does change corrosion behaviour locally, and a solution anneal would reverse it along with the strength gain.