Austenitic stainless steel is non-magnetic and stays tough in the cold because enough nickel, about 8% in the 18/8 grade with 18% chromium, holds iron in its face-centred cubic austenite form all the way down to room temperature. It cannot be hardened by quenching, but it hardens quickly when hammered or bent, and it is prone to sensitisation if its carbon rises above 0.08% or if heat lingers in the grain boundaries. A magnet separates it from ferritic and martensitic grades, though heavy cold work can make it faintly magnetic.
Part of Stainless Steel
Austenitic stainless steel is the chromium-nickel family whose iron stays in the face-centred cubic austenite structure at room temperature, which gives it ductility, toughness and non-magnetism.
Nickel Keeps Iron In Its Hot Structure
Ordinary steel is austenite only when hot, above roughly 725-850 °C according to Heat Treatment, and reverts to body-centred ferrite and pearlite on cooling (see Crystal Structure). Nickel stabilises the face-centred form, so with about 8% nickel the austenite never reverts. Manganese and nitrogen do the same job to a lesser degree. Three properties follow directly from that structure.
- Non-magnetic. Face-centred iron is not ferromagnetic. The Advanced Materials article lists the 18/8 grade as non-magnetic.
- Tough at low temperature. The tough-to-brittle transition that affects body-centred steels does not appear, so austenitic steel stays tough where Ferritic Steel becomes brittle.
- Not hardenable by quenching. There is no phase change on cooling for the quench to trap. Stainless Steel Production says it plainly: do not quench austenitic stainless, because it is already soft.
It Hardens Under The Hammer Instead
The same face-centred structure, with many slip planes, lets the steel deform a long way, and each bend tangles dislocations and makes the next bend harder. The production article notes stainless is tougher than carbon steel and needs more hammer blows, hot forged at 1,100-1,250 °C and never below 900 °C. Cold work raises strength a lot; a part bent back and forth will crack. In machining, a dull tool or a light, hesitant cut hardens the surface it is meant to remove, so use a sharp tool and a steady cut.
Sensitisation: The Carbon Limit Has A Reason
Carbide forms on the grain boundaries when austenitic steel spends time at roughly 450-850 °C, and that range is where welding heat lingers. The chromium at the boundary is pulled into the carbide, the strip beside it falls below the 10.5% needed for the film, and the steel corrodes along the boundaries. See Grain Boundaries.
That is why the production article sets carbon below 0.08% and calls above 0.12% the cause. The Carbon Control article shows the difficulty: carbon-reduced ferrochrome carries several percent carbon, so a 100 kg melt given 26 kg of 6-8% C ferrochrome ends at roughly 1.6-2.1% carbon, which is 1.6 / 0.08 = 20 to 2.1 / 0.08 = 26 times the limit. The low-carbon alloy is the hard part, not the chromium or nickel.
Remedies, from standard practice:
- Start from low-carbon material (recycled stainless scrap, not new ferrochrome).
- Weld with low heat input and let the joint cool quickly.
- For a part that has already been sensitised, solution anneal: heat well above the carbide range and cool fast, which dissolves the carbides and does not harden the steel. Slow cooling through the range makes the problem worse.
- Industrial grades add titanium or niobium, which bind carbon before chromium does. A settlement cannot count on these.
Telling Austenitic From Other Stainless In The Field
| Test | Austenitic 18/8 | Ferritic | Martensitic |
|---|---|---|---|
| Magnet | Not attracted (weak if cold worked) | Attracted | Attracted |
| Hardens on quench | No | No | Yes |
| Cold toughness | Good | Poor | Poor |
A bent or stamped part of austenitic steel may show weak attraction at the bend, because severe deformation converts a little austenite to martensite. A true non-attraction at an undeformed area confirms austenitic. A cold-worked sample does not prove ferritic.
Worked Example: How Far Over The Limit
The Carbon Control article’s case is 26 kg of ferrochrome carrying 6-8% carbon. The carbon it brings is 26 × 0.06 = 1.56 kg to 26 × 0.08 = 2.08 kg. Expressed per 100 kg of melt, as that article does, this is 1.6% to 2.1%. If the 26 kg is instead added to a 100 kg melt, the total is 126 kg, and the same carbon is 1.56 / 126 = 1.2% to 2.08 / 126 = 1.7%. Against the 0.08% limit, the first reading is 1.6 / 0.08 = 20 to 2.1 / 0.08 = 26 times too high, and the second is 1.2 / 0.08 = 15 to 1.7 / 0.08 = 21 times too high. Either way the carbon comes from the alloy, not from the base steel, and it is more than ten times the limit.
Common Mistakes
- Quenching for hardness — it produces nothing and may leave the part sensitised.
- Forging cold — stainless cracks if finished below 900 °C.
- Trusting the magnet after cold work — a bent austenitic part can mislead you.
What’s Next
- Learn next: Ferritic Steel — the nickel-free alternative and its trade-offs
- Combine with: Passivation Treatment — restoring the surface film after working