Ferritic stainless steel contains 12-17% chromium and no nickel, so it keeps iron’s body-centred cubic structure at every temperature: it is magnetic, cheaper than nickel grades, and cannot be hardened by quenching. The same structure makes it less tough in the cold than austenitic steel, and because it never changes phase on heating, coarse grain from overheating cannot be refined by heat treatment. It is the sensible first stainless for a settlement without nickel.
Part of Stainless Steel
Ferritic stainless steel is the chromium-only family whose iron remains body-centred cubic ferrite from room temperature to melting.
No Nickel Means No Phase Change To Use
In plain carbon steel, heating makes austenite and cooling can turn it into martensite or soft pearlite, which is how Hardening and normalising work. Chromium at the Advanced Materials article’s 12-17% stops that happening. The iron stays ferrite throughout, so there is nothing for a quench to trap and nothing for normalising to refine. Three consequences:
- Magnetic. Body-centred iron is ferromagnetic. The Advanced Materials article gives ferritic as magnetic.
- Not hardenable by heat treatment. Strength comes only from chromium and cold work.
- Grain growth is permanent. Plain steel resets its grain every time it goes through the austenite range. Ferritic stainless does not, so grain that grows at forging or welding temperature stays coarse (see Grain Boundaries). The only cure is cold work followed by a short anneal that recrystallises it, and a long soak undoes the benefit. Coarse grain makes the metal brittle.
The Case For Starting With Ferritic
Stainless Steel Production recommends starting with ferritic grades because they need only chromium, with no nickel supply, and tolerate less precise carbon control. Its 430-type entry gives 16-18% chromium, carbon below 0.08%, magnetic, with uses in kitchen equipment and water tanks. The article’s own caveat still applies: carbon ties up chromium as carbides, as covered in Carbon Control, so low carbon is still the target. “Tolerates” means a ferritic grade is a little more forgiving, not that the limit disappears.
Where It Falls Short
- Cold service. Body-centred steels pass from tough to brittle as temperature falls. Do not use ferritic stainless for parts that must absorb shock in freezing weather. Austenitic Steel does not have this transition.
- Welds. Welding heats a narrow strip hot enough to grow coarse grain that cannot be repaired, leaving a brittle zone. Thin sections are less affected than thick ones.
- Embrittlement while hot. Ferritic grades with higher chromium can become brittle after long periods at around 475 °C and in other intermediate ranges. Do not use them for parts that live in that range.
Worked Example: The Chromium Cost Without Nickel
To make 100 kg of ferritic steel at 17% chromium you need 17 kg of chromium. Ferrochrome carries 60-70% chromium according to the production article, so 17 / 0.70 = 24.3 kg to 17 / 0.60 = 28.3 kg of ferrochrome. At roughly 4,000 kWh per tonne, 24.3 kg (0.0243 t) costs 97 kWh and 28.3 kg costs 113 kWh. That is a lower bound, since it ignores chromium lost to oxidation and the energy to melt the steel. No nickel is needed, which is the reason this is the cheaper route.
Comparing The Two Main Stainless Families
| Property | Ferritic | Austenitic |
|---|---|---|
| Nickel needed | No | About 8% |
| Magnetic | Yes | No |
| Hardens on quenching | No | No |
| Cold toughness | Falls off | Stays tough |
| Weld toughness | Poor | Good |
| Forging | Grain grows when hot | Work-hardens when cool |
Common Mistakes
- Annealing ferritic steel at high temperature for a long time — it softens and also coarsens, and cannot be rescued.
- Expecting it to match 304-type behaviour — it is a different, cheaper, less tough material.
- Heat treating to harden — nothing happens; the martensitic grades are the hardenable ones.
What’s Next
- Learn next: Stainless Steel — the three families side by side
- Combine with: Metallography — seeing whether a sample is coarse-grained