Alloying elements and raw material selection in stainless steel production

Where stainless behaviour comes from

On contact with oxygen, chromium forms a dense, adherent chromium oxide film a few nanometres thick on the surface. This film separates the metal from its environment. Scratched or mechanically removed, it rebuilds itself within seconds as long as oxygen is present — and that is what separates stainless steel from painted or coated steel.

For the film to form at all, the chromium threshold has to be exceeded. Below 10.5% the film is not continuous and offers no protection. In more aggressive environments that proportion has to be raised.

The main families

The difference between 304 and 316

Both are austenitic stainless steels. The essential difference is molybdenum: 316 contains roughly 2–3% molybdenum by weight, 304 contains none.

Molybdenum gives resistance against chloride ions breaking through the passive film locally. That is why 316 is markedly superior to 304 in seawater, chloride-bearing chemicals and salt environments. Where there is no chloride, 304 is usually sufficient and more economical.

The PREN value is used to estimate resistance to pitting corrosion roughly: PREN = %Cr + 3.3 × %Mo + 16 × %N. Typical values come out around 18–20 for 304, 24–26 for 316 and 34–38 for duplex 2205; the higher the number, the greater the resistance in a chloride environment.

What each element does

Chromium

The element that builds the passive film. As its proportion rises, general corrosion and oxidation resistance rise.

Nickel

Stabilises the austenite phase; increases ductility, toughness and deep drawability. It is the most volatile component of the stainless steel cost.

Molybdenum

Resistance to pitting and crevice corrosion; strength at high temperature.

Nitrogen

Stabilises austenite, increases strength and pitting resistance; can replace part of the nickel.

Titanium and niobium

Tie up carbon themselves and so prevent chromium carbide formation; used in stabilised grades such as 321 and 347.

Sulphur and selenium

Improve chip breaking (free-machining grades) but lower corrosion resistance.

Sensitisation and welding

If stainless steel is held for a time between 450 and 850 °C, carbon combines with chromium and precipitates chromium carbide at the grain boundaries. The area immediately around the carbide is depleted of chromium and the passive film weakens there. This condition, called sensitisation, leaves the material open to intergranular corrosion and is seen most often in the heat-affected zone of a weld.

There are three remedies:

1. Lower the carbon. In 304L and 316L grades the carbon ceiling is 0.03%. That makes the use of low-carbon ferrochrome mandatory.
2. Stabilise. Carbon is tied up in advance by adding titanium or niobium (321, 347).
3. Solution annealing. The precipitated carbides are redissolved and rapid cooling prevents them re-precipitating.

The raw material side

Stainless steel production is largely scrap-based, but primary raw materials are needed to hit the composition and make up what the scrap lacks:

• Low-carbon ferrochrome: the chromium source, and the basic way of adding chromium without exceeding the carbon limit.
• Pure nickel or nickel pig iron: the nickel source in austenitic grades.
• High-purity chromium metal: used instead of ferrochrome in special grades where carbon must be kept very low.
• Ferromolybdenum: the molybdenum source in 316 and duplex grades.
• Ferrotitanium and ferroniobium: carbon fixing in stabilised grades.
• Ferrosilicon and lime: reduction and slag forming in the AOD process.

Trace elements matter in stainless steel too. Residual elements arriving with the scrap, such as copper, tin and lead, can cause trouble in hot forming; it is common to put upper limits on these in the specification as well.

Writing the specification

• Alongside the grade number, write the lower and upper limits of the critical elements.
• State the carbon ceiling explicitly; the price changes depending on whether you want an L grade.
• If chlorides are present, add the molybdenum and nitrogen content and, where needed, a target PREN value.
• If you will be welding, put the stabilised or low-carbon grade into the specification.
• Ask for a certificate of analysis for every lot and, where needed, EN 10204 3.1.