Austenitic Stainless Steel 304: Why Chromium Makes It "Stainless"
Type 304 (UNS S30400, EN 1.4301, GB 06Cr19Ni10) is the most widely used stainless steel, accounting for roughly half of all stainless production. Its corrosion resistance comes from a passive chromium-rich oxide film that self-heals in the presence of oxygen - the principle established by Monypenny (1931) in the first systematic treatise on stainless iron and steel.
304 contains about 18 percent Cr and 8 percent Ni, which stabilise the austenitic (face-centred cubic) structure at room temperature. This gives excellent formability, weldability and toughness down to cryogenic temperatures, plus oxidation resistance to about 870 C. The classic work of Bain and Aborn in the 1930s clarified how chromium (at least about 10.5 percent) and nickel control the austenite/ferrite/martensite balance. Lula (1986) later quantified the strain-induced martensite transformation that can harden 304 during cold work - useful for spun or drawn components but a cause of magnetic response in heavily worked parts.
Key limitations: 304 is prone to sensitisation (chromium carbide precipitation at grain boundaries) if welded or held in the 450-850 C range without low carbon (use 304L) or stabilised (321/347) grades. It is not suitable for chlorides (pitting/crevice corrosion) - switch to 316L (2-3 percent Mo) for marine or chemical duty. JLHSteel supplies 304/304L and 316/316L plate, bar, tube and pipe with EN 10204 3.1 mill certificates.
References
1. Monypenny, J.H.G. (1931). Stainless Iron and Steel. Chapman and Hall.
2. Bain, E.C., Aborn, R.H. (1940). Nature of the alloying elements in stainless steel. American Society for Metals.
3. Lula, R.A. (1986). Twinning and the austenite-to-martensite transformation in 304 stainless steel. JOM 38(8): 26-33.
4. ASM Handbook, Volume 1 (1990). ASM International.
304 contains about 18 percent Cr and 8 percent Ni, which stabilise the austenitic (face-centred cubic) structure at room temperature. This gives excellent formability, weldability and toughness down to cryogenic temperatures, plus oxidation resistance to about 870 C. The classic work of Bain and Aborn in the 1930s clarified how chromium (at least about 10.5 percent) and nickel control the austenite/ferrite/martensite balance. Lula (1986) later quantified the strain-induced martensite transformation that can harden 304 during cold work - useful for spun or drawn components but a cause of magnetic response in heavily worked parts.
Key limitations: 304 is prone to sensitisation (chromium carbide precipitation at grain boundaries) if welded or held in the 450-850 C range without low carbon (use 304L) or stabilised (321/347) grades. It is not suitable for chlorides (pitting/crevice corrosion) - switch to 316L (2-3 percent Mo) for marine or chemical duty. JLHSteel supplies 304/304L and 316/316L plate, bar, tube and pipe with EN 10204 3.1 mill certificates.
References
1. Monypenny, J.H.G. (1931). Stainless Iron and Steel. Chapman and Hall.
2. Bain, E.C., Aborn, R.H. (1940). Nature of the alloying elements in stainless steel. American Society for Metals.
3. Lula, R.A. (1986). Twinning and the austenite-to-martensite transformation in 304 stainless steel. JOM 38(8): 26-33.
4. ASM Handbook, Volume 1 (1990). ASM International.
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