How the Austenite Ferrite Balance Achieved

To understand how duplex stainless steel works, first compare the composition of two familiar steel austenitic 304 (1.4301) and ferritic 430 (1.4016).

The important elements in stainless steel can be classified into ferritisers and austenitisers. Each element favours one structure or the other: 

Ferritisers – Cr (chromium), Si (silicon), Mo (molybdenum), W (tungsten), Ti (titanium), Nb (niobium) 

Austenitisers – C (carbon), Ni (nickel), Mn (manganese), N (nitrogen), Cu (copper) 

Grade 430 has a predominance of ferritisers and so is ferritic in structure. Grade 304 becomes austenitic mainly through the use of about 8% nickel. To arrive at a duplex structure with about 50% of each phase, there has to be a balance between the austenitisers and the ferritisers. This explains why the nickel content of duplex steels is generally lower than for austenitics. 

Here are some typical compositions of duplex stainless steel:

In some of the recently developed grades, nitrogen and manganese are used together to bring the nickel content to very low levels. This has a beneficial effect on price stability. 

At present, we are still very much in the development phase of duplex stainless steel. Therefore, each mill is promoting its own particular brand. It is generally agreed that there are too many grades. However, this is likely to continue until the “winners” emerge.

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