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Ferrite is important in avoiding hot cracking in during cooling from welding of austenitic stainless steels. ‘Constitution diagrams’ are used to predict ferrite levels from the composition by comparing the effects of austenite and ferrite stabilising elements. The Schaeffler and Delong diagrams are the original methods of predicting the phase balances in austenitic stainless steel welds.
A ‘nickel equivalent’ is calculated for the austenite stabilising elements, and a ‘chromium equivalent’ ferrite stabilising elements. These are used as the axes for the diagrams, which show the compositional equivalent areas where the phases austenite, ferrite, martensite, (and mixtures of these), should be present. Although intended to show the phase balance of weld fillers, these diagrams can also be used to illustrate the phase balance of the ‘parent’ material. There are different diagrams for different alloy systems.
The nickel and chromium equivalents use the formulae.
This gives a diagram that is useful for the austenitic steels, except those with nitrogen additions. The values for typical 304(1.4301) and 316(1.4401) compositions are shown below.
|.||Ni (equiv)||Cr (equiv)|
The diagram, identifying the phase boundaries is shown below.
This refines the Schaeffler diagram by taking account of the strong austenite stabilising tendency of nitrogen. The chromium equivalent is unaffected, but the nickel equivalent is modified to
The diagram, identifying the phase boundaries is shown below. This shows the ferrite levels in bands, both as percentages, based on metallographic determinations and as a ferrite number ‘FN’, based on magnetic determination methods.