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Any reduction in alloy content of 304 / 316 type austenitics is likely to cause the formation of matensite on cooling. This could lead to cracking problems and poor ductility. To avoid this problem an overalloyed filler metal is used, such as a 309, which should still form austenite on cooling providing dilution is not excessive.
The Shaeffler diagram can be used to determine the type of microstructure that can be expected when a filler metal and parent metal of differing compositions are mixed together in a weld.
The Shaeffler Diagram
Nickel Equivalent = %Ni + 30%C + 0.5%Mn
Chrome Equivalent = %Cr + Mo + 1.5%Si + 0.5%Nb
Example, a typical 304L = 18.2%Cr, 10.1%Ni, 1.2%Mn, 0.4%Si, 0.02%C
Ni Equiv = 10.1 + 30 x 0.02 + 0.5 x 1.2 = 11.3
Cr Equiv = 18.2 + 0 + 1.5 x 0.4 + 0 = 18.8
A typical 309L welding consumable Ni Equiv = 14.35, Cr Equiv = 24.9
The main disadvantage with this diagram is that it does not represent Nitrogen, which is a very strong Austenite former.
The ferrite number uses magnetic attraction as a means of measuring the proportion of delta ferrite present. The ferrite number is plotted on a modified Shaeffler diagram, the Delong Diagram. The Chrome and Nickel equivalent is the same as that used for the Shaeffler diagram, except that the Nickel equivalent includes the addition of 30 times the Nitrogen content.
If the welds molten pool spans two different metals the process becomes more complicated. First plot both parent metals on the shaeffler diagram and connect them with a line. If both parent metals are diluted by the same amount, plot a false point B on the diagram midway between them. (Point B represents the microstructure of the weld if no filler metal was applied.)
However the presence of martensite is unlikely to cause hydrogen cracking, as any hydrogen evolved during the welding process will be absorbed by the austenitic filler metal.
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تاريخ التسجيل : 07/09/2007
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