Why is 25% ferrite enough for a weld, when higher ferrite content is specified for the base metal?
The base metal is specified with a composition that, for a fully annealed and quenched structure, will lead to about 40 to 50% ferrite, essentially the equilibrium structure. This chemistry is found to return rapidly to almost that balance after the thermal cycle that occurs in the HAZ during welding, retaining corrosion resistance and toughness. It is known that the oxygen associated with flux shielding reduces the toughness of the weld metal. Therefore, the compositions of the filler metal for flux-shielded welds have been adjusted to produce the highest austenite that can be accepted while still retaining the benefits of the duplex structure. If there is significant dilution from the base metal, then the weldment will have slightly more ferrite. The 25% ferrite represents the minimum that will be achieved in there is essentially no dilution, as in a capping pass.
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