Austenitic stainless steels resist solidification cracking much better if ferrite (δ), not austenite (γ), is the primary solidification phase, but why? The curve of temperature T vs. fraction of solid fS was calculated for 304, which solidifies as primary δ-ferrite, and for 310, which solidifies as primary γ. Back diffusion, being much faster in bcc (δ) than in fcc (γ), flattens the curve much more in 304 than 310. Consistent with quenched 304 mushy-zone microstructure, back diffusion narrows the freezing temperature range, depletes the residual liquid, and lets δ dendrites bond together earlier to better resist cracking. It also reduces the crack susceptibility index |dT/d(fS)1/2| near fS = 1, consistent with the much less cracking observed in 304 than 310.
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