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 f S 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(f S ) 1/2 | near f S = 1, consistent with the much less cracking observed in 304 than 310.