Analysis of microstructural evolution during friction stir welding of ultrahigh-strength steel

Friction stir lap-welded advanced high-strength M190 steel exhibited formation of lath martensite at weld nuggets, a ferrite layer at faying surfaces and a ferrite–pearlite microstructure at the bottom of the stir zone. The phase transformation was governed by severe plastic deformation in the austenitic region followed by cooling. The strain rate and peak temperature played key roles in controlling the prior-austenitic grain size and were correlated to the Zener–Hollomon parameter. An empirical relationship has been obtained for dynamically recrystallized austenite grains.