In Situ Observation of Phase Transformations in the Coarse-Grained Heat-Affected Zone of P91 Heat-Resistant Steel During Simulated Welding Process

The solid-state phase transformation in the coarse-grained heat-affected zone of P91 steel under simulated welding thermal cycle has been investigated in situ by confocal scanning laser microscope. It is found that γ-austenite initiates from the triple point of δ-ferrite grain boundaries (GBs), then at ordinary δ-GBs, and eventually from grain interiors. Upon further cooling, martensite laths are observed to nucleate and growth instantaneously, with the growth rate being estimated ranging from 175 to 454 μm/s.