Self refining grain behavior of X80 pipeline steel in the heat affected zone was studied systematically
by physical simulation method. The sizes and distribution of the inclusions which promote intra-granular ferrite
nucleation were measured using the automatic image analyzer, and the figures of the intra-granular ferrite which
has self refining grain behavior were observed using transmission electron microscopes with energy dispersive
X-ray spectroscopy, the fracture toughness values of the coarse grain zone in the HAZ were also measured. The
results show that the welding thermal cycle has nearly no effect on the sizes and distribution of the inclusions in
the HAZ. The reasons of self-refining grain in the HAZ are that the small non-metallic inclusions such as MnO、
TiO、MnS、CuS and （Mn，Cu）S in X80 pipeline steel ，which can induce the nucleation of intra-granular ferrite
in the welding process, and sympathetic nucleation of intra-granular ferrite makes the coarse grains refined in the
HAZ. First intra-granular ferrite has large boundary energy and high density dislocation, which induce the
sympathetic nucleation of intra-granular ferrite.
Physical simulation as a tool to understand friction stir processed X80 pipeline steel plate complex microstructures