Abnormal Grain Growth in the Heat Affected Zone of Friction Stir Welded Joint of 32Mn-7Cr-1Mo-0.3N Steel during Post-Weld Heat Treatment

Abstract

The abnormal grain growth in the heat affected zone of the friction stir welded joint of 32Mn-7Cr-1Mo-0.3N steel after post-weld heat treatment was confirmed by physical simulation experiments. The microstructural stability of the heat affected zone can be weakened by the welding thermal cycle. It was speculated to be due to the variation of the non-equilibrium segregation state of solute atoms at the grain boundaries. In addition, the pressure stress in the welding process can promote abnormal grain growth in the post-weld heat treatment.

Relationship between welding conditions, abnormal grain growth and mechanical performance in friction-stir welded 6061-T6 aluminum alloy

“…This undesirable phenomenon occurs during post-weld heat treatment and involves catastrophic growth of a few grains which finally consume almost entire weld zone. It has been observed in various aluminum alloys including 1 xxx [3][4][5][6], 2 xxx [7][8][9][10][11][12][13][14][15][16], 5 xxx [17][18][19][20][21][22][23][24][25][26][27][28][29], 6 xxx [30][31][32][33], and 7 xxx [11,[34][35][36][37][38][39][40][41] series, as well as in magnesium alloys [42][43][44][45], some steels [46][47][48][49], and even metal-matrix composites [50,…”

“…According to this theory, the abnormal annealing behavior is attributed to a combined effect of drastic grain refinement and dissolution of second-phase particles both occurring during FSW. Alternative explanations for the catastrophic grain growth include inhomogeneous microstructure distribution within the stir zone [20,31,40,48], precipitation of grain-boundary segregations [47,49], and increased mobility of particular grain-boundaries [6,44].…”