| Variation of HAZ strength of a fine-grained ferrite-pearlite steel with peak temperature (T_p) and cooling time (△t) was investigated, Using microstructural evolution model and rule of mixture, HAZ strength was also calculated and compared with measured one. It showed that variation of HAZ tensile and yield strength with T_p was different each other. At △t is 40s, for example, HAZ tensile strength decreased continuously with decrease of T_p to 1050℃ and varied little below it. Yield strength, however, increased again below 1050℃. Calculation of microstructure volume fraction and ferrite grain size showed that even though all HAZ below 1050℃ showed same ferrite and pearlite microstructure, ferrite grain size decreased continuously with decrease of T_p, resulting in increase of yield strength. Therefore, HAZ to base metal yield strength ratio showed minimum at 1050℃. With increase of △t, the minimum value becomes smaller; 0.88 at 20s and 0.80 at 40s, showing softened HAZ in certain welding conditions. Calculated HAZ yield strength showed a good agreement with measured one. |
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