Effect of tempering temperature on microstructure and mechanical properties of 690 MPa grade engineering machinery steel
A low-carbon Fe-Mn-Mo-Nb-Cu-B 690 MPa grade structural steel for construction machinery was designed and the effects of tempering temperature on microstructure and mechanical properties of the steel were investigated by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM).The results show that tempering temperature has considerable influence on both the yield strength and the tensile strength.The yield strength and the tensile strength decrease first,and increases to the maximum value of 840 MPa,and then decline with the increasing tempering temperature.In comparison with the as-received steel,the yield strength of the steel tempered at 600 ℃ increases by 145 MPa.The changing trend of the yield/tensile ratio and hardness with the tempering temperature is the same with the variation of tensile and yield strength.The change of mechanical properties after tempering is relative to the dispersive precipitates with the size of less than 20 nm,the recovery of dislocations and the decomposition of large M-A islands.
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