Notch Tensile Properties Prediction of Low-alloy Steel Processed by Different Tempering Temperatures
The microstructure and notch tensile fracture morphologies at different stress concentra-tion factors of the low alloy steel 35CrMo for the head cover bolts of the pump turbine of a storage power station were investigated by electron backscatter diffraction microscopy and scanning electron microsco-py.The effects of tempering temperature on the relationship between tensile properties,damage mecha-nism and mechanical properties of 35CrMo steel were studied.The results show that the microstructures of tempered state at 150～200℃ and quenched state are composed of lath martensite.After tempering at 400℃,the microstructure of tempered troostite is more uniform.The final fracture of the notched speci-mens is a mixture of ductile and brittle fracture.At the two stress concentration factors(Kt=3,5),the notch tensile strength has the same changing trend with the tempering temperature.With the tempering temperature increasing,the notch tensile strength first increases and then decreases.For Kt=3,the high-est tensile strength is 2626 MPa at tempering temperature of 150℃;when Kt=5,the highest tensile strength is 2450 MPa at tempering temperature of 200℃.Notch sensitivity ratio(NSR)is greater than 1,that is,notch strengthening effect occurs after different tempering temperatures,and notch sensitivity tends to decrease with the increase of tempering temperature.With the increase of stress concentration factor,the notch strengthening effect of specimens treated at different tempering temperatures shows an increasing trend first and then a decreasing trend,and the notch strengthening efficiency was the most obvious when tempering at 400℃.Finally,based on the relationship between hardness and notch tensile strength,a fast prediction method of notch tensile strength was proposed,and the prediction error was less than 8％.
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