鍛造焼入処理した機械構造用鋼の焼もどし温度と機械的性質

Abstract

Investigation was made on the effect of tempering temperature on the mechanical properties of carbon steel, Mn steel, Cr steel and Cr-Mo steel which had been forge-quenched. The results obtained were as follows:(1) The temper brittleness of forge-quenched Mn steel, Cr steel and Cr-Mo steel was more clearly observed than normally quenched steels when the specimens were tempered in the temperature range of 300°∼400°C or 500°∼600°C. From the Load-Time Diagrams of Charpy impact tests, it could be deduced that the increase in brittleness of 300°∼400°C tempered specimens was caused by the reduction in the maximum loads (fracture resistance), and that of 500°∼600°C tempered specimens was due to the reduction in time to fracture (deformation to fracture). The increase in temper brittleness was not observed with forge-quenched plain carbon steel.(2) When the specimens either normally quenched or forge-quenched were subsequently tempered at 350°C for 50 hr, there was observed no distinct difference in the structures of carbides and the matrix between these differently quenched specimens of the steels studied. However, when they were subjected to the tempering at 550°C for 50 hr, the carbides of forge-quenched steels were more needle-like than those normally quenched, and more lath-shaped martensites were retained in the matrix.(3) Comparison of impact strengths was made with those steels which had almost the same hardness but quenched and tempered in different ways. As for the specimens harder than Hv 300, the impact value of the normally quenched steel was higher than the forge-quenched or high-temperature quenched steel. As for the specimens of less than Hv 300, the impact strength became lower in the order of the forge-quenched, normally quenched and high-temperature quenched steels.