The effect of tempering temperature on the impact properties of 30cr13nb0.1 steel is shown in Fig. 1. It can be seen from Fig. 1 that with the increase of tempering temperature, the impact absorbed energy of 30cr13nb0.1 steel first increases and then decreases, which corresponds to the change of hardness. The results of microstructure and carbide extraction analysis show that martensite decomposes and carbon precipitates in the form of cr15.58fe7.42c6 and NBC when tempering temperature increases from 250 ℃ to 350 ℃. At about 350 ℃, the quenched martensite is decomposed into tempered martensite with good toughness and maximum impact energy absorption.

When the tempering temperature is further increased to 450 ℃, the amount of carbides precipitated in the steel increases. Some carbides precipitate from granular to strip and flake and aggregate, which is easy to cause brittleness. Compared with granular carbides, the strip and flake carbides precipitated along the grain boundary will reduce the impact properties of the steel. In addition, Nb forms NBC instead of cementite to strengthen the steel, but the toughness decreases.

The fracture morphology of the impact specimen was analyzed by SEM, and the results are shown in Fig. 2. It can be seen from Fig. 2 that the fracture morphology of impact samples tempered at different temperatures is different. The fracture surface of the impact specimen tempered at 250 ℃ has a small number of dimples and a lot of dense dissociation platforms, which belongs to brittle fracture; The size of dimples of the steel tempered at 350 ℃ and 450 ℃ is similar to that of the steel tempered at 450 ℃, but the number of dimples is more than that of the steel tempered at 450 ℃. The number of dimples is large and dispersed, and some dimples change from small to shallow to large and deep with dark color. Dimple is a typical characteristic of micro pore aggregation and fracture. The size of micro pore is related to the size and density of second phase particles and the plastic deformation property of matrix. Fig. 2 shows that with the increase of tempering temperature, the fracture of 30cr13nb0.1 steel quenched at 1020 ℃ transits from dissociation fracture to ductile fracture, and then develops into mixed fracture.