It can be seen from the figure that with the increase of tempering temperature, the hardness of 30cr13nb0.1 steel first decreases and then increases. Combined with SEM analysis, it can be seen that the change of hardness is closely related to the formation of carbides. When the tempering temperature increases from 250 ℃ to 350 ℃, the hardness of the steel decreases obviously, but the hardness of the steel tempered at 350 ～ 450 ℃ increases on the contrary, showing a secondary hardening effect, which may be related to the formation of cr15.58-fe7.42c6 carbide and the increase of NBC carbide content.
When the diffraction peak of cr15.58fe7.42c6 carbide increases, the carbon content in the matrix decreases and the hardness decreases; when the content of cr15.58-fe7.42c6 carbide decreases, the hardness increases. This aspect is worthy of further study. The hardness of 30cr13nb0.1 steel decreased from 50.5 HRC to 47.0 HRC when tempering temperature increased from 250 ℃ to 350 ℃. As the quenched martensite is supersaturated solid solution, with the increase of tempering temperature, the carbon atom desolvation is enhanced, and the solid solution strengthening effect is greatly weakened, so the hardness decreases sharply.
However, the hardness of the steel tempered at 450 ℃ rises to 53.0 HRC, which is closely related to the increase of transformation from retained austenite to tempered martensite and the formation of highly dispersed NBC by Nb in a specific temperature range.