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本文试验研究了45 Cr 钢淬水,200℃回火态和40 MnB 钢淬水,200℃、500℃回火态的疲劳裂纹在第一、第二扩展阶段的扩展。试验结果表明,当疲劳裂纹处于第一扩展阶段时,两种钢在200℃回火态的疲劳裂纹主要是穿晶扩展,它们的疲劳裂纹扩展速率(da/dN)和门槛值(△K_(th))基本相同。当疲劳裂纹进入第二扩展阶段后,晶界状态对疲劳裂纹扩展的影响明显起来。硼具有韧化200℃回火态组织晶界的作用,致使40MnB 钢的疲劳裂纹不优先沿晶界扩展。而45Cr 钢,疲劳裂纹优先沿高畸变能的晶界扩展,疲劳裂纹沿晶扩展速率加快,所以得到40 MnB 钢的da/dN<45Cr 钢的da/dN 的结果。当40MnB 钢500℃回火态的疲劳裂纹处于第一、第二扩展阶段时,均是穿晶扩展,断口上有明显疲劳条纹,裂纹扩展途径弯曲度大,从而使其在两个扩展阶段的da/dN 均低于两种钢200℃回火态的da/dN,△K_(th)值高于两种钢200℃回火态的△K_(th)值。
In this paper, 45 Cr steel quenching, 200 ℃ tempered and 40 MnB steel quenched, 200 ℃, 500 ℃ tempering state fatigue crack in the first and second expansion stage of the expansion. The results show that when the fatigue crack is in the first stage of expansion, the fatigue cracks of the two steels at 200 ℃ are mainly transgranular expansion, and the fatigue crack growth rate (da / dN) and the threshold value (ΔK_ ( th)) is basically the same. When the fatigue crack enters the second stage of expansion, the influence of the grain boundary state on the fatigue crack growth is obvious. Boron has the effect of toughening the grain boundary of the tempered microstructure at 200 ℃, so that the fatigue crack of 40MnB steel does not preferentially extend along the grain boundary. In 45Cr steel, the fatigue crack propagates preferentially along the grain boundary with high distortion energy and the fatigue crack growth rate increases along with the grain growth rate. Therefore, da / dN of da / dN <45Cr steel of 40 MnB steel is obtained. When the fatigue crack of 40MnB tempering steel at 500 ℃ is in the first and the second extension stages, all of them are transgranular expansion with obvious fatigue streaks on the fracture surface, and the crack propagation path has a large degree of curvature so that in the two expansion stages da / dN are lower than the da / dN of the 200 ℃ tempering state of the two steels, and the △ K th value is higher than the ΔK th value of the 200 ℃ tempering state of the two steels.