In this study, a NiCrFeBSi alloy layer with a thickness of about 140 μm was cladded on the AISI4340 steel and then annealed at 700 °C, and the influences of cladding and annealing on tensile and fatigue damage mechanisms of the AISI4340 steel were investigated. The fatigue stress ranges were set from 0 to 0.30–0.75 of the yield strength, and the fatigue deformation and cracking morphologies after different cycles were in-situ compared to reveal the dynamic damage mechanisms. The results reveal that the yield strength of the substrate shows little change after cladding, while the tensile fracture strain and fatigue life decrease significantly due to the high cracking susceptibility of the cladding layer. After the annealing treatment, the yield strength decreases obviously, while the fracture strain and fatigue life recover somewhat. For the original AISI4340 steel specimen, a few fatigue cracks initiate around the stress concentration region at relative high stress level, and the crack initiation cycles are high. After cladding, more fatigue cracks initiate along the cladding track boundaries at a stress far lower than the yield strength, resulted by the residual stress and microdefects, and some of the microcracks propagate into the substrate, makes the crack initiation cycle decreases sharply. For the annealed specimen, fewer fatigue cracks appear and the main crack initiates at the edge of the cladding layer, and the crack initiation and propagation cycles recover. All the fatigue fracture surfaces are composed of a flat area, a radial area and a fibrous area, and the fracture mode transforms from quasi-cleavage to microporous polymerization with crack propagation.

本研究在 AISI4340 钢上熔覆约 140 μm 的 NiCrFeBSi 合金层，然后在 700 °C 下退火，研究熔覆和退火对 AISI4340 钢拉伸和疲劳损伤机制的影响。疲劳应力范围设置为屈服强度的 0 到 0.30-0.75，并原位比较不同循环后的疲劳变形和开裂形态，以揭示动态损伤机制。结果表明，熔覆后基体的屈服强度变化不大，而熔覆层的高裂纹敏感性导致拉伸断裂应变和疲劳寿命显着降低。退火处理后，屈服强度明显下降，而断裂应变和疲劳寿命有所恢复。对于原始AISI4340钢试件，在应力集中区域周围出现少量疲劳裂纹，应力水平相对较高，裂纹萌生周期较高。熔覆后，残余应力和微缺陷导致更多的疲劳裂纹沿熔覆轨道边界以远低于屈服强度的应力萌生，部分微裂纹扩展到基体中，使裂纹萌生周期急剧下降。退火后的试样疲劳裂纹较少，主要裂纹萌生在熔覆层边缘，裂纹萌生和扩展循环恢复。所有疲劳断裂面均由平坦区、径向区和纤维区组成，断裂模式从准解理转变为微孔聚合，裂纹扩展。