Effects of thin cladding layer and annealing treatments on mechanical properties of AISI4340 steel

Abstract 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.