Abstract: Gradient microstructure and mechanical properties of the surface induction hardened crankpins of actual industrial reciprocating compressor crankshafts made of the 45 steel and 40Cr steel were investigated. The gradient microstructure of 45 steel and 40Cr steel was examined by using optical microscope (OM) as well as scanning electron microscope (SEM), and the hardness, the tensile strength and the fatigue property were tested by using Vickers hardness tester, universal testing machine, instrumented indentation system and resonant testing machine. The results show that the microstructure of the 45 steel crankpin gradually transits from the surface martensite to the inner troostite+granular ferrite, sorbite+small strip ferrite and pearlite+intergranular massive ferrite; consequently, the surface hardness keeps a high level, and with depth increasing the hardness has sharp decrease before continuously slow decrease. The microstructure of the 40Cr steel crankpin gradually transits from the surface martensite to the inner tempered sorbite and sorbite+intergranular massive ferrite; it has a thicker surface layer with high hardness, and subsequently sudden drop occurs followed by slow increase and then slow decrease in the hardness curve with the increase of distance from surface. The tensile strength of crankpin materials progressively decreases with depth increasing, and has nearly linear relationship with Vickers hardness. Combined with stress analysis of crankshaft, the subsurface regions of crankpins affected by induction hardening heat are considered to be at high risk of fatigue failure, and the fatigue lives can be easily obtained based on the intersection of the fatigue life surface and the applied stress plane.

Key words: reciprocating compressor crankshaft, 45 steel, 40Cr steel, induction hardening, microstructure and properties