Microstructure evolution and strength-toughness matching mechanism of Fe-Cr-Ni gradient alloy steel prepared by direct laser deposition

Abstract

A novel 70Cr8Ni2Y alloy steel powder was designed and prepared to meet the requirement for composition design and preparation of gradient alloys with strength-toughness matching the preparation of nuclear power camshafts by direct laser deposition (DLD). A three-layer gradient structure alloy sample with strength-toughness matching was successfully prepared by DLD using 70Cr8Ni2Y, 50Cr6Ni2Y and 12CrNi2Y alloy steel powder. The microstructure evolution and strength-toughness matching mechanism of the gradient sample were studied. The interfaces between the gradient layers were metallurgical bonding and had no defects such as cracks and pores. With the change of the alloy composition of the three gradient layers, the main microstructure of the sample changed from granular bainite to lath bainite, and to martensite in turn. The mechanism of strength-toughness matching was that the changes of the three alloy components made the strength of microstructure increase, and a metallurgical bond formed at the interfaces of the gradient layers. The gradient sample demonstrated strength-toughness matching of high-strength surface and good toughness core through adjusting of the gradient layers and change of microstructure. The high-strength 70Cr8Ni2Y surface layer ensured the high hardness and wear resistance of the gradient sample, and the 50Cr6Ni2Y gradient layer acted as an intermediate layer to support the surface and protect the bottom layer. The 12CrNi2Y gradient layer with good toughness played a role in delaying fracture and hindering crack propagation. The microhardness of the gradient sample showed a gradient distribution of 739-534-321 HV. The average tensile strength was 1076 MPa, the elongation was 1.6%, and the specific wear rate (SWR) of the sample surface was 9.5 × 10−5 mm3/(N·m). This paper can provide a useful reference for the preparation of nuclear power camshaft parts with gradient properties of good toughness of the core and high hardness and wear resistance of the surface by DLD.