Reverse Analysis for Material Parameters of Hardening Layer on Laser Quenched U71Mn Steel Rail Surface

Nano-indentation tests were conducted on hardening layer and base metal of laser-quenched U71Mn steel rail, and then the elastic modulus and nanohardness were calculated from the load-depth curves. An axisymmetric finite element model for nano-indentation of the hardening layer was established, and then the material parameters for isotropic power-hardening model were determined by the reverse analysis of the load-depth curves on the basis of the isotropic power-hardening model combined with iterative method. The validity of the reverse analysis was verified. The results show that the average elastic modulus and nanohardness of the hardening layer were 220.3, 11.8 GPa, improved by 4.9%, 187.8% those of the base metal, respectively. The elastic modulus and nanohardness distributed relatively homogeneously in the hardening layer and only changed sharply at the boundary. By the reverse analysis, the representative stress of the hardening layer was 3 146.0 MPa, the representative strain was 0.038, the strain hardening index was 0.64, and the nominal yield strength was 498.3 MPa. The stress-strain curves obtained by the reverse analysis and determined by given parameters were highly consistent, indicating the validity of the reverse analysis method.