Anisotropic elastic–plastic properties of surface-modified layers of 18CrNiMo7-6 alloy steel after carburizing heat treatment determined by an indentation method

Abstract

This work considers the analytical solution of the contact problem between a conical indenter and a transversely isotropic material, and uses a finite element method to determine the correction coefficient to implement in the analytical solution involving a Vickers indenter. Meanwhile, an indentation characterization method was established to determine the elastic modulus of the transversely isotropic material. By applying the finite element model of indentation to evaluate transversely isotropic materials, the dimensionless functions used to compute the yield strength, strain-hardening exponent, and yield strength ratio could be established based on the sensitivity analysis of the plastic parameters. The anisotropic elastic–plastic properties of surface-modified layers (SMLs) were characterized by treating the SML as several layers of transversely isotropic material. Indentation tests were carried out to study the SMLs at different depths, and their elastic–plastic parameters were obtained using the proposed method. The results indicate that the elastic modulus in the longitudinal direction and the isotropic plane at the same depth are distinct, and there is a small difference between the yield strengths in the longitudinal and vertical directions of the heat-treated 18CrNiMo7-6 alloy steel. However, the yield strength ratios in both directions are generally constant along the depth of the SML. As the depth of the SML increases, the yield strength and hardening index also decrease gradually.

Keywords

• 18CrNiMo7-6 alloy steel
• Surface-modified layer
• Transverse isotropic material
• Elastic–plastic properties
• Dimensionless function
• Indentation characterization