Investigation of thermal residual stress of borided steel: finite-element simulation

Abstract

Although a technologically interesting characteristic of boriding is the production of a hard, wear-resistant layer on the treated steel parts, thermal residual stresses may lead to crack formation in both the iron boride (FeB) layer and the vicinity of the iron boride-diiron boride (Fe2B) interface. In this study, the thermal residual stress distribution in borided tool steel without temperature gradients was investigated systematically by using finite-element analysis. A transient analysis was performed to obtain the stress change with cooling time. The models were cooled down from 823 to 293 K under steady-state conditions. The maximum compressive residual stress is about 690 MPa and tends to decrease with increasing layer thickness. For the case of the double-layer model with 60 mu m thick diiron boride, the tensile residual stress occurs on the surface of the iron boride layer and its value changes within the range of 1870-2025 MPa with the iron boride layer thickness. However, the magnitude of the shear stress at the steel-diiron boride interface is not crucial in the model with a single boride layer. The shear stress at the iron boride-diiron boride interface in the double-layer model increases dramatically with the iron boride layer and exceeds 1050 MPa for a high percentage of iron boride (52%) in the total layer thickness.