The impact of post-treatments on the brittleness and wear resistance of borided 8 % Cr steel

Abstract

The implementation of post-treatments for the enhancement of the mechanical behavior of boride layers has been conducted in recent years. For this reason, the borided 8 % Cr steel underwent austempering-interrupted treatment and austempering-interrupted + cryogenic treatments. Resulting microstructures have been characterized using light optical microscopy, scanning electron microscopy and energy dispersive X-ray spectrometry, and X-ray diffraction. The brittleness grade and dry sliding wear resistance of the borided steel with and without post-treatments were achieved through a Vickers microindentation approach (at various loads of 2 N, 3 N, and 5 N) and the ball-on-flat wear test (at a constant load of 20 N for 100 m). In addition, a non-borided material (hardening + tempering) was tested employing the ball-on-flat wear test to demonstrate the improvement of the boride layers.As expected, the wear resistance of the 8 % Cr steel subjected to conventional boriding was 8 times greater than that of the non-borided material due to the surface hardening achieved through the thermochemical treatment. On the other hand, the austempering-interrupted treatment conducted after conventional boriding resulted in a 1.2 times improvement in wear resistance. This enhancement was attributed to the transformation of the FeB–Fe2B dual-phase layer into a Fe2B single-phase layer, which reduced the brittleness grade of the boride layer from 4 to 3.1. Nonetheless, the Fe2B single-phase layer achieved the highest wear resistance, which was 1.3 times greater, after undergoing austempering-interrupted + cryogenic treatments (resulting in a slight decrease in the material's brittleness grade from 3.1 to 2.9) compared to the borided steel treated with austempering-interrupted alone.Grooving and debris were observed on the entire wear tracks for both non-borided material (ductile) and boride layers (brittle). Additionally, brittle layers exhibited the development of cracks and spalling, while ductile non-borided steel showed plastic deformation.