Evaluation of Wear Resistance of AISI L6 and 5140 Steels after Surface Hardening with Boron and Copper

Abstract

(1) Background: Boriding is one of the most common methods of thermal-chemical treatment due to its excellent hardness and wear resistance of the produced diffusion layers. However, it has limited application compared to carburizing and nitriding because of fragility and chipping. Introducing another alloying element into the boron media helps avoid those drawbacks and improve other surface properties of the layer. The purpose of this work is to improve the surface mechanical properties of L6 and 5140 low alloy steels by two-component surface hardening with boron and copper. (2) Methods: The treatment was performed by means of a powder-pack method using boron, copper, and aluminum powders in the following proportions: 60% B4C + 20% Al2O3 + 16% CuO + 4% NaF. The time–temperature parameters of the treatment were four hours exposure at 950 °C. Microstructure, elemental, and phase composition were investigated as well as microhardness and wear resistance of the obtained layers. (3) Results: Layers of up to 180–200 μm thick are formed on both steels as a result of treatment. Needle-like structures similar to pure boriding was obtained. The maximum microhardness was 2000 HV on L6 steel and 1800 HV on 5140 steel. These values correspond to iron borides and were confirmed by XRD analysis revealing FeB, Fe2B, and Cr5B3. The wear resistance of both steels was about ten times higher after the treatment compared to non-treated samples. (4) Conclusions: Surface hardening with boron and copper significantly improves the mechanical properties of both alloy steels. The results obtained are beneficial for different tribo-pair systems or three-body wear with abrasion and minimum impact loads.