Effect of boriding on wear and fatigue life of WC-Co die inserts in cold forming

Abstract

Cold forging is a key manufacturing process in fastener production, utilizing multi-stage die systems to plastically deform materials under high compressive stresses. In cold forming operations such as extrusion, reduction, and head forming, WC-Co materials are widely employed in die applications, selected based on the specific requirements of the process. These die materials used in cold forming must exhibit high wear resistance and fatigue strength due to the substantial forming forces that generate significant stresses within the dies. Boriding, a surface treatment method, is applied to enhance these mechanical properties. This study investigates the effects of pack-boriding treatment on the fatigue life and wear properties of WC-Co material containing 19 % Co, which are used as die inserts in cold forging die systems. Three sets of samples were pack-borided using EKABOR-2 boron powder at 1000 °C, 950 °C, and 900 °C for 4 h under each temperature condition to accomplish this process. Following the pack-boriding process, pin-on-plate wear tests were conducted under both non-lubricated and lubricated conditions to evaluate the wear performance of the samples. Three-point bending fatigue tests were performed to assess the fatigue behavior of the borided samples. After the fatigue test, Goodman-Haigh diagrams were obtained from the experimental results to be utilized in predictive die-life calculations. X-ray diffraction (XRD) analysis confirmed the formation of CoB, Co₂B, WB₂ and W₂CoB₂ phases in the borided layer. Results indicate that increasing boriding temperature significantly improves wear resistance and surface hardness, with the highest microhardness (4104 HV₀.₁) and the lowest wear track width (183 µm non-lubricated, 70.76 µm lubricated) measured at 1000 °C. However, the thick and brittle boride layer induced stress concentrations, negatively affecting the material’s fatigue performance. According to the Goodman-Haigh diagrams, the highest fatigue life after boriding was measured as 5,000,000 cycles at 950 °C, while the lowest was 135,356 cycles at 450 MPa after boriding at 1000 °C. Although the increased hardness of the WB₂ phase at 1000 °C enhanced wear resistance, the formation of a brittle boride layer led to stress concentrations, facilitating crack initiation and reducing die life. This indicates that while higher boriding temperatures improve surface hardness, they also increase brittleness, negatively impacting the material’s fatigue performance.