Effects of tool path strategies and thermochemical treatments on the surface roughness of hardened punches for hot stamping

The surfaces of molds and dies need to present a good quality because their roughness profiles are transferred to either cast or formed products. Dies and molds are frequently made of complex surfaces, and consequently, an adequate milling strategy is important to result in a proper workpiece surface. To evaluate the adequacy of a surface to be used as forming tool involves the use of roughness parameters like Sa, Ssk, Sku, Sp and Sv. These parameters influence some surface properties like wear resistance and lubrication capacity. In this work, different tool paths of milling and turning processes were chosen to machine a typical spherical surface used as die (punch) of hot stamping, in order to analyze the surface parameters Sa, Ssk, Sku, Sp and Sv of hardened steel samples. The used milling strategies were circular (upward and downward), radial (upward and downward), parallel contours and spiral (upward). Punches machined by all strategies were submitted to thermochemical treatments, plasma nitriding and nitrocarburizing Tenifer® process. The cited roughness parameters were measured in both moments, before and after the thermochemical treatments. There was similarity between milled and turned results, and thermochemical treatments presented significant influence on measured surface parameters. Machining marks were smoothed by thermochemical treatments which altered surface parameters. Thermochemical treatments effects were affected by the combination of machining marks and micro-burrs from machining processes. All tests resulted in Sku parameter either close or above 3. At 45° position of the workpiece, plasma nitriding tended to decrease roughness Sa, Sp and Sv values and present positive values of parameter Ssk. Nitrocarburizing process tended to increase roughness Sa, Sp and Sv values and present negative values of parameter Ssk, while not treated samples presented positive and negative Ssk values, depending on the machining strategy.