Influence of the Coating on the Radiative and Conductive Heat Transfer of 22MnB5 Steel in Hot Stamping

Abstract

In hot stamping of Al-Si coated 22MnB5 steel, the heat transfer coefficient (HTC) during quenching is critical for determining the microstructure and mechanical properties of the formed part. Additionally, the radiative properties elucidate how the surface transforms as the steel is heated before quenching. Knowledge of the surface transformations is paramount for understanding the damage caused by the molten Al-Si coating to ceramic rollers in a production environment. This work investigates the effect of the coating on the HTC during quenching and explores the link between radiative properties and surface state changes, including the melting of the Al-Si coating and oxide layer growth. Experiments were performed using a hydraulic press fitted with cooled dies to study the impact of interfacial pressure, coating weight, and dwell time on the HTC. The HTC increased with interfacial pressure, before saturating between 6 and 10 MPa. Specimens with higher coating weights had lower HTCs, which was corroborated by a higher arithmetic roughness for specimens with higher coating weights. Furnace dwell time did not significantly affect the HTC or the roughness of the specimen. Ex situ reflectance measurements of hot stamped specimens revealed minima and maxima between 200 and 1000 nm, due to thin film interference. Wave optics analysis on the reflectance spectra suggested that the oxide layer grew with dwell time. This was confirmed using high resolution – scanning electron microscopy, wherein the measured oxide layer thicknesses were within 50 nm of the estimated oxide layer thicknesses. Additional samples were heated in a muffle furnace for between three and sixty minutes. Wave optics analysis on the reflectance spectra suggested that the oxide layer grew parabolically, as per Wagner’s law. Microscopy measurements revealed that the interdiffusion layer grew linearly simultaneously with the oxide layer. In situ specular reflectance measurements of specimens during heating were performed using a laser-driven light source. The specular reflectance peaked twice; the first peak was attributed to initial coating liquefaction, and the second peak was attributed to subsequent intermetallic reactions. In situ measurements performed on specimens coated with Thermoboost® and iron nitrate revealed a significantly lower specular reflectance peak and higher heating rates.