High-Temperature Phase Transformation and Corrosion Behavior of Zn-Ni Coated Press Hardenable Steels

Hot stamping of steel is an innovative process for producing components of higher strength with a significant weight reduction. Conventional hot stamping process is carried out at an elevated temperature in the range of 800 to 950 °C in atmospheric condition. So, a coating is a must to restrict high temperature oxidation and decarburization of the steel substrate. Majorly, Al-Si coating and Zn-based coatings are used for the purpose. However, Al-Si-based coatings are not able to provide sacrificial cathodic protection to steel substrate and the Zn-based coating suffer from the problem of microcracking. In the present study, the pre-Ni coated galvanized boron steels are investigated to understand high temperature phase transformation during austenitization step. Thermodynamic calculations along with Gleeble heat treatment experiments and coating characterizations with SEM, EDS, WDS and XRD shows initial melting of the coating. Subsequent Fe-enrichment in the coating has led to formation of BCC phase which is high melting as well as softer phase compared to Ni-Zn intermetallic. Moreover, the corrosion performance of the prior Ni coated galvanized steel has been evaluated. The presence of zinc in the coating shows sacrificial corrosion protection behavior and the Ni and Fe in the coating have further improved the corrosion rate owing to their barrier protection to the steel.