Effect of hot isostatic pressing on the microstructure and mechanical properties of vitreous enamel coatings on low carbon steel

A commercially produced vitreous enamel coating applied to low carbon steel was additionally treated in a hot isostatic press (HIP), and investigated in terms of the evolution of the microstructure and mechanical properties before and after the treatment. The coatings were analyzed using OM, SEM equipped with EDS and XRD, and tested for the evaluation of their mechanical properties using a micro-hardness tester and an erosion tester. According to the analysis results, a fair number of bubbles sized to 70 μm were found in the as-received coatings. After the treatment at 700 °C and 150 MPa for 2 h, bubbles in the coating have almost disappeared, resulting in the reduced thickness of the coating by about 15%. After the treatment the hardness of the coating increased by 37%, and the rate of erosion reduced by half. These improved mechanical properties were mostly attributed to the densification and homogenization of the coating. XRD patterns confirmed such a microstructural evolution of the coating. The results for the HIPed coating at 727 °C and 150 MPa for 2 h were not differentiated from those of the HIPed at 700 °C, but several vertical cracks through the whole coating thickness were observed, which possibly evolved out of the stress induced by the phase transformation of the steel substrate, and of the thermal mismatch due to the change in the mechanical properties of the coating itself by HIPing.