Application of the Micro-Tensile Testing to Investigate the Adhesion of Thermal Oxide Scales Grown on AISI 441 Stainless Steel Sheet Oxidised in Air and Water Vapour

Abstract

A micro-tensile testing has been developed to investigate the adhesion behaviour of the oxide scale thermally grown on AISI 441 stainless steel sheet oxidised at 800 °C in different atmospheres - synthetic air and water vapour. In the test, a sample was placed in a tensile testing machine sitting in the chamber of a scanning electron microscope at room temperature. Evolution of the failure of the oxide scale was monitored in function of the imposed strain. It was found that the scale formed on steel oxidised in synthetic air exhibited the drastically lower spallation ratio in function of strain comparing to the scale on steel oxidised in 20 %v/v H2O/N2. For the sample oxidised in water vapour, it was clearly observed that the scale was primarily failed by the crack perpendicular to the tensile loading direction, followed by the spallation due to the compressive stress generated by the Poisson effect. After the test, precipitates rich in Nb, Si, and possibly Ti were observed at the internal interface between scale and steel substrate. For the oxidised samples that the final polishing direction paralleled to the main sample axis, the strain provoking the first spallation of the samples oxidised in synthetic air and 20%H2O/N2 were 6.23 and 3.52 % respectively. The theoretical model was developed in our previous work to quantify the mechanical adhesion energy. These values were 357 and 68 J.m–2 for the steels oxidised in synthetic air and 20%H2O/N2 respectively.