Effect of shot peening on high temperature oxidation behaviour of boiler steel: Experimental results and simulation

Abstract

A prediction of the service life of boiler steels in aggressive atmospheres requires a full understanding of the degradation mechanisms of the material due to high temperature oxidation. The developed model is a useful tool to simulate such degradation processes under complex conditions and hence, to contribute to new mechanism based life prediction methods. In the present study the effect of shot peening on the oxidation behaviour is studied and given importance to model the behaviour and simulate the effect. Thermogravimetric measurements were carried out by using a microbalance with a resolution of 10^–5 g at 750°C. The scale morphology was examined by using scanning electron microscopy (SEM) in combination with energy dispersive X-ray (EDX) and X-ray diffraction (XRD). The oxidation of the alloys was modelled by a mechanism based computer simulation that has been developed in order to predict the corrosion rate of alloys depending on the chemical composition and microstructure. The computer simulation is based on the numerical Crank–Nicholson solution for the diffusion differential equation in combination with the powerful thermodynamic subroutine ChemApp. The program deals with the grain boundary diffusion and bulk diffusion in a different way and calculates the individual localised equilibrium state using ChemApp with the help of parallel processing units. Considering the effect of the grain size of the substrate, the developed model gives the distribution of alloying elements, oxide phases, the oxidation kinetics and the concentration of penetrating oxygen in a two-dimensional manner. The numerical description of oxide formation is in agreement with experimental observations.