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 25 g at 750uC. 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.

Oxidation of a Commercial Nickel-Based Superalloy under Static Loading

“…[27][28][29][30][31][32] The isotopic tracer experiments indicated that shot-peening and applied stresses have very little influence on the inward oxygen diffusion, thus eliminating enhancement due to a greater contribution from inward anionic growth. This suggests that the shot-peening and mechanical loads applied to polished samples act to influence the diffusivity of the oxidized solute species (i.e., chromium and titanium) in the parent and recrystallized alloy, [33][34][35] and this is proposed to result from defects induced from creep deformation. This could explain the increased internal oxidation kinetics realized in the polished specimens under load.…”

Dislocation engineering and its effect on the oxidation behaviour

“…Consequently, a more effective alloy diffusion coefficient is obviously of benefit in rapidly achieving and maintaining a steady growth of protective chromia. In recent studies, shotpeening martensitic steels show an improvement in the oxidation behaviour, but this is not yet sufficient to replace austenitic steels [7,8]. The present study reports an improvement of passification of these steels by 50%.…”

Effect of Shot-peening on the Oxidation Behaviour of Boiler Steels

“…2a shows a small dislocation density near the metal sub surface (area next to the surface). It is known that the shot-peening influences the microstructure by introducing dislocations in the effected area [7]. TEM studies showed the high dislocation density at the sub surface of SPCr-18 as in Fig.…”

“…It has been shown by several authors that cold work and surface finish (e.g. shot blasting) can strongly influence the oxidation behaviour and promotes the formation of protective layers on the surface of the material [4][5][6][7]. The reason can be attributed to the fact that the cold work influences the microstructure near to the surface, i.e., dislocations and grain boundaries are introduced, which can act as easy diffusion paths for Cr supporting the formation of Cr-oxide at the surface.…”