Microstructural Characterisation of Austenitic Heat Resistant Sanicro 25 Steel after Steam Oxidation.

Bogdan Rutkowski

doi:10.3390/ma13153382

Abstract

Microstructural and morphological observations of the surface scale on a high Cr and Ni austenitic heat resistant steel oxidised in water vapour at 700 °C are reported. Analysis of microstructure was carried out by analytical techniques of transmission- and scanning electron microscopy. Investigation of M23C6 nucleated at the interface between matrix and the Z-phase precipitates after exposing to high temperature showed semicoherency between M23C6 and the matrix and no coherency with the Z-phase. Plates developed on the oxide scale surface consist of Cr2O3 crystals separated by amorphous SiO2.

Highlights

In high efficiency conventional power stations, less coal combustion takes place as to obtain higher amount of energy in comparison to older generation power plants
Since the cross-section was cut across the largest surface of the plate, according to the geometry shown in Figure 3C, it can be concluded that large surfaces of Cr2 O3 plates are (0001) planes
Presence of glassy SiO2 in oxide scale of austenitic steel was found by Rutkowski et al [24] and later confirmed on other steel grades by Yoo et al [25]

Summary

Introduction

In high efficiency conventional power stations, less coal combustion takes place as to obtain higher amount of energy in comparison to older generation power plants. This is the result of increases in the operating pressure and temperature of steam [1]. At higher temperature, corrosion processes will take place faster or the corrosion mechanism might change, since various oxides have different free energies of formation [3]. This might lead to failure due to a decrease of the tube wall thickness. Mainly high-resolution scanning transmission electron microscopy (HRSTEM), scanning electron microscopy (SEM) with low accelerating voltage and energy dispersive x-ray spectroscopy (EDS) were used for complex structural analysis

Chemical composition of Sanicro

Results showing depth depth of of Lα

As-Received

Conclusions