Thermal Stability of Titanium and Niobium Stabilized Stainless Steel

Abstract: This master thesis examined the microstructure after heat treatments in stainless steel, two conventional stainless steel (304 and 316) and two novel high temperature stainless steel. These stainless steels are for the use in high temperature heat exchangers for extended temperature range. The positive characteristics in stainless steel are inversely correlated to grain size, characteristics such as strength and mechanical isotropy. Therefore, the grain size will be investigated after heat treatment at several high temperatures (1020℃, 1130℃ and 1230℃). Further, the study contained an investigation of the oxide layer with X-ray diffraction, since the oxide plays a vital role in protecting the stainless steel from corrosion. The grain size was investigated through the ASTM E112-10 methodology, which measures the amount of grain boundaries per unit length. This is performed three times to obtain an average. This was compared to conventional cross-sectional measurements. This showed a difference in maximum 10 µm, which is deemed adequately representative. ASTM E112-10 method shows that the high temperature stainless steels (321H and 347H) have a smaller grain size up to 1030℃ heat treatment. The method also shows anisotropy in the grain size between the different directions, planar and cross-sectional. This anisotropy was most extreme in the conventional stainless steel 304. The grain size of these stainless steels was also investigated with and without exposure to nitrogen gas during heat treatment. Nitrogen in the matrix acts as a grain refiner, which should mean smaller grains in the samples exposed to nitrogen gas. The results followed the theory, since most of the samples and directions display smaller grains when exposed to nitrogen gas. This is with exception to three samples; 304 planar, 316 cross-sectional and 321 planar. 321H planar and 316 cross-sectional displays small differences, inadequate for any conclusions. 304 planar does however display a large difference. What makes the high temperature steels’ (321H and 347H) more temperature stable are their particles. 321H has precipitation of titanium carbide, TiC, and 347H has precipitation of niobium carbide, NbC. TiC does convert (at least somewhat) into titanium nitride, TiN, even before any exposure to nitrogen gas. The plastic deformation effect on the heat-treated samples is also investigated. The texture change generates larger grain size in all samples except 347H, which exhibits smaller grains. This means that heat exchangers will have larger grains in the deformed parts, except for heat exchangers made from 347H. For the plastically deformed samples of 321H there were no main difference in particles after heat treatment, neither size nor amount. For the NbC particles in 347H an increased precipitation is observed as well as a stagnation of growth after a certain point. X-ray diffraction which was performed on the 316 powder generates a signal. There is no signal at the point where the oxide should be placed on the detector, however there is a clear signal from the stainless steel. Further investigation is needed since most of the beam was blocked, however that was hindered due to the Covid-19 pandemic.