Influence of heat treatment processes on the stress-rupture properties of stainless steels

This thesis work was carried out in collaboration and with the courtesy of the Vicenza plant of Acciaierie Valbruna S.p.A., and discusses the influence of heat treatment processes on the stress-rupture properties of two different stainless steel grades employed in the power generation market, with the aim of tracing the materials’ behaviour in different treatment conditions. In particular, the A-286 austenitic precipitation hardenable stainless steel (Valbruna grade AN5) and the grade 422 martensitic stainless steel (Valbruna grade VAL2W) were analysed. In order to evaluate the effect of thermal processing on the high temperature load-bearing characteristics of the two materials, multiple stress-rupture tests were performed on different specimens, from which the different rupture times and elongations were collected and compared. In addition to the stress-rupture tests, tensile tests, metallographic and chemical analyses were also performed. All the samples tested were machined from several round-section bars provided by the company, that underwent different thermal treatments, depending on the analysis to be performed. As of that, it was analysed how different solid solution and aging temperatures (for the AN5), and different quenching and tempering temperatures (for the VAL2W) affected the materials’ behaviour under a constant load at high temperatures. Concerning the AN5, an increase in the solid solution temperature resulted in a slight increase in the rupture times, mainly due to better homogenization and slightly bigger grains. Instead, by varying the aging temperature, the material suffered a decrease in strength in the 720-730°C range, due to the instability of the γ’-phase that at higher temperatures and for long treating times, dissolves and precipitate in the non-coherent η-phase. Regarding the VAL2W, an higher quenching temperature helped reducing segregated δ-ferrite bands and improved carbides distribution, resulting in better room-temperature and high-temperature strength. Varying the tempering temperature, in contrast, did not influence the material’s behaviour by much, with a minor decrease in strength at higher tempering temperatures.