The effect of grain boundary engineering on the properties of incoloy 800H/HT

Abstract

The following document details an investigation into grain boundary engineering (GBE) and its applicability to a high nickel stainless steel known as alloy 800H, which is used for high temperature piping in industrial applications such as methanol reformers. The grain boundary engineering process comprises several repeated cycles of deformation and intermediate annealing, with the aim of increasing the relative fraction of "special" grain boundaries described generally by their lattice misorientation. In the current work, the 𝚺3ⁿⁿ (n ﹤ 4) boundary fraction of plate samples has been optimised by the adjustment of parameters such as deformation per cycle, annealing temperature, annealing time and number of cycles. The measurement of grain size in GBE materials has been investigated and performed for all GBE samples. Several GBE conditions were subsequently examined in terms of their grain boundary spatial distribution, using a model of two-dimensional transport which was derived as part of this work. GBE materials were also tested for their room-temperature mechanical properties (yield strength, ultimate tensile strength, elongation at failure) and their creep rates at high temperature, low stress conditions. It was also showed that a GBE state could be replicated in tube by employing a swaging operation in place of the plate rolling process.