Thesis

Investigation of microstructural evolution and recrystallisation mechanisms in 316L and 316Nb austenitic stainless steels during early stages of industrial open-die forging

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Awarding institution
  • University of Strathclyde
Date of award
  • 2025
Thesis identifier
  • T17544
Person Identifier (Local)
  • 201891274
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Department, School or Faculty
Abstract
  • The ingot-to-billet conversion process is a critical thermomechanical stage in the manufacturing of high-value engineering components, where the refinement of coarse, columnar as-cast microstructures into fine, equiaxed grains ensures optimal mechanical properties and structural integrity. This PhD research focuses on the deformation and recrystallisation behaviour of 316L and 316Nb austenitic stainless steel grades during the ingot-to-billet conversion process. While existing studies largely focus on either laboratory-scale experiments or industrial-scale observations, this work bridges the gap between both scales by employing intermediate-scale experiments that balance industrial representativeness while ensuring experimental control. Compared to laboratory-scale studies, intermediate-scale experiments involve larger samples containing a significantly higher number of grains. As a result, they offer a statistically more representative depiction of the deformation and recrystallisation behaviour of as cast materials under industrial-scale processing conditions. An experimental campaign involving small-scale hot upsetting tests and large-scale forging trials (i.e., cogging and upsetting) was designed to investigate material behaviour in the early stages of the ingot-to-billet conversion process. The experimental work was supported by finite element (FE) simulations incorporating anisotropic material models and an innovative quantitative characterisation methodology developed to capture microstructural evolution over large surface areas, enabling the effective tracking of recrystallisation progress at large length scales and allowing for meaningful analysis of microstructural changes in partially recrystallised as-cast material. The results demonstrated the significant influence of crystallographic texture on anisotropic deformation behaviour of the as-cast material, and the role of the coarse as-cast grain size on its heterogeneous deformation and recrystallisation during forging. The presence of Niobium in the 316Nb grade was shown to result in a finer initial grain structure and promoted the retention of secondary phases after homogenisation. Through an increased number of nucleation sites and reduced grain boundary mobility, both characteristics promoted enhanced recrystallisation in 316Nb compared to the 316L grade. This research highlights the complementary roles of laboratory and intermediate-scale studies in the design and optimisation of ingot-to-billet manufacturing routes. It also demonstrates the value of larger scale experimental and characterisation approaches in understanding material and process interactions for efficient and quality-driven ingot-to-billet conversion processes, with direct relevance to industrial sectors including energy, aerospace, and transportation.
Advisor / supervisor
  • Dumont, Christian
  • Langlois, Laurent
  • Nouveau, Sébastien
  • Rahimi, Salaheddin
  • Konkova, Tatyana
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