A Thermal Cycling Route for Processing Nano-grains in AISI 316L Stainless Steel for Improved Tensile Deformation Behaviour

Abstract

<p>The present work significantly improved the mechanical strength of AISI 316L stainless steel by producing nano-sized grains. Steel was subjected to cold rolling followed by repetitive thermal cycling to produce ultra-fine/ nano-sized grains. The optimum processing parameters including extent of cold deformation, annealing temperature for thermal cycling, soaking period during each thermal cycle, and number of thermal cycles were determined through a systematic step-by-step procedure. After conducting thermal cycling under optimum conditions, a significant amount of grain size reduction was achieved. The effect of nano-sized grains on tensile deformation behavior was analysed. High cold deformation resulted in increased amount of stored strain energy. The stored strain energy accelerated the re-crystallisation kinetics during the thermal cycling process. Every thermal cycle resulted in irregular dispersal of stored energy. This irregular dispersal of stored energy favoured recrystallisation rather than grain growth and led to refinement of grains, in the absence of strain induced martensite. Repetitive thermal cycling promoted grain refinement and resulted in very significant grain size reduction with resultant grain size in the range of 800–1200 nm as compared to initial size of 90–120 μm. The resultant microstructure improved tensile strength by<br />106.8 per cent, from 590 MPa to 1220 MPa.</p>

Deformation-induced martensite in austenitic stainless steels: A review

“…15d. It was reported that during thermal cycling, strain heterogeneity develops between the newly recrystallized and the remaining unrecrystallized regions and results in irregular dispersal of stored energy, which favors recrystallization against grain growth and thus leads to grain refinement [216,217]. In a more recent study, a two-step annealing treatment after cold rolling was proposed for more intense grain refinement of AISI 316L grade.…”

Refinement of Banded Structure via Thermal Cycling and Its Effects on Mechanical Properties of Dual Phase Steel

“…Thermal cycling is another general processing route for refinement of grain size . This was practiced for steels in early publications by Grange .…”

“…Interestingly, Kayali et al have used this method for grain refinement of ultrahigh carbon steels for development of fine structure superplasticity. In another front, Nanda et al and Ravi Kumar et al have used the concept of strain heterogeneity during thermal cycling of cold‐worked austenitic stainless steels. They reported that during thermal cycling, strain heterogeneity develops between the newly recrystallized and the remaining unrecrystallized regions and results in irregular dispersal of stored energy, which favors recrystallization against grain growth and thus leads to grain refinement.…”

Cyclic re-austenitization of copper-bearing high-strength low-alloy steels fabricated by laser powder bed fusion

“…The grain size saturates eventually, by attaining a balance between the refinement due to nucleation and the growth due to the completion of reversion [1]. An alternative theory was postulated for the grain refinement based on strain heterogeneity during cyclic re-austenitization of cold-worked austenitic stainless steels [18,19]. It was reported that a strain heterogeneity develops between the newly recrystallized and unrecrystallized regions leading to an irregular dispersion of the strain energy [18,19].…”

“…An alternative theory was postulated for the grain refinement based on strain heterogeneity during cyclic re-austenitization of cold-worked austenitic stainless steels [18,19]. It was reported that a strain heterogeneity develops between the newly recrystallized and unrecrystallized regions leading to an irregular dispersion of the strain energy [18,19]. Hence, recrystallization is preferred against grain growth, thus, leading to grain refinement.…”