Recrystallization In Austenitic Stainless Steel Research Articles

Geometric dynamic recrystallization of austenitic stainless steel through linear plane-strain machining

ABSTRACTType 316L austenitic stainless steel was severely plastically deformed at room temperature using linear plane-strain machining in a single pass that imparted shear strains up to 2.2 at strain rates up to 2 × 103 s−1. The resulting microstructures exhibited significant grain size refinement and improved mechanical strength where geometric dynamic recrystallization was identified as the primary microstructural recrystallization mechanism active at high strain rates. This mechanism is rarely observed in low to medium stacking fault energy materials. The critical stress required for twin initiation is raised by the combined effects of refined grain size and the increase in stacking fault energy due to the adiabatic heating of the chip, thus permitting geometric dynamic recrystallization. The suppression of martensite formation was observed and is correlated to the significant adiabatic heating and mechanical stabilisation of the austenitic stainless steel. A gradient of the amount of strain induced martensite formed from the surface towards the interior of the chip. As the strain rate is increased from 4 × 102 s−1–2 × 103 s−1, a grain morphology change was observed from a population of grains with a high fraction of irregular shaped grains to one dominated by elongated grain shapes with a microstructure characterised by an enhanced density of intragranular sub-cell structure, serrated grain boundaries, and no observable twins. As strain rates were increased, the combination of reduction in strain induced martensite and non-uniform intragranular strain led to grain softening where a Hall-Petch relationship was observed with a negative strengthening coefficient of −0.08 MPa m1/2.

Investigation on Grain Boundary Character Distribution During Dynamic Recrystallization of Austenitic Stainless Steel During Hot Deformation

Abstract The phenomenon of dynamic recrystallization (DRX) is central to the hot working and thermomechanical processing of austenitic stainless steel. This study aims to relate two distinct parameters that influence DRX: the content of interstitial atoms, such as nitrogen, and character of grain boundaries in the steel. 316LN austenitic stainless steel has been subjected to thermomechanical processing at 1,423 K through uniaxial compression at different strain rates. At these, the microstructure is fully recrystallized. The nature, character, and growth of DRX grains are studied using Electron Backscatter Diffraction (EBSD). EBSD is used to determine grain boundary character distribution and the Effective Grain Boundary Energy (EGBE) parameter. The evolution of EGBE and its relation to grain growth are analyzed. The concomitant development of special low-energy boundaries is correlated with the interstitial content in the steel, thus linking interstitial content to EGBE. The correlations so made are tested by examining the corrosion resistance of different boundaries formed at different deformation conditions.

Dynamic Recrystallization in Austenitic Stainless Steel during Hot Working with Decreasing Deformation Temperature

Abstract The deformation microstructures and the mechanical properties of an advanced austenitic stainless steel subjected to hot forging with a decrease in deformation temperature from 1,323 to 1,073 K were studied. The deformation microstructures were characterized by the development of dynamic recrystallization (DRX), leading to a decrease in the mean grain size from 11 to 1 μm as temperature decreased. Two DRX mechanisms, i.e., discontinuous and continuous ones, operated concurrently during the hot compressions. Corresponding true flow stress gradually increased approaching a steady-state deformation during each compression. The grain refinement was accompanied by an increase in the dislocation density, resulting in significant strengthening. The yield strength increased from about 600 MPa to almost 1,000 MPa as revealed by tensile tests at room temperature. Both the grain-size strengthening and the dislocation strengthening contributed to the overall strength of the processed steel samples with DRX microstructures.

Dynamic Recrystallization of Austenitic Stainless Steel under Multiple Peak Flow Behaviours

The hot working behaviour of a Fe-0.02C-8.2Ni-18.5Cr (mass%) austenitic stainless steel was studied at high temperatures and low strain rates. These conditions led to a multiple peak stress-strain curve, where the second peak had a much larger strain and stress than the first peak. The first peak showed evidence of some grain refinement, indicative of conventional dynamic recrystallization. However, after this there was significant grain coarsening, similar to abnormal grain growth, with the formation of substructure within the grains. The strain to the first peak in the multi peak tests was much lower strain than expected from extrapolation of the single peak data.

Continuous recrystallization in austenitic stainless steel after large strain deformation

Static restoration mechanisms operating during annealing were studied in a 304 steel with strain-induced submicron grain structures. The initial microstructure with an average grain size of about 0.3 μm was developed by large strain deformation at 873 K. Early annealing leads to a full relaxation of high internal stresses associated with non-equilibrium strain-induced grain boundaries, while their boundary misorientations and the average grain size barely change. Further annealing results in a transient recrystallization followed by a normal grain growth. The average grain boundary misorientation increases up to around 45° in the former and becomes constant in the latter. This is associated with the change in the grain boundary misorientation distribution from a characteristic strain-induced one to a near random distribution corresponding to a fully recrystallized state. The annealing processes operating in the strain-induced fine grains take place homogeneously in the whole matrix and can be called continuous recrystallization.