Softening mechanisms in two duplex stainless steels deformed in hot torsion

The aim of the present work was to undertake a detailed investigation of the softening mechanisms during hot deformation of a 21Cr-10Ni-3Mo (steel A) and a 21Cr-8Ni-3Mo (steel B) austenite/ferrite duplex stainless steels containing about 60% and 30% of austenite, respectively. The steels were subjected to hot deformation in torsion performed at 900 ºC and 1200 ºC using a strain rate of 0.7 s-1 to several strain levels. Quantitative optical and transmission electron microscopy were used in the investigation. Austenite was observed to soften via dynamic recovery (DRV) and dynamic recrystallisation (DRX) accompanied by DRV for the deformation temperatures of 900 °C and 1200 °C, respectively, for the both steels studied. DRX of austenite largely occurred through strain-induced grain boundary migration, complemented by (multiple) twinning, and developed significantly faster in steel A than in steel B, indicating that considerably larger strains partitioned into austenite in the former steel during deformation at 1200 °C. The above softening mechanism was accompanied by the formation of DRX grains from subgrains along the austenite/ferrite interface and by large-scale subgrain coalescence. At 900°C, stressassisted phase transitions between austenite and ferrite were observed, characterised by dissolution of the primary austenite, formation of Widmanstätten secondary austenite and gradual globularisation of the microstructure with increasing strain. These processes appeared to be significantly more widespread in steel B. The softening mechanism within ferrite for the both steels studied was classified as “continuous DRX”, characterised by a gradual increase in misorientations between neighbouring subgrains with strain, for the both deformation temperatures.

Evolution of hot working stress and microstructure of Mg-3Al-1Zn

The deformation behaviour of magnesium alloy AZ31 was examined. The work found that dynamic recrystallisation operates during hot deformation. The influence that different process variables have on this mechanism were quantified. The optimisation of dynamic recrystallisation allows magnesium alloys to be formed into products more easily whilst developing enhanced final properties.

Variant selection during the c-to-ab phase transformation in hot-rolled bainitic TRIP-aided steels

The variant selection phenomenon during the austenite to bainite phase transformation in hot-rolled TRIP-aided steels was quantitatively characterized at the level of individual austenite grains. The reconstruction of the electron backscatter diffraction maps provided evidence that bainite grows by packets of laths sharing a common {1 1 1}_y plane in the austenite. The affect of hot deformation is to reduce the number of packets that form. It is suggested that slip activity is important in understanding this effect.

Microstructure evolution modeling during and after deformation in 304 austenitic stainless steel through cellular automaton approach

A 2D cellular automation approach was used to simulate microstructure evolution during and after hot deformation. Initial properties of the microstructure and dislocation density were used as input data to the cellular automation model. The flow curve and final grain size were the output data for the dynamic recrystallization simulation, and softening kinetics curves were the output data of static and metadynamic recrystallization simulations. The model proposed in this work considered the effect of thermomechanical parameters (e.g., temperature and strain rate) on the nucleation and growth kinetics during dynamic recrystallization. The dynamic recrystallized microstructures at different strains, temperatures, and strain rates were used as input data for static and metadynamic recrystallization simulations. It was shown that the cellular automation approach can model the final microstructure and flow curve successfully in dynamic recrystallization conditions. The postdeformation simulation results showed that the time for 50% recrystallization decreases with increasing strain for a given initial grain size and that dynamic recrystallization slows the postdeformation recrystallization kinetics compared to a model without dynamic recrystallization.

The role of strain on the recrystallization behaviour of hot worked and annealed magnesium alloy Mg-3Al-1Zn

The present work examines the microstructure that evolves during the hot deformation and subsequent annealing of magnesium alloy AZ31. In particular, the role of strain on the progression of dynamic recrystallization (DRX) and post-deformation recrystallization is investigated. It is found that the grain size developed after post-deformation recrystallization is larger when the deformation strain, and hence the degree of DRX, is low (for strains up to 0.4). Also, the kinetics of post-deformation recrystallization are found to be independent of strain for strain values of 0.4 and above. Whilst increasing strain alters the texture of the un-recrystallized microstructure (for the deformation mode examined), the texture does not change significantly during post-deformation recrystallization.

Deformation induced phase transformation during machining of Ti-5553

Ti-5553 is a relatively new titanium alloy with applications particularly in the aerospace industry for such key structural components as landing gear. However, during machining of Ti-5553, the elevated temperature and high strain at tool-workpiece interface may alter workpiece microstructure and result in ß to a phase transformation. During phase transformation, some intermediated phase such as w phase may form which is brittle and hard to machine, and it could reduce the fatigue life of machined components. The aim of this research work is to optimize the machining condition for Ti-5553, in which its hot deformation behavior in terms of ß to a phase transformation could be fully understood. Analysis of variables such as micrographs of phase components and cutting zone temperature demonstrates that the cutting temperature governs the formation of final phase components and to some extent this variation has been quantified to allow for further and more detailed investigation.

Microstructure evolution and softening processes occurring during annealing of hot deformed Ni-30Fe austenite

The current work investigates the microstructure evolution and softening processes that take place during annealing of an austenitic Ni-30Fe model alloy subjected to hot deformation in the dynamic recrystallization (DRX) regime. The substructure of the deformed matrix grains largely comprised organized microband arrays, though that of the DRX grains consisted of more random, complex subgrain/cell arrangements. This substructure disparity was also reflected by the distinct difference in the mechanism of post-deformation softening taking place during annealing of the deformed matrix and DRX grains. In the former, the recrystallization process took place through nucleation and growth of new grains fully replacing the deformed structure, as expected for the classical static recrystallization (SRX). The corresponding texture was essentially random, in contrast to that of the DRX grains dominated by low Taylor factor components. The microbands originally present within the deformed matrix grains displayed some tendency to disintegrate during annealing, nonetheless, they remained largely preserved even at prolonged holding times. During annealing of the fully DRX microstructure, a novel softening mechanism was revealed. The initial post-dynamic softening stage involved rapid growth of the dynamically formed nuclei and migration of the mobile boundaries in correspondence with the well-established metadynamic recrystallization (MDRX) mechanism. However, in contrast to the deformed matrix, SRX was not observed and the sub-boundaries within DRX grains rapidly disintegrated through dislocation climb and dislocation annihilation, which led to the formation of dislocation-free grains already at short holding times. Consequently, the DRX texture initially became slightly weakened and then remained largely preserved throughout the annealing process.

Data gathered during investigations into the evolution of hot working stress and microstructure in Mg-3Al-1Zn

The data is the result of hot deformation tests conducted on magnesium alloy AZ31. It includes stress strain data for a range of deformation conditions and different initial microstructures. It also includes data for the developed grain size and the degree of dynamic recrystallisation.

Recrystallization in 304 austenitic stainless steel

A 304 austenitic stainless steel was deformed using hot torsion to study the evolution of dynamic recrystallization (DRX). The initial nucleation of dynamically recrystallization occurred by the bulging of pre-existing high angle grain boundaries at a strain much lower than the peak strain. At the
peak stress, only a low fraction of the prior grain boundaries were covered with new DRX grains. Beyond the peak stress, new DRX grains formed layers near the initial DRX and a necklace structure was developed. Several different mechanisms appeared to be operative in the formation of new high angle boundaries and grains. The recrystallization behaviour after deformation showed a classic transition from strain dependent to strain independent softening. This occurred at a strain beyond the
peak, where the fraction of dynamic recrystallization was only 50%.

Effect of carbon content on the recrystallization kinetics of Nb-steels

A rapid method was used to study the effect of carbon content on the kinetics of post-deformation softening, t50, in Nb-steels. The hot deformation behaviour of austenite was not affected by carbon. However, the t50 was influenced by the carbon with different effects in different temperature regimes. At deformation temperatures above the non-recrystallization temperature, Tnr, carbon produced a small change in the softening behaviour. However, the t50 was significantly retarded with increasing carbon content at deformation temperatures lower than Tnr, due to Nb(C,N) precipitates.

Dependency of recrystallization mechanism to the initial grain size

The effect of initial grain size on the recrystallization behavior of a type 304 austenitic stainless steel during and following hot deformation was investigated using hot torsion. The refinement of the initial grain size to 8 μm, compared with an initial grain size of 35 μm, had considerable effects on the dynamic recrystallization (DRX) and post-DRX phenomena. For both DRX and post-DRX, microstructural investigations using electron backscattered diffraction confirmed an interesting transition from conventional (discontinuous) to continuous DRX with a decrease in the initial grain size. Also, there were unexpected effects of initial grain size on DRX and post-DRX grain sizes.

The mechanism of metadynamic softening in austenite after complete dynamic recrystallization

A novel mechanism of post-dynamic softening during annealing of a fully dynamically recrystallized (DRX) austenitic Ni–30Fe alloy is proposed. The initial softening stage involves rapid growth of the dynamically formed nuclei and migration of the mobile boundaries. The sub-boundaries within DRX grains progressively disintegrate through dislocation climb and dislocation annihilation, which ultimately leads to the formation of dislocation-free grains, and the grain boundary migration gradually becomes slower. As a result, the DRX texture largely remains preserved throughout the annealing process.

Effect of orientation stability on recrystallization textures of deformed aluminium single crystals

High purity Al single crystals of the Cube (0 0 1)[1 0 0] and rotated Cube (0 1 1)[0 1 ¯ 1] orientations have been deformed in plane strain compression in a channel die. Deformation was carried out at temperatures between 25 and 600 8C up to strains of 1.2. The as-deformed microstructure has been characterised using electron microscopy and electron backscattered diffraction (EBSD).
Annealing was carried out for various times and temperatures. The recrystallized microstructure has been studied using electron microscopy, and the orientation of recrystallized grains determined using EBSD. After cold deformation and annealing both orientations exhibited a random recrystallization texture component. After hot deformation both orientations retained a similar annealing texture to their starting deformation texture. The annealing texture of deformed single crystals was found to be more sensitive to the temperature of deformation than the stability of the orientation.

Effect of isothermal dissolution and re-precipitation of austenite on the mechanical properties of duplex structures

The hot deformation behaviour of a duplex ferritic/austenitic stainless steel was studied after different deformation conditions. The results showed a strange and interesting behaviour in the strength of the material during post-deformation studies. For most deformation conditions, the flow stress of the material was un-expectedly increased after annealing of deformed structures. This phenomenon implied that microstructural hardening occurred in the material during the interpass annealing rather than the expected softening. Also, an interesting change was observed where the morphology of the austenite phase changed from stringers or layers of austenite to a widmanstätten structure. The microstructural studies suggest that the austenite was dissolved and re-precipitated during the annealing process and the hardening was mostly associated with the change in the morphology of austenite.