Papers by Keyword: Hot Torsion

Abstract: Double-twist torsion tests were used to determine static softening in the hot working range of three tool steels – W1, a carbon steel (1.03% C - 0.8% other elements), A2 and D2, a medium and a high alloy steel, containing 8.45% and 14.82% alloying elements. The carbon steel, that was single-phase austenite in the hot-working range, experienced rapid static recrystallization due to increased diffusion rate caused by C in hot austenite, very little alloying solute and no carbides. Carbides in alloy tool steels, which exist throughout the hot-working range, have a retarding effect on the progress of recrystallization but are responsible for enhancing initiation due to formation of nuclei at the strain concentration near the particle/matrix interface. Static recrystallization (SRX) of the alloy tool steels was compared with austenitic stainless steels, with similar strengths but much greater alloying content, and with microalloyed steels, as well as with the dynamic recrystallization kinetics.

Abstract: High silicon steel (up to 6.5 wt.-%Si) is important for the electrical industry because of its magnetic properties. However, its production in low thickness by cold rolling is difficult due to extreme brittleness, mainly caused by ordering processes, making dislocation motion more complex. Nevertheless, these materials appear to be deformable at higher temperatures. The cooling rate after hot deformation, the temperature from which it is cooled and the time delay prior to cold deformation are important elements for the understanding of their workability. Hot torsion tests were performed on Fe-Si steel (4.2 and 5.6 wt.-%Si) under continuous cooling to study the influence of strain and interpass time on ordering and non-recrystallization temperatures. Compression tests at a constant strain rate were used to study the effect of continuous cooling to RT and the delay time between deformations for series of silicon alloys (from 3.3 to 6.3 wt.-% Si) with different thermomechanical treatments. An aging phenomenon due to an ordering reaction at RT was observed. Finally, extrapolating the hot torsion and compression tests parameters to the rolling mill a suitable schedule for hot rolling was found guaranteeing good conditions for further cold rolling.

Abstract: The present work deals with the influence of niobium in solid solution on the dynamic recrystallization of pure nickel. High-purity nickel and two model nickel-niobium alloys were deformed to large strains via torsion at temperatures between 800 and 1000°C. Niobium additions considerably increased the flow stress, while they lowered the strain-rate sensitivity and increased the apparent activation energy. EBSD of the steady-state microstructures revealed strong grain refinement. Substructure development was favored, whereas thermal twinning was reduced by niobium. More generally, discontinuous recrystallization kinetics were considerably decreased.

Abstract: The high temperature deformation behavior of AISI 430 ferritic stainless steel has been studied over a temperature range of 800 to 1000°C and strain rate of 0.05-5.0/sec. The evolution of flow stress and microstructures showed the characteristics of continuous dynamic recrystallization (CDRX). The flow stress curves gradually decreased with increasing strain over the peak stress until 500% of strain without any steady state shown in typical austenitic stainless steel. Sub-grains of low angle firstly formed along the original high angle grain boundary were propagated into the inside of original grain and transformed to high angle. The CDRX grain sizes of AISI 430 deformed at 1000 °C and 0.5/sec was about 30-35㎛.

Abstract: 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 °C and 1200 °C 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.