Ali Dehghan-Manshadi

Microstructural evolution during hot deformation of duplex stainless steel

Abstract The microstructure evolution during hot deformation of a 23Cr–5Ni–3Mo duplex stainless steel was investigated in torsion. The presence of a soft δ ferrite phase in the vicinity of austenite caused strain partitioning, with accommodation of more strain in the δ ferrite. Furthermore, owing to the limited number of austenite/austenite grain boundaries, the kinetics of dynamic recrystallisation (DRX) in austenite was very slow. The first DRX grains in the austenite phase formed at a strain beyond the peak and proceeded to <15% of the microstructure at the rupture strain of the sample. On the other hand, the microstructure evolution in δ ferrite started by formation of low angle grain boundaries at low strains and the density of these boundaries increased with increasing strain. There was clear evidence of continuous dynamic recrystallisation in this phase at strains beyond the peak. However, in the δ ferrite phase at high strains, most grains consisted of δ/δ and δ/γ boundaries.

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 microstructural morphology on the mechanical properties of titanium alloys

Different morphologies of ?+? microstructures were obtained in a commercial Ti-6Al-4V alloy by cooling at different rates from the single ?-phase region into the two phase region. The effect of such morphologies on mechanical properties was studied using hot compression tests in a Gleeble thermomechanical simulator. A variety of complex morphologies could be obtained since the cooling rate has a significant influence on the ? to ? phase transformation and the resulting morphological development. While most of the ? phase transformed to colonies of ? at high cooling rates, it was possible to obtain a complex mixture of a colonies, grain boundary a and lamellar structure by decreasing the cooling rate. These complex morphologies each exhibited distinctive mechanical properties and characteristic dynamic phase transformation behaviour during deformation as a function of strain rate.

Effects of Processing Parameters on Microstructure Development in X70 Pipeline Steel

Achieving fine and uniform grains is the most effective way to enhance strength and toughness, which are required properties for pipeline steels. Steels microalloyed with Nb can exhibit a mixed grain structure, which can deteriorate low temperature toughness. In this work the effects of the thermomechanical processing parameters on the prior austenite grain structure before ferrite transformation have been investigated.

Resistance heated pressing (RHP): A novel technique for fabrication of titanium alloys

A novel powder metallurgical technique for the fabrication of titanium alloys has been developed by utilizing a pressure-assisted, resistance-heating sintering technique. In this technique, the high electrical resistance of oxide layers present on the surface of powder particles has been exploited to ensure effective resistance heating of green compacts. Ti-6Al-4V pre-alloyed powders of 100 µm size were compressed while being heated under a variety of conditions of sintering temperature, pressure and time. The outcomes of our experiments have proven that resistance heating can be a very effective means of heating during powder consolidation. The results have indicated that the required sintering time and temperature in the new resistance-heated sintering technique are much reduced in compared to sinter-press and/or hot isostatic pressing techniques, resulting in a refined microstructure with a concomitant improvement in mechanical properties.

Dissolution of precipitates in hot rolled low Mn, Ti added pipeline steels

The dissolution of different sulphides, carbides, carbo-sulphides and nitrides during re-heating of hot rolled low carbon, low manganese, titanium added steel have been studied using transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) analysis. In addition, the chemical composition and size distribution of the different precipitates have been determined before and after reheating to analyze the modification of these precipitates in the course of the reheating cycle. The TEM and EDS analyses showed the presence of a wide variety of simple and/or complex precipitates in as rolled samples. The reheating of these samples to temperatures as high as 1350 °C, caused dissolution of most particles, although titanium nitride (TiN) did not dissolve even after reheating. By decreasing the reheating temperature more and more precipitates remained un-dissolved, but some spherodization occurred at higher temperatures.

Effect of initial grain size on the dynamic recrystallization behaviours of austenite

The effect of initial grain size on the recrystallization behaviour of a type 304 austenitic stainless steel was investigated using hot torsion. Refinement of the initial grain size to 8 μm had a considerable effect on the flow curve shapes, peak and critical strains and also on DRX kinetics, compared with an initial grain size of 35 μm. Microstructural investigations using EBSD suggest a transition from conventional (discontinuous) to continuous dynamic recrystallization with decreasing initial grain size. Also, there was a moderate effect of initial grain size on the DRX grain size.

Thermo-Mechanical Processing of Low Manganese, Titanium Added Pipeline Steels

Thermo-mechanical processing of a new family of low carbon, low manganese, titanium added steels was studied under a variety of simulated conditions. In this paper emphasis is placed on developing a better understanding of the effect of Ti and N on the microstructural development and mechanical properties of these new steel types. Also, the behavior of precipitates during hot deformation was examined. The results indicated that steel with a Ti/N ratio lower than the stoichiometric value provide best mechanical properties.

Weldability of Ti-6Al-4V Alloys Formed via Various Powder Consolidation Techniques

This study considers the weldability of Ti-6Al-4V alloys formed by various powder consolidation methods. Samples were prepared from commercially sourced pre-alloyed Ti-6Al-4V powder using both conventional press-and-sinter (PS) and the new novel resistance-heated pressing (RHP) methods. Fusion welding was executed by the gas tungsten arc (GTA) process with arc stability assessed in-situ by observations of the arc as well as monitoring of transient arc voltage. Results indicated equivalent arc stability between samples of RHP and commercially sourced wrought material while samples formed by PS showed high instability in arc initiation, attributed to high levels of porosity. Post weld analysis of mechanical hardness in powder based samples revealed no significant deviation in weld metal properties from welds conducted on commercially sourced wrought material. In all cases weld microstructures typical of Ti-6Al-4V alloys were observed with significant grain growth in the fusion and heat affected zones. Samples prepared by PS methods showed internal porosity due to gas evolution upon solidification, which may again be attributed to the highly porous initial microstructure.

Hot deformation and recrystallisation of low Mn steels with different Ti/N ratio

Abstract The effects of small titanium and nitrogen additions on the mechanical properties and microstructural characteristics of four different low carbon, low manganese steels have been studied using a Gleeble thermomechanical simulator and TEM investigations. The Ti/N ratio in these steels as well as the size and distribution of TiN particles play a major role in determining the mechanical properties, the resulting austenite grain size as well as the size of inclusion precipitates. The best mechanical properties were obtained in the steel with a Ti/N ratio lower than the stoichiometric value. This steel also had the smallest austenite grain size after reheating as well as the highest rate of dynamic recrystallisation during hot deformation. The nature and rate of recrystallisation seem to be determined by an intricate interplay between the size and distribution of titanium nitrides and the prevailing austenite grain size.