Influence Of Thermomechanical Processing Research Articles

Influence of thermomechanical processing on the grain size, texture and mechanical properties of Mg-Al alloys

The work carried out by the authors over the last decade on the processing, microstructural characterization and the mechanical behaviour of Mg alloys is reviewed. In particular, the potential for grain refinement and for the development of specific textures of large strain hot rolling (LSHR), equal channel angular pressing (ECAP) and accumulative roll bonding (ARB) is discussed. The recrystallization and the deformation mechanisms predominant in Mg alloys are analyzed as a function of the grain size and the texture in a wide range of stresses, strain rates and temperatures. Finally, the the feasibility of superplatic forming of Mg alloys was examined, taking into account the influence of factors such as grain size stability and microstructural heterogeneities.

Influence of thermomechanical processing on shear bands formation and magnetic properties of a 3% Si non-oriented electrical steel

The effect of thermomechanical processing on the formation of shear bands and on the magnetic properties of a 3.0wt% silicon non-oriented steel was investigated by hot rolling samples with different thicknesses at different temperatures, in order to obtain a variation in hot band grain size and cold strain. All the samples were processed in a single-stage cold rolling and finally annealed at 1020°C. It was found that the increase of the hot band grain size decreases the γ fiber volume fraction and increases the η fiber volume fraction after the final annealing. The increase of the cold strain strongly contributed to this result. A good combination of intense generation of shear bands, and proper crystallographic texture, due to higher nucleation of grains with favorable orientations to magnetization in these bands, can be obtained for the samples hot rolled at 1000 and 1120°C and submitted to cold strain of 64.3% and 72.2% respectively. However the best combination of B50, W15/60 and μr can be obtained by hot rolling the samples at 1000°C to the thickness of 1.4mm, corresponding to 64.3% of cold strain.

Influence of thermomechanical processing and heat treatments on tensile and Charpy impact properties of B and Cu bearing high-strength low-alloy steels

High-strength low-alloy steels containing different B and Cu contents were fabricated by thermomechanical processing and heat treatments such as accelerated cooling, intercritical annealing, and tempering, and then tensile and Charpy impact tests were conducted on them in order to investigate the mechanical properties in terms of yield strength, deformability, and toughness. The test results revealed that the B-added acceleratedly cooled specimens composed mostly of lath martensite had relatively high yield strength, poor deformability and low-temperature toughness in contrast to the B-free acceleratedly cooled specimens consisting mainly of granular bainite and degenerate upper bainite. When the B-added acceleratedly cooled specimens were tempered, the yield strength increased to a certain degree and the low-temperature toughness was remarkably improved, but the deformability severely deteriorated. However, the B-added intercritically annealed specimens provided an attractive balance of yield strength, deformability, and low-temperature toughness due to the dual-phase microstructure of fine ferrite and martensite with lath type morphology.

Influence of Thermomechanical Processing on the Martensitic Transformation Temperatures of NiTi SMA Wire

Most of the applications of NiTi SMA are as a wire form. In this sense it is important to know the effects of thermo-mechanical processing such as reduction per pass and intermediate annealing on the wire drawing process. For this work they were produced wire by cold drawing using 15 % area reduction per pass with and without intermediate annealing. The starting ingot was produced by VIM process. The influence of thermo-mechanical processing will be related to the martensitic transformation temperatures.

Influence of Thermomechanical Processing on the Austenite–Pearlite Transformation in High Carbon Vanadium Microalloyed Steels

Multipass torsion tests were carried on with several V-microalloyed high carbon steels, using different deformation sequences in order to modify the austenite state prior to transformation. Both recrystallized and deformed austenite microstructures were studied. After deformation, different cooling rates were applied. The results show that accumulating strain in the austenite before transformation seems to slightly increase the interlamellar spacing for a given cooling rate, this increase being related to the pearlite transformation taking place at higher temperatures because of the increase in the austenite grain boundary area per unit volume (SV). On the other hand, the retained strain significantly contributes to a refinement of the “ferrite units”, this effect being more significant as vanadium and nitrogen contents rise. A relationship between the mean “ferrite unit” size with SV and cooling rate was determined. Similarly, empirical expressions to predict strength as a function of vanadium microalloying addition, SV and cooling rate were derived.

Influence of thermomechanical processing on superplastic forming of Mg–Al alloys

The aim of this paper is to study the influence of the initial microstructure of several Mg–Al alloys on their superplastic formability and on their post-forming microstructure and mechanical properties. Various thermomechanical processing routes, such as annealing, conventional rolling, severe rolling and cross rolling, were used in order to fabricate AZ31 and AZ61 alloys with different grain sizes. These materials were then blow formed into a hat shaped die. It was found that the processing route has only a small effect in the formability of Mg–Al alloys or on the post-forming microstructures and properties due to rapid dynamic grain growth taking place at the forming temperatures. Nevertheless, good formability is achieved as a result of the simultaneous operation of grain boundary sliding and crystallographic slip during forming.

Influence of thermomechanical processing on the superelastic properties of a Ni-rich Nitinol shape memory alloy

Ni-rich NiTi shape memory alloys usually undergo a series of thermomechanical treatment in their production and component fabrication processes. Comprehension of property changes and microstructural evolutions induced by these treatments is essential for quality control and improvement of these materials. The effect of controlled ageing for prolonged times (of the order of several hours) has been extensively investigated in the literature. This study investigates the effect of actual production processes of NiTi tubing, which often involve thermomechanical treatment for multiple periods of short durations and which are typical of industrial production routines of devices like stents, on the transformation and deformation behaviour of a Ti–50.8 at%Ni alloy. Evolution of the properties is characterized by DSC measurements and by tensile testing at 310 K.

Influence of thermomechanical processing on microstructural evolution in near-α alloy IMI834

The effect of processing parameters on the evolution of the microstructural characteristics of IMI834 were examined over the temperature range 950–1125 °C at constant strain rates of 0.001–1 s −1 and true strains up to 1.2. During isothermal deformation, differences in the flow softening behavior were observed for processing in the single-phase beta region, as compared to the two-phase alpha–beta region, which has been related to differences in the structural changes occurring in IMI834. In particular, deformation processing of the alpha–beta structure was observed to work the lamellar alpha structure of the transformed beta grains without recrystallization of either phase, whilst microstructural development in a predominately beta structure resulted in the formation of dynamically recrystallized grains that were necklaced around the deformed and elongated beta phase. For the later case, the variation in the dynamically recrystallized grain size was determined to follow a Zener–Hollomon relationship between a strain rate range 0.001 and 1 s −1.

Influence of thermo-mechanical processing on the microstructure of Cu-based shape memory alloys produced by powder metallurgy

Abstract Cu–Al–Ni shape memory alloys processed by powder metallurgy show very good thermo-mechanical properties, much better than those found in alloys produced by conventional casting. In this paper, we present the microstructural characterisation of these powder metallurgy alloys in order to find the microscopic mechanisms, linked to the powder metallurgy processing method, which are indeed responsible of such good thermo-mechanical behaviour. Electron microscopy studies [scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM)] show that powder metallurgy processing creates a sub-grain structure characterised by the presence of low angle sub-boundaries. These sub-boundaries are found to be lying on {1 1 0} and {1 1 2} lattice planes and are composed by an arrangement of superdislocations. These sub-boundaries may improve ductility in two ways: acting as a sink of dislocations which promotes plastic deformation and decreasing stress concentration at grain boundaries. Moreover, since sub-boundaries act as weak obstacles for the movement of martensite plates, the improvement on ductility is accomplished by an adequate thermo-mechanical behaviour.

An ultra low carbon Cu bearing steel: influence of thermomechanical processing and aging heat treatment on structure and properties

An ultra low carbon Cu bearing steel, micro alloyed with Nb was thermo-mechanically processed. Variation in microstructures and associated mechanical properties at different finish rolling temperatures was studied. Microstructures in hot rolled steels showed a mixture of acicular ferrite and bainitic ferrite with inter-lath or intra-lath precipitation of microalloying NbC/Nb(CN) precipitates. Second phase particles were observed, mostly at lath boundaries and were identified as retained austenite, twinned martensite islands or martensite austenite constituents. Variation in strength at different finish rolling temperatures was due to variation in volume fraction of bainitic ferrite and dislocation substructure in the matrix. Precipitation behaviour of Cu was studied by differential scanning calorimetry and transmission electron microscopy. At peak age hardening condition, coherent bcc Cu particles were formed and activation energy for this reaction was evaluated. At higher aging temperature, growth of fcc e-Cu particles and recovery of structure occurred which decreased the strength value.

Intercritical Deformation of Low Carbon Steel

Improving the strength of steel sheets by increasing the carbon content decreases the formability and highly impairs the weldability. Thermomechanical processing (TMP) is one of the most important mechanisms that simultaneously enhances the strength and ductility while keeping the excellent weldability of low carbon steel. The influence of TMP in the (α + γ) region on the microstructure and mechanical properties of low carbon steel (LC) is investigated. The TMP was carried out by rolling at 760, 810 and 860 C with different amounts of reductions followed by water quenching. Rolling was maintained at 5 m/min rolling speed. The results showed that high strength with good elongation was obtained using intercritical deformation at both 810 °C and 70% total rolling reduction.L’amélioration de la résistance mécanique des feuilles d’acier par augmentation du contenu en carbone diminue la formabilité et affecte beaucoup la soudabilité. Le traitement thermomécanique (TMP) est l’un des mécanismes les plus importants qui augmentent simultanément la résistance mécanique et la ductilité tout en conservant une excellente soudabilité de l’acier à faible teneur en carbone. On étudie l’influence du TMP dans la région (α + γ) sur la microstructure et les propriétés mécaniques de l’acier à faible teneur en carbone (LC). On a effectué le TMP par laminage à 760, 810 et 860 °C, avec différentes quantités de réduction, suivi par la trempe à l’eau. On a maintenu la vitesse de laminage à 5 m/min. Les résultats ont montré qu’on obtenait une résistance mécanique élevée avec une bonne élongation en utilisant la déformation intercritique à 810 °C et 70% de réduction totale par laminage.

The influence of thermo-mechanical processing on the microstructure of steel 20MoCrS4

The influence of thermo-mechanical processing (TMP) on the microstructure and mechanical properties of 0.22%C–0.87%Mn–0.73Cr–0.40Mo steel was investigated. The transformation CCT diagram and CCCT diagram were determined by dilatometric measurements. Hot deformation before austenite decomposition slightly accelerates ferritic transformation, retards bainitic reactions and decreases the bainite start temperature. Special methods of TMP were performed consisting of hot and/or warm compression deformations and dwell at an elevated temperature. The microstructure was studied using metallography and transmission electron microscopy. The compression deformation results in a remarkable refinement of the microstructure and an improvement of mechanical properties. Warm deformation followed by dwell at 470°C was found to be suitable for an increase of tensile strength and notch toughness; the corresponding microstructure is a fine lath-like bainitic microstructure with a relatively homogeneous distribution of carbide particles.

Influence of thermo-mechanical processing and different post-cooling techniques on structure and properties of an ultra low carbon Cu bearing HSLA forging

An ultra low carbon Cu bearing HSLA steel, micro alloyed with Nb and Ti was forged in two stages and subsequently cooled at different cooling rates. Variation in microstructures and the mechanical properties at different cooling rates have been studied. Volume fraction of bainitic phase has been found to vary with different cooling rates. Maximum strength was achieved in this steel at an intermediate cooling due to precipitation hardening of very fine microalloying carbides, carbonitrides and ε-Cu precipitates. Finer polygonal ferrite and granular bainite structure obtained at the slowest cooling rate resulted in high impact toughness values at ambient and sub ambient temperatures.

Influence of Thermomechanical Processing on the Microstructure and Properties of a Cu-Cr-P Alloy

The microstructure and properties of a Cu-0.55 wt. % Cr-0.07 wt. % P alloy were studied by using optical microscopy, transmission electron microscopy, and the measurements of Vickers hardness and electrical conductivity after it was subjected to conventional aging and two thermomechanical treatments. The hardness increment resulting from the thermomechanical treatments was 50% higher than the increment produced by conventional aging. The thermomechanical procedure, including two aging steps (double aging), produced a 5% International Annealed Copper Standard (IACS) higher increase in conductivity than the procedure including a single aging step. However, the former procedure did not lead to more efficient hardening in the studied alloy than the latter procedure because during the second aging step extensive recovery or even the onset of recrystallization tended to suppress precipitation hardening to some extent. After being subjected to a thermomechanical treatment that included three cold-drawing steps and one aging step, the studied alloy showed a tensile strength of 550 MPa with a conductivity of 74% IACS. Based on the obtained results as well as on the comparison with other Cu-Cr type alloys, some suggestions were given for improving the thermomechanical processing route of the studied alloy.

The influence of thermomechanical processing on recrystallisation and precipitation in austenitic alloys with particular reference to the effects of deformation and ageing conditions

The recrystallisation and precipitation behaviour of an austenitic stainless steel has been studied with particular emphasis on the role of the experimental parameters in controlling the precipitation mechanisms of sigma phase. The alloy, containing 23 wt% chromium and 23 wt% nickel and with negligible carbon (0.004 wt%), was aged isothermally at different temperatures following hot-, warm-, or cold-working. Three distinct behaviours during ageing were found: static recrystallisation on ageing at 900°C; conventional precipitation of sigma phase after static recrystallisation at 800°C; and ‘discontinuous recrystallisation’ (conjoint recrystallisation and precipitation) at 700°C and 600°C. The conditions of thermomechanical processing interacted strongly with ageing history, hot-working having least influence and cold-working the greatest significance, a consequence of the combination of the amount of strain energy available for recrystallisation and the degree of supersaturation for sigma phase. Recrystallisation at adequate supersaturation caused discontinuous recrystallisation, yielding very rapid and heavy precipitation, apparently approaching thermodynamic equilibrium after only a few hours of ageing. By contrast, conventional precipitation after static recrystallisation gave only light precipitation by the same time. The distribution, shapes, and sizes of precipitates were influenced by the mechanism.

Influence of thermomechanical processing and accelerated cooling on microstructures and mechanical properties of plain carbon and microalloyed steels

A thermomechanical control process consisting of slab reheating, controlled rolling, and accelerated cooling has been adopted at the plate mill, Bhilai Steel Plant, India for achieving high strength and toughness in C–Mn and microalloyed steels while keeping the mechanical properties and the flatness constant. A mathematical model has been developed to predict the temperature distribution in the plate during accelerated cooling, taking into account the heat generation of the phase transformation. Effects of chemistry and mill parameters on ferrite grain refinement are explained in terms of nucleation and growth rate.

The quasi-static response and fracture behavior of a thermomechanically processed aluminum alloy metal–matrix composite

In this paper, the influence of thermomechanical processing on the tensile response and fracture behavior of a disintegrated melt deposited aluminum alloy metal–matrix composite is presented and discussed. The uniaxial tensile properties of the as-processed composite is compared with an extruded counterpart. The tensile properties and fracture behavior of the of the as-processed and extruded composites are rationalized in light of the mutually interactive influences of stress state and intrinsic composite microstructural effects.

Modelling of the influence of thermomechanical processing of Nb-microalloyed steel on the resulting mechanical properties

Abstract Two-stage compression tests were performed at various temperatures and reductions. The influence of finish deformation temperature in the range of 650–820°C and strain on microstructure and final mechanical properties of Nb steel is investigated. Starting from detailed simulation of microstructure development, i.e. recrystallization, precipitation and grain growth kinetics, the material behaviour was modelled. An empirical model of yield strength, developed in previous work, is employed to predict mechanical properties of microalloying steel during hot deformation above and below the γ–α transformation temperature. The comparison of simulated and measured results shows that the real thermomechanical behaviour as well as the microstructure development of Nb-steel has been correctly implemented. The results provided by both finite element analyses and microstructure modelling were compared with earlier similar investigations of C–Mn steel.

The influence of thermomechanical processing on the smooth fatigue properties of Ti–15V–3Cr–3Al–3Sn

This study has examined the potential for enhancing the fatigue performance of Ti–15V–3Cr–3Al–3Sn sheet through variations in heat treatment and thermomechanical processing procedures. Processing variants included conventional solution treatment and aging, solution treatment with duplex- and triplex-ages, and cold and warm rolling followed by aging. Tensile and smooth fatigue performance of each processing route were evaluated and compared to ‘standard’ commercial practice. These processing avenues provided a wide range of microstructures with tensile strengths ranging from 1070 to 1610 MPa. Enhancements in fatigue performance were dependent on processing and stress level. At high stresses, i.e. within the low cycle regime, a 15% improvement in the maximum stress for a fixed 30 000 cycle lifetime was achieved. At lower stresses, i.e. within the high cycle fatigue ‘limit’ regime, a 50% improvement relative to the standard heat treatment was achieved. A good correlation of fatigue and tensile strengths was observed; generally, the smooth fatigue properties improved as the tensile strength increased.

Processing-property-microstructure relationships in TiAl-based alloys

The influence of thermomechanical processing on the microstructure of a range of TiAl-based alloys has been assessed using optical and electron microscopy, and the room-temperature mechanical properties have been determined. Long-term exposure at high temperatures has been used to assess the thermal stability of some of the structures generated through the different processing routes, and it has been found that the (gamma and alpha 2) lamellar structures, in some of the alloys, are unstable at 700°C, a likely operating temperature. Addition of boron increases the stability of the lamellar structure. The influence of the difficulty of slip transfer between gamma and alpha 2 has been assessed as one of the factors limiting ductility in samples with this lamellar structure. In addition to the alloys produced via the ingot route, some atomized material has been produced and the microstructure and properties of hot-isostatically pressed “hipped” material assessed. Regions, high in titanium, are present in all atomized powders that have been examined, and these regions are found to initiate fracture at very low strains. These results are briefly discussed in terms of the factors that control the room-temperature strength and fracture behavior of TiAl-based alloys.