Microstructure evolution in adiabatic shear band in α-titanium

The microstructure and microtexture of adiabatic shear bands (ASBs) on the titanium side of a titanium/mild steel explosive cladding interface are investigated by means of optical microscopy (OM), scanning electron microscopy/electron back-scattered diffraction (SEM/EBSD) and transmission electron microscopy (TEM). Highly elongated subgrains and fine equiaxed grains with low dislocation density are observed in the ASBs. Recrystallization microtextures (28°, 54°, 0°), (60°, 90°, 0°) and (28°, 34°, 30°) are formed within ASBs. The grain boundaries within ASBs are geometrical necessary boundaries (GNBs) with high-angles. Based on the relations between temperature and the engineering shear strain, the temperature in the ASBs is estimated to be about 776–1142 K (0.4–0.6 T _m). The rotation dynamic recrystallization (RDR) mechanism is employed to describe the kinetics of the nano-grains’ formation and the recrystallized process within ASBs. The small grains within ASBs are formed during the deformation and do not undergo significant growth by grain boundary migration after deformation.     

Influence of crystallographic textures on tensile properties of 316L austenitic stainless steel

Role of cold rolling texture on the tensile properties of the cold rolled and cold rolled and annealed AISI 316L austenitic stainless steel is described here. The solution-annealed stainless steel plates were unidirectionally cold rolled to 50, 70 and 90% of reduction in thickness. The cold rolled material was annealed at 500–900 °C annealing temperatures. X-ray diffraction technique was employed to study the texture evolution in cold rolled as well as cold rolled and annealed conditions. The texture components that evolved were translated into slip transmission number ‘λ’ and Schmid factor ‘μ’. These two parameters were correlated with the tensile properties of the material. The tensile properties were evaluated under all processing conditions. Softening of the cold rolled material was observed after annealing with increasing annealing temperatures. From the stress–strain curves, strain hardening coefficient ‘n’ and strain hardening rate ‘θ’ were determined. It was found that the effect of texture on tensile behaviour could be understood clearly by strain hardening rate. Out of the two parameters, ‘n’ and ‘θ’, strain hardening rate was found to be more sensitive to type of texture in the material.     

Influence of hot working on microstructure and mechanical behavior of high nitrogen stainless steel

The relationship between microstructures and mechanical properties of a high nitrogen stainless (HNS) steel (0.65N–1.8Ni, wt%) manufactured by argon oxygen decarburization and continuous casting was investigated in this article. The plates with different thicknesses were obtained by thermo-mechanical control process. The results revealed that the plates prepared by hot rolled and solution treated possessed good balanced mechanical properties, i.e., satisfactory strength and higher toughness. Compared with hot-rolled and solution-treated plates, the hot-rolled plate had much higher tensile strength, but its impact toughness was extremely low. Furthermore, with the increase of deformation strain, the plate with finer grains, more precipitates and higher strength was achieved, but its plasticity decreased obviously. The worm-like carbides formed along the grain boundary during the finish rollings at 850 °C, which are detrimental to the toughness of hot-rolled plates. In addition, small amount of ferrite precipitated in the steel due to the non-equilibrium solidification during continuous casting, and to ensure full austenitic structure, composition design method was recommended.     

Evolution of textures and microstructures in IF-steel sheets during continuous confined strip shearing and subsequent recrystallization annealing

Interstitial free (IF) steel sheets were deformed by continuous confined strip shearing (CCSS) based on the equal channel angular pressing (ECAP). The samples were deformed by CCSS up to three passages and subsequently recrystallized at 700°C for 1 h. The strain history of IF steel sheets in the CCSS die-channel was tackled by finite element method (FEM) simulations. The deformation by CCSS led to the shear deformation and consequently the formation of shear texture components. With increasing number of CCSS passages, the intensity of the deformation texture was hardly increased. The recrystallization texture resembled the deformation texture. The orientation stability was discussed by mean of Taylor deformation model and the formation of recrystallization textures was discussed by occurrence of the discontinuous recrystallization. Observations by transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) revealed the formation of ultra-fine grains in IF sheets deformed by CCSS.     

Role of strain-induced martensite on microstructural evolution during annealing of metastable austenitic stainless steel

Metastable austenitic stainless steel of type AISI 304L was cold rolled to 90% with and without inter-pass cooling. Inter-pass cooling produced 89% of strain-induced martensite whereas no inter-pass cooling resulted in the formation of 43% of martensite in the austenite matrix. The cold-rolled specimens were annealed at various temperatures in the range of 750–1000 °C. The microstructures of the cold-rolled and annealed specimens were studied by the electron microscope. The grain size and low angle boundaries were determined from the orientation maps recorded by the scanning electron microscope-based electron backscattered diffraction technique. The observed microstructural changes were correlated with the reversion mechanism of martensite to austenite and volume fraction of martensite. It was noted that large volume fractions of martensite at low annealing temperatures, below 900 °C, were most suitable for the formation of fine grains. On the contrary, reversion of small volume fractions of martensite at critical annealing temperature of 950 °C resulted in grain refinement.     

深冲IF钢再结晶{111}纤维织构形成机制探讨

为了探讨深冲IF钢再结晶织构与退火温度之间的关系及{111}再结晶织构形成机制,采用X射线衍射三维取向(ODF)和背散射电子衍射(EBSD)分析技术并结合金相组织观察,利用Gibbs-Thom son方程对冷轧IF钢在不同退火温度下的再结晶织构演变规律及形成机制进行研究.实验结果表明:随着退火温度的增加,再结晶量逐渐增多,γ纤维织构强度亦相应增强,同时,α纤维织构强度则逐渐降低;冷轧IF钢再结晶初期的织构转变主要发生在γ纤维织构之间.研究表明,再结晶核心的形成主要以"显微择优形核"为主,晶核的长大则主要以择优生长为主,而Σ重位点阵晶界在再结晶γ纤维织构形成过程中起着重要作用.     

Effects of pulsed magnetic annealing on Goss texture development in the primary recrystallization of grain-oriented electrical steel

As the condition of Goss component in primary structure of grain-oriented electrical steel plays a significant role for its abnormal growth in the subsequent secondary recrystallization, pulsed magnetic annealing was used to affect the development of Goss texture in primary recrystallized structure in this study. Specimens of one-stage cold-rolled electrical steel were annealed under pulsed magnetic field with the maximum strength 1T from three different directions—rolling direction, transverse direction, and normal direction at the temperature about 700 °C for 16 min, and then electron backscattering scanning diffraction technology was used to measure grains for texture determination. It was found that pulsed magnetic field not only decreases the average grain size of recrystallized structure but also affects the development of primary recrystallized texture varied with the change of applying direction due to that the magnetic field induces extraneous driving forces causing preferential grain growth and the preferential order can be listed from those with crystal orientation 〈100〉//H, 〈112〉//H, 〈110〉//H to 〈111〉//H. This research can be a guide to control the Goss texture development in primary recrystallization with pulsed magnetic annealing for deformed grain-oriented electrical steel.     

Electron backscatter diffraction study of deformation and recrystallization textures of individual phases in a cross-rolled duplex steel

The evolution of microstructure and texture during cross-rolling and annealing was investigated by electron backscatter diffraction in a ferritic–austenitic duplex stainless steel. For this purpose an alloy with nearly equal volume fraction of the two phases was deformed by multi-pass cross-rolling process up to 90% reduction in thickness. The rolling and transverse directions were mutually interchanged in each pass by rotating the sample by 90° around the normal direction. In order to avoid deformation induced phase transformation and dynamic strain aging, the rolling was carried out at an optimized temperature of 898 K (625 °C) at the warm-deformation range. The microstructure after cross warm-rolling revealed a lamellar structure with alternate arrangement of the bands of two phases. Strong brass and rotated brass components were observed in austenite in the steel after processing by cross warm-rolling. The ferrite in the cross warm-rolling processed steel showed remarkably strong RD-fiber (RD//< 011 >) component {001}< 011 >. The development of texture in the two phases after processing by cross warm-rolling could be explained by the stability of the texture components. During isothermal annealing of the 90% cross warm-rolling processed material the lamellar morphology was retained before collapse of the lamellar structure to the mutual interpenetration of the phase bands. Ferrite showed recovery resulting in annealing texture similar to the deformation texture. In contrast, the austenite showed primary recrystallization without preferential orientation selection leading to the retention of deformation texture. The evolution of deformation and annealing texture in the two phases of the steel was independent of one another.     

High-temperature hydrogen resistance of stainless steels

At elevated temperatures, the influence of hydrogen on various mechanical characteristics of martensitic and disperse-hardened austenitic steels is different. The maraging steel has better characteristics of durability and plasticity and the critical values of static and cyclic crack resistance at temperatures of 450–600°K than the austenitic steel with intermetallic hardening. As a result of the intense temperature softening, its ultimate and yield strengths are much lower than for the austenitic steel. The austenitic steel has higher resistance in terms of the threshold value ∆K _th. At room temperature, the low-cycle fatigue limit proves to be most sensitive to the action of hydrogen, whereas at 673°K, the parameter K _fc for the maraging steel decreases.     

Experimental characterization and modeling of misorientations induced by plastic deformation at boundaries of annealing twins in austenitic steel

The evolution of misorientations at twin boundaries during hot deformations of austenitic steel was studied using electron backscatter diffraction and computer modeling with the purpose to explore the interrelation of this evolution with lattice rotations in grain interior. It was shown that a deformation-induced angular deviation from the initial twin misorientation turns out to be outside Brandon interval at many segments of the boundaries. The length fraction of these segments increases with strain, so that some twin boundaries transform to random ones at all their length. Statistical study shows that the most probable angular deviations are 9° and 13°, while the largest deviations are about 36° and 45° for the strain of 0.36 and 0.7, respectively. The experimental misorientation distributions were compared with theoretical ones computed using Taylor-type model. It was shown that deformation-induced misorientation changes are well described by the theory of polycrystals plasticity. At the same time, a discrepancy with the experiment occurs, which was proposed to be due to non-uniformity of grain lattice rotations.     

Influence of texture on the short fatigue crack growth in austenitic stainless steel

We study the influence of texture on the propagation of short cracks on the basis of the evolution of a family of microcracks in 316L austenitic stainless steel under the conditions of symmetric tension-compression. It is shown that the process of crack initiation is concentrated on grain boundaries and strongly depends on the mesostructure. In the process of fatigue fracture, the transcrystalline propagation of the largest crack (affected mainly by the global texture) is predominant. We discuss two approaches to the prediction of the influence of texture on the fatigue life. According to the concept of equivalent crack, this influence can be estimated depending on the size of the largest crack. On the contrary, the Zhurkov criterion deals with the state of the defect characterized by the presence of a family of microcracks. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 4, pp. 84–94, July–August, 2006.     

Influence of texture on twin boundary cracks in fatigued austenitic stainless steel

It is well-known that crack initiation in fatigued austenitic steel (316L) specimens is dominated at lower deformation amplitudes by twin boundaries (TBs). For medium plastic strain amplitudes, it is shown here that the propagation of short cracks starting at TBs can be explained when both the surface tractions caused by elastic anisotropy as well as the related slip processes are considered. This conclusion has been obtained from grain orientation measurements along damaged TBs using the electron backscatter diffraction technique in the scanning electron microscope. The frequency of the damaged TBs strongly depends on the meso-texture given by the distribution of 60° 〈111〉 rotation axes of the twins in the pole figure. The texture was determined by automatic orientation mapping. Specimens, which were machined transverse to the rolling direction of the plate, show more damaged TBs than those machined parallel. Consequently, the risk of TB cracks can be reduced by favorable alignment of the specimens with respect to the rolling direction.     

Effect of reversion of strain induced martensite on microstructure and mechanical properties in an austenitic stainless steel

Effect of reversion of strain induced α′ martensite on mechanical properties of an austenitic stainless steel has been examined. The α′ martensite formed by cold rolling (40%) at 0 °C has been reverted to austenite by carrying out annealing in the temperature range of 300–800 °C for 1 h. Microstructural investigation has demonstrated the enhanced reversion with increasing annealing temperature without any perceptible grain growth up to 800 °C. X-ray diffraction (XRD) analysis has revealed that 40% cold rolling has resulted in the formation of 32% martensite. The residual martensite content has been found to be about 8% after reversion at 800 °C. Different stages of reversion behavior has been examined by differential scanning calorimetric measurement. The variation of dσ/dε with ε is examined to identify different stages of work hardening of the investigated steel. Both tensile strength and percent elongation values increase with increasing annealing temperature up to 500 °C. Beyond that annealing treatment results in the drop of tensile strength value with the consequent increase in percent elongation. Attractive strength–ductility combination (22.80 GPa%) has been achieved after annealing of the 40% cold deformed specimen at 800 °C for 1 h. The fractographic observation corroborates the tensile results.     

The role of low-angle grain boundaries in multi-temperature equal channel angular pressing of Mg–3Al–1Zn alloy

Equal channel angular pressing was used to process an AZ31B magnesium alloy (nominally Mg–3Al–1Zn in wt%) at temperatures decreasing from 200 to 150 °C. The resulting microstructure was characterized by electron backscattered diffraction to reveal the role of low-angle grain boundaries in grain refinement. It was found that low-angle grain boundaries with misorientation angles lower than 5° are surrounded by regions of increased strain gradients, which can stimulate the generation of non-basal slip dislocations during the equal channel angular pressing at temperatures of approximately 150 °C. The strain gradients in the vicinity of the grain boundaries with misorientation angles in the range of 5°–10° were less frequent or were completely absent for high-angle grain boundaries with misorientation angles higher than 10°. This article also discusses the importance of low-angle grain boundaries for the generation of non-basal 〈c+a〉 dislocations needed for successful equal channel angular pressing of AZ31B at temperature of 150 °C.     

Microstructural evolution analysis of grains and tensile properties of tin bronze in hot extrusion at different temperatures

In this paper, Electron Back-Scattered Diffraction is adopted to analyze the tin bronze microstructural changes of grains and grain boundaries in hot extrusion from 620 to 720°C. The result shows that the size of grains grows as the temperature rises with a competitive status between recrystallisation and subgrains and deformed grains, and shows a random distribution with no obvious preferred orientation in structure. High angle grain boundaries (>30°) are most common with a large quantity of Σ3 boundaries at 60°. Twin grains firstly appear in recrystallisation and grow continuously at subgrains. In the tensile test, re-crystallised grains had certain influence on the plasticity of tin bronze. Along with the elongation of tin bronze at temperatures above 700°C, its plasticity also declined.     

Dynamic recrystallization in adiabatic shear band in α-titanium

The microstructure and microtexture in adiabatic shear bands (ASBs) on the titanium side of the titanium/mild steel explosive cladding interface are investigated by means of OM, SEM/EBSD and TEM. The highly elongated subgrains and fine equiaxed grains with low dislocation density are observed in ASBs. Recrystallization microtextures (28°, 54°, 0°), (60°, 90°, 0°) and (28°, 34°, 30°) are formed within ASBs. According to the misorientation distribution, the grain boundaries in ASBs are geometrical necessary boundaries (GNBs) with high-angles. The temperature in the ASBs is estimated to be about 776∼1142 K (0.4∼0.6 T_m). Based on the rotational dynamic recrystallization (RDR) mechanism, the fine equiaxed grains in the core of ASBs are formed during the deformation processing. These results indicate that the dynamic recrystallization take place in ASBs.     

Deformation Behavior of Fe-Mn-Cr-N Austenitic Steel

The deformation-induced microstructures and fracture appearances of Fe-Mn-Cr-N austenitic steel have been studied by using metallography, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. ϵ-Martensite and slip bands are the deformation-induced products on the {111}_γ planes and appear as thin straight laths with a 60–80° alignment difference between them. ϵ-Martensite and slip bands were found to be kinked at face-centered cubic twin boundaries with a kink angle of 35–40°. The slip bands in the tensile deformation zone appear as a set of equilateral triangles. Slip-band traces with an intersecting angle of 60° between the bands are also observed on the transgranular facets of both impact and tensile fractures.     

Precipitation in 25Cr20NiNbN austenitic steel after ageing at 750°C

Abstract

The heat-resistant austenitic steel 25Cr20NiNbN (HR3C) has been widely used in super heater and re-heater tubes in ultra-supercritical (USC) boilers. The precipitation processes in the steel are complicated because of its high alloy element contents. Precipitation in a 25Cr20NiNbN steel has been investigated after thermal aging at 750°C for period from 500 to 3000 h. The results indicate that M23C6 carbides generally precipitate along grain boundaries and that the major intragranular precipitates are NbCrN nitrides and M23C6 carbides. More precipitates are observed in the specimens after long term aging treatment: NbCrN nitrides on grain boundaries, as well as Cr3Ni2SiC carbides, have been found in the sample after aged for 2000 h at 750°C. The formation of σ phase has also been confirmed in the current steel after long term aging based on the results of X-ray diffraction (XRD), SEM and chromogenic methods.     

Effect of precipitates on subgrain growth during continuous recrystallisation in Al–4Cu

Abstract

It was found that on annealing supersaturated and cold rolled Al–4Cu at 280°C subgrains were initially formed as a result of the rearrangement of dislocations. Concurrent with the formation of subgrains having well defined boundaries, θ particles were formed, situated mostly on the subgrain boundaries, and the growth of subgrains was therefore severely hindered by the particles, leading to continuous recrystallisation for annealing times up to 10 000 min at 280°C. The pinning force due to particles located on the subgrain boundaries was much greater than that due to randomly dispersed particles. Subgrain growth was found to be arrested for particle parameters f/r of greater than 0·25 μm^?1, where f is the particle volume fraction and r is the mean particle radius.

MST/1572     

Influence of heat treatment on creep of a Mn–N stabilised austenitic stainless steel

Creep at 700 °C/196 MPa and 900 or 925 °C/27.4 MPa of 21Cr–4Ni–9Mn austenitic stainless steel is determined as a function of the heat treatment. The heat treatment variation involves altering the solution heat treatment cooling rate from water quenching to cooling at 6 or 4 °C/min causing: serrated grain boundaries versus planar grain boundaries, coarser intergranular carbides, and discontinuous precipitation of grain boundary reaction zones. Water quenching causes improved creep resistance. Creep fracture and cracking is intergranular. Coarse intergranular carbides and grain boundary reaction zones cause premature void formation and cracking, this damage leading to an accelerating creep rate and lowering creep resistance of the more slowly cooled conditions. During creep, grain boundary serrations, which may otherwise contribute to improved creep, are eliminated. Determining the individual influence of grain boundary serrations on creep requires a detailed investigation of various heat treatment parameters to prevent concurrent formation of grain boundary reaction zones and serrations.