Grain size effect on high-speed deformation of Hadfield steel

The influence of the microstructure on the tensile properties and fracture behavior of Hadfield steel at high strain rate were studied. Hadfield steel samples with different mean grain sizes and carbon phases were prepared by rolling at medium temperatures and subsequent annealing. A sample with an average grain size larger than 10 μm, and a small number of carbides shows ductility with local elongation (post uniform elongation) at a high-speed tensile deformation rate of 10³ s⁻¹. In addition, the fracture surface changes from brittle to ductile with increasing strain rate. In contrast, a fine-grained sample with carbides undergoes brittle fracture at any strain rate. The grain size dependence is discussed by considering the dynamic strain aging as well as the emission of dislocation from cracks. The accelerated diffusion of carbon due to grain refinement is considered as one of the important reason for brittle fracture in the fine-grained Hadfield steel.     

Dislocation configurations through austenite grain misorientations

In the present investigation, many strain controlled low cycle fatigue experiments of metastable austenitic stainless steel were performed at various total strain amplitudes under ambient temperature where the strain rate was kept constant. Dislocation cell developed due to strain cycling was measured through extensive analytical transmission electron microscopic investigation and the deformed austenite grains’ misorientation was measured through electron back scattered diffraction experiments. A strong connection has been established with the dislocation substructures’ configurations, the deformed austenite grains’ misorientation and the extents of induced phase transformation occurs while cyclic plastic deformation of metastable austenite at various total strain amplitudes. It has been investigated that with the increase in strain amplitude, dislocation cells are getting more uniform. It has also been found that with the increase in strain amplitude, dislocation cell size decreases drastically towards the higher strain amplitudes.     

Deformation twinning in AISI 316L austenitic stainless steel: role of strain and strain path

Abstract

AISI 316L austenitic stainless steel was deformed at different strain and strain paths. The twin boundaries in the deformed microstructure had two possible origins: decay of original annealing twins and generation of deformation twins. Assuming that rotations of grains, specifically grains on both sides of a twin boundary, are responsible for the twin decay, a simple model was proposed to bring out the domain of relative twin generation. A biaxial strain path, in general, was associated with strong twin generation – an association or dependency linked to the texture estimated values of Taylor factor. Formation of strain induced martensite was also observed to be strain and strain path dependent and was more in biaxial strain path.     

Comparison of microstructures in electroformed copper liners of shaped charges before and after plastic deformation at different strain rates

Transmission electron microscopy observations of the recovered slugs of electroformed copper liner materials that had undergone high-strain-rate deformation show the existence of a wide range of crystal defects, including vacancy clusters and porosity. Cellular structures formed by tangled dislocations and subgrain boundaries consisting of dislocation arrays were also detected. Electron backscattering Kikuchi pattern technique analysis reveals that the fibrous texture observed in the as-formed copper liners of shaped charges disappeared after explosive detonation deformation. In a specimen that had been plastically deformed at a normal strain rate (4×10⁻⁴ s⁻¹), a high density of dislocations was observed within grains. These experimental results indicate that dynamic recovery and recrystallization play an important role during high-strain-rate deformation by virtue of a temperature increase in the deformation process, whereas the conventional slip mechanism operates during deformation at the normal strain rates.     

Microstructural evolution during formation of ultrafine grain structures by severe deformation

Abstract

High resolution electron backscattered diffraction analysis has been used to compare the evolution of the deformed state during severe deformation of two model materials: an Al–0.13 wt-%Mg alloy and an interstitial free steel. The alloys were deformed by equal channel angular extrusion up to a total effective strain of 10, at 20 and 500°C, respectively. At strains of <2, new high angle boundaries were formed from primary deformation bands. At strains of 2–5 (corresponding to high conventional strains) the new high angle grain boundaries, associated with the deformation bands, rotated towards the billet axis creating a lamella structure. Narrow bands of fine grains were also formed in unstable crystal orientations. With increasing strain the separation of the lamella high angle boundaries reduced until at very high strains their spacing was equal to the subgrain width and the long ribbon grains broke up into shorter segments. After an effective strain of 10 the microstructures for both materials consisted of submicron grains with an aspect ratio of ～3. In general it was observed that the interstitial free steel refined more rapidly with strain than the aluminium alloy.     

Characterization of different work hardening behavior in AISI 321 stainless steel and Hadfield steel

In order to distinguish the difference between AISI 321 stainless steel and Hadfield steel in work hardening behavior, both the Hollomon analysis and the differential Crussard–Jaoul analysis were used to determine the strain hardening exponent as a function of the strain. The results showed that the differential Crussard–Jaoul analysis characterized the discrepancy between AISI 321 steel and Hadfield steel in work hardening behavior more accurately than the Hollomon analysis. The work hardening of AISI 321 stainless steel resulted mainly from interactions of dislocations. When the true strain was rather low, the work hardening of Hadfield steel also resulted mainly from interactions of dislocations. At high strains, twinning would occur in Hadfield steel. It was the occurrence of twins that led to unusual work hardening at larger strains in Hadfield steel.     

高锰钢-碳钢复合板的制造和应用

介绍了一种高锰钢与结构钢的爆炸焊接工艺，包括表面处理方法、表面处理参数及条件、所采用的炸药和复合板的性能、爆炸焊用材料的准备。进行了成品复板（复合板）的质量检验，给出了该技术的应用实例。     

A novel high manganese austenitic steel with higher work hardening capacity and much lower impact deformation than Hadfield manganese steel

To tackle the problem of poor work hardening capacity and high initial deformation under low load in Hadfield manganese steel, the deformation behavior and microstructures under tensile and impact were investigated in a new high manganese austenitic steel Fe18Mn5Si0.35C (wt.%). The results show that this new steel has higher work hardening capacity at low and high strains than Hadfield manganese steel. Its impact deformation is much lower than that of Hadfield manganese steel. The easy occurrence and rapid increase of the amount of stress-induced ε martensitic transformation account for this unique properties in Fe18Mn5Si0.35C steel. The results indirectly confirm that the formation of distorted deformation twin leads to the anomalous work hardening in Hadfield manganese steel.     

An etching characterization method for revealing the plastic deformation zone in a SUS303 stainless steel

Etching characteristics used to reveal localized plastic deformation zones in a SUS303 stainless steel have been examined. The etching was conducted on a sample using an etchant consisting of 5-g ferric chloride, 50-mL hydrochloric acid, and 100-mL water. The sample was deformed severely and heated to various temperatures before the etching process. With this etching technique, the plastically deformed area is clearly observed even at low magnification. This is due to a change of the microstructural characteristics in the plastic deformation zone. There are different microstructure patterns that reveal the plastic zone in the sample with and without heating, e.g., plastic zone in the sample with heating to 800 °C is observed clearly due to randomly oriented crystals and recrystallized small grains with precipitated nano-size particles. Details of the etching characteristics that reveal the plastic deformation zone are further discussed.     

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.     

The role of grain size in static and cyclic deformation behaviour of a laser reversion annealed metastable austenitic steel

Different grain sizes were created in a metastable 17Cr‐7Mn‐7Ni steel by martensite‐to‐austenite reversion at different temperatures using a laser beam. Two fully reverted material states obtained at 990°C and 780°C exhibited average grain sizes of 7.7 and 2.7 μm, respectively. The third microstructure (610°C) consisted of grains at different stages of recrystallization and deformed austenite. A hot‐pressed, coarse‐grained counterpart was studied for reference. The yield and tensile strengths increased with refined grain size, maintaining reasonable elongation except for the heterogeneous microstructure. Total strain‐controlled fatigue tests revealed increasing initial stress amplitudes but decreasing cyclic hardening and fatigue‐induced α′‐martensite formation with decreasing grain size. Fatigue life was slightly improved for the 2.7‐μm grain size. Contrary, the heterogeneous microstructure yielded an inferior lifetime, especially at high strain amplitudes. Examinations of the cyclically deformed microstructure showed that the characteristic deformation band structure was less pronounced in refined grains.     

Effect of stacking fault energy on work hardening behaviors in Fe–Mn–Si–C high manganese steels by varying silicon and carbon contents

To improve the low work hardening capacity of Hadfield steel at low stress, the effect of stacking fault energy (SFE) on the microstructures and the work hardening behaviors of the Fe–Mn–Si–C high manganese steels were investigated by varying the silicon and carbon contents. The work hardening rates of the Fe–17Mn–Si–C steels with lower SFE were higher than that of the Hadfield steel at the strain below 0.28. The reason was that the amount of deformation-induced ε-martensite or mechanical twins was higher in the Fe–17Mn–Si–C steels than in the Hadfield steel due to their earlier onset. The work hardening rate of the Fe–17Mn–Si–C steels increased with decreasing the SFE because the rate of the formation and the amounts of martensite and twins increased with lowering the SFE.     

晶粒尺寸对304奥氏体不锈钢组织演变和性能的影响

研究了初始织构相近而晶粒尺寸不同的304奥氏体不锈钢在后续10%压缩变形和热处理过程中微观组织、力学和耐蚀性的变化。结果表明,具有相似织构而晶粒尺寸不同的样品变形热处理后其织构不同,粗晶在变形中织构的变化更大;织构相近时抗拉强度对晶粒尺寸的依赖较大;织构不同时,织构对硬度和抗拉强度的影响大于晶粒尺寸和微应变的影响;变形热处理后普通大角度晶界和晶内微应变的增大降低了试样的耐腐蚀性能;初始晶粒尺寸较小的试样在变形热处理后出现四种密排面平行于外表面的织构,其耐点蚀的性能更优。     

Microstructure and mechanical properties of Hadfield steel matrix composite reinforced with oriented high-chromium cast iron bars

To obtain a compatible material of high hardness and high toughness, Hadfield steel matrix was reinforced by oriented high-chromium cast iron bars. The mechanical behaviors of the as-cast and water-quenched composites were comparatively studied with a Hadfield steel substrate. The experimental results showed that the alloy powders inside the flux-cored welding wires could be melted by the heat capacity of Hadfield steel melt and became high-chromium cast iron bars. The impact toughness of the water-quenched composite was higher than that of the as-cast composite and lower than that of the Hadfield steel. The wear rate of the water-quenched composite was 1.23 mg/h m² at 0.3 kg and 2.93 mg/h m² at 1.2 kg, which was lower compared with those of the as-cast composite and Hadfield steel. The impact toughness and wear resistance of the water-quenched composite were related not only to the combining actions of the Hadfield steel matrix and high-chromium cast iron bars but also to the effect of heat treatment. The wear behavior of the water-quenched composite was industrially tested as pulverizer plate.     

Recrystallised grain size of commercial purity aluminium after hot torsion within steady state regime

Abstract

Hot deformation within the steady state regime of commercial purity aluminium has been carried out under wide ranges of deformation parameters: temperature, strain, and strain rate. The microstructure developed after deformation exhibited dynamically formed grains having volume fraction increasing with strain and of size inversely dependent on flow stress. The effects of the parameters of deformation and of the annealing temperature after deformation on statically recrystallised grain size and on the rate of grain boundary migration during recrystallisation were studied and can be satisfactorily described by equations in which each of the variables is considered to act independently. The effects of the deformation parameters on the grafnsize produced after static recrystallisation are interpreted in terms of their influence on the number of dynamically formed grains that can act as pre-existing nuclei. A decrease in the final recrystallised grain size with an increase in annealing temperature, which may be related to the number of dynamically formed grains that become viable nuclei, was observed. The growth rate during recrystallisation was found to decrease with increasing time, probably as a result of the distribution of stored energy within the deformed structure.

MST/1169     

Microstructural change of ultrafine-grained aluminum during high-speed plastic deformation

Effect of strain rate on microstructural change in deformation of the ultrafine grained (UFG) aluminum produced by severe plastic deformation (SPD) was studied. Commercial purity 1100 aluminum sheets were highly strained up to an equivalent strain of 4.8 by the Accumulative Roll-Bonding (ARB) process at ambient temperature. The ARB-processed sheets were found to be filled with pancake-shaped ultrafine grains surrounded by high-angle grain boundaries. The ultrafine grains had a mean grain thickness of 200 nm and a mean grain length of 1100 nm. The ultrafine-grained aluminum sheets were deformed at various strain rates ranging from 2 to 6.0×10⁴ s⁻¹ by conventional rolling, ultra-high-speed rolling, and impact compression. High-speed plastic deformation generates a large amount of heat, inducing coarsening of the ultrafine grains during and after deformation. On the other hand, it was also suggested that high-speed plastic deformation is effective for grain-subdivision, in other words, ultra-grain refinement, if the effect of heat generation is extracted.     

Effect of Nb on dynamic strain induced austenite to ferrite transformation

Abstract

Dynamic strain induced transformation (DSIT) is an interesting processing route to obtain ultrafine ferrite grains. In the present work, the effect of Nb on DSIT was investigated. Samples of low C–Mn steels, with and without Nb, were intensively deformed in hot torsion, aiming at the production of ultrafine ferrite grains. After soaking at 1200°C, the samples were cooled to 1100°C, submitted to hot torsion deformation to decrease the grain size and then cooled to 900, 850 or 800°C for further hot torsion deformation. In the steel without Nb, recrystallisation took place before enough deformation could be accumulated to induce ferrite formation, so DSIT would only take place at the lowest temperature investigated, 800°C. In the Nb steel, Nb addition delayed austenite recrystallisation, allowing DSIT ferrite to form at higher temperature than in the steel without Nb, 850°C.     

高锰钢整铸辙叉爆炸硬化实践与研究

针对目前爆炸硬化所用炸药的性能不足,研制出一种高聚物粘结塑性炸药,利用该塑性炸药对高锰钢辙叉进行爆炸硬化。试验表明,经过该炸药表面硬化后的高锰钢辙叉,在铸造基体硬度为HB170～190时,一次硬化后,表面硬度达到HB260—280;两次硬化后,表面硬度达到HB310～330。并进一步结合金相分析,探讨当前国内在高锰钢爆炸硬化中硬度测量方面存在的问题。最后,运用动力有限元软件对高锰钢爆炸硬化工艺过程进行了数值模拟,模拟结果与实验情况基本吻合。     

Microstructural evolution in 42CrMo steel during compression at elevated temperatures

In order to study the workability and to optimize the processing parameters for 42CrMo steel hot forming, the hot compressive tests of 42CrMo steel were performed in the temperature range of (850-1150) °C at strain rates of (0.01-50) s^− 1 and deformation degrees of (10-60)% on Gleeble-1500 thermo-simulation machine. The effects of processing parameters, including the strain rate, forming temperature and deformation degree, on the microstructures of 42CrMo steel were investigated by metallurgical analysis. The results show that the average grain size of the deformed 42CrMo steel increases with the forming temperature and decreases with the deformation degree and strain rate. In a word, the grain size of hot compressive 42CrMo steel is dependent on the forming temperature, strain and strain rate, also on Zener-Hollomon parameter.     

Microstructure and mechanical properties of UFG medium carbon steel processed by HPT at increased temperature

Using the high pressure torsion (HPT) deformation method the medium carbon steel (AISI 1045) was the experimental material used to conduct the deformation process. The torsion deformation experiment was performed at increased temperature of 400 °C. The influence of deformation processing parameters, resolved shear strain γ (number of turns N = 1–6) and applied pressure p (constant pressure of 7 GPa), was evaluated by microstructure analysis and mechanical properties. The strength behaviour was assessed by microhardness measurements across the disc to detect the positional hardening, by tensile tests and in situ measured torque. In situ measurement of torque during deformation allows characterizing the changes in mechanical properties due to the large shear deformation developed across the disc. To obtain absolute values of strength the ultimate tensile strength was measured in radial direction with respect to the deformed sample. From each deformed disc two sub-sized tensile test specimens with gauge length of 2.5 mm were machined. The tensile strength in samples increased markedly with the number of turns. The hardness measured at disc edge gradually increases as straining increases until it saturates after 2–3 turns. However, the hardness values at edge were different from those measured in disc centre and for applied straining no saturation was reached across the disc. The SEM and TEM investigations were carried out to analyze the fine microstructure evolution regarding the strain introduced. To follow the difference in strain distribution across the deformed disc the microstructure analysis was performed at edge and central site of the disc in order to evaluate the effect of the strain distribution. TEM investigation confirmed the increasing misorientation even in very small grains, the fragmentation and dissolution of the cementite lamellae, (diffuse cementite/ferrite boundaries), the alignment of the fragments to the shear plane with increasing deformation. Indistinct deformation of ferrite and preserved cementite lamellae morphology were found at the centre of the disc.