Thermal Stability in Nanocrystalline Fe-30?Wt?Pct Ni Alloy Induced by Surface Mechanical Attrition Treatment

By means of surface mechanical attrition treatment (SMAT), a nanocrystalline surface layer is produced in Fe-30 wt pct Ni alloy, accompanying the formation of the strain-induced martensite. The thermal stability of nanocrystalline martensite and parent phase austenite in Fe-30 wt pct Ni alloy is studied by X-ray diffraction (XRD) and transmission electron microscope (TEM). The grain growth kinetics parameters, time exponent, n, and activation energy, Q, for both martensite and austenite, are determined, respectively. The TEM observations indicate that abnormal grain growth occurs during annealing at high temperatures.     

Strain Rate Dependent Flow Stress and Energy Absorption Behaviour of Cast CrMnNi TRIP/TWIP Steels

Modern steel developments often use additional deformation mechanisms like the deformation induced martensitic transformation (TRIP‐effect) and mechanical twinning (TWIP‐effect) to enhance elongation and strength. Three high‐alloyed cast CrMnNi‐steels with different austenite stabilities were examined. Dependent on the austenite stability, TRIP‐effect and TWIP‐effect were found. A low austenite stability causes a distinctive formation of deformation induced α'‐martensite and therefore a strong strain hardening. The increase of strain rate leads to an increase in yield strength and flow stress, but also to a counteractive adiabatic heating of the specimen. Dependent on the degree of deformation, low austenite stabilities and high strain rates lead to excellent values in specific energy absorption.     

A multi-scale grained microstructure of the surface nanocrystallized 304 stainless steel sheets after warm-rolling

An ultrafine grained microstructure was obtained for 304 stainless steel(304SS)sheets by using surface nanocrystallization and warm-rolling.The microstructure and mechanical properties were determined by X-ray diffraction(XRD),transmission electron microscope(TEM)and a test on microhardness.Experimental results were shown that the microstructure was featured by a continuous distribution from the nanocrystalline on the surface to micro-grains in the center,in which the volume fraction of the micro-sized grains is about 40% in the surface layer.This multi-scale grained microstructure was composed of austenite and martensite phases with a gradient increasing volume fraction of austenite from the surface to the centre.The microhardness of the resultant steel was higher than 150% of that as received,due to the refined grains and strain-induced martensitic transformation.The hardness distribution was consistent with the microstructural variation,suggesting a good combination of high strength and improved ductility.     

Nanocrystallization and magnetic properties of Fe-30 weight percent Ni alloy by surface mechanical attrition treatment

A nanostructured surface layer was formed in Fe-30 wt pct Ni alloy by surface mechanical attrition treatment (SMAT). The microstructure of the surface layer after SMAT was investigated using optical microscopy, X-ray diffraction, and transmission electron microscopy. The analysis shows that the nanocrystallization process at the surface layer starts from dislocation tangles, dislocation cells, and subgrains to highly misoriented grains in both original austenite and martensite phases induced by strain from SMAT. The magnetic properties were measured for SMAT Fe-30 wt pct Ni alloy. The saturation magnetization (M _s ) and coercivity (H _c ) of the nanostructured surface layers increase significantly compared to the coarse grains sample prior to SMAT. The increase of M _s for SMAT Fe-30 wt pct Ni alloy was attributed to the change of lattice structure resulting from strain-induced martensitic transformation. Meanwhile, H _c was further increased from residual microstress and superfined grains. These were verified by experiments on SMAT pure Ni and Co metal as well as liquid nitrogen-quenched Fe-30 wt pct Ni alloy.     

Temperature Dependent Deformation Mechanisms of a High Nitrogen‐Manganese Austenitic Stainless Steel

The influence of temperature on the deformation behaviour of a Fe‐16.5Cr‐8Mn‐3Ni‐2Si‐1Cu‐0.25N (wt%) austenitic stainless steel alloy was investigated using transmission electron microscopy and X‐ray diffraction measurements. Recrystallized samples were deformed under tension at ?75°C, 20°C, and 200°C and the microstructures were characterized after 5% strain and after testing to failure. Deformation to failure at ?75°C resulted in extensive transformation induced plasticity (TRIP) with over 90% α′‐martensite. The sample deformed to 5% strain at ?75°C shows that the austenite transformed first to ?‐martensite which served to nucleate the α′‐martensite. Transformation induced martensite prohibits localized necking providing total elongation to failure of over 70%. At room temperature, in addition to some TRIP behaviour, the majority of the deformation is accommodated by dislocation slip in the austenite. Some deformation induced twinning (TWIP) was also observed, although mechanical twinning provides only a small contribution to the total deformation at room temperature. Finally, dislocation slip is the dominant deformation mechanism at 200°C with a corresponding decrease in total elongation to failure. These changes in deformation behaviour are related to the temperature dependence on the relative stability of austenite and martensite as well as the changes in stacking fault energy (SFE) as a function of temperature.     

Stress‐Temperature‐Transformation and Deformation‐Temperature‐Transformation Diagrams for an Austenitic CrMnNi as‐cast Steel

Stress‐Temperature‐Transformation (STT) and Deformation‐Temperature‐Transformation (DTT) diagrams are well‐suited to characterize the TRIP (transformation‐induced plasticity) and TWIP (twinning‐induced plasticity) effect in steels. The triggering stresses for the deformation‐induced microstructure transformation processes, the characteristic temperatures, the yield stress and the strength of the steel are plotted in the STT diagram as functions of temperature. The elongation values of the austenite, the strain‐induced twins and martensite formations are shown in the DTT diagram. The microstructure evolution of a novel austenitic Cr‐Mn‐Ni (16%Cr, 6% Mn, 6% Ni) as‐cast steel during deformation was investigated at various temperatures using static tensile tests, optical microscopy and the magnetic scale for the detection of ferromagnetic phase fraction. At the temperatures above 250 °C the steel only deforms by glide deformation of the austenite. Strain‐induced twinning replaces the glide deformation at temperatures below 250 °C with increasing strain. Below 100 °C, the strain‐induced martensite formation becomes more pronounced. The kinetics of the α'‐martensite formation is described according to stress and deformation temperatures. The STT and DTT diagrams, enhanced with the kinetics of the martensite formation, are presented in this paper.     

冲击载荷对10Mn钢磨损性能的影响及耐磨机制分析

通过水韧处理配合450℃时效热处理工艺,研究10Mn钢在2和5 J冲击载荷下耐磨性能.结果表明,10Mni钢在5J冲击载荷下失重量更少,加工硬化的速率更快,并且在不同冲击载荷下磨损失重量都出现了周期性变化.磨损表面SEM结果表明,表面存在犁削磨损、凿削磨损、裂纹.通过XRD数据对亚表面奥氏体和马氏体体积分数进行定量计算...     

等温压缩对高强度汽车钢WHT1300HF的马氏体相变及微观组织的影响

通过等温形变研究了形变参数(形变温度、形变速率、形变量)对高强度汽车钢WHT1300HF的微观组织转变和形貌的影响规律。研究结果表明:增加奥氏体等温形变量,有利于铁素体的缺陷形核,促进了形变奥氏体向铁素体转变;奥氏体的形变强化导致马氏体相变阻力增大,马氏体相变开始温度(Ms)下降,细小晶粒数量和小角度晶界数量增多;增加奥氏体等温形变(40%)速率能同时促进马氏体和铁素体相变,但马氏体体积分数和小角度晶界数量减少,细小晶粒数量略有提高;降低等温形变温度加剧奥氏体的形变强化,导致Ms温度下降,马氏体体积分数、小角度晶界比例减少,细小晶粒数量增多,铁素体含量明显增加。     

Mechanical Behaviors of Ultrafine-Grained 301 Austenitic Stainless Steel Produced by Equal-Channel Angular Pressing

The technique of equal-channel angular pressing (ECAP) was used to refine the microstructure of an AISI 301 austenitic stainless steel (SS). An ultrafine-grained (UFG) microstructure consisting mainly of austenite and a few martensite was achieved in 301 steel after ECAP processing for four passes at 523 K (250 °C). By submitting the as-ECAP rods to annealing treatment in the temperature range from 853 K to 893 K (580 °C to 620 °C) for 60 minutes, fully austenitic microstructures with grain sizes of 210 to 310 nm were obtained. The uniaxial tensile tests indicated that UFG 301 austenitic SS had an excellent combination of high yield strength (>1.0 GPa) and high elongation-to-fracture (>30 pct). The tensile stress–strain curves exhibited distinct yielding peak followed by obvious Lüders deformation. Measurements showed that Lüders elongation increased with an increase in strength as well as a decrease in grain size. The microstructural changes in ultrafine austenite grains during tensile deformation were tracked by X-ray diffraction and transmission electron microscope. It was found that the strain-induced phase transformation from austenite to martensite took place soon after plastic deformation. The transformation rate with strain and the maximum strain-induced martensite were promoted significantly by ultrafine austenite grains. The enhanced martensitic transformation provided extra strain-hardening ability to sustain the propagation of Lüders bands and large uniform plastic deformation. During tensile deformation, the Lüders bands and martensitic transformation interacted with each other and made great contribution to the excellent mechanical properties in UFG austenitic SS.     

The Effect of Size and Shape of Austenite Grains on the Mechanical Properties of a Low‐Alloyed TRIP Steel

The effects of size and shape of austenite grains on the extraordinary hardening of steels with transformation induced plasticity (TRIP) have been studied. The deformation and transformation of austenite was followed by interrupted ex situ bending tests using electron backscatter diffraction (EBSD) in a scanning electron microscope (SEM). A finite element model (FEM) was used to relate the EBSD based results obtained in the bending experiments to the hardening behavior obtained from tensile experiments. The results are interpreted using a simple rule of mixture for stress partitioning and a short fiber reinforced composite model. It is found that both, the martensite transformation rate and the flow stress difference between austenite and martensite significantly influence the hardening rate. At the initial stage of deformation mainly larger grains deform, however, they do not reach the same strain level as the smaller grains because they transform into martensite at an early stage of deformation. A composite model was used to investigate the effect of grain shape on load partitioning. The results of the composite model show that higher stresses develop in more elongated grains. These grains tend to transform earlier as it is confirmed by the EBSD observations.     

Martensite Transformation and Its Control in DP,TRIP and TWIP Steels

Designing of alloy concept and process for DP,TRIP and TWIP steels stressing at martensite transformation are analyzed.For DP steel,austenite volume percent and its carbon content at different intercritical temperatures are calculated as well as the tensile strength of the steel,which meet well with the experimental result.The condition for dissolution of carbide is discussed by experiments and predicted by kinetic estimation.Several sample TRIP steels are prepared and their concentration profiles are calculated showing different diffusion characteristics of elements.Calculation also shows carbon enrichment is successful in this stage through the quick diffusion of carbon from ferrite to austenie.In order to maintain the austenite stability or to prevent precipitation of cementite,minimum cooling rate from the intercritical zone to over aging stage is obtained through kinetic simulation.Bainite transformation is estimated,which indicates the carbon rerichment from ferrite of bainite structure to austenite in this stage is also successful.Thermal HCP martensite transformation and the strain induced martensite transformation in TWIP steel is introduced.Relationship between transformation and mechanical properties in the steel is also mentioned.     

基于退火路径的中锰钢组织转变与力学性能

为了更好地指导中锰钢工业试制,利用扫描电镜、电子背散射衍射、透射电镜和拉伸试验机等研究了不同退火路径下低碳中锰钢的组织转变及合金元素配分行为,并评价了其对力学性能的影响.结果表明,热轧中锰钢的显微组织主要由铁素体、板条马氏体、粒状贝氏体和残余奥氏体构成.经冷轧变形后,原组织中的铁素体和马氏体晶粒破碎,残余奥氏体和M/A...     

形变强化和逆相变细晶强化对Fe-18Cr-8Ni钢组织和性能的影响北大核心CSCD

奥氏体不锈钢较低的屈服强度限制了它在结构件中的使用。采用形变和相逆转变方法分别制备出了高屈服强度的奥氏体不锈钢。利用X射线衍射仪、光学显微镜、扫描电子显微镜、透射电子显微镜、电子背散射衍射技术和万能试验机分别对奥氏体钢进行组织表征和力学性能测试,结果表明粗大的奥氏体晶粒在形变过程中形成位错、剪切带、应变诱导马氏体等组织,相逆转变方法获得了超细的无缺陷等轴奥氏体晶粒。形变强化和细晶强化均能明显提高奥氏体不锈钢屈服强度(280 MPa提升至550 MPa)的同时保持较好的塑性(伸长率46%和55%)。     

Influence of laser- electromagnetic field composite strengthening on hardness of 45 steel

The influence of rotating magnetic field on hardness of 45 steel was studied. The 45 steel which was under the process of laser transformation hardening was magnetized by self- made magnetic enforcement device. The effect of magnetic frequency, magnetic time on hardness of 45 steel was studied. The microstructure of 45 steel laser transformation hardening layer was analyzed by metallographic microscope, and its microscope mechanism of properties?? improvement was considered. Experiment results show that surface hardness values of 45 steel samples are all improved under proper magnetic processing parameters. A part of retained austenite is transformed into martensite because of decrease of martensite activation energy when rotating magnetic field is applied. Dislocation redistributes and becomes more uniform. Microstructure of the sample is more compact.     

On the Significance of Nature of Strain-Induced Martensite on Phase-Reversion-Induced Nanograined/Ultrafine-Grained Austenitic Stainless Steel

We recently described the reversal of strain-induced martensite to the parent austenite phase in the attempt to produce nanograins/ultrafine grains via controlled annealing of heavily cold-worked metastable austenite. The phase-reversion-induced microstructure consisted of nanocrystalline (d < 100 nm), ultrafine (d ≈ 100 to 500 nm), and submicron (d ≈ 500 to 1000 nm) grains and was characterized by high strength (800 to 1000 MPa)–high ductility (30 to 40 pct) combination, which was a function of cold deformation and temperature-time annealing sequence.[1] In this article, we demonstrate that the success of the approach in obtaining nanograined/ultrafine-grained (NG/UFG) structure depends on the predominance of dislocation-cell–type structure in the severely deformed martensite. Electron microscopy and selected area electron diffraction analysis indicated that with an increase in the degree of cold deformation there is transformation of lath martensite to dislocation-cell–type martensite, which is a necessary prerequisite to obtain phase-reversion-induced NG/UFG austenite. The transformation of lath-type to dislocation-cell–type martensite involves refinement of packet and lath size and break up of lath structure. Based on detailed and systematic electron microscopy study of cold-deformed metastable austenite (~45 to 80 pct deformation) and constant temperature-time annealing sequence, when the phase reversion kinetics is rapid, our hypothesis is that the maximization of dislocation-cell–type structure in lieu of lath-type facilitates NG/UFG structure with a high strength–high ductility combination. Interestingly, the yield strength follows the Hall–Petch relation in the NG/UFG regime for the investigated austenitic stainless steel.     

Microstructure Evolution and Mechanical Behavior of a Hot-Rolled High-Manganese Dual-Phase Transformation-Induced Plasticity/Twinning-Induced Plasticity Steel

The microstructures and deformation behavior were studied in a high-temperature annealed high-manganese dual-phase (28 vol pct δ-ferrite and 72 vol pct γ-austenite) transformation-induced plasticity/twinning-induced plasticity (TRIP/TWIP) steel. The results showed that the steel exhibits a special Lüders-like yielding phenomenon at room temperature (RT) and 348 K (75 °C), while it shows continuous yielding at 423 K, 573 K and 673 K (150 °C, 300 °C and 400 °C) deformation. A significant TRIP effect takes place during Lüders-like deformation at RT and 348 K (75 °C) temperatures. Semiquantitative analysis of the TRIP effect on the Lüders-like yield phenomenon proves that a softening effect of the strain energy consumption of strain-induced transformation is mainly responsible for this Lüders-like phenomenon. The TWIP mechanism dominates the 423 K (150 °C) deformation process, while the dislocation glide controls the plasticity at 573 K (300 °C) deformation. The delta-ferrite, as a hard phase in annealed dual-phase steel, greatly affects the mechanical stability of austenite due to the heterogeneous strain distribution between the two phases during deformation. A delta-ferrite-aided TRIP effect, i.e., martensite transformation induced by localized strain concentration of the hard delta-ferrite, is proposed to explain this kind of Lüders-like phenomenon. Moreover, the tensile curve at RT exhibits an upward curved behavior in the middle deformation stage, which is principally attributed to the deformation twinning of austenite retained after Lüders-like deformation. The combination of the TRIP effect during Lüders-like deformation and the subsequent TWIP effect greatly enhances the ductility in this annealed high-manganese dual-phase TRIP/TWIP steel.     

Mechanical stability of retained austenite in tempered 9Ni steel

The behavior of the thermally stable austenite in the ductile fracture surface layer of a grain-refined and tempered 9Ni steel broken at 77 K was studied through use of Möss-bauer spectroscopy and transmission electron microscopy. Thin foils revealing the mi-crostructural profile of the fracture surface layer were prepared by electroplating a thick pure iron layer on the fresh fracture surface, then thinning a profile sample through a combination of conventional twin-jet electropolishing and ion milling techniques. The re-sults of both Mössbauer spectroscopy and TEM studies showed that the thermally stable austenite transforms to a dislocated martensite in the deformed zone adjacent to the duc-tile fracture surface. This result suggests that transformation of the retained austenite present in tempered 9Ni steel is compatible with low temperature toughness, at least when the transformation product is a ductile martensite.     

Selective Oxidation and Sub‐Surface Phase Transformation of TWIP Steel during Continuous Annealing

The selective oxidation of Twinning Induced Plasticity (TWIP) steel during annealing at 800 °C in a N₂ + 10%H₂ gas atmosphere with a dew point of ?17 °C and ?3 °C was investigated by means of high resolution transmission electron microscopy of cross‐sectional samples. The annealing resulted in the selective oxidation of Mn and Si and the austenite‐to‐ferrite phase transformation of the sub‐surface region. In the low dew point atmosphere, the annealing resulted in the formation of a MnO layer at the surface. Crystalline c –xMnO · SiO₂ (x ≥ 2) particles and amorphous a –xMnO · SiO₂ (x < 0.9) particles were found at the interface between the MnO layer and the steel matrix. In a narrow zone of the sub‐surface, the Mn depletion resulted in the transformation of the initial austenite. In the high dew point atmosphere, a thicker MnO layer was formed on the surface and no mixed manganese‐silicon oxides particles were observed at the MnO/steel matrix interface. In the sub‐surface, Mn was significantly depleted in the range of 2–3 µm below the surface and the initial austenite in this zone was transformed to ferrite. MnO particles were found at the grain boundaries and in the interior of grains.