Fabrication and Microstructural Control of Nano-structured Bulk Steels： A Review

There exist strong interests of developing nano-grained steels because of the outstanding properties including high strength/weight ratio, wear resistance, excellent toughness, and favorable cellular activity. This article reviews the main fabrication process and microstructural control of nano-structured steels over the last decades. Severe plastic deformation is considered as an effective route of obtaining the nano-grained microstructures. The process of cold-rolling and annealing of martensitic steel is a viable method to obtain bulk nano-structured low carbon steel, while the final thickness of the cold-rolling plate is limited. According to the theoretical results of the thermal simulation studies, a novel alloy design combined with the rapid transformation and rolling process is proposed to successfully fabricate nano-grained high strength bulk steel. The refinement mechanisms are expected to be taking advantage of increase in the transformation nucleation sites and inhibiting the grain coarsening. Moreover, corresponding mechanical properties are summarized.     

Strain mapping of crack extension in pseudoelastic NiTi shape memory alloys during static loading

Crack extension in pseudoelastic binary NiTi shape memory alloy (SMA) compact tension (CT) specimens was examined during static loading. The material composition of 50.7 at.% Ni (austenitic, pseudoelastic) was investigated using high-energy synchrotron X-ray diffraction. A miniature CT specimen was developed, which is small enough to allow in situ testing in a synchrotron beam line to identify phases, textures and lattice strains in front of a crack tip. Stress-induced martensite in pseudoelastic NiTi SMAs was mapped in front of the crack of a CT specimen during static loading using synchrotron radiation. The phase volume fraction and lattice microstrain results are discussed and compared with results from thermographic measurements. The Poisson effect is observed by comparing the lattice strains in the loading direction and transverse to the loading direction.     

Evolution of lattice strain and phase transformation of β III Ti alloy during room temperature cyclic tension

An in situ high-energy X-ray diffraction cyclic tension test was carried out on a β III Ti alloy to study its micromechanical behavior and the stress-induced phase transformation. Pre-strained material showed a microscopic multi-stage re-loading behavior following the sequence of elastic deformation, stress-induced martensite (SIM) transformation, a second stage of elastic deformation followed by a final stage of SIM transformation. Based on the relationship of internal strains and diffraction intensities between the β phase and the SIM, it is concluded that after a small strain deformation, the austenite is divided into two different sets of grains with different properties. Those that previously experienced phase transformation have a lower critical stress for the SIM transformation due to residual martensite and dislocations, while the rest have a higher trigger stress and only transform to martensite after the stress is back to levels comparable to where transformation was seen in the previous cycle. The different properties within the same austenite grain family cause the multistage re-loading behavior. The reverse phase transformation during unloading was impeded by the combination of increased dislocation density in the austenite and the increased tensile strain in the martensite prior to unloading.     

Strain-induced martensitic transformation in stainless steels: A three-dimensional phase-field study

A three-dimensional elastoplastic phase-field model is developed to study the microstructure evolution during strain-induced martensitic transformation in stainless steels under different stress states. The model also incorporates linear isotropic strain hardening. The input simulation data is acquired from different sources, such as CALPHAD, ab initio calculations and experimental measurements. The results indicate that certain stress states, namely uniaxial tensile, biaxial compressive and shear strain loadings, lead to single variant formation in the entire grain, whereas others, such as uniaxial compressive, biaxial tensile and triaxial strain loadings, lead to multivariant microstructure formation. The effects of stress states, strain rate as well as temperature on the mechanical behavior of steels are also studied. The material exhibits different yield stresses and hardening behavior under different stress states. The equivalent stress is higher at low strain rate, whereas a higher elongation is obtained at high strain rate. The deformation temperature mainly affects the hardening behavior of the material as well as the transformation, i.e. martensite volume fraction decreases with increasing temperature. Some of the typical characteristics of strain-induced martensite, such as the formation of thin elongated martensite laths, shear band formation and nucleation of martensite in highly plasticized areas, as well as at shear band intersections, are also observed.     

82B高碳钢盘条中马氏体成因

对82B盘条中马氏体产生区域进行了统计分析,得知其基本分布在1/2半径的盘条心部。利用有限元法对82B斯太尔摩冷却线进行了温度场模拟,模拟结果与实际生产符合较好,风冷线上盘条心部和边部温度变化趋势基本一致,其冷速小于动态CCT曲线中测定的马氏体开始出现的最低冷速,排除了本次控冷工艺设置不合理导致马氏体的产生。利用扫描电镜能谱仪和电子探针波谱仪对盘条心部马氏体进行了定量分析和线扫描分析,结果表明心部马氏体区域存在严重的Cr、Mn元素偏析,此为马氏体组织产生的根本原因。     

Effect of post-weld heat treatment on the wear resistance of hardfacing martensitic steel deposits

The effect of different post-weld heat treatments on the microstructure and wear resistance of martensitic deposits were studied. The deposit was welded using a metal-cored tubular wire, in the flat welding position, on a 375 × 75 × 19 mm SAE 1010 plate, using 98% Ar–2% CO₂ shielding gas mixture and with an average heat input of 2.8 kJ/mm. The samples were heat treated at temperatures between 500 and 680°C for 2 h. Chemical composition, Vicker's microhardness and wear properties with AMSLER tests in a sliding condition were determined. In the as welded condition, the microstructure was principally composed of martensite and retained austenite. Significant variations in wear resistance and hardness were measured for different tempering temperatures. For the different heat-treated conditions, it was observed that the decomposition of retained austenite to martensite and carbide precipitation was associated with the tempering of martensite. A secondary hardness effect was detected with maximum hardness of 710 HV for 550°C heat treatment temperature. The best performance in wear test was obtained for this condition. Wear rates for the different conditions were obtained and mathematical expressions were developed. For each case, wear mechanisms were analyzed.     

A thermodynamic approach for the development of austenitic steels with a high resistance to hydrogen gas embrittlement

The CALPHAD method was employed to assess the austenite stability of model alloys based on the Cr–Mn–Ni–Cu system. Stability was evaluated as the difference in Gibbs free energy between the austenite and ferrite phases. This energy difference represents the chemical driving force for the martensitic transformation and is employed as a design criterion. Six novel alloys featuring a lower driving force compared to the reference material AISI 316L were produced in laboratory. The susceptibility of all alloys to hydrogen gas embrittlement was evaluated by slow strain-rate tensile testing in air and hydrogen gas at 40 MPa and −50 °C. The mechanical properties and ductility response of four of the six alloys exhibited an equivalent performance in air and hydrogen. Thermodynamic calculations were in agreement with the amount of α′-martensite formed during testing. Furthermore, a 4.5 wt.% reduction in the nickel content in comparison to 316L promises a cost benefit for the novel materials.     

Relationship between mechanical property and microstructure of NiTi shape memory alloy

1IntroductionNi Ti shape memory alloy had superordinary superelasticity,shape memory property,biocompatibility,corrosion resistance and they have been applied extensively in aeronautics,aerospace,medical apparatusand instruments,and many other fields[1].The transformation behavior,shape memory effect,superelasticof Ni Ti shape memory alloy have been studied[2].It was founded that shape memory alloy had not onlyshape recovery but also recovery stress.However,studies of microstructure and the …     

轴承外圈裂纹原因分析

对某轴承套圈裂纹产生原因进行了分析,通过化学和金相检测,对套圈的材料质量和热处理质量进行全面分析,对裂纹和脱碳的产生进行整个工艺过程讨论和论证,并经摸拟试验,证明套圈裂纹是锻造湿裂。     

一种Fe-C-Si-Mn-Cr-Mo钢碳配分行为对组织的影响

 利用Formastor-FⅡ全自动相变仪模拟研究了一种Fe-0.24C-0.3Si-1.0Mn-0.56Cr-0.17Mo（质量分数,%）钢在冷却过程中的碳配分行为及其对马氏体和残余奥氏体的影响,用扫描电镜、透射电镜进行微观组织表征,用X射线衍射法和电子背散射衍射法测定残余奥氏体体积分数。结果表明,试验钢分别经末段慢冷和直接快冷工艺冷却后均获得马氏体＋残余奥氏体两相组织,其中直接快冷工艺所得马氏体相对杂乱,尺寸较小,残余奥氏体体积分数较少;而末段慢冷工艺所得马氏体板条较长,且发生了碳的配分,残余奥氏体体积分数较多,以薄膜状分布在马氏体板条间,板条内部含有高密度位错。