Effect of Nitrogen on the Deformation Behaviour of High‐Nitrogen Austenitic Stainless Steels

The deformation behaviour of high‐nitrogen austenitic steels with the base composition of Fe‐18Cr‐10Mn containing various contents of nitrogen was investigated. Two deformation modes including deformation‐induced martensitic transformation (DIMT) and deformation twinning (DT) were observed depending on the nitrogen content. In the alloys with lower nitrogen contents, γ→?→α' martensitic transformation sequentially occurred, whereas DT acted as a main deformation mode and DIMT was suppressed in the alloys with increasing nitrogen content. Both DIMT and DT showed strong crystallographic orientation dependence. The competing mechanism between them was discussed in terms of the variation of stacking fault energy with nitrogen content.     

Spontaneous and Deformation‐Induced Martensite in Austenitic Stainless Steels with Different Stability

The fraction and microstructure of spontaneous and deformation‐induced martensite in three austenitic stainless steels with different austenite stability have been investigated. Samples were quenched in brine followed by cooling in liquid nitrogen or plastically deformed by uniaxial tensile testing at different initial temperatures. In‐situ ferritescope measurements of the martensite fraction was conducted during tensile testing and complemented with ex‐situ X‐ray diffractometry. The microstructures of quenched and deformed samples were examined using light optical microscopy and electron backscattered diffraction. It was found that annealing twins in austenite are effective nucleation sites for spontaneous α'‐martensite, while deformation‐induced α'‐martensite mainly formed within parallel shear‐bands. The α'‐martensite formed has an orientation relationship near the Kurdjumov‐Sachs (K‐S) relation with the parent austenite phase even at high plastic strains, and adjacent α'‐martensite variants were mainly twin related (<111> 60° or Σ3).     

Sensitization Behaviour of Manganese‐Alloyed Austenitic Stainless Steels

Manganese is an essential alloying element in advanced austenitic stainless steels with specific properties such as high resistance to harsh corrosive environments, high strength or low material costs. These materials are often used for welded constructions which have to be highly corrosion resistant. Hence it has to be ensured that the heat input during welding does not initiate the precipitation of chromium carbide resulting in a susceptibility to intergranular corrosion. This leads to the question whether the sensitization behaviour of manganese‐alloyed austenitic stainless steels is comparable to that of the well‐known conventional chromium nickel austenites. In the present work the effect of heat‐input on the susceptibility of the CrNi‐steel 1.4301 and the CrNiMn‐steel 1.4376 to intergranular corrosion (IGC) was considered. Investigations were carried out by corrosion testing in the so‐called Strauss‐Test to elucidate the effect of the annealing temperatures on the microstructure. Furthermore, the influence of heat treatment on the mechanical properties was evaluated by tensile testing. As a result, it could be demonstrated that manganese‐alloyed austenitic stainless steels like grade 1.4376 exhibit a sensitization behaviour very similar to the conventional austenitic steel grades. The same kinds of tests on intergranular corrosion resistance can be applied for both types of materials.     

Microstructure and Local Strain Fields in a High‐Alloyed Austenitic Cast Steel and a Steel‐Matrix Composite Material after in situ Tensile and Cyclic Deformation

The tensile and cyclic deformation behaviour of a new metastable austenitic stainless cast TRIP (TRansformation Induced Plasticity) steel and a composite material consisting of austenitic steel matrix (AISI 304) with 5% MgO partially stabilized ZrO₂ (MgO‐PSZ) was studied in‐situ in a scanning electron microscope (SEM). In‐situ tests in the SEM show the evolution of the microstructure with the strain for uniaxial deformation and the number of cycles during fatigue, respectively. Initially, deformation bands develop. In these bands, the face‐centred cubic austenite transforms into the hexagonal ε‐martensite and subsequently to the body‐centred cubic α'‐ martensite. This evolution was studied by different SEM techniques. Electron backscatter diffraction (EBSD) was applied for phase and orientation identification. The dislocation arrangement was investigated applying the electron channelling contrast imaging (ECCI) technique to different deformation stages. The studies are completed with measurements of local displacement fields using digital image correlation (DIC).     

Reverse Martensitic Transformation and Resulting Microstructure in a Cold Rolled Metastable Austenitic Stainless Steel

The reverse martensitic transformation in cold‐rolled metastable austenitic stainless steel has been investigated via heat treatments performed for various temperatures and times. The microstructural evolution was evaluated by differential scanning calorimetry, X‐ray diffraction and microscopy. Upon heat treatment, both diffusionless and diffusion‐controlled mechanisms determine the final microstructure. The diffusion reversion from α′‐martensite to austenite was found to be activated at about 450°C and the shear reversion is activated at higher temperatures with A_f′ ~600°C. The resulting microstructure for isothermal heat treatment at 650°C was austenitic, which inherits the α′‐martensite lath morphology and is highly faulted. For isothermal heat treatments at temperatures above 700°C the faulted austenite was able to recrystallize and new austenite grains with a low defect density were formed. In addition, carbo‐nitride precipitation was observed for samples heat treated at these temperatures, which leads to an increasing M_s‐temperature and new α′‐martensite formation upon cooling.     

Magnetic State of Deformed Austenite Before and After Martensite Nucleation in Austenitic Stainless Steels

 The effect of the increase in the paramagnetic susceptibility of austenite up to the true value of the deformation-induced martensite transition point εs has been experimentally established in steels X6CrNiTi18-10 (corresponding to AISI 321 steels). At this point nucleation and accumulation of martensite with the increase in the extent of deformation but at a constant magnetic state of austenite takes place.     

Effect of Texture and Temperature on Strain-Induced Martensitic Transformation in 304 Austenitic Stainless Steel

The austenitic stainless steel's remarkable mechanical properties are caused by twinning-induced plasticity and transformation-induced plasticity mechanisms. Numerous studies focus on stacking fault energy's effect, which is affected by various factors, to interpret and control these mechanisms. However, crystallographic orientation is also an important parameter for mechanical properties in metals. This study compares the mechanical properties and microstructural features of 304 austenitic stainless steel, focusing on the effect of initial texture and deformation temperature. Microstructural characterization is identified by an interrupted tensile test based on strain, tensile direction, and temperature conditions, and X-ray diffraction and electron back-scattered diffraction analysis are performed. The results show that the mechanical features and strain-induced martensitic transformation rate depend on the tensile directions. In addition, this trend is maintained irrespective of the temperature conditions. The attribute reason is that the difference in the Taylor factor and the formation rate of the deformed band structure is induced by the initial crystallographic orientations. Moreover, a decrease in temperature significantly increases the dislocation densities and abundant twins and transformed martensites formation. Furthermore, the yield and tensile strengths are enhanced while the elongation decreased with the tensile strains.     

301奥氏体不锈钢薄板拉伸强度与冷轧硬度之间的关系

 为对冷轧不锈钢薄板的产品硬度控制提供指导,尝试用一个新的方法来取代试轧,既达到控制冷轧板硬度的目的,又能降低成本、提高效率。对099mm厚的经过退火的301奥氏体不锈钢薄板进行冷轧减薄,并进行室温拉伸试验,测量其维氏硬度。通过观察金相和利用X射线衍射,验证了应变诱导马氏体相变是导致301奥氏体不锈钢冷轧和拉伸时产生加工硬化的主要原因。试验结果表明,冷轧和拉伸有着相似的加工硬化趋势,综合拉伸与轧制试验数据,确定了拉伸强度与冷轧硬度之间的关系,实现了通过拉伸强度来得到对应应变下的冷轧硬度,具有很好的预见性。冷轧可以提高301不锈钢的强度和硬度,显著改善其力学性能。     

In‐Situ Study of the Microstructure Development in a CrMnNi TRIP Steel During Plastic Deformation

The microstructure development in CrMnNi TRIP steel during the onset of the plastic deformation was investigated with the aid of in‐situ X‐ray diffraction experiments. The analysis of the shift and broadening of the X‐ray diffraction lines allowed the elastic and the plastic components of the lattice deformation to be separated from each other. This separation made possible to follow the formation of the microstructure features like stacking faults, deformation bands and local lattice rotations that were afterwards confirmed by X‐ray diffraction with high resolution, scanning electron microscopy and transmission electron microscopy.     

Fe-20Mn-0.6C TWIP钢力学性能和微观组织的演变

 研究一种Fe-Mn-C系新型TWIP钢的力学性能和微观变形机制。采用静态拉伸方法测试Fe-20Mn-0.6C钢在热轧和冷轧及热处理后的力学性能,通过金相、X-射线衍射、透射电镜观察等方法研究该钢的微观组织演变。结果表明：试验钢经过热轧后,表现出优异的综合力学性能,伸长率高达100%,抗拉强度达到924MPa。将热轧钢板经过适量冷轧后其抗拉强度提高到1210MPa。 热轧态组织为等轴的奥氏体基体及退火孪晶,拉伸变形后其微观组织中孪晶密度显著增加,晶粒内由一套孪晶系逐渐演化为两套孪晶系,而且因变形诱导马氏体相变产生大量马氏体组织。     

Dependence of Accelerated Creep Rate at 580 °C and Room Temperature Yield Stress for Two Creep Resistant Steels

Two creep resistant steels, P91 and X20, were tempered for 17520 h at 650 °C or 8760 h at 750 °C to study the growth and redistribution of carbide precipitates in martensite. On specimens annealed for a different time, yield stress at room temperature and accelerated creep rate at 580 °C were determined. With increasing yield stress in the range from 350 to 650 MPa the accelerated creep rate decreased continuously by about 2 orders of magnitude from 8·10^?7 s^?1 to 5·10^?9 s^?1. For equal yield stress, the creep rate was slightly lower for the steel P91 than for the steel X20.     

Effects of Solution Treatment and Test Temperature on Tensile Properties of High Mn Austenitic Steels

Tensile properties of high Mn austenitic Fe‐26.5Mn‐3.6Al‐2.2Si‐0.38C‐0.005B (HM1) and Fe‐18.9Mn‐0.62C‐0.02Ti‐0.005B (HM2, in mass%) steels after different solution treatments have been investigated. The results show that the solution treatment has a significant influence on microstructure and mechanical properties of the investigated steels. By appropriate solution treatment the product of tensile strength (R_m) and total elongation (A₅₀) of the hot rolled steel can be improved from ? 40000‐50000 MPa% to ? 55000‐65000 MPa% depending on the steel chemical composition. A solution treatment with a very high temperature, e.g. at 1100 °C for the Fe‐18.9Mn‐0.62C‐0.02Ti‐0.005B steel, results in a significant increase in the ?‐martensite fraction during quenching. This deteriorates the ductility of the steel. A solution treatment at low temperature in the austenitic range, e.g. at 700 °C for the Fe‐18.9Mn‐0.62C‐0.02Ti‐0.005B steel, results in a decrease in the grain size of the steel. This suppresses the ?‐martensite transformation during cooling. EBSD measurements revealed the mechanisms contributing to the overall plasticity of the investigated steels on the microscale. The plasticity of the 26.5Mn‐3.6Al‐2.2Si‐0.38C‐0.005B steel is produced mainly by TWIP mechanism under the examined experimental conditions, whereas for the Fe‐18.9Mn‐0.62C‐0.02Ti‐0.005B steel TWIP and TRIP mechanisms occur with different degrees depending on the test temperature of the tensile test.