Effect of Chromium-enhanced Diffusivity on Reverted Transformation in Metastable Austenitic Stainless Steels:Theoretical Calculation and Experiment

Effects of nucleation sites and diffusivity enhancement of chromium on reverted transformation of AISI 304 stainless steel during annealing process were investigated.Dynamics calculation revealed that the reverted transformation of strain-inducedα’-martensite→γaustenite could were closely associated with active nucleation sites and diffusivity enhancement of chromium in nanocrystallineα’-martensite.The experimental data and the results were in accordance with 2-grain austenite/α’-martensite junctions calculated theoretically,which could result from high chromium diffusion rate in nanocrystallineα’-martensite.In addition,low temperature is not conducive to reversed transformation,while high temperature and long annealing time will lead to inhomogeneous grain size distribution.

Martensite transformation induced by deformation and its phase electrochemical behavior for stainless steels AISI 304 and 316L

The martensite transformation induced by tensile elongation and its effect on the behavior of phase electrochemistry of AISI 304 and 316L in 3.5% NaCl solution were studied. The results show that the content of ((-martensite in stainless steel 304 increases with the true strain. As ((-martensite content increased, free corrosion potential and pitting potential of stainless steel 304 in 3.5% NaCl solution appeared the change trend of a minimum. It was also found that pitting nucleated preferentially at the phase interfaces between martensite and austenite. There existed apparent difference between electrochemical properties of austenite and of martensite for stainless steel 304 and 316L in 3.5% NaCl solution.

Deformation at Room and Low Temperatures and Martensite Transformation in Resistance Spot Welding Duplexγ&α(δ) Materials of 301L Stainless Steel

导致的粘性(旅行)和 twinning inducedplasticity (缇)完成的转变广泛地在双 gamma &alpha (三角洲)在单个奥氏体 steel.But 被学习了阶段例如焊接不锈钢的材料，他们是装载与 2 张公里 301L 表准备的抵抗点焊标本的实验的更少的 studied.Tensileshear ， wascarried 外面在 15 °C 和 -50°C.Optical 显微镜学和扫描电子显微镜学( SEM )，也，asX光线衍射( XRD )被用来调查

Effect of strain-induced martensitic phase transformation on the formability of nitrogen-alloyed metastable austenitic stainless steels

Strain-induced martensitic phase transformation and its influence on the formability of newly developed nitrogen-alloyed metastable austenitic stainless steels were systematically investigated. Yield strength for the asreceived steels bearing lownickel content was around 300 MPa and their elongation ratios varied from 55. 2% to61. 7%. Erichsen numbers of these samples differed from 13. 82 to 14. 57 mm. Although its Cu content was lower than that of other samples,steel D2 exhibited better plasticity and formability,which was attributed to γ→α'martensitic phase transformation. EBSD,XRD,and magnetism tests showed that increases in deformation ratio gradually increased the α' martensite phase of a sample,thereby contributing to its strain and inducing the optimal transformation-induced plasticity effect. An M_(d30/50) temperature of around 20 ℃,which is close to the deformation temperature,provided the austenite with adequate stability and gradually transformed it into martensite,thereby endowing lean ASS with better formability.

Effect of cold rolling on the microstructural, magnetic, mechanical, and corrosion properties of AISI 316L austenitic stainless steel

This study has evaluated the effect of different levels of cold rolling(from 0 to 50%)on the microstructural,magnetic,and mechanical properties and the corrosion behavior of 316L austenitic stainless steel in Na Cl(1 mol/L)+H_2SO_4(0.5 mol/L)solution.Microstructural examinations using optical microscopy revealed the development of a morphological texture from coaxial to elongated grains during the cold-rolling process.Phase analysis carried out on the basis of X-ray diffraction confirmed the formation of the ferromagneticα′-martensite phase under the stresses applied during cold rolling.This finding is in agreement with magnetic measurements using a vibrating sample magnetometer.Mechanical properties determined by tensile and Vickers microhardness tests demonstrated an upward trend in the hardness-to-yield strength ratio with increasing cold-rolling percentage,representing a reduction in the material’s work-hardening ability.Uniform and localized corrosion parameters were estimated via potentiodynamic polarization corrosion tests and electrochemical impedance spectroscopy.In contrast to the uniform corrosion,wherein the corrosion current density increased with increasing cold-working degree because of the high density of microstructural defects,the passive potential range and breakdown potential increased by cold working,showing greater resistance to pit nucleation.Although pits were formed,the cold-rolled material repassivation tendency decreased because of the broader hysteresis anodic loop,as confirmed experimentally by observation of the microscopic features after electrochemical cyclic polarization evaluations.

Softening interstage annealing of austenitic stainless steel sheets for stamping processes

To study the mechanics of work-hardening and annealing-softening,a series of experiments were conducted on samples of 304 austenitic stainless steel sheets.In addition,transmission electron microscopy(TEM),scanning electron microscopy(SEM),and tensile testing were carried out to study changes and mechanisms of the stainless steel structures and properties during work-hardening and annealing-softening.The results indicate that annealing at low temperatures(100-500 ℃) can only remove partial residual stresses in the sample and the softening via annealing is not obvious.Bright annealing and rapid cooling in a protective atmosphere can completely soften the cold-worked material.In addition,the low-temperature sample without a protective atmosphere only has a little oxidation on the surface,but at higher temperature the oxidized layer is very thick.Thus,high-temperature annealing should include bright annealing.

Researches on the problems in the production of low nickel austenitic stainless steel

Problems encountered in the production of low nickel austenitic stainless steel have been studied. These problems primarily include the changes to the microstructure of the slab during the heating process,the formation and removal of deformation- induced martensite during cold rolling,and the effects of the annealing process on the surface oxide structure. A reasonable manufacturing process has been proposed on the basis of the research results and highquality cold-rolled strips of low nickel austenitic stainless steel have been produced.

Nano-Twinning and Martensitic Transformation Behaviors in 316L Austenitic Stainless Steel During Large Tensile Deformation

The evolutions of nano-twins and martensitic transformation in 316L austenitic stainless steel during large tensile deformation were studied by electron backscatter diffraction(EBSD)technology and transmission electron microscopy(TEM)in detail.The results show that due to the low stacking fault energy of the steel,phase transformation induced plasticity(TRIP)and twinning induced plasticity(TWIP)coexist during the tensile deformation.The deformation firstly induces the formation of deformation twins,and dislocation pile-up is caused by the reduction of the dislocation mean free path(MFP)or grain refinement due to the twin boundaries,which further induces the martensitic transformation.With the increase of tensile deformation,a large number of nano-twins andα’-martensite appear,and the width of nano-twins decreases gradually,meanwhile the frequency of the intersecting deformation twins increases.The martensitic transformation can be divided into two types:γ-austenite→α’-martensite andγ-austenite→ε-martensite.α’-martensite is mainly distributed near the twin boundaries,especially at the intersection of twins,whileε-martensite and stacking faults exist in the form of transition products between the twins and the matrix.

Effect of martensitic transformation on nano/ultrafine-grained structure in 304 austenitic stainless steel

304 austenitic stainless steel was cold rolled in the range of 20%-80%reductions and then annealed at 700-900°C for 60 sto obtain nano/ultrafine-grained(NG/UFG)structure.Transmission electron microscopy,electron backscatter diffraction and X-ray diffraction were used to characterize the resulting microstructures.The results showed that with the increase of cold reduction,the content of martensite was increased.The steel performed work hardening during cold-working owing to the occurrence of strain induced martensite which nucleated in single shear bands.Further rolling broke up the lath-type martensite into dislocation-cell type martensite because of the formation of slip bands.Samples annealed at 800-960°C for 60 swere of NG/UFG structure with different percentage of nanocrystalline(60-100 nm)and ultrafine(100-500 nm)grains,submicron size(500-1000 nm)grains and micron size(>1000 nm)grains.The value of the Gibbs free energy exhibited that the reversion mechanism of the reversion process was shear controlled by the annealing temperature.For a certain annealing time during the reversion process,austenite nucleated first on dislocation-cell type martensite and the grains grew up subsequently and eventually to be micrometer/submicrometer grains,while the nucleation of austenite on lath-type martensite occurred later resulting in nanocrystalline/ultrafine grains.The existence of the NG/UFG structure led to a higher strength and toughness during tensile test.

Effects of Stabilization Heat Treatment on Microstructure and Mechanical Properties of Si‑Bearing 15Cr–9Ni–Nb Austenitic Stainless Steel Weld Metal

Two 15Cr–9Ni–Nb austenitic stainless steel weld metals with 2.5%Si and 3.5%Si(namely 2.5Si and 3.5Si samples,respectively)were designed and prepared through tungsten inert gas(TIG)welding and then hold at 800℃ or 900℃ for 3 h for stabilization.The microstructure and mechanical properties were investigated both for the as-welded and after-stabilization heat treatment(SHT)weld metals.There are 3.0–4.0%martensite and 2.5–3.5%δferrite in the 2.5Si as-welded weld metal and 6.0–7.0%δferrite in the 3.5Si as-welded weld metal.After SHT,a large amount of martensite formed in the 2.5Si weld metal,andδ→γtransition occurred during the SHT process both for the 2.5Si and 3.5Si weld metals.There were a large amount of coarse NbC and few nanoscale NbC in the as-welded weld metal.During the SHT,a large amount of nanoscale NbC formed in the matrix,while a large number of G phases formed at the austenite grain boundaries and theδ/γinterfaces.The decrease in solid solution C andδferrite content led to the decline of the yield strength of the weld metal after SHT.The martensite formed in 2.5Si weld metal after SHT had less effect on strength because of its low carbon content.The G phases formed during the SHT reduced the impact energy of the weld metal because it promoted the intergranular fracture,while theδ→γtransition reduced the amount of theδ/γinterfaces and avoided the intergranular fracture,which was beneficial for the impact toughness of the weld metals.

In Situ TEM Observation on Martensitic Transformation during Tensile Deformation of SUS304 Metastable Austenitic Stainless Steel

Through in situ transmission electron microscopy observation on SUS304 metastable austenitic stainless steel during stretching at room temperature,it is found that e martensite plates were induced preferentially from the sites of dislocation pile-ups.With increasing deformation,some of ε thin martensite platelets disappear and reversibly transform toγ austenite without heating treatment,which is different from the previous result that ε martensite can entirely transform toα'martensite.Then,some of deformation twins appear and grow along the vertical direction of ε martensite due to(111)_γ⊥(1010)_ε.Moreover,it is directly observed that multiple transformation mechanisms via γ→ε→γ,γ→ε,γ→α′,γ→ε→α′,γ→ deformation twins →α′ can co-exist.

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.

Effects of Strain Rate and Plastic Work on Martensitic Transformation Kinetics of Austenitic Stainless Steel 304

The martensitic transformation behavior and mechanical properties of austenitic stainless steel 304 were studied by both experiments and numerical simulation.Room temperature tensile tests were carried out at various strain rates to investigate the effect on volume fraction of martensite,temperature increase and flow stress.The results show that with increasing strain rate,the local temperature increases,which suppresses the transformation of martensite.To take into account the dependence on strain level,strain rate sensitivity and thermal effects,a kinetic model of martensitic transformation was proposed and constitutive modeling on stress-strain response was conducted.The validity of the proposed model has been proved by comparisons between simulation results and experimental ones.

Shape Memory Effect Induced by Stress-induced α' Martensite in a Metastable Fe–Cr–Ni Austenitic Stainless Steel

It is not clear whether a shape memory effect(SME) can be realized by stress-induced α' martensite in metastable austenitic stainless steels although the stress-induced ε martensite in these materials can result in the SME. To clarify this problem, the relationship between the shape recovery and the reverse transformation of the stress-induced ε and α' martensite in a 304 stainless steel was investigated. The results show that the stress-induced α' martensite can result in the SME when heating above 773 K. After deformation at 77 K and step heating or directly holding at 1073 K, two-stage shape recoveries below 440 K and above 773 K can be obtained due to the reverse transformation of the stress-induced e martensite and α' martensite, respectively. After deformation at room temperature, the α' martensite produced can result in the SME only when directly holding at 1073 K. The intrusion of more dislocations before the formation of the α' martensite at room temperature than at 77 K is the reason that the α' martensite induced at room temperature cannot result in the SME in the case of slow heating. The recovered strains resulting from the stress-induced ε and α' martensite are proportional to the amounts of their reverse transformation, respectively.

Controlled Laser Transformation Hardening of Martensitic Stainless Steel by Pulsed Nd:YAG Laser

Laser transformation hardening(LTH) was applied to the surface of the AISI 420 martensitic stainless steel by a pulsed Nd:YAG laser to obtain optimum hardness.The influences of process para-meters(laser pulse energy,duration time,and travel speed) on the depth and hardness of laser treated area were investigated.Image analysis of SEM micros-tructure of AISI 420 showed that plate-like carbide have almost fully and(30~40)% of globular carbide particles dissolved into the matrix after laser transformation hardening by pulsed laser and the microstructure was refined to obtain controlled tempered martensite microstructure with 450 VHN hardness.

Effect of Cold Deformation on Phase Evolution and Mechanical Properties in an Austenitic Stainless Steel for Structural and Safety Applications

The effects of cold deformation on the formation of strain induced α' martensite and mechanical properties of an austenitic stainless steel have been examined.X-ray diffraction analysis has revealed that 30% and 40% cold rolling have resulted in the formation of 24% and 31.5% martensite respectively.Microstructural investigation has demonstrated that the formation of martensite is enhanced with increase in the percent deformation at 0 ℃.Investigation of mechanical properties reveals that hardness,yield strength and tensile strength values increase where as percent elongation drops with increasing deformation.The fractographic observation corroborates the tensile results.Examination of sub-surface at the fractured end of the tensile sample manifests that void/microcrack nucleation occurs in the interfacial regions of the martensite phase as well as at the austenite-martensite interface.

Influence of Strain Rate on Tensile Characteristics of SUS304 Metastable Austenitic Stainless Steel

The microstructure characteristics and plastic deformation behavior of SUS304 metastable austenitic stainless steel sheets have been investigated during tensile process at diferent strain rates at room temperature.The yield stress continuously increases with strain rates due to low fraction of martensite transformed from austenite at 0.2%plastic stain.While the ultimate tensile stress(UTS)and elongation gradually decreases and then slightly increases with increase in strain rate from 0.0005 s 1to 0.1 s 1,which is attributed to the variation of the martensite fraction that is afected seriously by adiabatic heating.A higher temperature increase in the tensile specimens restricts the martensitic transformation at high strain rate.The strain rate of 0.1 s 1is considered as a transition deformation rate from quasi-static state to plastic forming,where the transformed martensitic content is very small in a higher strain rate range.Anomalous stress peaks in the later half stage of deformation occur at a very low strain rate(i.e.,0.0005 s 1)result from X-shaped strain localization repeatedly sweeping over the specimen.With increasing strain rates,the variation of dimple number density follows similar trend as that of UTS and ductility because martensite fraction mostly influences void nucleation and growth.

Effect of deformation temperature on transformation-induced plasticity effect of lean duplex stainless steel

Effects of deformation temperature on the mechanical properties and microstructure of lean duplex stainless steels B2102 and $32101 have been investigated.It was found that the strength decreased continuously with increases in temperature from-60 ℃ to 100 ℃.The strength of $32101 was higher than that of B2102 owing to its higher nitrogen content.Plasticity of B2102 increased with an increase in deformation temperature from-60 ℃ and reached the optimal elongation ratio of 49%-54% after deformation at 20-50 ℃.Martensite transformation was observed during deformation due to the transformation-induced plasticity effect.The optimal elongation was achieved at deformation temperatures close to the M_d(30/50) temperatures of 62℃ and 6℃ for B2102 and S32101,respectively.

Hardening after annealing in nanostructured 316L stainless steel

In this work,coarse-grained 316 L stainless steels were cold rolled with a thickness reduction of^83%(CR 83%).After annealing,the behaviors of the nanostructured stainless steel samples were systematically investigated in the temperatures range of 200C–650C.It was found that with increasing annealing temperature the volume fraction of theα0-martensite first increased to reach a maximum value at 400C,then the volume fraction decreased with further increases of the annealing temperature.The yield strength was increased from 1400 MPa to 1720 MPa after annealing;this strong hardening effect in cold rolled 316 L stainless steel was mainly attributed to the increase of the volume fraction ofα0-martensite.

Mechanical and Transformation Behaviors of a C-Mn-Si-Al-Cr TRIP Steel under Stress

导致的粘性(旅行) 钢高联合的转变力量和优秀韧性，使他们在汽车的工业在碰撞相关的部分适合申请。然而，在常规旅行钢的高 Si 内容将在热卷的长带上产生表面缺点，它是困难的在连续电镀处理线。为了解决上述问题，有代替 Si 的多数的艾尔的增加的 TRIP 钢被设计。在现在的纸，在 C-Mn-Si-Al-Cr 旅行钢的各种各样的阶段的体积部分被 metallographic 检查和 X 光检查衍射分析决定，并且 multi-phasemicrostructures 基于他们的高度差别用一台原子力量显微镜被描绘。张力的测试在从 -40 deg C 到 90 degC 的不同温度被执行。结果证明在现在的旅行钢的转变温度 M_s^( 西格马) 不能由于它保留的奥氏体的更低的体积部分是坚定的，与常规旅行钢不同。当在不同温度的收益应力和张力的强度比常规旅行钢的那些高时，它被归因于 Cr 的增加。以便在全部的延伸上评估马氏体的转变的效果，没有在液体氮熄灭获得的保留的奥氏体的样品在张力的测试下面被执行。Theresults 显示包含 9% 保留的奥氏体的原来的样品的延伸比在液体氮熄灭的样品的高是大约 20% ，它证明保留的奥氏体在改进 TRIP 钢的延伸起一个重要作用。