Microstructures and properties of friction stir welded 304 austenitic stainless steel

Friction stir welding (FSW) is a solid phase joining process which makes use of friction and was developed in 1991 by TWI (The Welding Institute) of Britain.¹ This welding process received attention as a joining process to solve the problems associated with fusion welding of Al alloys and active research into the applications of this technique has been progressed by numerous research institutions, at home and abroad. As a result, this process was made fit for practical use in less than ten years of development and has been applied extensively in the manufacture of aerospace equipments,² rolling stock³ and automobile components.⁴     

The effect of cold rolling regime on microstructure and mechanical properties of AISI 304L stainless steel

In this paper, the effect of different thickness reductions by cold rolling on the microstructure and mechanical properties of AISI 304L austenitic stainless steel were investigated. The hot rolled steel strips were subjected to cold rolling at 0 °C from 10 to 90% thickness reduction. Microstructures, strain-induced martensitic transformation and mechanical properties of the cold-rolled specimens were characterized by X-ray diffraction, Feritscope measurements, optical metallography, hardness and tensile tests. The resulting transformation curve showed a sigmoidal shape with the saturation value of strain-induced martensite of approximately 100%. A good agreement was found between the experimental results and the Olsen–Cohen model. The results indicated that formation of strain-induced martensite clearly resulted in a significant strengthening of the steel.     

Phase formation during solidification of AISI 304 austenitic stainless steel

Following casting of AISI 304 strip, both cellular and skeletal ferrite were observed within individual austenite grains. It is proposed that solidification commences with primary austenite, γ_p, to produce cellular ferrite and further solidification results in the formation of ferrite but its subsequent epitaxial transformation to austenite at the γ_p interface produces skeletal ferrite in addition to cellular ferrite.     

温度对AISI304奥氏体不锈钢离子渗氮的影响

对AISI304奥氏体不锈钢进行脉冲电流辉光离子渗氮处理,在不同处理温度(480 ℃、520 ℃、580 ℃)下渗氮8 h后,获得了一定厚度的渗氮层.通过对渗层进行金相分析和硬度测试表明,随着渗氮温度升高,渗层厚度增大,显微硬度先增大后减小.综合温度对渗层厚度与显微硬度的影响,AISI304奥氏体不锈钢卡套辉光离子渗氮温度可采用520 ℃,渗氮后渗层厚度为90 μm,显微硬度为1317 HV0.1.     

Plasma nitriding of AISI 304 austenitic stainless steel with pre-shot peening

AISI 304 austenitic stainless steel was plasma nitrided at the temperature ranging from 410 to 520 °C with pre-shot peening. The structural phases, micro-hardness and electrochemical behavior of the nitrided layer were investigated by optical microscopy, X-ray diffraction, micro-hardness testing and anodic polarization testing. The effects of shot peening on the nitride formation, nitride layer growth and corrosion properties were discussed. The results showed that shot peening enhanced the nitrogen diffusion rate and led to a twice thicker nitrided layer than the un-shot peening samples under the same plasma nitriding conditions (410 °C, 4 h). The nitrided layer was composed of single nitrogen expanded austenite (S-phase) when nitriding below 480 °C, which had combined improvement in hardness and corrosion resistance.     

SUS304奥氏体不锈钢板点焊接头的超声成像分析

利用超声波水浸聚焦入射法对1 mm厚的SUS304奥氏体不锈钢板点焊接头进行超声C扫描成像检测.分析了不同焊接工艺参数下的C扫描图像特征,甄别了飞溅、焊穿等典型焊接缺陷,并提取其对应的A扫描信号.基于C扫描图像对焊核直径进行了测量,并与焊核切口端面尺寸进行了比较.结果表明,基于超声波水浸聚焦入射法得到的C扫描图像,能有效观测焊核内部形貌特征.焊接电流超过8 kA,电极力小于2 700 N时,超声波C扫描图像中清晰反映出飞溅、焊穿等缺陷,其对应区域的A扫描信号与正常熔核区波形特征有明显差异;借助超声C扫描图像测得的焊核直径为4.39~5.25 mm.     

Artificial neural networks for pitting potential prediction of resistance spot welding joints of AISI 304 austenitic stainless steel

The aim of this work is to predict the influence of welding process on pitting corrosion behaviour (PCB) of resistance spot welding (RSW) joints of AISI 304 austenitic stainless steel (ASS). PCB is estimated from the pitting potential (E_pitt), which is predicted from three RSW parameters. The prediction is carried out by artificial neural networks (ANNs) because the phenomena that relate the heat generated in RSW process to the PCB are highly complex and nonlinear. The study of PCB of RSW joints of ASS by ANNs is a novel research field in which this work makes a significant contribution.     

Nitrogen interstitial diffusion induced decomposition in AISI 304L austenitic stainless steel

The nature of the near-surface γ_N phase produced by low-temperature (～400 °C) plasma-assisted nitriding of an austenitic stainless steel 304L is studied. A combination of global probes (X-ray diffraction, nuclear reaction analysis, glow discharge optical emission spectroscopy) and local probes (field ion microscopy, conversion electron Mössbauer, X-ray absorption near edge structure and extended X-ray absorption fine structure spectroscopies) is employed to reveal the morphology, phase structure, atomic ordering and chemical state of the obtained γ_N phase. The results consistently reveal the heterogeneous nature of the nitrided layer consisting of nanometric CrN precipitates embedded in a Fe₄N-like matrix. The size of the precipitates is found to be larger at the surface than at the nitrided layer–steel interface. The precipitates have irregular, sphere-like shapes. Moreover, X-ray spectroscopic investigation revealed three different intermetallic distances and different chemical environments for Fe, Cr and Ni, accompanied by a large static disorder. These findings suggest that the presence of the interstitial N destabilizes the homogeneous element distribution in 304L even at such low temperatures. This leads to the segregation into Cr-rich zones that are coherent with the Fe₄N matrix. Possible atomistic decomposition mechanisms are discussed. Based on the heterogeneous nature of the γ_N phase revealed in 304L, an alternative view of its remarkable combination of properties such as large hardness, induced ferromagnetism and preserved corrosion resistance is considered.     

Influence of deep cryogenic treatment on the microstructure and properties of AISI304 austenitic stainless steel A-TIG weld

Appropriate deep cryogenic treatment can improve comprehensive mechanical properties of the AISI304 austenitic stainless steel activating flux tungsten inert gas (A-TIG) welds. The microstructure of the welds before and after deep cryogenic treatment was all austenite with a small amount of δ-ferrite. The vermiform ferrite + austenite distributed in the whole weld, but the lath-shaped ferrite + austenite mixed components only distributed in the centre of the weld. The phases in the two welds were all Cr–Ni–Fe–C and Fe–Ni solid solutions, ferric carbide (i.e. Fe₃C) and chromic carbides (i.e. Cr₂₃C₆ and Cr₇C₃). After deep cryogenic treatment, the grain size of the weld was decreased a certain of degree, and the carbide phase content was increased. The strength and micro-hardness of the weld joints were increased due to the grain refinement. The intergranular corrosion resistance of the weld was reduced because the precipitation of chromium carbides at the austenite grain boundary.     

Magnetic characterization of cold rolled and aged AISI 304 stainless steel

Magnetic easy axis predicted by the orientation distribution of the maximum amplitude of magnetic Barkhausen emission (MBE), which is obtained by magnetization in radial directions from the center of the specimens has been applied to determine the magnetic anisotropy on cold rolled and aged 304 SS in two sets of specimen. The maximum of the MBE has been found to orient along the rolling direction (RD) compared to the transverse direction (TD), indicating the presence of magnetic easy axis along the rolling direction for both sets. The strain induced martensite phase transformation has been determined using X-ray diffraction technique. The orientation distribution function (ODF) analysis has been carried out to obtain the crystallographic texture with cold rolling. ODF analysis revealed the 110 texture as the major. The magnetic anisotropy factor has also been determined with cold deformation and noticed that the strength of magnetic anisotropy decreases above 50% deformation for both the sets. Results have been explained considering two competitive effects, formation of crystallographic texture in the martensite phase and presence of compressive residual stresses along RD during cold rolling.     

The investigation of corrosion behavior of borided AISI 304 austenitic stainless steel with nanoboron powder

In this study, corrosion behavior and mechanical properties of AISI 304 austenitic stainless steel, which was borided with Nanoboron powder, was investigated. The commercially available steel was subjected to a boriding treatment with a size of 10–50 nm Nanoboron powders, at the temperatures of 1223 K to 1273 K with boriding durations of 2 to 4 h. Microstructure characterization of the steel was carried out with optical microscopy, scanning electron microscopy, microhardness and X-ray diffraction analyses. Corrosion tests were made by static immersion into a 10% H₂SO₄ acid solution and weight loss calculations as well as salt spray tests were carried out in accord with the ASTM B-117 standard. Boriding thermal treatment, increased the corrosion resistance of the steel against the acid solution, up to about 4.3 times while in the salt spray tests, weight loss corrosion resistance increased up to tier 2. However, anti-corrosion resistance decreased by 40%, its untreated value.     

Influence of peening on the corrosion properties of AISI 304 stainless steel

The effects of peening treatment on the microstructure and corrosion behavior of AISI 304SS were investigated. Shot and ultrasonic peening were performed on the austenitic stainless steel, and peened specimens were compared in terms of microstructure, surface roughness and corrosion resistance. Nano-sized grains, multi-directional mechanical twins and strain-induced martensite were formed on the surfaces, and the volume fraction of strain-induced martensite in the ultrasonically peened specimen was higher than that of the shot-peened specimen. The ultrasonically peened specimen which had smoother surface and contained more strain-induced martensite showed superior general and localized corrosion resistance to the as-received and shot-peened specimens.     

Use of EPR test to study the degree of sensitization in resistance spot welding joints of AISI 304 austenitic stainless steel

The degree of sensitization (DOS) in resistance spot welding (RSW) joints is considered as the combined effect of intergranular corrosion (IGC) and transgranular corrosion (TGC) in the heat affected zone (HAZ) and of interdendritic corrosion (IDC) in the weld nugget (WN). The DOS is evaluated from electrochemical potentiokinetic reactivation (EPR) tests. The application of EPR test to RSW joints is optimized and an electrochemical minicell is used to study the effect of heat input on IDC in the WN.     

An interface study of various PVD TiN coatings on plasma-nitrided austenitic stainless steel AISI 304

The adhesion between a TiN coating and a plasma-nitrided austenitic stainless steel is influenced by using different process steps. In order to obtain some information about the effects of various parameters, different process steps are used during the coating of austenitic stainless steel AISI 304. The deposition of a TiN layer with a Ti intermediate layer results in bad adhesion, especially when a thicker TiN coating has been deposited. X-Ray diffraction measurements indicate the presence of a martensitic phase on top of the plasma-nitrided surface after deposition of the TiN coating. This martensitic phase is also observed if after plasma nitriding only an additional heat treatment is carried out. A transmission electron microscopy study is also performed to reveal the microstructure.     

304奥氏体不锈钢冷拔过程中局部磁性形成机理

将有磁试样与无磁试样进行合金成分对比,用金相显微镜、扫描电镜、透射电镜及X射线衍射仪对两种试样的显微组织、局部化学成分、物相组成等进行了分析,研究304奥氏体不锈钢冷拔过程中出现局部磁性的形成机理。结果表明,有磁试样化学成分中的Mn含量偏高而Ni含量偏低。冷拔过程中有磁试样的显微组织横向与纵向晶粒尺寸均小于无磁试样。有磁试样夹杂物中存在O、Al元素,且氧化物夹杂对局部磁性无影响。有磁试样的物相中α'-bcc马氏体的体积分数比无磁试样高7.98%。304奥氏体不锈钢的合金成分异常,造成奥氏体处于亚稳态;冷拔过程中晶格变形作用和位错增值引起马氏体相变,在材料中形成的应力集中使得马氏体转变量增大,当马氏体相含量超过临界值,出现局部磁性。     

Combined effect of resistance spot welding and post-welding sensitization on the degree of sensitization of AISI 304 stainless steel

This work studies the combined effect of resistance spot welding (RSW) and post-welding sensitization on the degree of sensitization (DOS) of AISI 304 stainless steel (SS) by using EPR and DLEPR tests. The combined effect of RSW and post-welding sensitization at 750 °C gives rise to an overall DOS that: (i) is lower than that of the state without prior welding; (ii) that decreases initially with increasing sensitization time but then increases. This behaviour is due to the fact that the interdendritic corrosion (IDC) located in the weld metal decreases with increasing sensitization time.     

2205双相不锈钢与304奥氏体不锈钢的焊接

采用焊条电弧焊(SMAW),以E2209作填充材料对2205双相不锈钢与304奥氏体不锈钢异种金属焊接工艺进行研究,通过优化焊接工艺参数,获得了具有良好力学性能和合适双相比例的焊接接头.接头力学性能测试表明,拉伸试样断裂发生在强度相对较低的304母材侧;2205母材侧热影响区的显微硬度值高于焊缝和2205母材,而304...     

低温冷却对AISI 304不锈钢铣削加工的影响

研究低温冷却对AISI304不锈钢铣削加工的影响。用内径为1mm的管子来喷淋液氮到铣刀、削口和材料界面起到低温冷却的作用。液氮的流量为5.2L/min;采用顺铣和逆铣2个铣削方向、干铣和低温冷却2种方式和80、120、160和200m/min 4种铣削速度。结果表明：低温冷却和铣削速度对切削力有影响;低温冷却过程中的切削力和扭矩比干铣过程中的高;切削力随着铣削速度的加快而增大。进入铣刀半径范围内的削屑影响低速顺铣的效果。     

Nondestructive assessment of tensile properties of cold worked AISI type 304 stainless steel using nonlinear ultrasonic technique

A new approach based on nonlinear ultrasonic (NLU) technique is presented for nondestructive evaluation of yield strength and tensile strength of cold worked AISI type 304 stainless steel (SS). In this approach, the ultrasonic harmonics generated in cold worked stainless steel, as a result of interaction of ultrasonic wave with dislocations and substructural changes, are measured precisely and the nonlinear ultrasonic parameter, β is determined. A quadratic relationship, with a correlation coefficient better than 0.99, is found between the yield strength and the β parameter as well as between the tensile strength and β parameter of the cold worked specimens. The observed behavior is explained based on microscopy, variations in martensite volume fraction and dislocation density determined from X-ray diffraction profile analysis. The proposed approach can be effectively used for nondestructively ensuring the uniformity of tensile properties of steel components on shop-floor during various stages of manufacturing and also as feedback module in intelligent processing of materials concept.