Effect of water-shower cooling during welding on tensile residual stress improvement in multi-layer welding of austenitic stainless steel plates

Abstract

A new welding method that uses a water shower behind the welding torch has been developed in order to reduce tensile residual stress in a welded region. When this method is applied to the welding of austenitic stainless steel, the welding and cooling conditions mainly determine how much the residual stress can be reduced. To optimize these conditions, we first used the robust design technique to determine the effects of the interpass temperature, the heat input quantity and the water-shower area on the residual stress distribution of bead-on-plate. We found that, to decrease the tensile residual stress, the interpass temperature should be high, the heat input low, and the water-shower area large. Effect of the water-shower cooling on multi-layer welding was examined analytically and experimentally. It was found that the residual stresses were tensile without water-shower cooling, but compressive with water-shower cooling under the optimized conditions. It can therefore be concluded that the new welding method is appropriate for reducing tensile residual stress in multi-layer welding of austenitic stainless steel.     

一种新型核级奥氏体不锈钢焊接接头的晶间腐蚀研究

利用"不锈钢硫酸-硫酸铜腐蚀实验"、光学显微镜和扫描电镜等测试方法分别对未敏化和经敏化处理(650℃×100 h)的800H和新型Cr18Ni30Mo2Al3Nb合金焊接接头的抗晶间腐蚀性能进行研究,结果表明,800H和Cr18Ni30Mo2Al3Nb接头焊缝组织均为单一的奥氏体基体,800H合金中TiN缺陷处易引起点蚀,而Cr18Ni30Mo2Al3Nb无明显点蚀现象;对比腐蚀失重、腐蚀深度等实验结果,未敏化和敏化态Cr18Ni30Mo2Al3Nb焊接接头的抗晶间腐蚀性能明显优于相同状态的800H。     

Solidification cracking in austenitic stainless steel welds

Solidification cracking is a significant problem during the welding of austenitic stainless steels, particularly in fully austenitic and stabilized compositions. Hot cracking in stainless steel welds is caused by low-melting eutectics containing impurities such as S, P and alloy elements such as Ti, Nb. The WRC-92 diagram can be used as a general guide to maintain a desirable solidification mode during welding. Nitrogen has complex effects on weld-metal microstructure and cracking. In stabilized stainless steels, Ti and Nb react with S, N and C to form low-melting eutectics. Nitrogen picked up during welding significantly enhances cracking, which is reduced by minimizing the ratio of Ti or Nb to that of C and N present. The metallurgical propensity to solidification cracking is determined by elemental segregation, which manifests itself as a brittleness temperature range or BTR, that can be determined using the varestraint test. Total crack length (TCL), used extensively in hot cracking assessment, exhibits greater variability due to extraneous factors as compared to BTR. In austenitic stainless steels, segregation plays an overwhelming role in determining cracking susceptibility.     

Embrittlement of austenitic stainless steel welds

To prevent hot-cracking, austenitic stainless steel welds generally contain a small percent of delta ferrite. Although ferrite has been found to effectively prevent hot-cracking, it can lead to embrittlement of welds when exposed to elevated temperatures. The aging behavior of type-308 stainless steel weld has been examined over a range of temperatures 400–850C for times up to 10,000 hr. Upon aging, and depending on the temperature range, the unstable ferrite may undergo a variety of solid state transformations. These phase changes affect creep-rupture and Charpy impact properties.     

双电极奥氏体不锈钢焊条单弧焊工艺研究

本文研究了双电极奥氏体不锈钢焊条单弧焊电弧的静特性、焊接电流、电弧电压、焊芯间隙对双电极焊条单弧焊的工艺性能和焊缝成形的影响;通过热电偶测试技术,对双电极焊条单弧焊焊接过程中不同部位焊芯表面温升进行了测定.研究结果表明焊芯直径为φ4.0mm的双电极A102焊条,其合适焊接工艺参数为:焊接电流140～160A,电弧电压45V左右,焊条两芯间隙1.2～1.5mm,焊接板厚8mm的1Cr18Ni9Ti材料,焊缝成形良好.     

Creep of austenitic stainless steel welds

Austenitic stainless steels (SS) find extensive application in power, petrochemical and nuclear industries in view of their excellent elevated temperature mechanical properties, corrosion resistance, formability and weldability. However, they are susceptible to hot cracking during fusion welding. To avoid this problem, chemical composition of the welding consumable is generally adjusted to promote primary ferrite mode of solidification and retain about 3 to 10%δ-ferrite in the as-welded condition. The duplex microstructure of the weld metal undergoes transformation to carbides and a variety of intermetallic phases during elevated temperature service and causes deterioration in the mechanical properties. This paper presents a comprehensive review of the current understanding of the solidification microstructures, ageing processes and their influence on the creep behaviour of types 308 and 316 SS weld metals. The effects of varying chemical composition,δ-ferrite content, electrode coating and welding processes on creep strength and ductility are examined. Current trends in the design of welded components for creep application are also discussed.     

奥氏体铸体和18—8不锈钢在烧碱中的腐蚀行为

用失重法和电化学方法研究了奥氏体铸铁的１８－８不锈钢在高温烧碱中的腐蚀行为。用扫描电镜观察了两种材料泵件表面的腐蚀形貌,分析了奥氏体铸铁耐碱腐蚀性能优于１８－８不锈钢的原因。     

Laser dissimilar welding of CoCrFeMnNi-high entropy alloy and duplex stainless steel

High entropy alloys(HEAs)have superior mechanical properties that have enabled them to be used as structural materials in nuclear and aerospace applications.As a dissimilar joint design is required for these applications,we created a dissimilar joint between CoCrFeMnNi-HEA and duplex stainless steel(DSS)through laser beam welding;a technique capable of producing a sound joint between the two materials.Microstructure examination using SEM/EBSD/XRD analysis revealed that the weld metal(WM)exhibits an FCC phase regardless of the postweld heat treatment(PWHT)temperature(800 and 1000℃)without forming detrimental intermetallic compounds or microsegregation.The heat-affected zone of the CoCrFeMnNi-HEA showed CrMn oxide inclusions while that of the DSS showed no inclusions.Moreover,a lower hardness was recorded by the WM compared to the base metal after welding.After PWHT,the hardness of the WM,CoCrFeMnNi-HEA,and DSS decreased with an increase in the PWHT temperature.However,the decrease in the hardness of the HEA was more significant than in the WM and DSS.The cause for this reduction in hardness was attributed to recrystallization and grain growth.In addition,a strength of 584 MPa with low ductility was recorded after welding.The obtained strength was lower than that of the BMs,but comparable to that of the welded CoCrFeMnNi-HEA.The application of PWHT resulted in over a 20％increment in ductility,with only a marginal reduction in strength.The deformation mechanism in the as-weld joint was mainly dominated by dislocation while that for the PWHT joint was twinning.We propose laser beam offset welding as a technique to improve the mechanical properties of the dissimilar joint,which will be the subject of future studies.     

Rapid welding of stainless steel wires using ultra-high frequency induction heating

In this study, JIS SUS202 stainless steel wires are rapidly welded by induction heating using a current at a frequency of 2?MHz. The effects of the heating parameters on the mechanical properties and microstructures of the joint are investigated. After induction welding, the main structures in the joint of the 202 stainless steel wires are austenite and ferrite. The amount of ferrite increases by increasing the heating temperature or heating time. Moreover, the tensile strength of the joint is superior for a higher welding temperature.     

不锈钢—碳钢单、双面复合板的爆炸焊接及性能研究

研究了不锈钢－碳钢单面、双面复合板爆炸焊接质量,结合界面的微观结构,剪切强度及耐蚀性能,结果表明,单、双面不锈钢－低碳爆炸焊接复合板的结合界面均为波状结构,结合面两侧存在一定组织变形,近界面处为角结晶组织,稍远处为拉伸变形后的维状组织,结合界面碳钢－侧过渡区存在增碳区,不锈钢一侧有一个脱碳层,双面复合界面的结合过渡区的单面为１．５倍宽,脱碳区也接近单面的１．５倍,采用切割爆炸焊接法有利于改善不锈钢－低碳钢复合板的边缘焊合,在同一基板上进行了双面不锈钢复合时,第一面复合界面的剪切强度比第二面复合界面的差,还是双面复合板,其界面结面强度均由起爆端的末端逐渐降低,结合界面的脱碳层对复合的耐蚀性能无明显影响。     

Crack resistance of austenitic stainless steel pipe and pipe welds with a circumferential crack under monotonic loading

This paper describes the experimental studies carried out on cracked austenitic stainless steel pipe and pipe welds under bending loads. Pipe welds were produced by gas tungsten arc welding (GTAW) and shielded metal arc welding (SMAW). Fracture resistance curves for pipe and pipe welds were compared. Results indicate that the fracture resistance of pipe and pipe weld (GTAW) is comparable but that of pipe weld (GTAW+SMAW) is inferior. Cracks do not deviate from their original plane during propagation as observed in the cases of carbon steel pipe and pipe welds. The fracture resistance of pipe welds does not depend on the loading histories to which it has been subjected prior to fracture test. Initiation and crack propagation were observed prior to the maximum moment. An existing limit load expression is applicable for the pipe base material but gives non‐conservative results for the pipe welds. Multiplication factors have been suggested for the pipe welds for evaluation of limit loads using the existing expression. Fracture resistance for the pipe and compact tension specimens have also been compared for base material and welds.     

A new life extension method for high cycle fatigue using micro-martensitic transformation in an austenitic stainless steel

Most of the conventional strengthening methods for metals and alloys such as work hardening, precipitation hardening, cause a decrease in ductility and are not very effective for cyclic loading. In this study, a new strengthening method, which is effective for high cycle fatigue, has been developed. The intersections of dislocations in a stainless steel are freezed by very fine martensite particles, which are supposed to suppress dislocation motion at low stress amplitudes. Fatigue life in a high cycle regime increased >60 times, and no decrease in ductility was observed in tensile tests, as compared to a work-hardened stainless steel.     

High-nitrogen steel laser-arc hybrid welding in vibration condition

ABSTRACT

Welding porosity and nitrogen content are considered significant factors affecting the mechanical properties of fusion-welding joints of high-nitrogen steel. In this study, a method of applying mechanical vibration in the welding process to reduce weld porosity and increase weld nitrogen content was investigated. The effects of mechanical vibration on porosity, tensile, and impact properties were analysed. The results indicated that the bubble floating speed in the vibrating weld pool is faster than that in the general welding mode. With the increase of mechanical vibration frequency, the porosity of the weld decreased at first and then rose. The tensile strength and impact energy increased first and then decreased, and the fracture surface indicated a ductile fracture.     

Photoelectrochemical investigations on individual ferritic and austenitic grains of a duplex stainless steel oxidized in water vapour

Abstract

This work presents a multiscale photoelectrochemical characterization of oxide layers, and aims mainly at introducing the mesoscopic scale (ca 30 µm). For the first time, photocurrent energy spectra could be recorded on individual ferritic and austenitic grains of a unique 2205 duplex stainless steel sample oxidized for 5 minutes at 650°C under 20% water vapour in nitrogen. These results allowed us to explain contrasting issues in the photoelectrochemical images obtained from the oxidized sample. Moreover, the bandgap energies obtained by fitting these individual mesoscopic photocurrent energy spectra with a novel approach developed in our laboratory, showed that all oxide scales were constituted of Fe₂O₃ and Cr₂O₃ and of an Fe_2-xCr_xO₃ solid solutions, but that the x-value of the latter was different depending on the metallurgic phase of the oxidized substrate. The latter results were shown to be in agreement with those of additional Raman analyses of the oxidized austenitic and ferritic grains.     

Q-switch Nd:YAG laser welding of AISI 304 stainless steel foils

Conventional fusion welding of stainless steel foils (<100 μm thickness) used in computer disk, precision machinery and medical device applications suffer from excessive distortion, formation of discontinuities (pore, void and hot crack), uncontrolled melting (melt-drop through) and poor aesthetics. In this work, a 15 ns pulsed, 400 mJ Nd:YAG laser beam was utilized to overcome these barriers in seam welding of 60 μm thin foil of AISI 304 stainless steel. Transmission electron microscopy was used to characterize the microstructures while hardness and tensile-shear tests were used to evaluate the strengths. Surface roughness was measured using a DekTak profilometer while porosity content was estimated using the light microscope. Results were compared against the data obtained from resistance seam welding. Laser welding, compared to resistance seam welding, required nearly three times less heat input and produced welds having 50% narrower seam, 15% less porosity, 25% stronger and improved surface aesthetics. In addition, there was no evidence of δ-ferrite in laser welds, supporting the absence of hot cracking unlike resistance welding.     

基于焊道尺寸控制的电弧增材成形高氮奥氏体不锈钢与316L不锈钢交织结构EI北大核心CSCD

采用高氮奥氏体钢与316L不锈钢丝材,对高氮奥氏体不锈钢熔覆焊道、单道多层、单层多道表面成形特性进行分析,筛选适宜的工艺参数。通过控制两种材料焊道尺寸,获得最适用于成形异材交织结构的工艺参数。利用不同道间距实验得到异材焊道的最佳道间距,并采用合理路径,制备电弧增材成形交织结构。结果表明:工艺参数的变化对高氮奥氏体不锈钢焊道表面成形特性影响极大,易出现气孔;提出异材焊道几何尺寸、截面面积匹配误差法,获得最佳工艺参数:其中高氮奥氏体钢丝材的送丝速率为5.7 m/min,316L不锈钢丝材的送丝速率为5.6 m/min,焊接速率均为0.6 m/min。通过降低起弧次数,采用闭合路径,提高了异材交织结构成形精度,减少了后处理加工。     

Effect of cold deformation on corrosion fatigue behavior of nickel-free high nitrogen austenitic stainless steel for coronary stent application

Due to the excellent mechanical properties, good corrosion resistance, high biocompatibility and nickel-free character, the high nitrogen nickel-free austenitic stainless steel (HNASS) becomes an ideally alternative material for coronary stents. Stent implantation works in harsh blood environment after a balloon dilatation, i.e., the material is used in a corrosive environment with a permanent deformation. The present study attempts to investigate effects of pre-straining on high-cycle fatigue behavior and corrosion fatigue behavior of HNASS in Hank’s solution and the relevant mechanism for coronary stents application. It is found that higher pre-straining on HNASS results in higher strength and maintains almost same corrosion resistance. Fatigue limit of 0% HNASS is 550 MPa, while corrosion fatigue limit is 475 MPa. And improvement in fatigue limit of 20% and 35% pre-strained HNASS is in comparison with the 0% HNASS, while corrosion would undermine the fatigue behavior of HNASS. In a suitable range, the pre-straining had a beneficial effect on corrosion fatigue strength of HNASS, such as nearly 300 MPa improved with 20% cold deformation. This result provides a good reference for predicting the life of HNASS stent and as well its design.     

Dynamical behaviour and microstructural evolution of a nitrogen-alloyed austenitic stainless steel

The aim of this work is to study the mechanical properties of a nitrogen austenitic stainless steel (Uranus B66) and their relation to its microstructural evolution. Quasi-static (10⁻³ s⁻¹) and quasi-dynamic (1 s⁻¹) compression tests have been carried out with a universal servo-hydraulic testing machine. Dynamic (>10³ s⁻¹) compression tests have been performed on a classical split-Hopkinson bar apparatus. These tests, which cover a wide range of plastic strain, show that the material has a high-strain hardening rate, a good ductility and a great strain rate sensitivity. The temperature sensitivity has been determined over a large range, going from 77 K to 673 K. Transmission electron microscopy (TEM) observations have been conducted in order to correlate the microstructure to the mechanical behaviour. Uranus B66 undergoes basically the same structure evolution during both quasi-static and dynamic compression tests. The plastic deformation is governed initially by planar gliding, followed by mechanical twinning when the dislocation density is saturated.