不锈钢激光点焊接头组织和力学性能研究

目的 改善SUS301L–HT不锈钢激光点焊焊接性能。方法 以2 mm SUS301L–HT不锈钢为母材进行激光点焊试验,并分析焊接接头的金相组织、硬度、拉伸性能以及断口形貌等。结果 焊点表面无损坏、压痕均匀、无较大焊接变形,表面无飞溅、母材颜色无明显变化。焊核区的微观组织主要是柱状晶,柱状晶依附于未熔化母材晶粒向焊核中心生长。母材硬度最高,约为309HV;焊核中心附近区域硬度适中,约为255HV,热影响区硬度最小,不到220HV。点焊接头断裂形式多为纽扣式断裂且发生在热影响区。结论 不锈钢激光点焊整体质量较好,可用于轨道列车车体加工。     

Zum Einfluss des Schweißens auf das Kriechverhalten moderner Stähle für die thermische Energieerzeugung

The influence of welding on creep behaviour of modern steels for thermal power generation Un‐ and low alloyed ferritic/bainitic Chromium steels as well as high alloyed ferritic/martensitic 9–12 % Chromium steels are widely used for high temperature components in thermal power generation. Welding in all its variety is the major repair and joining technology for such components. The weld thermal cycle has significant influence on the base material microstructure and its properties. The Heat Affected Zone is often regarded as the weakest link during high temperature service. While weldments of un‐ and low alloyed ferritic Chromium steels can show significant susceptibility to Reheat Cracking in the coarse grained heat affected zone, weldments of high alloyed ferritic Chromium steels generally fail by Type IV Cracking in the fine grained heat affected zone during long term service. In this paper the influence of the weld thermal cycle on the base material microstructure is described. Long‐term creep behaviour of weldments is directly related to the main failure mechanisms in creep exposed ferritic weldments and implications for industries using heat resistant ferritic steels are shown.     

The relationship between atomic partitioning and corrosion resistance in the weld-heat affected zone microstructures of UNS S32304 duplex stainless steel

Wrought material as well as physically simulated welding heat affected zone (HAZ) samples of an UNS S32304 duplex stainless steel were subjected to electrochemical corrosion tests and electron microscopy characterization. An impaired corrosion resistance of the HAZ microstructures compared to the wrought material microstructure was observed. Calphad-based numerical simulation of phase transformations and solute redistribution taking place during welding provided an explanation of the observed corrosion behavior. The poor corrosion resistance of the HAZ microstructures studied was mainly attributed to a decrease in corrosion resistance of ferritic grains after welding, which exhibited lower chromium content than ferritic grains in the wrought material.     

Resistance spot welding joints of AISI 316L austenitic stainless steel sheets: Phase transformations,mechanical properties and microstructure characterizations

In this paper, we aim to optimize welding parameters namely welding current and time in resistance spot welding (RSW) of the austenitic stainless steel sheets grade AISI 316L. Afterward, effect of optimum welding parameters on the resistance spot welding properties and microstructure of AISI 316L austenitic stainless steel sheets has been investigated. Effect of welding current at constant welding time was considered on the weld properties such as weld nugget size, tensile–shear load bearing capacity of welded materials, failure modes, failure energy, ductility, and microstructure of weld nuggets as well. Phase transformations that took place during weld thermal cycle were analyzed in more details including metallographic studies of welding of the austenitic stainless steels. Metallographic images, mechanical properties, electron microscopy photographs and micro-hardness measurements showed that the region between interfacial to pullout mode transition and expulsion limit is defined as the optimum welding condition. Backscattered electron scanning microscopic images (BE-SEM) showed various types of delta ferrite in weld nuggets. Three delta ferrite morphologies consist of skeletal, acicular and lathy delta ferrite morphologies formed in resistance spot welded regions as a result of non-equilibrium phases which can be attributed to the fast cooling rate in RSW process and consequently, prediction and explanation of the obtained morphologies based on Schaeffler, WRC-1992 and Pseudo-binary phase diagrams would be a difficult task.     

Effect of laser welding mode on the microstructure and mechanical performance of dissimilar laser spot welds between low carbon and austenitic stainless steels

This paper aims at investigating metallurgical and mechanical characterization of dissimilar laser spot welds between low carbon and austenitic stainless steel sheets. Microstructural examination, microhardness test and quasi-static tensile–shear test were performed. Mechanical properties of the welds were described in terms of peak load. The effects of laser mean power on the performance of dissimilar laser spot welds have been studied. It was found that increasing laser mean power leads to the transition of laser welding mode from conduction to keyhole. This transition causes a significant growth of the fusion zone size in the lower sheet, i.e. the low carbon steel sheet; since, the keyhole acts as an effective trap for the laser beam and will greatly increase the energy absorption from the incident laser beam.It is also shown that the fusion zone size in the weaker sheet, i.e. the low carbon steel sheet is the controlling factors in determination of the mechanical strength of dissimilar austenitic/ferritic laser spot welds.     

Microstructural analysis of friction stir welded ferritic stainless steel

High-quality, defect-free welds were successfully produced in 409 ferritic stainless steel by friction stir welding. A remarkably fine-grained microstructure was observed in the stir zone, and the fraction of low angle grain boundary in the stir zone significantly increased as compared to that in the base material. An increase in plunging depth led to an increase of the fraction of low angle grain boundary, a decrease in grain size, and an increase in hardness in the stir zone.     

Microstructure and failure behavior of dissimilar resistance spot welds between low carbon galvanized and austenitic stainless steels

Resistance spot welding was used to join austenitic stainless steel and galvanized low carbon steel. The relationship between failure mode and weld fusion zone characteristics (size and microstructure) was studied. It was found that spot weld strength in the pullout failure mode is controlled by the strength and fusion zone size of the galvanized steel side. The hardness of the fusion zone which is governed by the dilution between two base metals, and fusion zone size of galvanized carbon steel side are dominant factors in determining the failure mode.     

Microstructure and Fracture Morphology in the Welding Zone of Cr18Mo2 Ferritic Stainless Steel

Microstructure, precipitates and fracture morphology in the weld metal and the heat-affected zone (HAZ) of Cr18Mo2 ferritic stainless steel have been studied by means of metalloscope, SEM,TEM and X-ray diffractometer. Experimental results indicate that crystalline grain coarsening in HAZ is one of the reason resulting in the embrittlement fracture in the welding zone of the ferritic stainless steel. Some precipitates (TiC, TiN and Cr2N) in the steel promote production and development of the brittle cracks. In practical applications. the welding heat input should be as small as possible to prevent embrittlement caused by HAZ grain coarsening.     

Similar and dissimilar RSW of low carbon and austenitic stainless steels: effect of weld microstructure and hardness profile on failure mode

Abstract

In this paper, the failure behaviour of similar and dissimilar resistance spot welded joints of low carbon and austenitic stainless steel sheets was studied under tensile shear test with attention focused on the failure mode. Results showed that the microstructure of the fusion zone and the hardness distribution across the weld have a profound effect on the failure behaviour. Similar spot welds of stainless steel sheets exhibit the highest tendency to fail in interfacial failure mode, compared to low carbon steel similar spot welds and dissimilar low carbon and stainless steel spot welds. This behaviour is explained by the consideration of pullout failure location and hardness profile characteristics of each joint. It was shown that the failure mode transition is controlled by the hardness ratio of the fusion zone and the pullout failure location. In the case of dissimilar resistance spot welding, the hardness of the fusion zone which is governed by the dilution between two base metals, and the fusion zone size of the low carbon steel side are the dominant factors determining the failure mode of the joint.     

Microstructure and mechanical properties of friction stir welded 18Cr–2Mo ferritic stainless steel thick plate

In this study, microstructure and mechanical properties of a friction stir welded 18Cr–2Mo ferritic stainless steel thick plate were investigated. The 5.4 mm thick plates with excellent properties were welded at a constant rotational speed and a changeable welding speed using a composite tool featuring a chosen volume fraction of cubic boron nitride (cBN) in a W–Re matrix. The high-quality welds were successfully produced with optimised welding parameters, and studied by means of optical microscopy (OM), scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD) and standard hardness and impact toughness testing. The results show that microstructure and mechanical properties of the joints are affected greatly, which is mainly related to the remarkably fine-grained microstructure of equiaxed ferrite that is observed in the friction stir welded joint. Meanwhile, the ratios of low-angle grain boundary in the stir zone regions significantly increase, and the texture turns strong. Compared with the base material, mechanical properties of the joint are maintained in a comparatively high level.     

Austenitic and ferritic stainless steel dissimilar weld metal evaluation for the applications as-coating in the petroleum processing equipment

The current study presents some fundamental observations on the effects of the welding heat input in the chemical composition, microstructure, hardness and petroleum corrosion resistance of the fusion zone, formed by the AWS E309MoL austenitic stainless steel covered electrode and the AISI 410S ferritic stainless steel, being a dissimilar welding procedure. Such welding configurations are widely used as an overlay of equipment in the petroleum and gas industries. The welds were performed with the application of three different levels in heat inputs (6, 9 and 12 kJ/cm). Samples of the weld metals were conventionally prepared for the microstructural characterization by light microscopy and scanning electron microscopy. A corrosion test with samples immersed in heavy oil heated at 300 °C, was carried out for a period of 60 h. The corrosion rate was determined by the weight loss given after the aforesaid test. The fusion zone microstructure has a typical δ-ferrite acicular morphology, from which the level of δ-ferrite was duly altered with the increases of the welding heat input, due to the variations in the composition of the weld metal caused by dilution. It was also concluded that the chemical composition and the weld metal microstructure had a slight influence in the material’s corrosion rate. As a matter of fact, the corrosion rate of the weld metals evaluated herein, was considered satisfactory with few variations between the welding heat inputs duly applied.     

Comparison of Laser and Gas Tungsten Arc Weldments of Stabilized 17wt% Cr Ferritic Stainless Steel

One of the major factors influencing weldability of stabilized 17 wt% Cr ferritic stainless steels is extensive grain coarsening in the fusion zone (FZ) and in the heat affected zone (HAZ). This study aimed at exploiting low heat input characteristics of laser welding to effectively control grain coarsening in the FZ and HAZ of 1.2-mm thick stabilized 17 wt% Cr ferritic stainless steel weldment. The study demonstrated that as compared with gas tungsten arc welding (GTAW), laser welding brought about significant grain refinement in the FZ and HAZ. Laser weldments exhibited superior ductility over their GTA counterparts. An important observation of the study was that, in contrast to GTA weldment, laser weldment was associated with weaker interfaces along columnar grain boundaries in the FZ that served to arrest crack propagation normal to them.     

活性剂对铁素体不锈钢A-TIG接头熔深及组织性能研究

目的 选用430铁素体不锈钢作为研究对象,对比研究添加SiO2、TiO2、Cr2O3和未添加活性剂对A-TIG焊接接头显微组织和力学性能的影响。方法 采用3种活性剂涂覆在430铁素体不锈钢上进行A-TIG试验,分析活性剂对接头熔深、组织、性能、元素含量的影响情况。结果 同一焊接工艺参数下,活性剂的加入均能提高焊缝的熔深和深宽比,减少熔宽;其中,SiO2为活性剂时获得了最佳的焊缝几何形貌。同时,对比常规TIG焊接(未添加活性剂)接头的显微组织及力学性能可知,活性剂的加入并未改变焊接接头的显微组织且无新相的生成;活性剂的添加能够细化接头组织,从而使得接头硬度有所提高。结论 活性剂的加入能够显著增加铁素体不锈钢TIG焊缝熔深,改善接头组织,提高接头硬度。     

焊接参数对铁素体不锈钢搅拌摩擦焊接头组织及性能的影响

对4mm厚T4003铁素体不锈钢进行搅拌摩擦焊接工艺实验,研究焊接参数对接头组织特征、硬度分布及常温和低温冲击韧性的影响。结果表明:接头搅拌区和热力影响区由铁素体和马氏体双相组织构成;接头搅拌区组织沿试样厚度方向存在非均质性,且随转速的降低及焊接速率的增加越发显著;转速从150r/min增加至250r/min,前进侧热力影响区组织呈现小梯度过渡趋势,无明显变形拉长特征。焊缝硬度分布相对均匀,其最高硬度为290HV,约为母材的1.87倍。焊接参数和温度对接头的冲击吸收功有较大影响:常温(20℃)下,热影响区为母材的90%～92%,搅拌区为母材的85%～103%;低温(-20℃)下,热影响区为母材的87%～97%,搅拌区为母材的82%～95%,表明焊缝区仍具有较好强韧匹配。     

Fusion Zone Microstructures of Laser and Plasma Welded Dissimilar Steel Joints

Dissimilar steel joints between 13CrMo44 low-alloyed ferritic steel and A1SI 347 austenitic stainless steel were produced using laser beam and plasma arC welding. Both autogenous welding (without Filler) and welding with nickel-based filler wire were employed in each process. Fusion zone microstructures were characterized using both optical and scanning electron microscopy. Compositional analysis was performed using scanning electron microscopy. Hardness was measured to provide data for identifying the microstructures. A clear correlation was observed between the composition, microstructure and hardness. The results show that both autogenous laser and plasma welding produce fusion zones with a mixed martensite/austenite microstructure. Autogenous plasma welds, however, possess less martensite than the laser welds. The formation of martensite was attributed to the material combination and the rapid cooling rate of the welding processes. The results also indicate that both laser and plasma welding with nickel-based filler wire can produce fully austenitic fusion zones. This is mainly due to the high nickel content in the filler wire. According to the results, laser and plasma welding with nickel-based filler wire are recommended as potential industrial processes.     

Microstructure evolution in heat affected zone of T4003 ferritic stainless steel

In this work, microstructural characteristics and development within the heat affected zone (HAZ) of T4003 ferritic stainless steel (FSS) welded joint were investigated combining experimental measurement with finite element simulation of welding temperature field. The results indicate that the HAZ was characterized with heterogeneous microstructure due to the extensive peak temperature range which could be divided into three sub-zones named as HAZ1, HAZ2 and HAZ3. The HAZ1 (the region next to weld zone boundary) experienced peak temperatures of 1300–1500 °C during welding process. This region presented almost fully δ ferrite microstructure with irregular grain, which was attributed to the high element diffusion rate and the absence of elevated-temperature austenite. The HAZ2 (center region of HAZ) suffered the peak temperatures of 1150–1300 °C. It presented martensite + δ ferrite dual microstructure with limited grain growth due to the formation of γ phase at grain boundaries. The HAZ3 (the region closed to the base metal) was undergone the peak temperatures of 830–1150 °C and was characterized with both martensite and ferrite structure.     

TIG–MIG hybrid welding of ferritic stainless steels and magnesium alloys with Cu interlayer of different thickness

The joining of ferritic stainless steels and magnesium alloys is light and economic for weight reduction of automobiles. Unlike previous conventional welding method, a novel TIG–MIG hybrid welding is applied for the joint successfully in this study. The melted Mg weld metal wets the ferritic stainless steels surface to form a brazed Mg–Cu to steel connection when the interlayer thickness is 0.02 mm. When the interlayer thickness is 0.1 mm, the intermetallic compounds transition layer determined the tensile-shear strength of joints. Intermetallic compounds transition layer has been found in the 0.1 mm thick interlayer joints and no particle has been found in the 0.02 mm thick interlayer joints. Based on the analysis of microstructure and properties, joining and strengthen mechanisms of the joint were discovered. As the thickness of the Cu interlayer increases, the joining mechanism changed. The joining and strengthen mechanisms are mainly determined by the thickness of the interlayer. The tensile-shear strength of 0.1 mm thickness Cu interlayer joints is improved by 47% compared to 0.02 mm Cu.     

Effects of various tool plunge depths on microstructure evolution,mechanical properties and dome structure features of friction stir spot welded AA5052-H32 similar joints

The solid-state nature of friction stir spot welding process provides outstanding advantages for the sound joining of aluminum alloys. Within this study, 3 mm-thick AA5052-H32 sheets are successfully joined by friction stir spot welding using 2344 hot-worked steel pin to investigate the effects of various tool plunge depths on the microstructure, mechanical and metallurgical properties of similar welds. Therefore, the experiments are performed at different plunge depths in the range of 3 mm–4 mm. Accordingly, the relationships between the process parameter (tool plunge depth) and the responses (microstructure, dome structure, microhardness and lap shear tensile load) are established. Microstructure analyses demonstrate that the increase in the plunge depth leads to more grain refinement within the stir zone, which significantly affects the mechanical performance of the similar joints. This study also indicates that the tool plunge depth in friction stir spot welding process has a noteworthy influence on the characteristic features of the 5052 aluminum alloy joints, such as the dome structure. Moreover, an explicit increase in the microhardness towards the weld stir zone is observed in all specimens. It is found that the average maximum tensile-shear force enhances with the increment in the tool plunge depth from 3 mm to 4 mm.     

Super duplex stainless steels Part 1 Heat affected zone microstructures

Abstract

The development of heat affected zone microstructures in super duplex stainless steels (nominal composition Fe–25·0Cr–3·5Mo–7·5Ni–O·5Cu–0·25N–0·5W wt-%) has been examined on multipass welds, bead-on-plate welds, and simulated weld samples. Three-dimensional heat flow equations, for different plate thicknesses, have been applied to the grain growth kinetics of δ ferrite. The grain growth constants have been rationalised using an equation of the Arrhenius type. The level of agreement between the measured δ ferrite grain size and the predicted values is found to be satisfactory. The effect of heat input on the δ ferrite grain growth has been examined in weld metals and the results indicate that the large δ ferrite grain structure can be eliminated using low levels of heat input. The δ + γ → #x03B4; transformation kinetics and the degree of ferritisation have been studied as functions of the welding variables. The results are discussed in an attempt to quantify a minimum plate thickness and optimum heat input to prevent a high level of ferritisation. Particular emphasis is given to the effect of reheating on the formation of intermetallics, nitrides, and carbides. During the first thermal cycle, the heat input can be sufficient to decompose some of the austenite to a metastable ferritic microstructure. Competition between the reformation of austenite and the nucleation of nitrides and carbides takes place in this supersaturated structure on reheating by subsequent passes. The relative positions of the C curves for formation of austenite and nitrides, and the peak temperature of the second cycle, determine which phases form. The depletion of chromium adjacent to the nitride/δ ferrite interface is examined using a finite differences model and the implications of the results for the localised corrosion resistance are discussed. The compositions of phases have been measured and the experimental values are compared with a thermodynamic analysis which predicts the equilibrium compositions of both the δ and γ phases as a function of temperature and chemical composition. Deviation from equilibrium partitioning is observed owing to non-equilibrium cooling conditions, which result in the formation of configurationally frozen structures. Extensive partitioning is observed for high heat input welds but, conversely, low heat input eliminates the partitioning in the heat affected zone. The results are summarised in an attempt to rationalise the relationship between chemical composition, welding conditions, and heat affected zone microstructures.

MST/1598     

316L不锈钢焊缝腐蚀微电池的形成机理

为了探究316L不锈钢焊缝腐蚀微电池的形成机理,通过微观组织观察及电化学试验进行了分析。采用扫描电镜(SEM)对焊件三区(焊缝区、热影响区、母材区)的形貌及元素组成进行了观察分析,采用金相显微镜观察微观组织,并确定晶粒度及非金属夹杂物及其分布。结果表明:316L不锈钢焊缝形成腐蚀微电池的倾向与元素组成、非金属夹杂物和晶粒度大小等因素紧密相关;焊缝三区腐蚀电位、腐蚀电流不同,耐蚀性能差异较大,因而腐蚀电位最低的热影响区与腐蚀电位最高的母材区在电解液中比较容易形成腐蚀微电池,且热影响区可视为此微电池的阳极,腐蚀进程较快。