Microstructure and interface characterization of dissimilar friction stir welded lap joints between Ti–6Al–4V and AISI 304

The feasibility of dissimilar friction stir welding (FSW) in overlap configuration between Ti–6Al–4V alloy (Ti64) and AISI 304 austenitic stainless steels (304SS) was investigated. Sound joints were achieved when placing titanium as the upper workpiece. Joints were successfully produced by employing a welding speed of 1 mm/s and rotational speeds of 300 and 500 rpm. A lamellar microstructure was formed in the stir zone of Ti64, where grain size was found to increase with increasing rotational speed, and austenitic equiaxed grains were obtained near the interface of 304SS coupon. Energy dispersive X-ray spectroscopy (SEM-EDS) of the interface revealed a thin intermixed region and suggested intermetallic compound formation. Microhardness data in the titanium weld zone for both rotational speeds exhibited slightly lower values than the base material, with the lowest values in the heat affected zone, whereas the microhardness values in the stainless steel side around the weld center were found to be higher than those obtained for the base material.     

Impact behavior of different stainless steel weldments at low temperatures

A comparative study was made of the fracture behavior of austenitic and duplex stainless steel weldments at cryogenic temperatures by impact testing. The investigated materials were two austenitic (304L and 316L) and one duplex (2505) stainless steel weldments. Shielded metal arc welding (SMAW) and tungsten inert gas welding (TIG) were employed as joining techniques. Instrumented impact testing was performed between room and liquid nitrogen (?196 °C) test temperatures. The results showed a slight decrease in the impact energy of the 304L and 316L base metals with decreasing test temperature. However, their corresponding SMAW and TIG weld metals displayed much greater drop in their impact energy values. A remarkable decrease (higher than 95%) was observed for the duplex stainless steel base and weld metals impact energy with apparent ductile to brittle transition behavior. Examination of fracture surface of tested specimens revealed complete ductile fracture morphology for the austenitic base and weld metals characterized by wide and narrow deep and shallow dimples. On the contrary, the duplex stainless steel base and weld metals fracture surface displayed complete brittle fracture morphology with extended large and small stepped cleavage facets. The ductile and brittle fracture behavior of both austenitic and duplex stainless steels was supplemented by the instrumented load–time traces. The distinct variation in the behavior of the two stainless steel categories was discussed in light of the main parameters that control the deformation mechanisms of stainless steels at low temperatures; stacking fault energy, strain induced martensite transformation and delta ferrite phase deformation.     

Considerations for the weldability of types 304L and 316L stainless steel

The susceptibility of austenitic stainless steels to the formation of two distinct weld defects, solidification cracking and lack of penetration, is related to the chemical composition of the base and filler material. The propensity for cracking is determined primarily by the solidification mode and the amount of residual tramp elements such as phosphorous and sulfur. High sulfur levels can lead to weld centerline cracking and heat affected zone (HAZ) cracking while very low sulfur levels (less than ∼50 ppm) in types 304L and 316L are associated with lack of penetration weld defects and a distinct loss in puddle control during fusion welding. A calculated Cr_eq to Ni_eq ratio of 1.52 to 1.9 is recommended to control the primary mode of solidification and prevent solidification cracks in type 304L while the Cr_eq/Ni_eq ratio of 1.42 to 1.9 is recommended for type 316L stainless steel. A lower limit of 50 ppm sulfur is recommended to avoid possible lack of penetration. These ranges should be validated by welding trials for specific weld processes and applications.     

High-brightness laser welding of thin-sheet 316 stainless steel

Photolytic iodine laser (PIL), a new industrial laser in the market, offers much higher brightness than existing Nd:YAG and CO₂ lasers. PIL has also a unique wavelength (1315 nm) that has not yet been tested for welding applications. In this work, the capabilities of PIL for precision seam welding of 0.1-mm thick sheet of AISI 316 stainless steel in the lap-joint configuration were evaluated. The weld performance data of PIL laser were compared with Nd:YAG and CO₂ lasers. The astounding benefits of PIL weld are narrow seam, extremely fine solidification cell structure, fully austenitic microstructure, and small heat-affected-zone (HAZ). These benefits are attributed to the PIL's high brightness that in turn enables achieving small spot size and energy transport through plasma rather than by heat conduction. In contrast, the welds produced by Nd:YAG and CO₂ lasers exhibited wider seams, coarser solidification structures, duplex microstructures of austenite and ferrite, and larger HAZ due to slow cooling of the melt, and lateral heat diffusion. Despite the narrow seam, the PIL weld carried a high tensile load (92% that of base metal) and was harder than the base metal. Microstructural analysis revealed that PIL welds exhibited fully austenitic structures and were free from hot cracking. These advantages are consequences of the rapid solidification effects including large undercooling, minimal segregation of impurities to the grain boundaries, and fine grain size.     

Characterization of Weld Solidification Cracking in a Duplex Stainless Steel

Weld solidification cracking in the duplex stainless steel SAF 2205 has been investigated and compared with that of alternate duplex and austenitic stainless steels. Varestraint weld-ability testing showed SAF 2205 to exhibit a lower cracking susceptibility than that of the duplex stainless steel Ferralium 255 but greater than that of a Type 304 austenitic stainless steel which solidified as ferrite and exhibited Ferrite Number 8 (FN 8) in the weld fusion zone. The high augmented strain levels required to induce cracking in these three alloys during Varestraint testing indicated a high resistance to solidification cracking at strain levels normally encountered in structural weldments. Cracking susceptibilities of the duplex and Type 304/FN-8 stainless steels were appreciably lower than that of a Type 304L stainless steel which solidified entirely to austenite and exhibited less than FN 1 in the weld fusion zone.

Microstructural characterization of SAF 2205 using conventional black-and-white and two different color metallography techniques showed solidification cracks to be associated with ferrite grain boundaries. Color metallography was also effective in revealing the fusion zone solidification structure and delineating second phases, including inter- and intragranular austenite and fine Cr₂N precipitates. Fractographic analysis of solidification crack surfaces from SAF 2205 Varestraint samples revealed dendritic and flat topographies, and confirmed a solidification versus solid-state cracking mechanism.     

Heißrißgefahr beim Elektronenstrahl-Schweißen metastabiler austenitischer Werkstoffe

The Risk of Hot Cracking during Electron Beam Welding of Metastable Austenitic Steels Electron beam welding of austenitic steels is an economic process for joining heavy section structures. Beside the technological advantages as there are low weld distortion, small shrinkage ratio and deep weld effect resulting from the concentrated heat input several problems arise when a metastable austenitic steel is electron beam welded. The high cooling rate may lead to a primary austenitic solidification of the weld pool increasing the risk of hot cracking. Therefore investigations in electron beam welding with the tulip-shape seam technology were carried out. The results show that the primary austenitic solidification and in consequence the risk of hot cracking can effectively be interfered by the addition of ferrite forming elements. With regard to industrial application an additional alloying wire filler offers itself to be a suitable procedure.     

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.     

Friction stir welding of AISI 304 austenitic stainless steel

The objective of this work is to demonstrate the feasibility of friction stir welding (FSW) AISI 304 austenitic stainless steels. The tool used was formed of a tungsten‐based alloy. The specimens were welded on an 11 kW vertical milling machine. Defect‐free welds were produced on 2.5 mm plates of hot‐rolled AISI 304 austenitic stainless steels at travel speeds ranging from 40 to 100 mm/min with a constant rotating speed of 1000 rpm. Tensile strengths and hardness values of the weld interface were determined and microstructure features of these samples were investigated.     

304不锈钢局部干法水下激光填丝焊接工艺及焊缝性能研究

目的 采用自主研制的水下激光填丝焊接装备,在304奥氏体不锈钢板材表面进行U形坡口激光填丝焊接试验,为304不锈钢水下修复工作提供技术参考。方法 在功率为5 600 W、焊接速度为6 mm/s、送丝速度为205 cm/min、保护气体流量为15 L/min、排水气体流量为30 L/min的条件下进行焊接试验,并对空气和水下环境下的焊缝进行对比检测分析。通过光学显微镜分析2种环境下焊缝的显微组织;对2种焊缝进行拉伸、弯曲等力学性能测试;采用显微硬度计测试1 kg载荷下不同区域的显微硬度;使用VersaSTAT3F电化学工作站测定在3.5%（质量分数）的NaCl溶液中2种焊缝的开路电位和极化曲线。结果 2种环境下的焊缝均无明显裂纹、气孔等缺陷;显微组织主要由奥氏体和铁素体组成,但2种环境下焊缝的奥氏体晶粒大小和铁素体形状均略有差别,焊缝拉伸断口均为典型的韧性断裂形貌且抗拉强度符合304不锈钢标准。2种环境下焊缝的微观组织和晶粒大小不同,水下焊缝硬度高于空气的。通过分析2种环境下焊缝的开路电位和极化曲线,可知水下焊缝的耐腐蚀性略高。结论 所开发的局部干法水下激光填丝焊接工艺可以满足实际工程中...     

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.     

Characterizations of 21-4N to 4Cr9Si2 stainless steel dissimilar joint bonded by electric-resistance-heat-aided friction welding

A new welding process, electric-resistance-heat-aided friction welding (ERHAFW), was introduced in this study. To further improve the joint quality and energy-saving, electric resistance welding was combined with the conventional continuous-drive friction welding. 21-4N (austenitic stainless steel) and 4Cr9Si2 (martensitic stainless steel) valve steel rods of 4 mm diameter were used as base metals. The results show that electric-resistance-heat-aided friction welding can be applied to join thin rods within a relatively short time, which is very difficult for conventional friction welding (FW). The microstructure of ERHAFW bonded 21-4N to 4Cr9Si2 presents non-uniform across the joint. Different structure zones are observed from the weld line to both sides, which are the weld center, thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ). These regions exhibit different structures owing to the difference in the thermophysical and mechanical properties of these two steels under the fast heating and cooling during welding. The variation of microhardness in the joint is attributed to the microstructure change. The higher microhardness is obtained in the weld center and TMAZ of 4Cr9Si2 corresponding to the presence of fine tempered martensite and carbides.     

Metallographic analysis of embedded crack in electron beam welded austenitic stainless steel chemical storage tank

An AISI 304L austenitic stainless steel tank used for chemical storage showed cracks during the post weld qualification programme. A crack of 75 mm length embedded within the weld pool was subjected to detailed metallographic analysis. The results revealed that the cracking was due to a shift in the solidification mode from primary ferrite to primary austenite. The residual stress introduced during rolling and forming of material as well as additional contractional strain during welding under fixtured condition, are additional factors which caused cracking.     

奥氏体不锈钢凝固相转变规律曲线(SPT曲线)建立

利用自行设计的高温凝固相转变测定实验装置,研究了304奥氏体不锈钢在不同冷却速度下的高温凝固相转变过程,得到了凝固过程中液相（ L）到高温铁素体（δ）到奥氏体（γ）的相变温度。在此基础上分析了304奥氏体不锈钢在不同冷却速度下的高温凝固相转变规律,从而建立了304奥氏体不锈钢的低冷速凝固相转变规律曲线—SPT（Solidification phase transforma?tion）曲线。结果表明：对试样进行液氮酒精淬火有效地保留了试样高温时各相的状态。可以清楚的显示在不同冷速下的不同温度淬火时液相和固相的各相成份比例及在不同淬火温度下各成份体积比例的变化。通过研究体积比例变化,可以得到304奥氏体不锈钢在不同冷速下的液相线、固相线及各种反应开始和结束的转变温度（即SPT曲线）。由SPT曲线也可以看出,随着冷却速度的增大,相转变模式会发生变化,相图会向左移动,各相变反应的温度区间减小。     

Optimization of weld bead geometry of laser welded ANSI 304 austenitic stainless steel using grey-based Taguchi method

This paper investigates the quality characteristics of the welding geometry of the laser welding process for the ANSI 304 austenitic stainless steel, with the use of a pulsed Nd:YAG laser welding system. Laser welding of 2 mm thick ANSI 304 stainless steel is performed at three different levels of three factors, i. e., peak power, welding speed and pulse duration. In this study, a multi-response optimization problem is developed to achieve weld bead geometry with full penetration as well as a narrow bead width and minimum crater. Grey relational analysis based on Taguchi orthogonal array is used to present an effective approach for the optimization of laser welding process parameters. Regression equations between the welding parameters and the bead dimensions for laser welded austenitic stainless steels are developed, which are used in predicting the penetration, width and crater. Finally, the equations are tested for values different from the levels of the parameters in the orthogonal array. It will be beneficial to engineers for continuous improvement in laser welded product quality.     

Effects of the Process Parameters on Austenitic Stainless Steel by TIG-Flux Welding

The effects of the process parameters of TIG （tungsten inset gas）-flux welding on the welds morphology, angular distortion, ferrite content and hot cracking in austenitic stainless steel were investigated. Autogenous TIG welding process was applied to the type 304 stainless steel through a thin layer of activating flux to produce a bead on plate welded joint. TiO2, SiO2, Fe2O3, Cr2O3, ZnO and MnO2 were used as the activating fluxes. The experimental results indicated that the TIG-flux welding can increase the weld depth/width ratio and reduce the HAZ （heat affected zone） range, and therefore the angular distortion of the weldment can be reduced. It was also found that the retained ferrite content within the TIG-flux welds is increased, and has a beneficial effect in reducing hot cracking tendency for stainless steels of the austenitic type weld metals. A plasma column constriction increases the current density at the anode spot and then a substantial increase in penetration of the TIG-flux welds can be obtained.     

固溶处理对304/Q245R焊接接头组织及性能的影响

为研究固溶处理对304奥氏体不锈钢和Q245R碳钢异种金属焊接接头显微组织及性能的影响,采用E309-16奥氏体不锈钢焊丝对6 mm厚的304/Q245R板材进行手工电弧焊,焊后使用箱式电阻炉对焊接接头进行固溶处理,对焊接接头进行显微组织观察和力学性能测试.结果表明:相对于固溶处理前,固溶处理后的接头焊缝组织为灰色奥氏体和黑色铁素体,枝晶偏析程度明显降低,Cr、Ni等合金元素分布较为均匀;焊缝和碳钢侧热影响区硬度值均有所提高,最高硬度值为304.59 HV,出现在焊缝位置;接头抗拉强度较高,达570 MPa,拉伸断裂发生在母材Q245R碳钢部位.另外,对焊缝进行XRD测定,未检测到不利于接头性能的相,这表明固溶处理后的异种金属接头性能良好,能够满足工程中的实际需求.     

Effect of δ-ferrite co-existence on hot deformation and recrystallization of austenite

This work evaluates the effect of co-existence of a large volume fraction of δ-ferrite on the hot deformation and dynamic recrystallization (DRX) of austenite using comparative hot torsion tests on AISI 304 austenitic and 2205 duplex stainless steels. The comparison was performed under similar deformation conditions (i.e. temperature and strain rate) and also under similar Zener-Hollomon, Z, values. The torsion data were combined with electron backscatter diffraction (EBSD) analysis to study the microstructure development. The results imply a considerable difference between DRX mechanisms, austenite grain sizes and also DRX kinetics of two steels. Whereas austenitic stainless steel shows the start of DRX at very low strains and then development of that microstructure based on the necklace structure, the DRX phenomena in the austenite phase of duplex structure does not proceed to a very high fraction. Also, the DRX kinetics in the austenitic steel are much higher than the austenite phase of the duplex steel. The results suggest that at a similar deformation condition the DRX grain size of austenitic steel is almost three times larger than the DRX grains of austenite phase in duplex steel. Similarly, the ratio of DRX grain size in the austenitic to the duplex structure at the same Z values is about 1.5.     

Hybrid (plasma + gas tungsten arc) weldability of modified 12% Cr ferritic stainless steel

This paper deals with the hybrid (plasma + gas tungsten arc) welding properties of 12 mm thick modified 12% Cr ferritic stainless steel complying with EN 1.4003 and UNS S41003 steels with a carbon content of 0.01% to improve the weldability. The root passes of the butt welds were produced with plasma arc welding (PAW) without filler metal while gas tungsten arc welding (GTAW) was used to accomplish filler passes with 309 and 316 austenitic stainless steel type of consumables, respectively. The joints were subjected to tensile and bend tests as well as Charpy impact toughness testing at −20 °C, 0 °C and 20 °C. Examinations were carried out in terms of metallography, chemical analysis of the weld metal, ferrite content, grain size and hardness analyses. Although 309 consumables provided higher mean weld metal toughness values compared to 316 (90 J vs. 75 J), 316 type of consumables provided better mean HAZ toughness data for the joints (45 J vs. 20 J) at −20 °C. Toughness properties of the welds correspond with those of microstructural features including grain size and ferrite content.     

Experimental investigation on dissimilar pulsed Nd:YAG laser welding of AISI 420 stainless steel to kovar alloy

This paper presents the results of an investigation on autogeneous laser welding of AISI 420 stainless steel to kovar alloy using a 100 W pulsed Nd:YAG laser. The joints had a circular geometry and butt welded. The joints were examined by optical microscope for cracks, pores and for determining the weld geometry. The microstructure of the weld and the heat affected zones were investigatedby scanning electron microscope. The austenitic microstructure was achieved in the weld. The morphology of weld zone solidification was basically cellural, being influenced by the temperature gradient. It was found that the start of solidification in the kovar side of weld zone occurred by means of epitaxial growth. When the temperature gradient was high, the columnar grains were created in the fusion boundary of 420 stainless steel side toward weld zone. Measurements taken by X-ray spectrometry for dispersion of the energy in the weld zone indicated a significantly heterogeneous distribution of chromium element. The variations in chemical compositions and grains morphologies significantly alter the Vickers microhardness values in the weld zone.     

Hot cracking susceptibility of boron modified AISI 304 austenitic stainless steel welds

Abstract

The hot cracking susceptibility of welds made on AISI 304 stainless steel modified with from 0·2 to 1·0 wt-%B has been investigated. Varestraint tests showed that the hot cracking susceptibility is high for boron additions of about 0·2%, but is decreased when the boron content is increased to ≥0·5%. Steels containing about 0·2%B were found to have a wide solidification temperature range and their high temperature ductility was low compared with boron free AISI 304 steel and the other boron modified steels. Ferrite precipitation was inhibited in the 0·2%B steels and the formation of low melting point grain boundary films was thereby promoted. Increasing the boron content to ≥0·6% reduces the coefficient of thermal expansion and narrows the solidification temperature range. In addition, crack refilling was observed, resulting in improved hot ductility and high resistance to hot cracking. It is concluded that in structures where weld restraint forces are not high, hot cracking is not likely to occur if boron additions of >0·6% are made to AISI 304 stainless steel. In T-type and Fisco weld cracking tests, in which the weld restraint forces are close to those experienced by actual structural welds, the boron modified stainless steels show a low hot cracking susceptibility which is not significantly different from that of boron free AISI 304 steel.

MST/1548