Effect of Austenitic and Austeno-Ferritic Electrodes on 2205 Duplex and 316L Austenitic Stainless Steel Dissimilar Welds

This study addresses the effect of different types of austenitic and austeno-ferritic electrodes (E309L, E309LMo and E2209) on the relationship between weldability, microstructure, mechanical properties and corrosion resistance of shielded metal arc welded duplex/austenitic (2205/316L) stainless steel dissimilar joints using the combined techniques of optical, scanning electron microscope, energy-dispersive spectrometer and electrochemical. The results indicated that the change in electrode composition led to microstructural variations in the welds with the development of different complex phases such as vermicular ferrite, lathy ferrite, widmanstatten and intragranular austenite. Mechanical properties of welded joints were diverged based on compositions and solidification modes; it was observed that ferritic mode solidified weld dominated property wise. However, the pitting corrosion resistance of all welds showed different behavior in chloride solution; moreover, weld with E2209 was superior, whereas E309L exhibited lower resistance. Higher degree of sensitization was observed in E2209 weld, while lesser in E309L weld. Optimum ferrite content was achieved in all welds.     

Brazing of UNS S32101 and UNS S32304 lean duplex stainless steels and UNS S32750 super duplex with AWS A5.3 BNi-7 (Ni–Cr–P) filler metal

Summary

Weld metals solidified in the ferritic-austenitic solidification mode (FA mode) have dual phases of ferrite and austenite in their as-solidified condition, where ferrite exhibits different morphologies depending on the chemical composition and welding conditions. This paper describes an investigation of the effect of the solidification and transformation sequence on the formation of final ferrite morphologies. Austenite is formed through either a eutectic reaction or peritectic reaction at the dendrite boundaries after the primary formation of ferrite. During the eutectic formation of austenite, the <100>_δ direction of the primary ferrite and the <100>_γ direction of the eutectic austenite are parallel to each other and lie along the growth direction of the primary dendrites. However, any specific lattice plane relationship between the two phases is not identified. During cooling after solidification, the austenite extends into the primary ferrite via solid-state transformation, and the final morphology of the ferrite is vermicular without any coherent orientation relationship between the primary ferrite and eutectic austenite. During peritectic formation of austenite, the Kurdjumov-Sachs orientation relationship is established between the primary ferrite and peritectic austenite, and the <100>_γ direction of the peritectic austenite is not parallel to the growth direction of the primary dendrites. During cooling after solidification, the primary ferrite transforms into austenite, and the final morphology of the ferrite is lathy, since the primary ferrite and peritectic austenite have a favourable coherent orientation.     

Metallographic and OIM study of weld cracking in GTA weld build-up of polycrystalline,directionally solidified and single crystal Ni based superalloys

Abstract

The repair of gas turbine components is of importance both commercially and scientifically to ensure cost effective repair schemes that will extend the lives of hot end components such as blades and stators. The present communication reports the results of a metallographic and orientation imaging microscopy study of weld cracking observed in the gas tungsten arc repair welds of a polycrystalline (IN738LC), a directionally solidified (Rene 80) and a proprietary single crystal (SX) alloy. The three alloys were welded with low, intermediate and high strength weld fillers, using a weld build-up approach rather than a conventional weld repair of a through thickness crack. This procedure would be applicable for example to worn area on the tips of turbine blades. Inhomogeneous initial microstructures and those from solidification processes led to extensive heat affected zone microfissuring in the IN738LC alloy, associated with MC carbide liquation, liquation of gamma prime (γ′), segregation of boron and strain effects from precipitation of γ′ in both single and double pass welds. As observed previously in a V shaped weld preparation, the extent of microfissuring in alloy IN738LC increased substantially from the use of the low and intermediate strength weld fillers, to extensive heat affected zone microfissuring by using the high strength IN738 filler. In the directionally solidified Rene 80 welds, due to the reduction in grain boundary area per unit volume, only minor heat affected zone cracking was observed, while the SX alloy did not crack at all. The absence of any cracks in the SX alloy welds despite the presence of stray grains in the fusion zone appears to be related to reduced stress levels in the welds due to the choice of welding technique and the welding parameters.     

Effect of variant transformations in fusion zones of gas metal arc welds

Abstract

The macro- and microtextures of gas metal arc welds fabricated using both conventional ferritic and low transformation temperature (LTT) filler metals were examined. Both welds were largely composed of acicular ferrite. The weld textures were found to be distinct, which is observed to be due to variant selection during the austenite to ferrite transformation. In situ electron backscatter diffraction performed during thermal cycling of the LTT fusion zone into the austenite phase field confirmed that these transformations satisfy the expected crystallographic relationships. Implications for welding consumable design and finite element process modelling are drawn.     

Characterisation of inclusions found in C–Mn steel welds containing titanium

Abstract

The main goal of the present study was to identify the chemistry of the phases on the surface of inclusions found in steel welds, to enable a better understanding of their role in the formation of acicular ferrite. Transmission electron microscopy and parallel electron energy loss spectrum imaging of specimens prepared by ultramicrotomy have been used to characterise inclusions found in C–Mn welds having different titanium concentrations. The results indicate that, for a weld containing 28 ppm titanium, which is the tough est weld studied, a phase comprising MnTi₂O₄ is present on the surface of the inclusions. As the concentration of titanium in the weld is increased to 120 ppm, the MnTi₂O₄ phase is found more towards the centre of the inclusions. It is partially enveloped by a slag of MnO . SiO₂ . The presence of TiO, often cited as a phase promoting the formation of acicular ferrite, was found only in the specimen that contained 410 ppm titanium. Thermodynamic calculations using databases for oxide solutions corroborate the experimental findings. They show that, below 112 ppm titanium, the first phase to solidify in the liquid inclusions is MnO . SiO₂ , followed by MnTi₂O₄ . Above a concentration of 112 ppm titanium, the calculations indicate that there should be a reversal in the solidification sequence, i.e. MnTi₂O₄ should solidify first, followed by MnO . SiO₂ . Further more, these calculations indicate that the inclusions are still liquid after the solidification of the steel matrix.     

真空室窗口领圈316L厚板电子束焊接接头不均匀性

真空室是未来聚变工程实验堆的核心部件,其组件窗口领圈采用能一次焊透50 mm以上不锈钢且变形较小的电子束焊进行拼焊. 为了深入探索厚板焊接接头不均匀性,在焊接过程中应用扫描偏转并对50 mm厚的316L奥氏体不锈钢焊接接头厚度方向的微观组织和硬度进行分析. 结果表明,焊缝组织由奥氏体和铁素体组成,从焊缝中心线附近,上层到下层焊缝组织由粗大的板条状/骨架状铁素体依次变为更加紧密排列的骨架状铁素体和等轴晶状铁素体;带有扫描偏转的焊接接头在焊缝厚度方向更早出现等轴晶;扫描偏转能改善焊缝表面成形质量;焊缝显微硬度从上层到下层逐渐增加.     

焊缝金属对SUPER 304H奥氏体耐热钢焊接性的影响

综述了焊缝金属对SUPER 304H钢焊接性的影响.结果表明,SUPER 304H钢不同成分GTAW奥氏体焊缝的热裂纹倾向较大;焊缝中凝固裂纹倾向主要是受A凝固模式控制,而HAZ液化裂纹倾向的主要原因则与晶界析出相,以及铜的富集等因素有关.不同成分奥氏体焊缝接头的力学性能各异;合适的焊缝化学成分和优化的焊接工艺是获得满...     

Unmixing behaviour in dissimilar laser welds for duplex and austenitic stainless steels

ABSTRACT

Metallurgical characteristics of single-mode laser dissimilar welds between super duplex (UNS S32750) and austenitic (type 316L) stainless steels is the existence of an unmixed zone that originates from each base metal. It was confirmed that the unmixed zone that flowed out from the 316L had the microstructural morphology of primary austenite with secondary δ-ferrite solidification mode, while the morphology of a δ-ferrite single phase solidification mode was observed within the unmixed zone that flowed out from the S32750. Furthermore, it could be speculated that each unmixed zone that flowed out from 316L and S32750 coexisted independently in terms of crystallographic aspect (orientation distribution function and predominant orientation relationship) analysed by electron backscatter diffraction.     

Weldability of X100 linepipe

Abstract

Research has been carried out to identify weld metal compositions and microstructures capable of meeting high strength and toughness requirements for X100 seam welded linepipe. Single pass, multiwire submerged arc welds were made in experimental, high strength low alloy steel plates using consumables to give a wide range of weld metal alloying. Work has shown that the optimum strength and toughness are obtained in Mo–B–Ti alloyed weld metals with P _cm values between 0.218 and 0.250. Weld metal microstructures were almost fully acicular ferrite with an ultrafine grain size (1–2 µm). Dilatometric studies demonstrated that at typical weld cooling rates the optimised welds transformed at significantly lower temperatures than those reported for X65 plate deposits, which contain acicular ferrite in the form of idiomorphic primary ferrite and intragranular Widmanstätten ferrite. The maximum rate of transformation in the optimised welds occurred between 515 and 570°C, which indicates that the acicular ferrite in this case consisted of intragranular Widmanstätten ferrite and/or bainite. The ferrite would appear to have a fine plate morphology growing from large as well as small inclusions, but not very far before the onset of hard impingement, thereby ensuring an ultrafine grain size. Tensile strengths of 708–784 MPa were achieved with an 80 J Charpy impact transition temperature toughness between -68 and -115°C. More highly alloyed weld metals containing 2–3%Mn and 1.5%Si transformed at lower temperatures and showed increased strength, but there was a substantial loss of toughness, attributed to the relatively unimpeded growth of large ferrite plates from larger inclusions, and the replacement of ultrafine acicular ferrite between these plates by blocks of martensite–austenite. One pass per side, multiwire submerged arc welds manufactured to the optimum weld metal chemistry confirmed their applicability for thin section X100 linepipe.     

Effect of scandium and titanium–boron on grain refinement and hot cracking of aluminium alloy 7108

Abstract

The effect of scandium and titanium–boron (Tibor) additions on the solidification behaviour of castings and welds of aluminium alloy 7108 has been investigated. A circular patch test was adopted to evaluate the effects of these elements on the hot cracking suscepti bility of welds made on cast coupons treated with different grain refiner additions. It was observed that grain size, as well as cracking susceptibility, decreased with increasing amounts of scandium and that hot cracking was completely eliminated at scandium additions above 0·25 wt-%. A more pronounced grain refining effect in welds was observed with Tibor and, in addition, no hot cracking was observed with Tibor additions as low as 0·02 wt-%Ti (0·004 wt-%B). Castings, however, were more effectively grain refined with scandium, achieving a finer grain size than with Tibor.     

Effect of activating flux and shielding gas on microstructure of TIG welds in austenitic stainless steel

Abstract

The aim of this research is to study the effect of an activating flux, two shielding gases (100%Ar and 50%Ar z 50%He) and a range of weld currents on the microstructure of autogeneous A-TIG welds on an austenitic stainless steel. Metallographic, Mössbauer, X-ray diffraction and magnetic permeability methods were used in the study to evaluate ferrite content in the welds. The increase in welding current coarsened the microstructure and increased the retained ferrite content in welds made with and without flux. The activating flux increases the ferrite content and changes the distribution of ferrite in the welds. The influence of flux on ferrite content is less significant in Ar/He than in Ar shield welds. The process of filling steel samples, currently used in the Mössbauer method, drastically changes the microstructure of the parent and melted austenitic stainless steels.     

Effect of energy transfer modes on solidification cracking in pulsed laser welding

Abstract

In this study, solidification cracking in pulsed laser welding of fully austenitic, AISI Type 316 stainless steel has been analysed at different energy transfer modes. The pulse parameters have been selected appropriately to obtain conduction, transition and keyhole mode welds. Conduction and transition mode welds exhibit higher susceptibility to cracking than keyhole mode welds. It is observed that both heat input and energy transfer mode affect the cooling rate and hence influence solidification cracking. Microstructures of the fusion zone have been analysed, and the cooling rate experienced by the weld is estimated from the mean cell size in the weld. It is found that the critical cooling rate below which cracking does not occur is ～10⁴ K s^??1.     

Laser and laser assisted arc welding processes for DH 36 microalloyed steel ship plate

Abstract

A series of laser and laser assisted metal inert gas (MIG) welds was produced from a common plate. Each weld was mechanically tested, and the welds showed broadly similar properties, except for the autogenous CO₂ laser weld metal, which had poorer toughness. This was related to a harder weld metal microstructure. Toughness and hardness were related to the lath width of the ferrite, for the welds involved. The weld metal area/volume was used as an indicator of potential distortion. In this instance, the autogenous CO₂ laser weld was superior to the CO₂ laser assisted MIG weld which was better than the Nd:YAG laser assisted MIG weld. Each weld was examined using carbon extraction replicas in the TEM, and also using an SEM with an EDAX attachment. A number of inclusions and precipitates were observed, identified and sized. It was concluded that the particles observed were not detrimental in this specific case. A tentative relationship was established between parent plate inclusion size distribution and weld metal inclusion size distribution.     

Formation mechanism of vermicular and lacy ferrite in austenitic stainless steel weld metals

Abstract

Solidification and subsequent transformation of austenitic stainless steel weld metals that solidified in the ferritic–austenitic mode were investigated from the viewpoint of crystallography. The formation mechanisms for the vermicular and lacy ferrite observed in the weld metals were clarified. The ferrite morphology is determined by both the crystallographic orientation relationship between ferrite and austenite established at the stage of ferrite nucleation and the relationship between the welding heat source direction and the preferential growth directions of ferrite and austenite. In particular, for the formation of continuous lacy ferrite, it is necessary that the ferrite continues to grow with the Kurdjumov–Sachs orientation relationship with austenite that is established at the stage of ferrite nucleation.     

Microstructural characterisation of stir zone containing residual ferrite in friction stir welded 304 austenitic stainless steel

Abstract

Friction stir welding was applied to a 2 mm thick 304 austenitic stainless steel plate. The microstructural evolution and hardness distribution in the weld were investigated. The stir zone (SZ) and thermomechanically affected zone (TMAZ) showed dynamically recrystallised and recovered microstructures, respectively, which are typically observed in friction stir welds in aluminium alloys. The hardness of the SZ was higher than that of the base material and the maximum hardness was observed at the TMAZ. The higher hardness at the TMAZ was attributed to high densities of dislocations and subboundaries. Microstructural observations revealed that the ferrite was formed along grain boundaries of the austenite matrix in the advancing side of the SZ. It is suggested that the frictional heat due to stirring resulted in the phase transformation of austenite to ferrite and that upon rapid cooling the ferrite was retained in the SZ.     

Thermodynamic study of inclusion formation in low alloy steel weld metals

Abstract

The formation and stability of inclusions in low alloy steel welds were investigated using equilibrium calculations. Based on the results, the origin of inclusions effective in acicular ferrite production is discussed. Particular emphasis is placed on the effect of the Al/O ratio and titanium addition on inclusion formation since these two factors are experimentally critical to acicular ferrite production. Mullite (2SiO₂.3Al₂O₃) is found to be formed in addition to Ti₃O₅ in the steel melt at 1800 K when the melt has a wt-%Al/wt-%O ratio of ～0·6 optimum for acicular ferrite production. However, the mullite is unstable in the austenite and galaxite (MnO.Al₂O₃), having the spinel structure, becomes stable at the lower temperatures. Therefore, it is proposed that galaxite is responsible for the nucleation of acicular ferrite. The formation of the galaxite should be in the solid state after weld solidification, being associated with pre-existing mullite. Titanium additions are found to be beneficial to acicular ferrite production by decreasing the formation of ineffective glassy oxide.     

Nucleation process control of undercooled stainless steel by external nucleation seed

Competitive phase selection of undercooled melts between equilibrium ferrite and metastable austenite has been investigated as a function of undercooling. Stainless steel type 316 was undercooled up to 250 K using an electromagnetic levitation method. The microstructure showed different morphologies depending on the undercooling due to different solid phase transformation mechanisms. However, metastable austenite was not formed during the solidification for the undercooling up to 250 K due to the favorable nucleation kinetics of ferrite. The control of the phase selection has also been attempted using an external nucleation seed. Undercooled melts were touched by Fe–50 at.% Ni powders in the levitation coil, whose lattice constant is almost the same as that of metastable austenite. The microstructure showed a dramatic change in the solidification mode from equilibrium ferrite to metastable austenite during the first stage of the solidification.     

New model for prediction of ferrite number of stainless steel welds

Abstract

A new semi-empirical model for predicting the ferrite content of stainless steel welds has been developed. This model predicts the ferrite number of stainless steel welds as a function of composition. The model is based on an equation representing the free energy change between ferrite and austenite. This model has been derived from published data of experimental weld metal compositions and their corresponding ferrite numbers. The predictive capability of this model was found to be good and describes the effect of alloying elements on the ferrite number. This model is comparable in accuracy to currently available constitution diagrams but is applicable to a wider range of alloy compositions.     

温度诱导的液-液结构转变对Sn-10％Bi合金凝固的影响

以温度诱导的液-液结构转变为切入点,研究了Sn-10％Bi合金经不同过热处理后凝固行为和凝固组织的变化.首先利用自制的定向凝固装置进行了液-液结构转变对Sn-10％Bi合金的定向凝固影响的研究.然后测定经不同过热处理的合金熔体相应的凝固曲线,并观察凝固组织.结果表明,相对于在结构转变前保温处理的熔体,经历了结构转变的合金熔体,凝固时所需过冷度增大,并且凝固组织显著细化;揭示了温度诱导液-液结构转变对合金凝固行为及组织有着明显影响.     

Analysis of weld defects in similar and dissimilar resistance seam welding of aluminium,zinc and galvanised steel

Abstract

In the present investigation, a microstructural characterisation of welds performed by resistance seam welding was carried out, with special focus on weld defect analysis. In order to perform a comparative weldability analysis, the welds were performed using similar welding procedures. Similar welds in aluminium (5754-H22) and zinc (Zintek) alloys, as well as dissimilar welds between galvanised steel and zinc, were studied. The defective aluminium welds were found to be characterised by important grain growth inside an inhomogeneous nugget and by the presence of important voids and cracks. The zinc welds showed a well defined nugget, but with porosities and some cracks. In the dissimilar steel–zinc welds, important macroscopic defects were observed. Microstructural analysis evidenced the occurrence of melting at the zinc side of the welds; meanwhile, no microstructural modifications could be observed for the steel side. Defect formation, as well as weld morphologies, was related to the variation in welding parameters.