Evaluation of TIG flux welding on the characteristics of stainless steel

Abstract

The aim of the present study was to investigate the effect of specific oxide fluxes on the surface appearance, weld morphology, retained δ ferrite content, hot cracking susceptibility, angular distortion and mechanical properties obtained with the tungsten inert gas (TIG) process applied to the welding of 5 mm thick austenitic stainless steel plates. An autogenous gas tungsten arc welding process was applied to stainless steels through a thin layer of activating flux to produce a bead on plate welded joint. The MnO₂ and ZnO fluxes used were packed in powdered form. The experimental results indicated that the 80% MnO₂–20% ZnO mixture can give full penetration and also a satisfactory surface appearance for type 304 stainless steel TIG flux welds. TIG welding with MnO₂ and/or ZnO can increase the measured ferrite number in welds, and tends to reduce hot cracking susceptibility in as welded structures. It was also found that TIG flux welding can significantly reduce the angular distortion of stainless steel weldments.     

Interface microstructure of diffusion bonded austenitic stainless steel and medium carbon steel couple

Abstract

In the present study, an austenitic stainless steel and medium carbon steel were diffusion bonded. The effect of bonding temperature on microstructural changes and shear strength across the joint region was investigated by optical and scanning electron microscopy, energy dispersive spectroscopy and microhardness measurements. The results showed that the best joint free from microcrack and micovoids was obtained at 900°C with maximum shear strength of 475 MPa.     

Effect of active flux addition on laser welding of austenitic stainless steel

Abstract

The use of active flux in tungsten inert gas (TIG) welding is known to increase its weld depth. The present paper involves study of active flux laser beam welding (ALBW) of austenitic stainless steel sheets with respect to its effect on plasma plume, microstructure and mechanical properties of the resultant weldments. ALBW performed with SiO₂ as the flux significantly modified shape of the fusion zone (FZ) to produce narrower and deeper welds. Plasma plume associated with the process was considerably smaller and of lower intensity than that produced during bead on plate laser beam welding (LBW). Flux addition during LBW produced thin and rough weld bead associated with humping. The development of such a weld bead is cause by reversal in the direction of Marangoni flow by oxygen induced inversion of surface tension gradient, widely fluctuating plasma plume and presence of oxides on the weld pool surface preventing free flow of the melt. Active flux laser weldments exhibited lower ductility than that of bead on plate laser weldments.     

Structural and chemical evolution of super duplex stainless steel on activated tungsten inert gas welding process

Abstract

In the present work, the welding parameters of tungsten inert gas (TIG) and activated tungsten inert gas (ATIG) welding processes were compared on duplex stainless steel with two protective gases (Ar and ArHeN₂). The addition of an activating flux can improve the TIG welding process. The ATIG process involves an increase in the penetration as well as a reduction in the number of passes for a thickness higher than 2 mm for stainless steel or other metallic materials. Metallographic observations, chemical analyses and mechanical tests were performed. The results show the importance of the welding parameters, the protective gas and the use of a flux on the characteristics of the weld beads, its microstructure, its hardness and its behaviour in corrosion after welding.     

Effect of shielding gas and activating flux on weld bead geometry in tungsten inert gas welding of austenitic stainless steels

Abstract

The aim of this study was to investigate the influence of three shielding gases (argon and argon–hydrogen and argon–helium mixtures) and two activating fluxes (a commercial flux and a TiO₂ based flux) on the geometry of welds produced by the tungsten inert gas (TIG) welding process on several casts of austenitic stainless steel AISI 316, using currents ranging from 100 to 300 A. Penetration depth increases with increasing current for all shielding gases, but weld depth to width ratio is higher for argon than for argon–hydrogen shielded welds. Both activating fluxes produce a substantial increase in penetration depth and in depth to width ratio of the welds. No correlation was found between penetration depth and oxygen content in the melted material. Some interaction exists between activating fluxes and shielding gases, which can affect the weld geometry and/or the defect formation in the welds.     

Application of minielectrochemical cell to corrosion studies of welded joints of austenitic stainless steel

Abstract

A specific electrochemical cell for small scale electrochemical testing has been built. The electrochemical cell has been made bearing in mind the microstructural features of the stainless steel weldments and the performance of the design has been verified. Two polarisation methods (potentiodynamic anodic polarisation measurements and cyclic potentiodynamic polarisation measurements) and two reactivation electrochemical techniques (the electrochemical potentio-kinetic reactivation test, EPR, and electrochemical potentio-kinetic reactivation double loop test, EPRDL) have been applied to two austenitic stainless steels: AISI 304 and AISI 316L. The results are reliable and have been compared to large scale experiments. Scanning of the welding joints has been performed and the results have been correlated with their microstructural features. It, thus, makes it possible to study the intrinsic heterogeneous microstructures such as the heat affected zone of a welded joint.     

Diffusion bonding between high chromium white iron and austenitic stainless steel

Abstract

In the present study, a high chromium white iron was diffusion bonded to an austenitic stainless steel, AISI 316L. The effects of bonding temperature and holding time at the reached temperature on microstructural developments across the joint region were investigated. After diffusion bonding, microstructural analysis including metallographic examination, energy dispersive X-ray (EDX), X-ray, microhardness measurements and shear strength was performed. From the results, it was seen that bonding temperature with holding time was effective on the formation of carbide (M₃C) and width of the diffusion zone that affected the shear strength of the bonds.     

Welding of magnesium alloys with activating flux

Abstract

The effects of an activating flux on AZ31B alloy welding were investigated. Alternating current tungsten inert gas (ACTIG) welding was used to weld 5·0 mm thick AZ31B alloy plates with CdCl₂, AlF₃ and TiO₂ activating flux. Applying the activating flux on the AZ31B alloy surface led to an increase in weld penetration depth. Various welding conditions, such as welding current, welding arc length, welding shielding gas flowrate, welding speed and flux thickness, influenced to different extents the ability of the activating flux to increase weld penetration. Furthermore, a high speed camera was used to monitor the arc images during welding. It was found that the brightest region of the arc was broader when CdCl₂ and AlF₃ were used, while the stability of the arc was increased when TiO₂ was used, especially in the positive electrode period. In summary, it is important that a uniform flux layer is present at the alloy plate surface and suitable welding parameters are selected.     

Welding of titanium alloys with activating flux

Abstract

In the present paper, the effects of an activating flux on Ti–6Al–4V alloy welding were investigated. Tungsten inert gas welding was used to weld 8.0 mm thickness Ti–6Al–4V alloy plates. Results show that applying the activating flux on the Ti–6Al–4V alloy surface leads to an increase in weld penetration depth, whereas the corresponding weld bead width is reduced. It was also found that various welding conditions, particularly flux thickness, influence the effectiveness of the activating flux. Furthermore, a data acquisition system was used to monitor the current and voltage signals during welding. Results from monitoring of the welding current and voltage signals reveal that there is a clear correlation between the signals and the weld penetration when the welding arc is steady. Analysis of the acquired signals can be used to identify inconsistencies in weld penetration. In summary, to take advantage of the use of activating flux in Ti alloy welding, it is important that a uniform flux layer is present at the alloy plate surface and suitable welding parameters are selected.     

Impact toughness of 316 stainless steel weld metal on elevated temperatures aging

Abstract

Shielded metal arc welding electrodes of a modified E316-15 austenitic stainless steel, for service at 673–823 K with delta ferrite in the range of 3–7 ferrite number, have been developed indigenously for welding of 316L(N) stainless steel structural materials for the Indian Prototype Fast Breeder Reactor. Delta ferrite content in weld metals for high temperature service is restricted for limiting the formation of embrittling secondary phases during service. To study the effect of high temperature exposure on microstructure and mechanical properties, the 316 weld metal was aged at three different temperatures of 923, 973 and 1023 K, for various durations up to 500 h. The activation energy for the transformation of delta ferrite has been estimated to analyse the mechanism associated with the micro structural changes that led to the deterioration in toughness on elevated temperature aging of this weld metal.     

Novel method for quantitative assessment of slag detachability in austenitic stainless steels welds made by SMAW

Abstract

It is essential and important that slag which protects the molten metal from atmospheric contamination during shielded metal arc welding (SMAW) process gets removed as the weld metal gets solidified. During multipass welding in particular it is desirable that slag gets removed easily and totally before the next pass of welding. Otherwise it would lead to slag inclusions in the weld. Hence, the ease with which the slag gets detached from the weld termed as slag detachability is a vital parameter in deciding the weld quality. Till date, slag detachability has been assessed on qualitative terms only. Hence, it is necessary to develop test procedures and methodology for assessing the slag detachability in quantitative terms. In this work, the authors propose a novel slag detachability testing technique called slag detachability tester and a methodology for quantitative assessment of slag detachability. The test procedure and the methodology have been tested first on austenitic stainless steels. The slag on the weldment was removed by dropping varieties of hammers with different end shapes from a standard height with a custom designed drop weight tester. After the test, the adherent slag on the weldment was inspected and measured using infrared (IR) thermography. The acquired IR images were suitably processed using image processing software to identify the area of slag particles sticking on the weld. Testing parameters were standardised and using these data, an expression is proposed for assessing the slag detachability in quantitative terms.     

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.     

Friction welding of TiNi alloy to stainless steel using Ni interlayer

Abstract

Friction welding was carried out between TiNi alloy and austenitic stainless steel with and without a Ni interlayer. When TiNi alloy was welded to stainless steel without the Ni interlayer, a large amount of brittle Fe₂Ti intermetallic compound was formed at the weld interface. The formation of this brittle compound led to degradation of the joint strength. The Ni interlayer changed the microstructures at the weld interface and improved the joint strength. A fracture occurred at the interface between Ni and TiNi. The interface between Ni and TiNi was free from Fe₂Ti and consisted of mainly TiNi₃ and TiNi. After TiNi₃ was formed as the reaction layer, a eutectic reaction occurred between the TiNi₃ and TiNi base alloy. A reaction layer with a eutectic structure tends to form at the periphery, where the temperature would be higher than that of the central region.     

Control of nitrogen content and porosity in gas tungsten arc welding of high nitrogen steel

Abstract

In welding of high nitrogen steel (HNS), it is essential to control the nitrogen content and porosity in the weld metal. In this paper, the influence of shielding gas composition and heat input on the nitrogen content and porosity in the weld metal of HNS was investigated by gas tungsten arc welding. The experimental results indicate that the weld nitrogen content increases as N₂ in the shielding gas is increased in the same heat input of welding. The weld nitrogen content decreases with increasing the heat input for pure argon used as a shielding gas, whereas it increases with increasing the heat input for the shielding gas including some nitrogen. The nitrogen pore can be avoided when the nitrogen content in the shielding gas is <4% in the heat input range of 528–2340 J mm^–1.     

Microstructures and properties of laser/arc hybrid welds and autogenous laser welds in pipeline steels

Abstract

Lasers are capable of producing welds with deep penetration, low distortion and faster travel speeds, compared to arc welding. More recently, laser/arc hybrid welding processes have also been generating interest for industrial fabrication. In this paper, six carbon–manganese, mainly pipeline, steels were welded using both autogenous Nd:YAG laser welding, and Nd:YAG laser/MAG hybrid welding. The improvements in weld microstructures and weld metal toughness that are possible when using the hybrid process are described and illustrated. Laser/arc hybrid welding is shown to be a process that can generate good quality welds in commercially available pipeline steels. It also has the potential to complete girth welds in these steels with significantly fewer welding passes than are currently required for arc welded pipelines, reducing the joint completion time.     

Development of new additive for grain refinement of austenitic stainless steel

Abstract

Recently, a new additive for grain refinement of Ni based superalloys has been developed. In those studies, it was considered that niobium carbide in Ni–Nb–C alloy additives would act as nuclei on the solidification of Ni based superalloys. It is known that the crystallographic characteristics of iron are quite similar to those of nickel. Thus, it was expected that niobium carbide would act as a grain refiner for iron base alloys, especially for austenitic steels. In this study, the effect of the additives on the microstructure of SUS316 steel was examined in various experimental conditions. The grain size of SUS316 specimens without inoculation was ～2700 μm. On the other hand, when NbC containing alloy additives were added into the SUS316 melt, fine equiaxed grains were observed and the grain size of the specimen was significantly reduced to ～200 μm.     

Verification of numerical model to predict microstructure of austenitic stainless steel weld metal using synchrotron radiation and trans varestraint testing

Abstract

A numerical model to predict the microstructure of austenitic stainless steel weld metal is proposed, and spatially resolved X-ray diffraction measurements using synchrotron radiation have been carried out for Fe–20Cr–(9·8–14·4)Ni weld metals, quenched in liquid Sn, to verify the validity of the numerical model. X-ray diffraction analysis of Fe–20Cr–11·5Ni quenched weld metal, solidifying in the ferritic–austenitic mode, showed that the secondary γ phase crystallised in a eutectic growth mode down to a temperature drop of 6 K from the initiation of solidification. Also, from X-ray diffraction analysis of Fe–20Cr–12·7Ni quenched weld metal, which solidified in the austenitic–ferritic mode, it was found that the secondary δ phase crystallised in a eutectic growth mode within the temperature drop range between 15 and 21 K from the initiation of solidification. The crystallisation temperatures predicted by the numerical model for secondary γ and δ phases in Fe–20Cr–11·5Ni and Fe–20Cr–12·7Ni weld metals agreed with experimental data. Furthermore, it was found that the effect of Ni content on the solidification cracking susceptibility of Fe–20Cr–(9·8–14·4)Ni weld metal, determined via trans varestraint testing, agreed with the results calculated using the model. These agreements support the validity of the developed numerical model.     

Spatter reduction in gas metal arc welding of stainless steel sheets using controlled bridge transfer process

Abstract

In non-pulsed gas metal arc welding (GMAW), spatter can be reduced by controlling the short circuit current to a low level just before the re-arcing. The controlled bridge transfer (CBT) process, which optimises the accuracy of predicting the re-arcing in real time in response to the metal transfer, realises stable, low spatter level. In this research, the methods for controlling short circuit transfers to minimise spatter and realise stable arcs in GMAW of stainless sheet using argon rich shielded gases are investigated. The new CBT process has been developed by applying the specific arc length estimation method that is not affected by abnormal rise in arc voltage. This process can suppress the spatter generation caused by a fluctuation in the vibratory motion of the weld pool or inaccurate prediction of the re-arcing in the succeeding short circuit/re-arcing cycle, and thereby spatter free GMAW in the short circuit transfer mode can be carried out even on stainless steels.     

Mechanical properties of 422 stainless steel repaired via wire feed laser welding

Abstract

Laser welding with filler wire additions could be used in restoration of components that are of high cost or sometimes difficult to procure, such as steam turbine blades in fossil fuel power plants. In the present work, machined V groove specimens were employed to simulate laser repair of Carpenter 636 stainless steel (SS), which has a similar composition to a blade material, type 422 SS. Before repair welding, a heat treatment procedure including solution and temper treatments of the specimens was carried out according to the mechanical and microstructural analyses of a used blade after 20 years service at about 540° C. Tensile, impact, and fatigue crack growth tests of weld repairs using 410 SS filler wire were conducted. The weld repairs exhibited an impact toughness similar to that of the base metal and a lower fatigue crack growth rate than the base metal. However, the lower hardness associated with 410 SS filler metal led to tensile fracture in the weld metal of repaired specimens. Accordingly, the use of 410 SS filler metal for repair welding type 422 SS components should be limited to regions under low stress.     

Measurement of residual stresses in austenitic stainless steel weld joints using ultrasonic technique

Abstract

A methodology has been developed using a non-destructive ultrasonic technique for measuring surface/subsurface residual stresses in 7 mm thick AISI type 316LN stainless steel weld joints made by activated tungsten inert gas and multipass tungsten inert gas welding processes. Measurement of residual stresses using an ultrasonic technique is based on the effect of stresses on the propagation velocity of elastic waves. Critically refracted longitudinal L _CR wave mode was employed and accurate transit time measurements were made across the weld joints. Quantitative values of the longitudinal residual stresses across the weld joints were estimated from the measured transit times and predetermined value of acoustoelastic constant for AISI type 316LN stainless steel. The nature of the residual stress profiles and their variations across the two types of weld joints were compared and interpreted.