Microstructure and pitting corrosion of similar and dissimilar stainless steel welds

Abstract

Pitting Corrosion behaviour of similar and dissimilar metal welds of three classes of stainless steels, namely, austenitic stainless steel (AISI 304), ferritic stainless steel (AISI 430) and duplex stainless steel (AISI 2205), has been studied. Three regions of the weldment, i.e. fusion zone, heat affected zone and unaffected parent metal, were subjected to corrosion studies. Electron beam and friction welds have been compared. Optical, scanning electron microscopy and electron probe analysis were carried out to determine the mechanism of corrosion behaviour. Dissimilar metal electron beam welds of austenitic–ferritic (A–F), ferritic–duplex (F–D) and austenitic–duplex stainless steel (A–D) welds contained coarse grains which are predominantly equiaxed on austenitic and duplex stainless steel side while they were columnar on the ferritic stainless steel side. Microstructural features in the central region of dissimilar stainless steel friction welds exhibit fine equiaxed grains due to dynamic recrystallisation as a result of thermomechanical working during welding and is confined to ferritic stainless steel side in the case of A–F, D–F welds and duplex stainless steel side in the case of D–A welds. Beside this region bent and elongated grains were observed on ferritic stainless steel side in the case of A–F, D–F welds and duplex stainless steel side in the case of D–A welds. Interdiffusion of elements was significant in electron beam welding and insignificant in friction welds. Pitting corrosion has been observed to be predominantly confined to heat affected zone (HAZ) close to fusion boundary of ferritic stainless steel interface of A–F electron beam and D–F electron beam and friction weldments. The pitting resistance of stainless steel electron beam weldments was found to be lower than that of parent metal as a result of segregation and partitioning of alloying elements. In general, friction weldments exhibited better pitting corrosion resistance due to lower incidence of carbides in the microstructure.     

Corrosion properties of nickel free austenitic stainless steels in 0·5M H2SO4 and 0·5M H2SO4 plus 0·4M NaCl

Abstract

The corrosion behaviour of four nickel free austenitic stainless steels were investigated in 0·5M H₂SO₄ and 0·5M H₂SO₄ plus 0·4M NaCl solutions by means of potentiodynamic and potentiostatic anodic polarisation testing. The performances of the nickel free alloys are compared to those of an experimental intermediate nickel alloy (4%Ni) and a standard AISI 304 steel grade. Once passivity was reached all alloys displayed similar current densities i _p in 0·5M H₂SO₄, independent from alloying. Mo and Cu were shown to be beneficial in decreasing the active dissolution currents and i _crit values. The commercial AISI 304 steel displayed superior resistance to pit initiation during potentiodynamic testing, and AISI 304 steel displayed the highest E _pit value of all alloys tested. When tested potentiostatically the N and Mo alloyed nickel free alloys showed excellent resistance to pit initiation and growth. The dominant effect of N was associated with repassivation of incipient pits, while Mo appeared to act at an earlier stage, suppressing initiation.     

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.     

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.     

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.     

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.     

Novel laser surface treatment approach to suppress sensitisation in modified type 316(N) stainless steel weld metal

Abstract

Welded components are subjected to solution annealing heat treatment for achieving full stress relief and restoration of mechanical properties and corrosion resistance. During such heat treatments, optimum cooling rate has to be selected because very slow cooling rate will result in sensitisation and susceptibility to intergranular corrosion whereas fast cooling will result in reintroduction of residual stress. For 316 LN stainless steel which is welded using modified E316-15 electrodes (0·045–0·055%C), critical cooling rate above which there is no risk of sensitisation is 75 K h^?1. This paper presents a novel laser surface treatment which suppresses sensitisation in weld metal, even at a slower cooling rate of 65 K h^?1. Experiments involving laser surface melting were carried out with 150 W average power pulsed Nd:YAG laser and 10 kW CO₂ laser, in both continuous wave and pulse modulated (100 Hz) modes. Best results were obtained when surface melting was performed with high frequency pulse modulated CO₂ laser beam. The processed weld metal remained unsensitised after solution annealing followed by slower rate of cooling at 65 K h^?1. Numerical simulation study was performed with ANSYS 7·0 software to understand the physical reason behind the difference in sensitisation behaviour of CO₂ laser melted specimens under continuous wave and high frequency pulse modulated conditions and the predictions were validated using results of electron backscattered diffraction studies. Weld metal specimens treated with high frequency pulse modulated CO₂ laser clearly showed evolution of fine grains near the fusion boundary region which enhanced sensitisation resistance.     

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.     

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.     

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.     

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.     

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.     

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.     

Corrosion of Ni–Al high velocity oxyfuel (HVOF) thermal spray coating by fly ash and synthetic biomass ash deposits

Abstract

Corrosion of a thermal spray (HVOF) Ni–Al coating has been investigated at 600°C under both biomass generated fly ash and synthetic biomass ashes and has been monitored as a function of reaction time for up to 1000 h. Detailed microstructral and compositional analyses have been carried out to investigate the hot corrosion mechanism. Various microstructural defects and compositional inhomogeneities are found to play an important role in the initiation of hot corrosion. Deposition of molten reactants from the ashes creates the aggressive environment. Molten salts cause initial rapid hot corrosion via fluxing reactions between planar interlamellar porosity resulting in debonding of the surface lenticular splats followed by subsequent slow dissolution. Comparison between the performance of the coating in the fly ash and the synthetic ashes provides information for improved laboratory corrosion tests.     

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.     

Electrochemical behaviour of Type 316 austenitic stainless steel weld metals with and without microfissures

Abstract

The corrosion performance of austenitic stainless weld deposits with and without microfissures has been evaluated by cyclic polarisation testing in 3·5% sodium chloride solution at room temperature to define the electrochemical behaviour. The corrosion test results revealed that the presence of microfissures in the weld deposit significantly degrades corrosion resistance. Corrosion attack preferentially occurred at the tips of microfissures in fissure-containing samples and at the interpass boundaries for fissure-free samples.     

Corrosion of 316L austenitic stainless steel under desalination plant conditions

Abstract

The AISI type 316L stainless steel liner plates in the flash chambers of a multistage flash desalination plant, located on the Arabian Gulf coast, developed severe corrosion within 6 years of operation. The mode and causes of corrosion and the effect of heating (during annealing or welding) and temperature of salt solution on this attack were investigated. Specimens of liner plate were studied in the as received condition and after being heat treated at 900°C in air and air cooled to room temperature. Potentiodynamic polarisation techniques were used to determine corrosion rates. It was found that commercial grade 316L is susceptible to pitting, crevice, and grain boundary corrosion under the operating conditions in the desalination plant. The heat affected zone had larger grains and corroded more severely than other parts of the liner plates. Randomly distributed inclusions containing titanium, chromium, molybdenum, manganese, and sulphur were found in the liner plates. The as received specimens showed the lowest corrosion rate, followed in ascending order by heat treated and heat affected zone specimens.     

奥氏体不锈钢焊接接头晶间腐蚀断裂分析

核电站管道大多采用奥氏体铁素体不锈钢制成,以主管道为例,多数采用焊接方式进行连接。在现场应用之前,需要进行工艺评定的焊接和检验,以验证焊接接头是否满足高温、高压、高腐蚀的性能要求。晶间腐蚀按照标准RCCM+RCCM2000补遗中MC1312.3中B法处理后,在弯曲过程出现裂纹,对比试样也出现裂纹,难以判定是否是晶间腐蚀造成的裂纹缺陷。利用金相方法观察出现裂纹的试样,结合力学性能试验和化学成分进行分析得出,弯曲受检面产生的裂纹不是由晶间腐蚀造成的。     

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.     

Resistance welding of thin stainless steel sandwich sheets with fibrous metallic cores: experimental and numerical studies

Abstract

This paper concerns resistance spot welding (RSW) of two types of thin stainless steel sandwich sheet. The cores of these materials, made of stainless steel fibres, are highly porous (> around 85 vol.-%) and have low thermal and electrical conductivities. However, these conductivities change during the compression and heating associated with RSW. A sequentially coupled finite element model has been developed, in which the crushed core is treated as a continuum, with properties which vary throughout the process. It is shown that a constitutive relationship of the type commonly used for crushable foams can be successfully employed to simulate the deformation of the sandwich sheets. The thermoelectrical part of the model incorporates the effects of the associated phase transformations and changes in interfacial conductance. It is shown that the predictions are broadly consistent with data obtained during welding experiments. The model is used to explore the effects of welding parameters on weld characteristics (weld pool formation and weld nugget shape).