Effects of Welding Procedure on Corrosion Resistance and Hydrogen Embrittlement of Supermartensitic Stainless Steel Deposits

The effects of shielding gas and post weld heat treatment on the pitting resistance, stress corrosion cracking and hydrogen embrittlement of supermartensitic stainless steel deposits were studied. Two all-weld-metal test coupons were prepared using a metal-cored wire under Ar+5% He and Ar+18%CO₂ gas shielding mixtures. Solubilizing and solubilizing plus double tempering heat treatments were done with the objective of achieving different microstructural results. The samples welded under Ar+5% He showed higher pitting corrosion resistance, for all post weld heat treatments, than those welded under Ar+18%CO₂. The different post weld heat treatments generated higher susceptibility to this corrosion mechanism. None of the samples presented signs of stress corrosion cracking, but in those subjected to the heat treatment, grain boundary selective attack was observed, on the surfaces of all the samples studied. The samples with highest hardness were more susceptible to hydrogen damage, thereby leading to reduced tensile strength on this condition.     

Influence of CO2-Ar Mixtures as Shielding Gas on Laser Welding of Al-Mg Alloys

In this study, AA5083 samples were butt welded under a conduction regime with high-power diode laser (HPDL). Various mixtures composed of Ar and CO₂ were used as a shielding gas. The influence of the shielding gas composition on the microstructure and on the properties of laser welds was analyzed. The weld beads were deeply characterized by metallographic/microstructural studies, X-ray diffraction (XRD), X-ray energy dispersive spectrometry (X-EDS) chemical analyses, X-ray photoelectron spectra (XPS), microhardness, and tensile strength. The corrosion resistance of laser-remelted surfaces with different CO₂/Ar ratios was also estimated by means of electrochemical tests. The addition of CO₂ to the shielding gas results in a better weld penetration and oxidizes the weld pool surface. This addition also promotes the migration of Mg toward the surface of weld beads and induces the formation of magnesium aluminates spinel on the welds. The best corrosion resistance result is achieved with 20 pct CO₂. The overall results indicate that the addition of small percentage of CO₂ to Ar leads to improvements of the mechanical and corrosion properties of the aluminum welds.     

Influence of Mode of Metal Transfer on Microstructure and Mechanical Properties of Gas Metal Arc-Welded Modified Ferritic Stainless Steel

This article describes in detail the effect of the modes of metal transfer on the microstructure and mechanical properties of gas metal arc-welded modified ferritic stainless steel (SSP 409M) sheets (as received) of 4 mm thickness. The welded joints were prepared under three modes of metal transfer, i.e., short-circuit (SC), spray (S), transfer, and mix (M) mode transfer using two different austenitic filler wires (308L and 316L) and shielding gas composition of Ar + 5 pct CO₂. The welded joints were evaluated by means of microstructural, hardness, notched tensile strength, Charpy impact toughness, and high cycle fatigue. The dependence of weld metal microstructure on modes of metal transfer and filler wires has been determined by dilution calculation, WRC-1992 diagram, Cr_eq/Ni_eq ratio, stacking fault energy (SFE), optical microscopy (OM), and transmission electron microscopy (TEM). It was observed that the microstructure as well as the tensile, Charpy impact, and high cycle fatigue of weld metal is significantly affected by the mode of metal transfer and filler wire used. However, the heat-affected zone (HAZ) is affected only by the modes of metal transfer. The results have been correlated with the microstructures of weld and HAZ developed under different modes of metal transfer.     

Marangoni convection in weld pool in CO<Subscript>2</Subscript>-Ar-shielded gas thermal arc welding

Small CO₂ additions of 0.092 to 10 vol pct to the Ar shielding gas dramatically change the weld shape and penetration from a shallow flat-bottomed shape, to a deep cylindrical shape, to a shallow concave-bottomed shape, and back to the shallow flat-bottomed shape again with increasing CO₂ additions in gas thermal arc (GTA) welding of a SUS304 plate. Oxygen from the decomposition of CO₂ transfers and becomes an active solute element in the weld pool and reverses the Marangoni convection mode. An inward Marangoni convection in the weld pool occurs when the oxygen content in the weld pool is over 80 ppm. Lower than 80 ppm, flow will change to the outward direction. An oxide layer forms on the weld pool in the welding process. The heavy oxide layer on the liquid-pool surface will inhibit the inward fluid flow under it and also affects the oxygen transfer to the liquid pool. A model is proposed to illustrate the interaction between the CO₂ gas and the molten pool in the welding process.     

Effect of Process Parameters on Clad Quality of Duplex Stainless Steel Using GMAW Process

In recent years, weld cladding are being applied in numerous industries as cost effective engineering solution to use a surface protection layer to protect carbon steel against corrosion attack. The desirable characteristics of cladding alloy are reasonable strength, weldability, resistance to general and localized corrosion attack. The duplex stainless steel having all the desirable characteristic is the candidate material for cladding. However, duplex weld metals have not been studied in detail as duplex stainless steels. Consequently, the properties of duplex weld metals are less well known and only partially understood. In the present study, the properties of duplex weld deposits of the 22 % Cr, 10 % Ni, 3 % Mo, and 0.12 % N type using GMAW process have been investigated. In particular, the influence of welding heat input and shielding gas composition in GMAW process on weld deposit microstructure, impact toughness and resistance to pitting corrosion have been studied. It is observed that concentration of nitrogen of weld deposits influenced by both heat input and shielding gas composition exerted significant effect on microstructure, low temperature toughness and resistance to pitting corrosion.     

Decomposition of ferrite in commercial superduplex stainless steel weld metals; microstructural transformations above 700 °C

The microstructural stability at temperatures above 700 °C of weld metal of type 29Cr-8Ni-2Mo-0.39N and weld metal of type 25Cr-10Ni-4Mo-0.28N has been compared. Multipass welding was employed using the gas tungsten arc welding technique with a shielding gas of Ar+2 pct N₂. The quantitative assessment of the intermetallic phase was performed using automatic image analysis in the light optical microscope (LOM). Detailed microanalysis was also performed using scanning and transmission electron microscopy. A computer program developed by the authors was used to calculate a continuous cooling-temperature (CCT) diagram on the basis of the experimentally determined time-temperature-transformation (TTT) diagram. Thermodynamic calculations for estimating phase stabilities and for interpreting experimental observations were performed. It was found that weld metal of type 29Cr-8Ni-2Mo-0.39N was microstructurally more stable than weld metal of type 25Cr-10Ni-4Mo-0.28N. A lower molybdenum concentration and a higher nitrogen concentration in the former alloy could explain the higher stability with respect to the intermetallic phase. The higher nitrogen concentration also provides a rationale for the higher stability against the formation of secondary austenite in weld metal of type 29Cr-8Ni-2Mo-0.39N. This effect, which is associated with a lower thermodynamic driving force for precipitation of secondary austenite during multipass welding, can be explained by nitrogen-enhanced primary austenite formation.     

Effect of GTAW Parameters on Structure and Mechanical Properties of C86300 Weld Joint

In this study, the effects of heat input variation in gas tungsten arc welding (GTAW), on structure and mechanical properties of commercially C86300 (containing addition of 0.6 wt% silicon) weld joint were investigated. Following microstructural characterization of Base metal, GTAW has been performed at welding currents 50 and 60 A and flow rates of argon shielding gas (10, 14 and 18 l/min) using the same filler metal composition. Therefore six GTAW samples were performed with various welding specifications. By structural investigations and hardness profiles, effects of increasing heat input on increasing average grain size in weld zone, heat affected zone width, penetration depth and alloying element losses were indicated. However increasing heat input increases penetration depth and has a positive effect on hardness and strength of the joint. In considering wear application of this alloy castings and probable GTAW for them, pin-on-disc wear test was performed and revealed better wear resistance of weld metal in comparison with base metal. Hence the optimum values of welding current and argon flow rates (in GTAW with the same composition filler) was determined for this alloy.     

Influence of nitrogen addition on the microstructure and mechanical properties of duplex stainless steel weld metals

Duplex stainless steels find extensive applications in off‐shore, paper/pulp, refineries and petrochemical industries owing to their excellent combination of properties. Because of the thermal cycle experienced during the manufacturing/processing operations like welding, the weldment developed in this material is generally inferior in properties (mechanical and corrosion) when compared to the base material. The problem is more severe when the welding is done in the autogenous condition. The current work correlates the microstructure and property of UNS 31803 duplex stainless steel (material no. 1.4462) welded by autogenous GTA process with and without nitrogen in the shielding gas. It has been found that the nitrogen addition to the weld metal through the shielding gas mixture not only helps in achieving the proper phase balance but also improves the impact toughness of the resultant weld metal.     

Pulsed Nd:YAG laser welding of copper using oxygenated assist gases

The effects of using oxygenated assist gases on the weldability and weld properties of Nd:YAG, pulsed laser welds in copper (Cu) have been evaluated. It was found that the effective absorptivity of the Cu increased as the oxygen content of the Ar assist gas was increased. This facilitated laser welding of Cu at much lower laser powers and increased weld penetration. The use of oxygenated assist gas promoted nucleation and growth of submicroscopic oxide particles within the weld metal. These particles dispersion-strengthened the weld metal, thereby increasing both weld metal hardness and strength. However, when O₂ concentrations in the assist gas were greater than 90 pct, weld metal embrittlement due to excessive volume fractions of oxides was observed. The use of oxygenated assist gas also led to excessive cold lapping and poor bead quality. The bead quality was improved, however, by ramping-down the laser power before terminating each pulse.     

Submerged Friction-Stir Welding (SFSW) Underwater and Under Liquid Nitrogen: An Improved Method to Join Al Alloys to Mg Alloys

Submerged friction-stir welding (SFSW) underwater and under liquid nitrogen is demonstrated as an alternative and improved method for creating fine-grained welds in dissimilar metals. Plates of AZ31 (Mg alloy) and AA5083 H34 were joined by friction-stir welding in three different environments, i.e., in air, water, and liquid nitrogen at 400?rpm and 50?mm/min. The temperature profile, microstructure, scanning electron microscopy (SEM)-energy-dispersive spectroscopy (EDS) analysis, X-ray diffraction (XRD), hardness, and tensile testing results were evaluated. In the stir zone of an air-welded specimen, formation of brittle intermetallic compounds of Al₃Mg₂, Al₁₂Mg₁₇, and Al₂Mg₃ contributed to cracking in the weld nugget. These phases were formed because of constitutional liquation. Friction-stir welding underwater and under liquid nitrogen significantly suppresses the formation of intermetallic compounds because of the lower peak temperature. Furthermore, the temperature profiles plotted during this investigation indicate that the largest amount of ?T is generated by the weld under liquid nitrogen, which is performed at the lowest temperature. It is shown that in low-temperature FSW, the flow stress is higher, plastic contribution increases, and so adiabatic heating, a result of high strain and high strain-rate deformation, drives the recrystallization process beside frictional heat.     

Nitrogen desorption by high-nitrogen steel weld metal during CO<Subscript>2</Subscript> laser welding

Nitrogen desorption by high-nitrogen steels (HNSs) containing 0.32 and 0.53 pct nitrogen during CO₂ laser welding in an Ar-N₂ gas mixture was investigated and the obtained data were compared with those for arc welding and at the equilibrium state predicted by Sieverts’ Law. Although the nitrogen content in the weld metal during CO₂ laser welding was lower than that in the as-received base material in all conditions, the nitrogen desorption was larger in the top part of the weld metal than in the keyhole region. The nitrogen desorption in the Ar atmosphere was less during CO₂ laser welding than during arc welding. With the increase in nitrogen partial pressure, the nitrogen content in the weld metal sharply increased during arc welding, but only slightly increased during CO₂ laser welding. The nitrogen absorption and desorption of the HNS weld metal were much smaller during CO₂ laser welding than during arc welding.     

Hard amorphous silicon carbonitride coatings produced by plasma enhanced chemical vapour deposition

Hard amorphous silicon carbonitride coatings for wear resistance have been produced by d.c. plasma‐enhanced chemical vapour deposition on pure iron at 573 K. Compared with most plasma assisted processes work was conducted under a relatively high pressure of 130 Pa. The advantages of this technique are an amorphous film structure, high deposition rates (up to 2 μm/min), high hardness and simple equipment. Hexamethyldisilazan (HMDSN) was used as precursor and argon, hydrogen, nitrogen and ammonia as process gases. The dependence of the coatings on the process parameters (process gas and d.c. power) was investigated. The characterization of the samples was carried out mainly by Fourier transform infrared spectrometry (FTIR), electron probe microanalysis (EPMA) and hardness measurement. Samples show clearly the infrared spectra absorption bands characteristic of SiC and Si₃N₄, with traces of hydrogen bonding. The material structure shows a strong dependence on the process gas and the d.c. power. However, for argon and hydrogen were deposited carbon‐rich SiC films with low nitrogen content. Nitridic films with low carbon content were deposited using nitrogen and especially ammonia. The hardness of the produced coatings was about 10 ‐ 55 GPa.     

Influence of Activating Flux and Helium Shielding Gas on an Austenitic Stainless Steel Weldment

Activating flux-assisted gas tungsten arc welding (GTAW) is a well-established method for enhancing weld penetration. In GTAW, steel is usually welded with a shielding gas that contains mostly argon. However, pure argon does not provide enough weld penetration. Argon-helium mixtures are inert and a greater concentration of helium would increase the arc voltage and the weld depth-to-width (D/W) ratio. There is a significant level of interest in the interaction between activating flux and shielding gas composition. Weld morphology, arc profile, retained δ ferrite content, angular distortion, and microstructure are extremely important in applying the activating flux combination argon-helium in GTAW; therefore, in this work, all these were studied.     

Argon-Hydrogen Shielding Gas Mixtures for Activating Flux-Assisted Gas Tungsten Arc Welding

Using activating flux for gas tungsten arc welding (GTAW) to improve penetration capability is a well-established technique. Argon is an inert gas and the one most widely used as a shielding gas for GTAW. For the most austenitic stainless steels, pure argon does not provide adequate weld penetration. Argon–hydrogen mixtures give a more even heat input to the workpiece, increasing the arc voltage, which tends to increase the volume of molten material in the weld pool as well as the weld depth-to-width ratio. Great interest has been shown in the interaction between activating flux and the hydrogen concentration in an argon-based shielding gas. In this study, the weld morphology, the arc profile, the retained delta ferrite content, the angular distortion, and the microstructures were examined. The application of an activating flux combining argon and hydrogen for GTAW is important in the industry. The results of this study are presented here.     

K-TIG焊接中厚板的工艺窗口改进

针对穿孔深熔氩弧焊(K-TIG)工艺焊接8 mm厚Q235低碳钢板时焊接过程不稳定、焊接工艺窗口小等突出问题,首次提出在焊接工件背部铺加保护焊剂的方法改善焊接过程。采用对接焊的方式,在不开坡口、焊接过程不添加焊丝的情况下,达到单面焊双面成形的效果。最终成功的采用430～480 A范围内的直流电流对8 mm厚的Q235低碳钢进行了焊接,将焊接电流窗口扩大到50 A同时也显著的提高了焊接过程的稳定性。同时,在扩大焊接电流窗口之后,系统研究了不同焊接电流下焊接接头的组织性能。研究结果表明:在不同焊接电流下得到的焊接接头中,组织分布以及力学性能分布呈现出相同的状态。焊缝区的组织均由铁素体+珠光体+魏氏组织组成;熔合区由魏氏组织组成;热影响区由铁素体+少量的珠光体组成;此外随着焊接电流的增加,焊接接头背部的熔宽有略微增加;在焊接接头中,熔合区处硬度值最高,其次是焊缝区,之后是热影响区,母材的硬度值最低;焊接接头最终的拉伸断裂位置是在热影响区处。      

Effect of Mechanical Arc Oscillation on the Grain Structure of Mild Steel Weld Metal

The effect of mechanical arc oscillation on the weld metal grain structure in mild steel gas tungsten arc welds has been studied. For welds made without arc oscillation, columnar grains were observed in the weld metal; however, for the same welding parameters, the weld made with arc oscillation had smaller sized relatively equiaxed grains in the weld metal. The strengths for weld made with arc oscillation was higher than that for weld made without arc oscillation, with appreciable increase in ductility; this could be attributed to the reduction in grain size diameter due to arc oscillation. Lower weld metal hardness and increase in heat affected zone hardness was observed in weld made with arc oscillation; this could be attributed to increase in pro-eutectoid ferrite formation with absence of Widmanstatten ferrite structures in the weld metal and less coarsening of grains in the heat affected zone due to increased cooling rate.     

ER70S-G焊丝焊接工艺性能的研究

为提高气保焊丝的焊接工艺性能,选用不同化学成分的ER70S-G焊丝进行焊接工艺试验。当焊丝含某种元素X质量分数为0.09%、w（Mn）为1.50%及w（Si）为1.0%时,CO2气体保护焊接飞溅极少,但焊缝宽高比较小,为3.5;当焊丝含某种元素X质量分数为0.01%、w（Mn）为1.70%及w（Si）为0.8%时,CO2气体保护焊接飞溅较少,焊缝宽高比较大,为4.5,显示出很好的焊接工艺性能。结果表明,焊丝的成分对气保焊缝的成形有很大的影响,在某种元素X质量分数、w（Ti）及w（Si）/w（Mn）比适当时,可得到焊接飞溅小、焊缝成形佳的焊接工艺性能。     

Numerical modeling of enhanced nitrogen dissolution during gas tungsten Arc welding

Weld-metal nitrogen concentrations far in excess of Sieverts-law calculations during gas tungsten arc (GTA) welding of iron are investigated both experimentally and theoretically. A transient, threedimensional mathematical model has been developed to calculate the residual nitrogen concentrations during GTA welding. This model combines calculations for the plasma phase with those for nitrogen absorption and for the transport of nitrogen by convection and diffusion in the weld metal and diffusion throughout the weldment. In addition, the model takes into account the roles of turbulence and the nitrogen desorption reaction in affecting the residual nitrogen concentration in the weldment. Autogeneous GTA welding experiments in pure iron have been performed and the resulting nitrogen concentrations compared with the modeling results. Both experimental and modeled nitrogen concentrations fall in a range between 2.7 and 4.7 times higher than Sieverts-law calculations at a temperature of 2000 K. Modeled nitrogen concentrations correlate well with the experimental results, both in magnitude and in the general trends, with changes in the travel speed and nitrogen addition to the shielding gas.     

CO2 laser beam welding of 6061-T6 aluminum alloy thin plate

Laser beam welding is an attractive welding process for age-hardened aluminum alloys, because its low heat input minimizes the width of weld fusion and heat-affected zones (HAZs). In the present work, 1-mm-thick age-hardened Al-Mg-Si alloy, 6061-T6, plates were welded with full penetration using a 2.5-kW CO₂ laser. Fractions of porosity in the fusion zones were less than 0.05 pct in bead-on-plate welding and less than 0.2 pct in butt welding with polishing the groove surface before welding. The width of a softened region in the-laser beam welds was less than 1/4 times that of a tungsten inert gas (TIG) weld. The softened region is caused by reversion of strengthening β″ (Mg₂Si) precipitates due to weld heat input. The hardness values of the softened region in the laser beam welds were almost fully recovered to that of the base metal after an artificial aging treatment at 448 K for 28.8 ks without solution annealing, whereas those in the TIG weld were not recovered in a partly reverted region. Both the bead-on-plate weld and the butt weld after the postweld artificial aging treatment had almost equivalent tensile strengths to that of the base plate.     

Microstructural factors governing hardness in friction-stir welds of solid-solution-hardened Al alloys

Microstructural factors governing hardness in friction-stir welds of the solid-solution-hardened Al alloys 1080 and 5083 were examined by optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The effect of grain boundary on the hardness was examined in an Al alloy 1080 which did not contain any second-phase particles. The weld of Al alloy 1080 had a slightly greater hardness in the stir zone than the base material. The maximum hardness was located in the thermomechanically affected zone (TMAZ). The stir zone consisted of recrystallized fine grains, while the TMAZ had a recovered grain structure. The increase in hardness in the stir zone can be explained by the Hall-Petch relationship. On the other hand, the hardness profiles in the weld of Al alloy 5083 were roughly homogeneous. Friction-stir welding created the fine recrystallized grains in the stir zone and recovered grains in the TMAZ in the weld of this alloy. The stir zone and the TMAZ had slightly higher dislocation densities than the base material. Many small Al₆(Mn,Fe) particles were detected in all the grains of the weld. The hardness profiles could not be explained by the Hall-Petch relationship, but rather by Orowan hardening. The results of the present study suggest that the hardness profile is mainly affected by the distribution of small particles in friction-stir welds of Al alloys containing many such particles.