S. Sundaresan

Effect of weld metal chemistry and heat input on the structure and properties of duplex stainless steel welds

Abstract The excellent combination of strength and corrosion resistance in duplex stainless steels (DSS) is due to their strict composition control and microstructural balance. The ferrite–austenite ratio is often upset in DSS weld metals owing to the rapid cooling rates associated with welding. To achieve the desired ferrite–austenite balance and hence properties, either the weld metal composition and/or the heat input is controlled. In the current work, a low heat input process viz., EBW and another commonly employed process, gas tungsten-arc welding have been employed for welding of DSS with and without nickel enhancement. Results show that (i) chemical composition has got a greater influence on the ferrite–austenite ratio than the cooling rate, (ii) and even EBW which is considered an immature process in welding of DSS, can be employed provided means of filler addition could be devised.

Microstructural features of dissimilar welds between 316LN austenitic stainless steel and alloy 800

Abstract For joining type 316LN austenitic stainless steel to modified 9Cr–1Mo steel for power plant application, a trimetallic configuration using an insert piece (such as alloy 800) of intermediate thermal coefficient of expansion (CTE) has been sometimes suggested for bridging the wide gap in CTE between the two steels. Two joints are thus involved and this paper is concerned with the weld between 316LN and alloy 800. These welds were produced using three types of filler materials: austenitic stainless steels corresponding to 316, 16Cr–8Ni–2Mo, and the nickel-base Inconel 182 1 . The weld fusion zones and the interfaces with the base materials were characterised in detail using light and transmission electron microscopy. The 316 and Inconel 182 weld metals solidified dendritically, while the 16–8–2 (16%Cr–8%Ni–2%Mo) weld metal showed a predominantly cellular substructure. The Inconel weld metal contained a large number of inclusions when deposited from flux-coated electrodes, but was relatively inclusion-free under inert gas-shielded welding. Long-term elevated-temperature aging of the weld metals resulted in embrittling sigma phase precipitation in the austenitic stainless steel weld metals, but the nickel-base welds showed no visible precipitation, demonstrating their superior metallurgical stability for high-temperature service.

Microstructure and Mechanical Properties of Weld Fusion Zones in Modified 9Cr-1Mo Steel

Modified 9Cr-1Mo steel finds increasing application in power plant construction because of its excellent high-temperature properties. While it has been shown to be weldable and resistant to all types of cracking in the weld metal and heat-affected zone (HAZ), the achievement of optimum weld metal properties has often caused concern. The design of appropriate welding consumables is important in this regard. In the present work, plates of modified 9Cr-1Mo steel were welded with three different filler materials: standard 9Cr-1Mo steel, modified 9Cr-1Mo, and nickel-base alloy Inconel 182. Post-weld heat treatment (PWHT) was carried out at 730 and 760 °C for periods of 2 and 6 h. The joints were characterized in detail by metallography. Hardness, tensile properties, and Charpy toughness were evaluated. Among the three filler materials used, although Inconel 182 resulted in high weld metal toughness, the strength properties were too low. Between modified and standard 9Cr-1Mo, the former led to superior hardness and strength in all conditions. However, with modified 9Cr-1Mo, fusion zone toughness was low and an acceptable value could be obtained only after PWHT for 6 h at 760 °C. The relatively poor toughness was correlated to the occurrence of local regions of untransformed ferrite in the microstructure.

Microstructural changes during welding and subsequent heat treatment of 18Ni (250-grade) maraging steel

Abstract Sheet material specimens from 18Ni (250-grade) maraging steel were gas tungsten-arc welded using two different filler wires: one matching the base material in composition and the other with higher cobalt but lower molybdenum and titanium contents. Welding was carried out both in constant-current and pulsed modes. Post-weld aging was performed at three different temperatures, viz., 425, 480 and 520°C. Metallographic characterization revealed pronounced segregation, presumably of Ti and Mo, along interdendritic and intercellular boundaries in the weld metal produced with filler of matching composition. This led, during subsequent aging, to austenite reversion at temperatures much lower than in wrought (unwelded) material. Segregation and austenite reversion were not noticed when Ti and Mo contents were reduced in the filler wire, except at the highest aging temperature used. The use of pulsed welding resulted in some grain refinement, but the effect was not considerable.

Optimization of compound layer thickness for wear resistance of nitrocarburized H11 steels

Abstract Nitrocarburizing improves the wear resistance of components and hence is advantageously applied to hot working dies. The improvement is related to the development of surfacial compound layer of epsilon carbonitride. H11 steels, by virtue of high chromium content, develop only a very thin compound layer and hence the benefit accrued is limited. In this study, higher treatment temperatures were employed in a Sursulf salt bath to increase the layer thickness and the wear resistance of treated specimens assessed using pin-on-disc test geometry. It was found that the wear rate was lower with increasing layer thickness, but because of loss of core strenght, the treatment temperature of 700°C could not be recommended. A maximum temperature of 650°C appears to be the near optimum for improved wear resistance without a serious compromise on the core strength required for forging dies. Another significant observation was that there was no spalling or debonding of the compound layer and one of the reasons was the jagged interface between the compound layer and the diffusion zone.

Effect of nitrogen addition on microstructure and fusion zone cracking in type 316L stainless steel weld metals

Abstract Nitrogen is known to have a significant effect on cracking behaviour of austenitic stainless steel during welding, although reports on its effects have often been controversial. A study was therefore undertaken to examine the effect of nitrogen on the weldability of two type 316L weld metals. Weldability was assessed using the longitudinal moving torch Varestraint test. The brittleness temperature range during solidification was calculated from crack length data. Nitrogen was added through the shielding gas to 316L (base N-0.036%) and 316LN (base N-0.073%) to produce weld metal nitrogen contents in the range 0.04–0.19%. In the primary austenitic solidification mode, nitrogen addition had little effect when the P+S levels were relatively low (316LN with 0.031%P, 0.001%S) while cracking increased for higher impurity levels (316L with 0.035%P, 0.012%S). Nitrogen additions also produced significant coarsening of the primary solidification structure. The study indicates that weldability effects of nitrogen may be influenced by the impurity levels, particularly S. The cracking data showed good correlation with the WRC Cr eq /Ni eq ratio.

Use of inoculants to refine weld solidification structure and improve weldability in type 2090 AlLi alloy

Abstract The refinement of weld solidification structures will be useful not only because it is known to reduce the tendency to hot cracking, but also because the mechanical properties of the fusion zone are likely to be improved. In the current work, welds were produced in a type 2090 AlLi alloy using two filler materials based on types 2319 and 4043. Grain refinement was induced in the weld fusion zones through inoculation with Ti, Ti+B and Zr. Microstructural characterization showed that all three reduced grain size considerably, with Zr being the most effective. The structural refinement was found to reduce hot cracking susceptibility (measured by Varestraint testing), enhance post-weld age-hardening response and also improve tensile properties, especially ductility. The results showed that in the peak-aged condition the tensile elongations in the inoculated welds equalled or exceeded that in the base material.

Scuffing resistance of salt bath nitrocarburized medium carbon steel

Abstract The usefulness of the ferritic nitrocarburizing treatment for improving the tribological properties of ferrous components is well established. However, the thin compound layer of beneficial epsilon carbonitride that develops at the surface lacks the ability to bear high Hertzian stresses. Nitrocarburizing in the austenite phase field of the FeNC system overcomes this disadvantage since the formation of a hard zone of martensite-bainite below the compound layer provides the back-up to withstand point loads. Results of the Falex scuffing test indicate that the failure load increases on account of the thicker compound layer at the surface. The results indicate further that the frictional heat generated during the test coupled with working by the jaws convert the retained austenite present in the sub-compound layer to harder microconstituents. The study suggests that the austenitic treatment could be useful for applications involving simultaneous heat and work such as hot working dies.

Effect of nitrogen addition on formation of secondary austenite in duplex stainless steel weld metals and resultant properties

Abstract The gas tungsten arc (GTA) welding process is widely employed for fabricating duplex stainless steel (DSS) components. The microstructural phase balance is disturbed in DSS welds owing to the thermal cycles associated with welding and the desired phase balance is generally achieved either through control of heat input or by altering the composition. In GTA welding, the phase balance is obtained either via the use of nickel fortified fillers or by introduction of nitrogen. Control of ferrite in the welds is essential, since metastable ferrite may transform to secondary austenite during multiple pass welding. The present work describes the influence of nitrogen on the phase balance of DSS welds and also its controlling effect on the secondary austenite formation in multiple pass welds. It was observed that neither the nitrogen addition nor the secondary austenite formation influenced the hardness of the welds, but formation of secondary austenite led to a deterioration in corrosion resistance. The corrosion resistance of welds containing secondary austenite (SA) was observed to be inferior to that of SA free weld metals.

Fatigue crack growth behavior in a welded α-β Ti-Al-Mn alloy in relation to micro structural features

Although the fatigue behavior of titanium alloys in various heat-treated conditions has been studied in detail, welded joints in these alloys have not received similar attention. In the current work, fatigue crack growth studies have been conducted in weld fusion zones of a dilute structural α-β Ti-Al-Mn alloy, with particular reference to their microstructural features. Crack growth rates were measured in various welded and post-weld annealed conditions. It has been shown that in all these conditions the weld fusion zones exhibit crack growth rates lower than that in the parent material. Heat treatment after welding decreases crack growth resistance in relation to the as-welded condition. The results are explained in terms of the lamellar microstructures of the weld metals, structural coarsening during annealing and the residual stresses that arise during welding.