Microstructural modification of austenitic stainless steels by rapid solidification

The microstructural modifications in three austenitic stainless steels (types 308, 310, and 312) were evaluated after rapid solidification. These three steels are commonly used weld filler metals. Two methods of rapid solidification were investigated, autogenous laser welding and arc-hammer splat quenching. The structure of 310 stainless steel was found to be 100 pct austenite, and did not vary over the range of conditions studied. On the contrary, the structures of types 308 and 312 steels were very sensitive to the cooling rates and solidification conditions. With the highest cooling rates, the type 308 structure was fully austenitic while the type 312 structure was fully ferritic. At lower cooling rates, the structures were duplex ferrite plus austenite. The results were interpreted in terms of faster kinetics of solidification of austenite compared to ferrite under the conditions examined. A comparison of the structures produced by the two rapid solidification techniques indicated the cooling rates are comparable.     

The effect of quenching on the solidification structure and transformation behavior of stainless steel welds

Weld solidification structure of three different types of stainless steel,i.e., 310 austenitic, 309 and 304 semiaustenitic, and 430 ferritic, was investigated. Welds of each material were made without any quenching, with water quenching, and with liquid-tin quenching during welding. The weld micro-structure obtained was explained with the help of the pseudobinary phase diagrams for Fe-Cr-Ni and Fe-Cr-C systems. It was found that, due to the postsolidification 5 → γ phase transformation in 309 and 304 stainless steels and the rapid homogenization of microsegregation in 430 stainless steel, their weld solidification structure could not be observed unless quenched from the solidification range with liquid tin. Moreover, the formation of acicular austenite, and hence, martensite, at the grain boundaries of 430 stainless steel welds was eliminated completely when quenched with liquid tin. The weld solidification structure of 310 stainless steel, on the other hand, was essentially unaffected by quenching. Based upon the observations made, the weld microstructure of these stainless steels was summarized. The effect of cooling rate on the formation of primary austenite in 309 stainless steel welds was discussed. Finally, a simple method for determining the relationship between the secondary dendrite arm spacing and the solidification time, based on welding speeds and weld pool configurations, was suggested.     

Dendrite Morphological Analysis on SMA,GMA, and PGMA Welding of Dissimilar 304LN Austenitic Stainless Steel and Micro-alloyed Steel

Dissimilar metal welding between the austenitic stainless steel and micro-alloyed steel was widely used in high-temperature applications in power stations and petrochemical plants. In the current research, the dissimilar metals between austenitic stainless steel and micro-alloyed steel have been joined by shielded metal arc welding (SMA), gas metal arc welding (GMA), and pulse gas metal arc welding (PGMA) processes. Welded samples of the aforementioned processes were subjected to comparative studies pertaining to the dendrite morphological characteristics. The study reveals that the process parameters affect the growth of dendrite arm because of the variation in the coefficient of thermal conductivity, expansion, and metallurgical incompatibility of the metals. In the PGMA welding process, the dendrite length decreases, while its width increases in all the locations of the weld by varying dimensionless factors ϕ (0.05, 0.15, and 0.25) and keeping its heat input as constant (Ω—11.2 kJ/cm). Among the welded joints, the PGMA weld joint comparatively exhibit shorter length (20 µm) and width (4 µm) of dendrite arm than the welded joints of the GMA and SMA processes. The change in the dendrite dimension is observed to be due to the variation in the dimensionless factor ϕ and the quantity of heat transfer to the weld (QT). The studies have been systematically planned in order to gain advanced scientific knowledge to establish superior technique for multi-pass PGMA welding of thick section of austenitic stainless steel to micro-alloy steel with respect to that used with conventional welding process.     

304L不锈钢焊缝在混凝土模拟孔隙液中的点蚀行为

采用动电位极化曲线、电化学阻抗谱、Mott-Schottky曲线等电化学方法研究了以308 L为焊丝的304 L不锈钢焊接接头在不同氯离子含量的混凝土模拟孔隙液中腐蚀行为和电化学规律.随Cl-增加,304 L不锈钢焊接接头的三个区域(母材、焊缝和热影响区)在混凝土模拟孔隙液中的自腐蚀电位、点蚀电位及电荷转移电阻降低,钝化膜中载流子密度和焊接接头的点蚀坑数量增加.在同浓度的腐蚀溶液中,308 L的焊缝区域耐蚀性最佳,热影响区次之,304 L基体表现出低的电荷转移电阻和高的掺杂浓度使得母材的耐蚀性最差.      

Fatigue crack propagation in austenitic stainless steel weldments

The fatigue crack propagation rate (FCPR) in 316L austenitic stainless steel (ASS) and its weldments was investigated, at two loading amplitudes, 7 and 8.5 kN, under tension-tension mode. Two welding techniques, submerged arc welding (SAW) and manual arc welding (MAW), have been used. Magnetic δ-ferrite, depending upon Ni and Cr content in the metal, in the weld zone upon solidification was considered. The ferrite number (FN) of δ-ferrite formed in the SAW zone was much higher (maximum 9.6) compared to the corresponding value (maximum 0.75) in the MAW zone. A fatigue starter notch was positioned at different positions and directions with respect to the weld zone, in addition to the heat-affected zone (HAZ). Regions of high and low FCPRs as the fatigue crack propagated through and across the weld zone have been noticed. This is related to the direction of the tensile residual stresses present in weld zone, resulting from solidification of the weld metal. The FCPR was higher along through the HAZ and weld zone because of the microstructural change and direction and distribution of tensile residual stresses. The FCPR was much lower when crack propagated perpendicular to the weld zone, particularly in the case of SAW in which higher δ-ferrite volume fraction was noticed. A lower FCPR found across the weld zone, in both SAW and MAW, was accompanied by rubbed areas in their fractures.     

Effect of Welding Processes and Consumables on Tensile and Impact Properties of High Strength Quenched and Tempered Steel Joints

Quenched and tempered steels are prone to hydrogen induced cracking in the heat affected zone after welding. The use of austenitic stainless steel consumables to weld the above steel was the only available remedy because of higher solubility for hydrogen in austenitic phase. In this investigation, an attempt was made to determine a suitable consumable to replace expensive austenitic consumables. Two different consumables, namely, austenitie stain less steel and low hydrogen ferritic steel, were used to fabricate the joints by shielded metal are welding （SMAW） and flux cored arc welding （FCAW） processes. The joints fabricated by using low hydrogen ferritic steel consumables showed superior transverse tensile properties, whereas joints fabricated by using austenitic stainless steel consumables exhibited better impact toughness, irrespective of the welding process used. The SMAW joints exhibited superior mechanical and impact properties, irrespective of the consumables used, than their FCAW counterparts.     

The constitutive behavior of laser welds in 304L stainless steel determined by digital image correlation

A digital image correlation (DIC) method has been used to characterize the constitutive tensile stress-strain response in 304L austenitic stainless steel weldments produced by both continuous-wave (CW) and pulsed-wave (PW) laser welding. The method provides quantitative two-dimensional (2-D) strain maps of the deformation field across the transverse weld samples throughout the tensile test. Local stress-strain response was extracted from regions within the fusion zone and compared to base metal response. The weldments were found to have a higher yield strength than the base metal. The metallurgical origin for the fusion zone strengthening was largely attributed to Hall-Petch and ferrite content effects. While failures localized in the fusion zone with little appreciable necking, the material within the fusion zone retained considerable local ductility: more than 45 pct strain at failure. Significant weld root porosity found in the PW condition and absent in the CW condition appeared to have no deleterious effect on the mechanical performance under the present test conditions in this very ductile, flaw-tolerant alloy.     

On Characterizing a Composite Microstructure in 316LN Stainless Steel Weld Metal and a New Damage Micromechanism During Creep

The austenitic stainless steel weld metal fabricated by multipass welding exhibits a composite microstructure. Microstructural characterization of the weld metal revealed that there are two distinct regions on either side of the weld-pass interface. The variations in dislocation substructure and delta ferrite morphology are the two microstructural attributes which delineate these regions. The generation of subsequent thermal cycles during the fabrication of multipass weld joint is the paramount factor influencing the formation of the composite microstructure. During creep exposure, the extent of creep cavitation and propagation varies substantially in these two regions due to differences in their microstructures. This results in preferential damage during creep exposure of austenitic stainless steel weld metal.     

Study of welding ultra‐fine grained ferrite steel by CO2 laser

Ultra‐fine grained ferrite steels have higher strength and better toughness than the normal ferrite steels because of their micrometer or sub‐micrometer sized grains. In this paper the ultra‐fine grained steel SS400 is welded by CO₂ laser. The shape of weld, cooling rate of HAZ, width of HAZ, microstructures and mechanical properties of the joint are discussed. Experimental results indicate that laser beam welding can produce weld with a large ratio of depth to width. The cooling rate of HAZ of laser beam welding is fast, the growth of prior austenite grains of HAZ is limited, and the width of weld and HAZ is narrow. The microstructures of weld metal and coarse‐grained HAZ of laser beam welding mainly consist of B_L + M (small amount). With proper laser power and welding speed, good comprehensive mechanical properties can be acquired. The toughness of weld metal and coarse‐grained HAZ are higher than that of base metal. There is no softened zone after laser beam welding. The tensile strength of a welded joint is higher than that of base metal. The welded joint has good bending ductility.     

Effect of welding consumables on hydrogen induced cracking of armour grade quenched and tempered steel welds

Abstract

Armour grade quenched and tempered (Q&T) steels are susceptible to hydrogen induced cracking (HIC) in the heat affected zone after welding. Austenitic stainless steel (ASS) consumables are selected for welding Q&T steels as they have higher solubility for hydrogen in the austenitic phase and it is the most beneficial method for controlling HIC in Q&T steel welds. Recent studies reveal that high nickel steel and low hydrogen ferritic steel consumables can be used to weld Q&T steels, which can give very low hydrogen levels in the weld deposits. In this investigation, an attempt has been made to study the effect of welding consumables on hydrogen induced cracking of Q&T steel welds by implant testing. Shielded metal arc (SMAW) welding process has been used for making welds using three different consumables, namely austenitic stainless steel, low hydrogen ferritic steel (LHF) and high nickel steel (HNS) to assess HIC by implant testing. The high nickel steel consumables exhibited a higher value of lower critical stress (LCS) and thus they offered a greater resistance to hydrogen induced cracking of armour grade Q&T steel welds than other consumables. The diffusible hydrogen content and the value of the LCS meets the specified limit for armour grade Q&T steel welds and hence, the LHF consumables can be accepted as an alternative to the to the traditionally used ASS consumables and the proposed HNS consumables.     

Modelling and Characterisation of the Texture Development in the Fusion Zone of An Austenitic Weld

The relation between texture and microstructure and the macroscopic solidification direction has been investigated for an austenitic 316L stainless steel weld. The fusion zone has been characterised by EBSD. Texture development has been simulated by thermal Finite Element Analysis coupled with a simple model of preferential <100>‐oriented solidification following the solidification direction. The thermal part is validated by means of thermocouple measurements at several locations on the specimen. The predicted texture is in very good agreement with the measured one.     

Influence of post-weld heat treatment on the microstructure and hardness of laser weld seams on hot-rolled TRIP 800 steel

The transformation induced plasticity (TRIP) steels effect occurs because of the martensitic transformation of retained austenite during plastic deformation,and it provides the steel with excellent strength and ductility.While welding remains a vital part of auto body manufacturing,the weldability of TRIP steels is problematic,and this prevents its adoption for many applications in the automotive industry.This present work studies the effects of welding and post-weld heat treatment on the microstructure of TRIP steels.It is found that the microstructures of the fusion zone and the heat affected zone (HAZ) are changed after high-temperature heat treatment.Hardness tests revealed that fusion zone hardness decreased with increasing of temperatures in the post-weld heat treatment on the laser weld seam.The rolling performance of the welding seam and the seam of post-weld heat treatment were also studied.     

Influence of high-temperature exposure on the microstructure and mechanical properties of dissimilar metal welds between modified 9Cr-1Mo steel and alloy 800

Transition joints between ferritic steel and austenitic stainless steel are commonly encountered in high-temperature components of power plants. Service failures in these are known to occur as a result, mainly, of thermal stresses due to expansion coefficient differentials. In order to mitigate the problem, a trimetallic configuration involving an intermediate piece of a material such as Alloy 800 between the ferritic and austenitic steels has been suggested. In our work, modified 9Cr-1Mo steel and 316LN stainless steel are used as the ferritic and austenitic components and the thermal behavior of the joints between modified 9Cr-1Mo steel and Alloy 800 is described in this article. The joints, made using the nickel-base filler material INCONEL 82/182 (INCONEL 82 for the root pass by gas-tungsten arc welding and INCONEL 182 for the filler passes by shielded-metal arc welding), were aged at 625 °C for periods up to 5000 hours. The microstructural changes occurring in the weld metal as well as at the interfaces with the two parent materials are characterized in detail. Results of across-the-weld hardness surveys and cross-weld tension tests and weld metal Charpy impact tests are correlated with the structural changes observed. Principally, the results show that (1) the tendency for carbon to diffuse from the ferritic steel into the weld metal is much less pronounced than when 2.25Cr-1Mo steel is used as the ferritic part; and (2) intermetallic precipitation occurs in the weld metal for aging durations longer than 2000 hours, but the weld metal toughness still remains adequate in terms of the relevant specification.     

Metallurgical characterisation of dissimilar welds between modified 9Cr‐1Mo steel and Alloy 800

Dissimilar metal welds between ferritic low‐alloy and austenitic stainless steels commonly occur in power plant application. In order to overcome some of the problems encountered here, a trimetallic configuration using an intermediate piece (such as Alloy 800) between the austenitic and ferritic steels has been suggested. This paper describes some features of the joints between modified 9Cr‐1Mo steel and Alloy 800, produced with Inconel 82/182 filler material. The joints require heat‐treatment after welding and the results have shown that a treatment at 760 °C for 2 h would be optimal. Although most tensile failures occurred in the weld metal the welds were found to exhibit strength properties that are at least equal to those of Alloy 800, with a tensile elongation lying between those of the two base materials. Similarly, while the weld metals are slightly less tough than the two base materials, the weld metal toughness at 120 J is still quite adequate for the intended application.     

The microstructural response of delta-stabilized plutonium to pulsed laser welding

The effects of the welding parameters on weld-bead morphology and the microstructural evolution of autogenous conduction-mode melted laser welds in gallium delta-stabilized plutonium were investigated. This investigation demonstrates that delta-stabilized plutonium is easily welded with a pulsed Nd:YAG laser. Variations in the welding parameters created a range of heat inputs that had a significant effect on the size and morphology of the resultant weld and associated heat-affected-zone (HAZ) microstructure. Unlike other low-melting-temperature materials such as aluminum alloys, neither fusion-zone porosity nor hot cracking was observed. The reactivity of plutonium promotes the rapid formation of surface oxides that inhibit complete weld-joint fusion. The high viscosity of the plutonium molten pool impeded the breakup of the surface oxide by fluid motion, thus further impeding joint fusion. Modifications to input parameters to affect changes in the weld-pool flow were only partially successful in improving joint fusion. Analysis of the weld region using optical and electron microscopy and electron microprobe analysis revealed fusion-zone microstructures consisting of fine acicular delta grains, with a limited amount of retained alpha phase. The Pu₆Fe, the low-melting-point intermetallic phase, was observed in the base material and the partially melted HAZ. Further investigation into the solid/solid phase transformations, specifically the effect of the cooling rate, during the cooling of the weld will be required to describe fully the microstructural development of the fusion zone.     

Gas Tungsten Arc Welding of 316L Austenitic Stainless Steel with UNS S32205 Duplex Stainless Steel

In the present work, dissimilar welding between UNS S32205 duplex stainless steel (DSS) and 316L austenitic stainless steel (ASS) was performed by using gas tungsten arc welding and ER2209 filler at two different heat inputs (0.52 and 0.98 kJ/mm). Microstructures were characterized using reflected light optical microscope and scanning electron microscope. Micro-hardness and tensile properties were measured across the weld for both the heat inputs. The microstructure of the welded region was primarily austenitic (for both heat inputs) with Widmanstätten morphology. The grain size of the heat affected zone on DSS side was very large (~200 µm) for the high heat input sample with the presence of partially transformed austenite and acicular austenite. The precipitation of intermetallic phases and carbides was not observed for both the heat inputs. The proportion of ferrite in the weld metal (as measured by feritscope) was higher for the high heat input sample than the low heat input sample. During the tensile test, fracture occurred in 316L ASS base metal (because of its lower strength) in ductile manner. For high heat input welds, the impact tested sample showed the presence of fine spherical precipitates rich in Cr, Mn and Fe in the fracture surface of weld metal.     

An Investigation of the Massive Transformation from Ferrite to Austenite in Laser-Welded Mo-Bearing Stainless Steels

A series of 31 Mo-bearing stainless steel compositions with Mo contents ranging from 0 to 10 wt pct and exhibiting primary δ-ferrite solidification were analyzed over a range of laser welding conditions to evaluate the effect of composition and cooling rate on the solid-state transformation to γ-austenite. Alloys exhibiting this microstructural development sequence are of particular interest to the welding community because of their reduced susceptibility to solidification cracking and the potential reduction of microsegregation (which can affect corrosion resistance), all while harnessing the high toughness of γ-austenite. Alloys were created using the arc button melting process, and laser welds were prepared on each alloy at constant power and travel speeds ranging from 4.2 to 42 mm/s. The cooling rates of these processes were estimated to range from 10 K (°C)/s for arc buttons to 10⁵ K (°C)/s for the fastest laser welds. No shift in solidification mode from primary δ-ferrite to primary γ-austenite was observed in the range of compositions or welding conditions studied. Metastable microstructural features were observed in many laser weld fusion zones, as well as a massive transformation from δ-ferrite to γ-austenite. Evidence of epitaxial massive growth without nucleation was also found when intercellular γ-austenite was already present from a solidification reaction. The resulting single-phase γ-austenite in both cases exhibited a homogenous distribution of Mo, Cr, Ni, and Fe at nominal levels.