Single pass laser joining of Inconel 718 superalloy with filler

Abstract

Microstructure and mechanical property of CO₂ laser beam welded IN 718 superalloy were studied by electron microscopy and hardness testing. The use of a welding filler wire produced a sound fusion zone with no cracking but grain boundary microfissuring occurred in the heat affected zone (HAZ) and was observed to be significantly influenced by pre-weld heat treatment and laser welding speed. Crack-free weld was produced by a pre-weld heat treatment that minimised non-equilibirum grain boundary boron segregation and inhibited grain growth. While post-weld heat treatment (PWHT) reduced the difference between the hardness values of the base alloy, HAZ and the fusion zone, it resulted in increased HAZ cracking, which was likely aided by pre-existing cracks. The PWHT cracking was, however, avoided by subjecting pre-weld material to the heat treatment condition that produces crack-free weld during welding process.     

HAZ microstructure simulation in welding of a ultra fine grain steel

In the present work the evolution of grain structure in the weld HAZ (heat affected zone) under welding thermal cycle was simulated. Especially the grain growth in the HAZ of a SS400 ultra fine grain steel was investigated. An integrated 3-D Monte Carlo (MC) simulation system for grain growth of the weld HAZ was developed based on Microsoft Windows. The results indicate that MC simulation is an effective way to investigate the grain growth in weld HAZ. The method not only simulates the non-isothermal dynamics process of the grain growth in the weld HAZ, but also visualizes the austenite grains realistically. Moreover, the thermal pinning effect can be easily included in the simulation process. The grain sizes of the CGHAZ (coarse grain heat affected zone) obtained from MC simulation are basically in agreement with the experimental measurement of the real welded joints under different heat input. Furthermore, the simulation indicates that the grain growth degree is higher for the SS400 ultra fine grain steel compared to conventional steel. With the increase in the heat input, the grain growth of the CGHAZ rapidly increases. Because the activation energy of the grain growth is lower for the SS400 ultra fine grain steel, austenite grains can grow at a relatively lower temperature, hence the range of the CGHAZ becomes wider.     

The effect of rejuvenation heat treatments on the repair weldability of wrought Alloy 718

Extensive precipitation of needle- and plate-shaped δ-phase in the γ-nickel matrix of wrought Alloy 718 is the major microstructural change resulting from multiple weld repair/post weld heat treatment (PWHT) cycles. Isothermal heat treatments at 954 °C for times up to 100 h were used to simulate the multiple PWHTs in laboratory samples. Grain size did not change appreciably during these heat treatments owing to δ-phase pinning of the grain boundaries (GBs). The susceptibility of Alloy 718 to heat-affected-zone (HAZ) liquation cracking degraded as a result of these heat treatments. This degradation is due to the short time, high temperature GB liquation caused by the combined effects of δ-phase dissolution, boron carbide constitutional liquation, and GB segregation. A rejuvenation treatment (1010 °C/2 h) effectively restored the degraded weldability by removing the adverse influence of δ-phase through dissolution above the δ-phase solvus. This heat treatment also promoted a spontaneous grain refinement and increase in the fraction of special GBs owing to the elimination of δ-phase pinning of GBs. The combined effects of δ-phase, grain size and fraction of special GBs are discussed in the context of HAZ liquation cracking that occurs during repair welding of Alloy 718.     

Stress relief cracking in high strength structural steel BS 4360 grade 55F

Abstract

The stress relief cracking susceptibility of a commercial cast of BS 4360 55F, a high strength structural steel, has been investigated by short term creep rupture testing of crossweld specimens. The weld was made with a heat input of 1·75 kJ mm^?1, which is typical of that encountered during fabrication. The creep specimens fractured in the coarse grained heat affected zone with low deformation over a wide range of test conditions. In all instances failure was by intergranular cracking at prior austenite grain boundaries, generally by the formation of creep cavities. This indicates that the steel is susceptible to stress relief cracking for the high welding heat inputs used. Special precautions may be necessary when welding high strength steels of this type when postweld heat treatment is specified.

MST/745     

Welding heat input effect on the hydrogen permeation in the X80 steel welded joints

In this study, the effect of microstructure at the base metal (BM), the fine grain heat affected zone (FGHAZ), the coarse grain HAZ (CGHAZ) and weld metal (WM) under different welding heat input on hydrogen permeation in X80 steel weldments have been investigated. Base metal showed the highest effective diffusivity. With each heat input, the effective hydrogen diffusivity in FGHAZ is comparable to that of the base metal. The effective hydrogen diffusivity in weld metal was lower than that in CGHAZ. With increasing the welding heat input, the effective diffusivity in different zones of the weldment decreased correspondingly. Non-metallic inclusions were not detected in each specimen. Constituents in microstructure under low heat input are likely to agglomerate during accelerated cooling. The retained hydrogen may create an unpredictable susceptibility to hydrogen cracking at the CGHAZ even existing during service.     

Mechanism of intergranular reheating crack in butt-welded joint of 12Cr1MoV tube served for 40,000 hours

Cracking failure of butt-welded joint of 12Cr1MoV tube was comprehensively studied. Results show that both of initiation and propagation of the primary crack were circumferentially intergranular in the coarse-grained heat affect zone (CGHAZ). Many isolated and intergranular micro-cracks and cavities were observed near the primary crack．Neither oxide or corrosion products were observed in the isolated cavities or micro-cracks. According to the microstructure, location, propagation mode and morphology of the crack,the primary crack in butt-welded joint is concluded to be the intergranular reheating cracking (IRC). The crack failure is mainly due to poor welding quality, characterised by high residual stress and coarsened grain size. Mechanisms on the IRC based on previous laboratory research were studied on the failed tube sample, and results showed that the IRC is accumulation of high-stress induced creep damage, such as cavities along prior austenite grain boundaries (PAGBs). Neither segregation of alloys elements nor trace impurities were detected.     

Mechanism of post-weld heat treatment cracking in Rene 80 nickel based superalloy

Abstract

Microstructural studies carried out on Rene 80 (approximate composition 60Ni–14Cr–9.5Co–4Mo–5Ti–3Al–0.17C–Zr–B, wt-%) weldments before and after post-weld heat treatment (PWHT) revealed abundant evidence of constitutionally liquated and resolidified grain boundaries extending from the mushy zone into the heat affected zone (HAZ). While total dissolution of γ' occurred along such grain boundaries, a much lesser degree of γ' dissolution was noted in the adjacent material. During the PWHT, a high density of γ' precipitated out both within the mushy zone and in the constitutionally liquated and resolidified grain boundary regions in the HAZ. As the dissolution and reprecipitation of γ' occurred fairly uniformly throughout the mushy zone, the ensuing aging contraction stress/ strain was fairly uniformly distributed in the region. In contrast, in the adjacent part of the HAZ, an extreme volume of γ' precipitation occurred locally along the grain boundary regions, a result of the highest concentration of γ' forming solutes and the complete dissolution of γ' during welding in these regions. This, combined with the much stronger adjacent grain matrix, caused the aging contraction stress and strain to become highly concentrated along the grain boundary regions in the HAZ. This promoted the formation of PWHT cracks along such grain boundaries, which then propagated along the grain boundary into the mushy zone and beyond.     

Post-weld heat treatment cracking in autogenous GTA welded cast Inconel 738LC superalloy

Abstract

The post-weld heat treatment (PWHT) cracking in autogenous gas tungsten arc (GTA) welded Inconel 738LC superalloy, which was given two different preweld heat treatments, was studied. One of the preweld heat treatments, designated as SHT, consisted of solution heat treatment at 1120°C for 2 h in vacuum followed by argon quenching. The second preweld heat treatment, designated as UMT, consisted of solution treatment at 1120°C for 2 h followed by air cooling and then aging at 1025°C for 16 h followed by water quenching. The welded specimens were given the same conventional PWHT, which consisted of SHT at 1120°C for 2 h in vacuum followed by argon quenching and subsequent aging at 845°C for 24 h in vacuum. Microstructural examination of the welded SHT and UMT treated material showed that intergranular microfissuring occurred during welding only in the heat affected zone (HAZ) with some cracks extending into the adjoining base metal (BM), whereas after the PWHT microfissures were observed in the fusion zone (FZ), HAZ and the BM far removed from the HAZ. The crack width ranged from 5 to 10 μm in the PWHT specimens as compared with 1–2 μm in the as welded sections. Although similar type of cracks was observed in samples given the two preweld heat treatments, the UMT preweld heat treatment was found to result in a significant reduction in average total crack length and average crack length, both during welding and during the subsequent PWHT. After PWHT, SHT samples had ～43% more cracking than the UMT samples. It is suggested that a larger particle size of γ′ precipitates in the HAZ and a smaller size of HAZ in the as welded samples, combined with a softer BM of the UMT material (hardness 280 ± 12 HV10, as compared with 380 ± 10 HV10 of the SHT material) resulted in an improved capability of the material to absorb the strain–aging stresses, and hence a reduced incidence of cracking during PWHT.     

Austenite grain growth modelling in weld heat affected zone of Nb/Ti microalloyed linepipe steel

Abstract

A model to predict the austenite grain size in an Nb/Ti microalloyed steel weld heat affected zone (HAZ) was developed. The present work investigates grain growth behaviour under the influence of pinning carbonitrides. The steel has been subjected to austenitising heat treatments to selected peak temperatures at various heating rates that are typical for thermal cycles in the HAZ. The effect of temperature and heating rate on the grain size is studied. A model is proposed for the dissolution of NbCN precipitates. This model has been coupled to an austenite grain growth model in the presence of pinning particles. This coupling leads to accurate prediction of the austenite grain size along the heating path simulating selected thermal profiles of the HAZ.     

Fatigue crack growth behavior of the simulated HAZ of 800 MPa grade high-performance steel

The present study focuses on the fatigue properties in the weld heat-affected zone (HAZ) of 800 MPa grade high-performance steel, which is commonly used in bridges and buildings. Single- and multi-pass HAZs were simulated by the Gleeble system. Fatigue properties were estimated using a crack propagation test under a 0.3 stress ratio and 0.1 load frequencies. The microstructures and fracture surfaces were analyzed by optical microscopy, scanning electron microscopy, and transmission electron microscopy. The results of the crack propagation test showed that the fatigue crack growth rate of coarse-grained HAZ (CGHAZ) was faster than fine-grained HAZ (FGHAZ), although both regions have identical fully martensite microstructures, because FGHAZ has smaller prior austenite grain and martensite packet sizes, which can act as effective barriers to crack propagation. The fatigue crack growth rate of intercritically reheated CGHAZ (ICCGHAZ) was the fastest among local zones in the HAZ, due to rapid crack initiation and propagation via the massive martensite-austenite (M-A) constituent.     

High-brightness laser welding of thin-sheet 316 stainless steel

Photolytic iodine laser (PIL), a new industrial laser in the market, offers much higher brightness than existing Nd:YAG and CO₂ lasers. PIL has also a unique wavelength (1315 nm) that has not yet been tested for welding applications. In this work, the capabilities of PIL for precision seam welding of 0.1-mm thick sheet of AISI 316 stainless steel in the lap-joint configuration were evaluated. The weld performance data of PIL laser were compared with Nd:YAG and CO₂ lasers. The astounding benefits of PIL weld are narrow seam, extremely fine solidification cell structure, fully austenitic microstructure, and small heat-affected-zone (HAZ). These benefits are attributed to the PIL's high brightness that in turn enables achieving small spot size and energy transport through plasma rather than by heat conduction. In contrast, the welds produced by Nd:YAG and CO₂ lasers exhibited wider seams, coarser solidification structures, duplex microstructures of austenite and ferrite, and larger HAZ due to slow cooling of the melt, and lateral heat diffusion. Despite the narrow seam, the PIL weld carried a high tensile load (92% that of base metal) and was harder than the base metal. Microstructural analysis revealed that PIL welds exhibited fully austenitic structures and were free from hot cracking. These advantages are consequences of the rapid solidification effects including large undercooling, minimal segregation of impurities to the grain boundaries, and fine grain size.     

Overview Inclusion assisted microstructure control in C–Mn and low alloy steel welds

Abstract

Inclusion assisted microstructure control has been a key technology to improve the toughness of C–Mn and low alloy steel welds over the last two to three decades. The microstructure of weld metals and heat affected zones (HAZs) is known to be refined by different inclusions, which may act as nucleation sites for intragranular acicular ferrite and/or to pin austenite grains thereby preventing grain growth. In the present paper, the nature of acicular ferrite and the kinetics of intragranular ferrite transformations in both weld metals and the HAZ of steels are rationalised along with nucleation mechanisms. Acicular ferrite development is considered in terms of competitive nucleation and growth reactions at austenite grain boundary and intragranular inclusion nucleation sites. It is shown that compared to weld metals, it is difficult to shift the balance of ferrite nucleation from the austenite grain boundaries to the intragranular regions in the HAZ of particle dispersed steels because inclusion densities are lower and the surface area available for ferrite nucleation at the austenite grain boundaries tends to be greater than that of intragranular inclusions. The most consistent explanation of high nucleation potency in weld metals is provided by lattice matching between ferrite and the inclusion surface to reduce the interfacial energy opposing nucleation. In contrast, an increase in the thermodynamic driving force for nucleation through manganese depletion of the austenite matrix local to the inclusion tends to be the dominant nucleation mechanism in HAZs. It is demonstrated that these means of nucleation are not mutually exclusive but depend on the nature of the nucleating phase and the prevailing transformation conditions. Issues for further improvement of weldment toughness are discussed. It is argued that greater numbers of fine particles of a type that preferentially nucleate acicular ferrite are required in particle dispersed steels to oppose the austenite grain boundary ferrite transformation and promote high volume fractions of acicular ferrite and thereby toughness.     

Heat affected zone cracking in cast inconel 718

Electron beam welding parameters have been shown to influence heat-affected zone (HAZ) microfissuring behavior in cast Inconel 718 (Inconel is a trademark of INCO Alloys). Travel speed was shown to be the most significant factor in influencing HAZ cracking behavior by modifying the thermal gradient(s) and subsequently the stress state in the HAZ. This has been shown previously to be related to a change in weld pool geometry and to the effect of speed on heat flow in the weld zone.

Microfissuring in the HAZ was observed to occur intergranularly in areas containing Laves phase. Liquation of the Laves phase and subsequent wetting of the grain boundary is believed to be but one element of the complex interactions occuring in the HAZ during the weld thermal cycle.     

785MPa级焊接接头消应力热处理后韧性研究

研究不同消应力热处理制度后785MPa级气保焊接头的韧性.试验结果表明,冷却速率是影响消应力热处理后焊接接头韧性的关键因素；组织分析表明,消应力热处理后焊缝中晶界偏聚主要是P偏聚,马氏体板条束间存在少量细小的逆转变奥氏体,对改善焊缝的冲击韧性有利.     

Effects of post-weld heat treatment on microstructure and mechanical properties of friction stir welded joints of 2219-O aluminium alloy

Abstract

Post-weld heat treatment (PWHT) of 2219-O aluminium alloy friction stir welding joints was carried out at solution temperatures of 480, 500 and 540°C for 32 min followed by aging at 130°C for 9 h. The effects of PWHT on the microstructure and mechanical properties of the joints were investigated. Experimental results show that PWHT causes coarsening of the grains in the weld, and the coarsening degree increases with increasing solution temperature. The tensile strength of the heat treated joints increases with increasing solution temperature. The maximum tensile strength can reach 260% that of the base material at the solution temperature of 540°C. PWHT has a significant effect on the fracture locations of the joints. When the solution temperature is lower than 500°C, the joints fracture in the base material; when the temperature is higher than 500°C, the joints fracture in the weld. The change of the fracture locations of joints is attributed to the presence of precipitate free zones beside the grain boundaries and coarsening equiaxed grain structures in the weld.     

Microstructural response to heat affected zone cracking of prewelding heat-treated Inconel 939 superalloy

The microstructural response to cracking in the heat-affected zone (HAZ) of a nickel-based IN 939 superalloy after prewelding heat treatments (PWHT) was investigated. The PWHT specimens showed two different microstructures: 1) spherical ordered γ′ precipitates (357–442 nm), with blocky MC and discreet M₂₃C₆ carbides dispersed within the coarse dendrites and in the interdendritic regions; and 2) ordered γ′ precipitates in “ogdoadically” diced cube shapes and coarse MC carbides within the dendrites and in the interdendritic regions. After being tungsten inert gas welded (TIG) applying low heat input, welding speed and using a more ductile filler alloy, specimens with microstructures consisting of spherical γ′ precipitate particles and dispersed discreet MC carbides along the grain boundaries, displayed a considerably improved weldability due to a strong reduction of the intergranular HAZ cracking associated with the liquation microfissuring phenomena.     

Weldability and properties of martensitic/austenitic stainless steel joints

Abstract

A series of studies has been carried out to examine the weldability and properties of dissimilar steel joints using martensitic and austenitic stainless steels F6NM (OCr13Ni4Mo) and AISI 347, respectively. This type of joint requires good mechanical properties, corrosion resistance, and a stable magnetic permeability in addition to a good weldability. Weldability tests include weld thermal simulation of the martensitic steel to investigate the influence of weld thermal cycles and post-weld heat treatment (PWHT) on the microstructure and mechanical properties of the heat affected zone (HAZ); implant testing to examine the tendency for cold cracking of martensitic steel; and rigid restraint testing to determine hot crack susceptibility of the multipass dissimilar steel joints. The simulation results indicated that the toughness of the martensitic steel HAZ did not change significantly after the weld thermal cycles. The implant test results indicated that welds produced using nickel based filler show no tendency for cold cracking, whereas welds produced using martensitic or ferritic filler show such a tendency. Based on the weldability tests, a welding procedure (tungsten inert gas welding for root passes with HNiCrMo-2B wire followed by manual metal arc welding using ENiCrFe-3B coated electrode) was developed and a PWHT at 600°C for 2 h was recommended. Joints produced using the developed welding procedure are not susceptible to hot and cold cracking. After PWHT the joints exhibit both satisfactory mechanical properties and stress corrosion cracking resistance.

MST/1955     

Grain boundary segregation of phosphorus and molybdenum in A533B steel

Abstract

Phosphorus and molybdenum segregation to grain boundaries in a commercial grade A533B steel subjected to a variety of heat treatments has been examined using a field emission gun scanning transmission electron microscope (FEGSTEM) with energy dispersive X-ray micro-analysis. The results indicate that P and Mo concentrations at prior austenite grain boundaries increase with aging time. This follows the prediction of McLean's equilibrium segregation model, when modified to take account of the interaction energy between phosphorus and molybdenum.     

Microstructure Characterization of the Fusion Zone of an Alloy 600-82 Weld Joint

Characterization of the microstructure of the fusion zone of an Alloy 600-82 weld joint was conducted,with focus on the weld residual strain distribution and the comparison of the microstructure of heat affected zone(HAZ) with that of cold worked alloy. Peak of the residual strain was observed to approach to the fusion boundary in HAZ while the strain increased from the top of the weld to the root. Strain distribution in the HAZ was found to be concentrated adjacent to grain boundaries(GBs), with a peak of approximately three times of that in grain. Further, triple junctions of the GB appear to cause a higher strain concentration than single GBs. The microstructure of HAZ consists of partially tangled dislocations, which is different from slip bands of high density dislocations in cold worked alloy. This may cause a relatively higher intergranular cracking resistance of HAZ due to the difficulty in transferring tangled dislocations to GB in HAZ under deformation.     

Effects of solution treatment temperature on grain growth and mechanical properties of high strength 18%Ni cobalt free maraging steel

Abstract

The effects of solution treatment (ST) temperature (1073–1473 K) on the prior austenite grain size, microstructure, and mechanical properties of a 2000 MPa grade 18%Ni Co free maraging steel have been investigated. The results show that prior austenite grain size normally increases with increase of ST temperature. Strength and ductility in the solution treated condition are independent of both ST temperature and prior austenite grain size due to constant martensite lath spacing and dislocation tangles. In the solution treated + aged condition, the relationship between yield strength and prior austenite grain size follows the Hall- Petch equation, and ductility improves until the ST temperature used is >1373 K. Accordingly, the fracture mode transforms from intergranular to transgranular at a critical prior austenite grain size of ~ 150 μ m, because of severe segregation of Ni₃(Mo,Ti) and reverted austenite at prior austenite grain boundaries and martensite lath boundaries. The variation of Charpy V notch impact energy with increase of ST temperature in both the solution treated and solution treated + aged conditions is similar to that of the tensile ductility. The fracture toughness K_IC, however, increases with increase of ST temperature. No thermal embrittlement resulted from the Ti(C,N,S) inclusion segregation at prior austenite grain boundaries and martensite lath boundaries in the high temperature solution treatment.