Study of the fusion zone and heat-affected zone microstructures in tungsten inert gas-welded INCONEL 738LC superalloy

The fusion zone and heat-affected zone (HAZ) microstructures obtained during tungsten inert gas (TIG) welding of a commercial superalloy IN 738LC were examined. The microsegregation observed during solidification in the fusion zone indicated that while Co, Cr, and W segregated to the γ dendrites, Nb, Ti, Ta, Mo, Al, and Zr were rejected into the interdendritic liquid. Electron diffraction and energy-dispersive X-ray microanalyses using a transmission electron microscope (TEM) of secondary phases, extracted from the fusion zone on carbon replicas, and of those in thin foils prepared from the fusion zone showed that the major secondary solidification constituents, formed from the interdendritic liquid, were cubic MC-type carbides and γ-γ’ eutectic. The terminal solidification reaction product was found to consist of M₃B₂ and Ni₇Zr₂ formed in front of the interdendritic γ-γ’ eutectic. Based on the knowledge of the Ni-Ti-C ternary system, a pseudoternary solidification diagram was adapted for IN 738 superalloy, which adequately explained the as-solidified microstructure. The HAZ microfissuring was observed in regions surrounding the fusion zone. Closer and careful microstructural examination by analytical scanning electron microscopy revealed formation of re-solidified constituents along the microfissured HAZ grain boundaries, which suggest that HAZ cracking in this alloy involves liquation cracking. Liquation of various phases present in preweld alloy as well as characteristics of the intergranular liquid film contributing to the alloy’s low resistance to HAZ cracking were identified and are discussed.     

Dispersoid-free zones in the heat-affected zone of aluminum alloy welds

The local microstructure in the heat-affected zone 1 (HAZ1) of a laser beam-welded Al-Mg-Si-Cu aluminum alloy is investigated closely. Dispersoid-free zones (DFZs), where the dispersoids of the base material (BM) are dissolved, are found in the vicinity of the fusion line (FL). They are not uniformly surrounding a grain, but oriented toward the FL. Their width can be as large as 10 μm. Detailed analysis has revealed a decreased silicon concentration as well as a decreased hardness of the oriented dispersoid-free zones (ODFZs). Two mechanisms for the formation of this welding metallurgical feature are discussed.     

Dilution control in gas-tungsten-arc welds involving superaustenitic stainless steels and nickel-based alloys

Fusion welds were prepared between a superaustenitic stainless steel, (the AL-6XN alloy) and two Ni-based filler metals (IN625 and IN622) using the gas-tungsten-arc welding (GTAW) process. Fusionzone compositions over the full range of dilution levels (0 to 100 pct) were produced by varying the independent welding parameters of arc power and volumetric filler-metal feed rate. Microstructural characterization of the welds was conducted via light optical microscopy, with quantitative chemical information obtained through electron-probe microanalysis (EPMA). The dilution level of each weld was determined from the EPMA data as well as through geometric measurements of the weld cross-sectional areas. The dilution level was observed to decrease with increasing filler-metal feed rate and decreasing arc power. These effects are quantitatively interpreted based on a previously proposed processing model. The model is used to demonstrate that, in terms of welding parameters, the dilution level can be correlated exclusively to the ratio of the volumetric filler-metal feed rate (V _fm) to arc power (VI), i.e., the individual values of V _fm and VI are not important in controlling the dilution and resultant weld-metal composition. Good agreement is obtained between experimental and calculated dilution values using the model. It is also demonstrated that the melting enthalpies of the filler metal and substrate have only a minor influence on dilution at dilution levels in the range from 40 to 100 pct. This knowledge facilitates estimates of dilution levels in this range when the substrate and fillermetal thermal properties are not accurately known. The results presented from this study provide guidelines for controlling the weld-metal composition in these fusion-zone combinations.     

Effect of boron segregation at grain boundaries on heat-affected zone cracking in wrought INCONEL 718

Susceptibility to heat-affected zone (HAZ) cracking during electron-beam welding was studied in two INCONEL 718-based alloys doped with different levels of boron. By lowering the carbon, sulfur, and phosphorous concentrations to be “as low as possible,” the occurrence of HAZ cracking was related directly to the level of segregation of boron at grain boundaries, which occurred by nonequilibrium segregation during a preweld heat treatment. The study has demonstrated a direct correlation between the amount of boron segregated at grain boundaries and their susceptibility to HAZ cracking, in terms of the total crack length and number of cracks observed in the HAZ. The analysis of results suggests that both the melting and resolidification temperatures of the boron-segregated grain boundaries can be about 100 °C to 200 °C lower than those of the grain boundaries that were susceptible to constitutional liquation of Nb carbides on them, making boron more deleterious in causing HAZ cracking.     

Spatially resolved X-ray diffraction mapping of phase transformations in the heat-affected zone of carbon-manganese steel arc welds

Phase transformations that occur in the heat-affected zone (HAZ) of gas tungsten arc welds in AISI 1005 carbon-manganese steel were investigated using spatially resolved X-ray diffraction (SRXRD) at the Stanford Synchrotron Radiation Laboratory. In situ SRXRD experiments were performed to probe the phases present in the HAZ during welding of cylindrical steel bars. These real-time observations of the phases present in the HAZ were used to construct a phase transformation map that identifies five principal phase regions between the liquid weld pool and the unaffected base metal: (1) α-ferrite that is undergoing annealing, recrystallization, and/or grain growth at subcritical temperatures, (2) partially transformed α-ferrite co-existing with γ-austenite at intercritical temperatures, (3) single-phase γ-austenite at austenitizing temperatures, (4) δ-ferrite at temperatures near the liquidus temperature, and (5) back transformed α-ferrite co-existing with residual austenite at subcritical temperatures behind the weld. The SRXRD experimental results were combined with a heat flow model of the weld to investigate transformation kinetics under both positive and negative temperature gradients in the HAZ. Results show that the transformation from ferrite to austenite on heating requires 3 seconds and 158°C of superheat to attain completion under a heating rate of 102°C/s. The reverse transformation from austenite to ferrite on cooling was shown to require 3.3 seconds at a cooling rate of 45 °C/s to transform the majority of the austenite back to ferrite; however, some residual austenite was observed in the microstructure as far as 17 mm behind the weld.     

Fracture toughness analysis of heat-affected zones in high-strength low-alloy steel welds

This study is concerned with a correlation of fracture toughness with microstructural factors in heat-affected zones (HAZs) of a normalized high-strength low-alloy (HSLA) steel. In order to explain weld joint performance, tensile and plane strain fracture toughness tests were conducted for the simulated coarse-grained HAZ microstructures. The micromechanisms of fracture processes involved in void and microcrack formation are identified byin situ scanning electron microscopy (SEM) fracture observations and void initiation study. The fracture toughness results are also interpreted using simple fracture initiation models founded on the basic assumption that a crack initiates at a certain critical strain or stress developed over some microstructurally significant distance. The calculated K_Ic values are found to scale roughly with the spacing of the stringer-type martensite islands associated with voids, confirming that martensite islands play an important role in reducing the toughness of the coarse-grained HAZs. These findings suggest that the formation of martensite islands should be prevented by controlling the chemical compositions and by using the proper welding conditions to enhance fracture toughness of the welded joints of the HSLA steel. Formerly Research Assistant with the Department of Materials Science and Engineering, Pohang Institute of Science and Technology     

A model for the formation and solidification of grain boundary liquid in the heat-affected zone (HAZ) of welds

The rapid thermal cycle experienced in most welding operations can promote the constitutional liquation of precipitates in certain alloy systems. Grain boundary liquid films form in the subsolidus portion of the heat-affected zone (HAZ) as a consequence of constitutional liquation. Rapid cooling limits the extent of solute diffusion into the matrix from the grain boundary liquid and hence extends its solidification temperature range. Liquation cracking can occur in the HAZ if the grain boundary film exists at a time when the local thermal stresses become tensile. Hence, in order to predict the liquation cracking susceptibility of an alloy under a given welding condition, both the microstructural evolution which centers around grain boundary liquation and the stress generation have to be modeled. This article addresses the issue of microstructural evolution and attempts to present a model for the formation of grain boundary liquid during the heating cycle and its solidifiction during the cooling cycle. The variables which increase the life of the transient grain boundary liquid during the thermal cycle are identified. The onedimensional (1-D) model presented here is an important first step toward the ability to predict liquation cracking susceptibility of an alloy during welding.     

The effect of grain boundary orientation on creep and rupture of IN-738 and nichrome

Creep rupture specimens taken from directionally solidified ingots of IN-738 and Nichrome in which the grain boundaries were oriented longitudinally, transversely, and 45 deg to the stress axis have been tested over a range of temperature and stress. For both alloys, the ductility was appreciably higher in the longitudinal orientation; but in IN-738, the creep strength was higher in the other two orientations. The net effect on rupture life was small for the superalloy. The nichrome showed much greater scatter which was due partly to inhomogeneous deformation and local recrystallization at the higher temperatures. Because of the recrystallization, even the longitudinal specimens showed intergranular failure for nichrome. The microstructural features of intergranular cracking both internally and on the surface are documented. It is suggested that surface cracking may be an important contributory factor in leading to reduced life with decreased section size which is commonly observed in conventionally cast superalloys.     

A Study on the Effect of Process Parameters on the Properties of Joint in TLP-Bonded Inconel 738LC Superalloy

Optimization of transient liquid phase (TLP)-bonding variables is essential to achieve a joint free from deleterious intermetallic constituents and with appropriate mechanical properties. In this study, TLP bonding of IN-738LC superalloy was performed using AMS 4777 filler metal. The influence of gap size and bonding parameters (temperature and time) was investigated on the joint microstructure and its properties. In cases where the holding time was insufficient for complete isothermal solidification, the residual liquid transformed to non-equilibrium eutectic microconstituents consisting of nickel-rich boride, chromium-rich boride, and γ solid solution phases. The eutectic width decreased with the increase of holding time and the increase in initial gap size resulted in thicker eutectic width in the samples bonded at the same temperature and for equivalent holding times. The time of complete isothermal solidification decreased with the increase in bonding temperature to 1100°C, which was consistent with the models based on the diffusion-induced solid/liquid interface motion. Microhardness and shear strength tests were used to investigate the mechanical properties of the bonds. In the bonding condition in which isothermal solidification was not accomplished completely, the eutectic constituent with the highest hardness in the bond region was the preferential failure source. The results showed that homogenized joints had the highest shear strength.     

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.     

Simulation study on heat-affected zone of high-strain X80 pipeline steel

The microstructure evolution and impact-toughness variation of heat-affected zone(HAZ)in X80 highstrain pipeline steel were investigated via a welding thermal-simulation technique,Charpy impact tests,and scanning electron microscopy observations under different welding heat inputs and peak temperatures.The results indicate that when heat input was between 17 and 25kJ·cm~(-1),the coarse-grained heat-affected zone showed improved impact toughness.When the heat input was increased further,the martensite-austenite(M-A)islands transformed from fine lath into a massive block.Therefore,impact toughness was substantially reduced.When the heat input was 20kJ·cm~(-1) and the peak temperature of the first thermal cycle was between 900 and 1300°C,a higher impact toughness was obtained.When heat input was 20kJ·cm~(-1) and the peak temperature of the first thermal cycle was 1300°C,the impact toughness value at the second peak temperature of 900°C was higher than that at the second peak temperature of 800°C because of grain refining and uniformly dispersed M-A constituents in the matrix of bainite.     

Influence of various heat treatments on the microstructure of polycrystalline IN738LC

IN738LC is a modern, nickel-based superalloy utilized at high temperatures in aggressive environments. Durability of this superalloy is dependent on the strengthening of γ′ precipitates. This study focuses on the microstructural development of IN738LC during various heat treatments. The 1120 °C/2 h/accelerated air-cooled (AAC) solution treatment, given in the literature, already produces a bimodal precipitate microstructure, which is, thus, not an adequate solutionizing procedure to yield a single-phase solid solution in the alloy at the outset. However, the 1235 °C/4 h/water quenched (WQ) solution treatment does produce the single-phase condition. A microstructure with fine precipitates develops if solutionizing is carried out under 1200 °C/4 h/AAC conditions. Agings at lower temperatures after 1200 °C/4 h/AAC or 1250 °C/4 h/AAC or WQ conditions yield analogous microstructures. Agings below ∼950 °C for 24 hours yield nearly spheroidal precipitates, and single aging for 24 hours at 1050 °C or 1120 °C produces cuboidal precipitates. Two different γ′ precipitate growth processes are observed: merging of smaller precipitates to produce larger ones (in duplex precipitate-size microstructures) and growth through solute absorption from the matrix. Average activation energies for the precipitate growth processes are 191 and 350 kJ/mol in the ranges of 850 °C to 1050 °C and 1050 °C to 1120 °C, respectively, calculated using the precipitate sizes from microstructures in the WQ condition, and 150 and 298 kJ/mol in the analogous temperature ranges, calculated from precipitate sizes in the microstructures in the slow furnace-cooled condition.     

Spatially resolved X-ray diffraction phase mapping and α → β → α transformation kinetics in the heat-affected zone of commercially pure titanium arc welds

Spatially resolved X-ray diffraction (SRXRD) is used to map the α → β → α phase transformation in the heat-affected zone (HAZ) of commercially pure titanium gas tungsten arc welds. In situ SRXRD experiments were conducted using a 180-μm-diameter X-ray beam at the Stanford Synchrotron Radiation Laboratory (SSRL) (Stanford, CA) to probe the phases present in the HAZ of a 1.9 kW weld moving at 1.1 mm/s. Results of sequential linear X-ray diffraction scans made perpendicular to the weld direction were combined to construct a phase transformation map around the liquid weld pool. This map identifies six HAZ microstructural regions between the liquid weld pool and the base metal: (1) α-Ti that is undergoing annealing and recrystallization; (2) completely recrystallized α-Ti; (3) partially transformed α-Ti, where α-Ti and β-Ti coexist; (4) single-phase β-Ti; (5) back-transformed α-Ti; and (6) recrystallized α-Ti plus back-transformed α-Ti. Although the microstructure consisted predominantly of α-Ti, both prior to and after the weld, the crystallographically textured starting material was altered during welding to produce different α-Ti textures within the resulting HAZ. Based on the travel speed of the weld, the α → β transformation was measured to take 1.83 seconds during heating, while the β → α transformation was measured to take 0.91 seconds during cooling. The α → β transformation was characterized to be dominated by long-range diffusional growth on the leading (heating) side of the weld, while the β → α transformation was characterized to be predominantly massive on the trailing (cooling) side of the weld, with a massive growth rate on the order of 100 μm/s.     

High-cycle fatigue-life of the cast nickel base-superalloys in 738 LC and IN 939

The high cycle fatigue (HCF) properties of two cast nickel base-superalloys, IN 738 LC and IN 939, were investigated using both fracture mechanics samples and smooth specimens. The crack propagation behavior was studied in terms of linear fracture mechanics at RT and at 850 °C. In addition to the influence of temperature, the influences of frequency, mean stress, and environment (vacuum, air, sulfidizing atmosphere) were studied. At 850 °C, the fatigue thresholds were found to be higher in air than in vacuum. This could be explained by crack branching. The high scatter of fatigue crack propagation rates could be related also to this phenomenon. The S/N curves at 850 °C can be predicted treating crack growth from casting pores as the predominant failure mechanism. At RT the same method is not as successful. The reason for this may be that crack growth laws measured on long, branched cracks are not applicable to short, unbranched cracks. At RT, no significant influence of frequency on S/N-curves and fatigue crack growth rates was observed for frequencies up to 20 kHz.     

Microstructural evolution in the heat-affected zone of a friction stir weld

The microstructure and crystallographic texture spanning the soft region at the thermomechanically affected zone/heat-affected zone (TMAZ/HAZ) boundary of a friction stir weld in 2519 Al were systematically investigated to determine their contributions to the properties of that region. The microstructure was shown to be the primary cause of softening at the TMAZ/HAZ boundary. During welding, fine ϑ′ precipitates responsible for much of the strength in this alloy coarsen and transform to the equilibrium ϑ phase in the HAZ and into the TMAZ, accounting for the observed softening through the HAZ region. The higher temperatures achieved in the TMAZ partially resolutionize the precipitates and allow the subsequent formation of Guinier-Preston (GP) zones during cooling. These two processes are responsible for the variation in microhardness observed in the TMAZ/HAZ region. Texture analyses revealed significant differences in the crystallographic texture across this region that were primarily due to macroscopic rigid-body rotations of the grains, but do not account for the observed softening. The effect of the observed microstructural evolutions on the friction stir welding (FSW) deformation field and on the fracture behavior of the weld are also discussed.     

Effect of residual elements on radiation strengthening in iron alloys,pressure vessel steels,and welds

Embrittlement of pressure vessel steels and welds during irradiation at service temperatures around 290°C is known to be related to the presence of residual elements such as copper but the mechanism of embrittlement has not been clear. Pressure vessel steels and welds with high and low copper content and binary iron alloys with 0.3 at. pct of Cu, V, Ni, and P and 0.1 at. pct C were irradiated to fluences of 2.5 x 10¹⁹ and 4.5 x 10²⁰ n per sq cm > 1 MeV and compression tested to determine the radiation strengthening. Alloys containing copper had more rapid and greater radiation strengthening. It is concluded that the embrittlement is due to this radiation strengthening which in turn results from a higher density of defect aggregates. Vacancy trapping by the copper atoms which modifies the kinetics of defect aggregate nucleation is shown to be the most likely cause for the differences in microstructure.