Role of microstructural degradation in the heat-affected zone of 2.25Cr-1Mo steel weldments on subscale features during steam oxidation and their role in weld failures

Microstructural degradations in the base metal adjacent to the weld pool, i.e., the heat-affected zone (HAZ), caused during welding of 2.25Cr-1Mo steel, were characterized by electron and optical microscopy of different regions of the weldments. In order to study the influence of the microstructural degradations on scaling kinetics in steam and the resulting subscale features, samples of the base metal, the HAZ, and weld metal specimens were extracted from the weldment and oxidized in an environment of 35 pct steam+nitrogen at 873 K for 10 hours. Oxide scales formed in the three regions and the underlying subscales were characterized using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Influence of the “free” chromium content in the three weldment regions on protective scale formation and on the subscale features has been investigated. As the principal achievement, this study has clearly shown the occurrence of oxidation-induced void formation in the subscale zone and grain boundary cavitation in the neighboring area during steam oxidation of the HAZ. This article also discusses the possible role of oxidation-induced void formation and grain boundary cavitation in the inferior service life of welds in 2.25Cr-1Mo steel components.     

Role of gaseous environment and secondary precipitation in microstructural degradation of Cr-Mo steel weldments at high temperatures

This study is an attempt to understand the combined role of variations in oxidizing environment and secondary precipitation, in the microstructurally different regions of a standard Cr-Mo steel weldment, on the intensity of internal oxidation during high-temperature oxidation in air and steam environments. Samples of the weld-metal, heat-affected zone (HAZ), and base-metal regions were separated from the weldment of 2.25Cr-1 Mo steel and oxidized in the environments of air and steam at 873 K. The oxide scales and underlying subscales were characterized using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, and electron probe microanalysis (EPMA). Extensive internal oxidation and oxidation-induced void formation in the subscale zone and grain-boundary cavitation in the neighboring region were found to occur during oxidation in the steam environment. However, the internal oxidation and void formation were much more extensive in the subscale regions of the HAZ than in the subscales of the weld-metal and base-metal regions. As a result, the alloy matrix in the area neighboring the subscale region of the HAZ specimen suffered extensive grain-boundary cavitation. This behavior has been attributed to a rather specific combination and complex interplay of the environment, alloy microstructure, oxidizing temperature, and nature of the resulting external scale in causing and sustaining internal oxidation. The article also discusses the role of internal oxidation-assisted microstructural degradation in deteriorating the service life of components of 2.25Cr-1Mo steel.     

Fracture toughness behavior of ex-service 2-1/4Cr-1Mo steels from a 22-year-old fossil power plant

Elevated-temperature fracture toughness properties were developed on ex-service 2-l/4Cr-1Mo steel weldments. Fracture toughness was measured on both base and heat-affected zone (HAZ) metals. A composite specimen consisting of base, HAZ, and weld metals was used to develop fracture toughness properties in the HAZ area. It was observed that the J-R curve of the HAZ was significantly lower than that of the base metal. Increasing crack extension increased the difference between theJ-R curves of the base metal and the HAZ. Dimpled fracture was the prime fracture mode in the base metal specimen, and a mixed-mode (ductile and “granular”) fracture was found in the HAZ specimens. Scanning transmission electron microscopy (STEM) examination revealed significant intergranular carbide precipitation and agglomeration within the HAZ. The lower fracture toughness of the HAZ, as compared to the base metal, was attributed to the large accumulation of carbides in the grain boundaries of the HAZ, which weakened the grain boundaries and caused “granular” fracture.     

Effect of multiaxial stresses on creep damage of 316 stainless steel weldments

The difference in creep strength between a base metal and a weld metal always produces a multiaxial stress state even if the macroscopic loading is uniaxial. In this study, weldments were formed between wrought 316 stainless steel and two types of 316 weld metals with slightly different creep properties and chemical compositions. Full-size 316 weldments, including base metal, heat-affected zone (HAZ) and welds, were creep tested at 650 °C. The multiaxial stress distributions in full-size 316 weldments were simulated by the finite element method (FEM). Three stress parameters, namely, the maximum principal stress (MPS), the yon Mises effective stress (VMS), and the principal facet stress (PFS), were used to correlate the local multiaxial stresses with local creep damage distributions and failure lifetime. Metallographic examination and creep rupture data showed that the PFS parameter gave the best prediction of the creep damage distribution caused by the multiaxial stresses in 316 weldments. This approach may have application in the design, life prediction, and in-service evaluation of weldments. Performed this work at Ohio State This article is based on a presentation made at the “High Temperature Fracture Mechanisms in Advanced Materials” sympsosium as a part of the 1994 Fall meeting of TMS, October 2-6, 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow and Fracture Committee.     

Microstructural analysis of fracture in heat-affected zone of two ×70 pipeline steel weldments

In the previous study, different crack propagation behaviours (ductile fracture and brittle cleavage fracture) were observed in two ×70 pipeline steel weldments (13.4 and 17.8-mm-thick) during single-edge notched bend testing. To further understand these two fracture behaviours, detailed microstructures of the base metal (BM), fine-grained heat-affected zone (FGHAZ), and coarse-grained heat-affected zone (CGHAZ) of these two ×70 pipeline steel weldments have been analysed. The results show that the initial structure of the two pipe BMs and different welding cooling rates owing to different thicknesses contributed to structural variations of the correlated sub-regions of the HAZ. For both weldments, the FGHAZ close to the BM has the highest fraction of the high-angle grain boundaries, the finest grain size, the lowest local strain levels, and the highest fraction of recrystallised ferrite grains. The CGHAZ of the 17.8-mm-thick pipe welds exhibits the lowest toughness with the highest hardness, a high frequency of deformed grains, the highest local strain level, and the highest density of preferred {100} cleavage planes than the other sub-regions in the HAZ. The high density of the {100}<011> texture components in the HAZ may cause the cleavage micro-cracks to propagate toward the BM at an approximate 45° angle to the original crack plane during bending tests.     

Fracture toughness of modified P91 weldments

There are efforts to develop modified P91 steel (9Cr-1Mo-V) consumables to optimize strength and fracture toughness in weldments for similar and dissimilar welding of 9Cr-1Mo (modified P91) for both new construction and replacement of serviced components. Fracture toughness is an important consideration which plays a vital role in determining the performance and life of the materials under the given service conditions. Toughness characterization was done by the Crack Tip Opening Displacement (CTOD) method. Welding results in a variety of non-equilibrium microstructures in the HAZ of 9Cr-lMo-V, modified P91 steel. These variations of microstructures from wrought base material through transformed HAZ to cast weld metal, may give rise to considerable inhomogeneity with respect to tensile & creep strength and ductility across the weld joints. However the mechanical properties of the individual regions of HAZ are difficult to obtain because of the small extent over which each region exists. Welded joints are used as structural parts of boilers and pressure vessels working at high temperatures, hence the main requirement is creep resistance. In the present investigation, the fracture toughness characteristics of base metal and weld metal have been evaluated by CTOD method as per the standard BS 7448. The fracture surfaces of the CTOD tested specimens were examined under Scanning Electron Microscope (SEM). Fractographic studies revealed the mode of failure and the characteristics of the fracture surface.     

Creep Strength of Dissimilar Welded Joints Using High B-9Cr Steel for Advanced USC Boiler

The commercialization of a 973 K (700 °C) class pulverized coal power system, advanced ultra-supercritical (A-USC) pressure power generation, is the target of an ongoing research project initiated in Japan in 2008. In the A-USC boiler, Ni or Ni-Fe base alloys are used for high-temperature parts at 923 K to 973 K (650 °C to 700 °C), and advanced high-Cr ferritic steels are planned to be used at temperatures lower than 923 K (650 °C). In the dissimilar welds between Ni base alloys and high-Cr ferritic steels, Type IV failure in the heat-affected zone (HAZ) is a concern. Thus, the high B-9Cr steel developed at the National Institute for Materials Science, which has improved creep strength in weldments, is a candidate material for the Japanese A-USC boiler. In the present study, creep tests were conducted on the dissimilar welded joints between Ni base alloys and high B-9Cr steels. Microstructures and creep damage in the dissimilar welded joints were investigated. In the HAZ of the high B-9Cr steels, fine-grained microstructures were not formed and the grain size of the base metal was retained. Consequently, the creep rupture life of the dissimilar welded joints using high B-9Cr steel was 5 to 10 times longer than that of the conventional 9Cr steel welded joints at 923 K (650 °C).     

Microstructure,Composition, and Impact Toughness Across the Fusion Line of High-Strength Bainitic Steel Weldments

This paper analyzed the evolution of microstructure, composition, and impact toughness across the fusion line of high-strength bainitic steel weldments with different heat inputs. The main purpose was to develop a convenient method to evaluate the HAZ toughness quickly. The compositions of HAZ were insensitive to higher contents of alloy elements (e.g., Ni, Mo) in the weld metal because their diffusion distance is very short into the HAZ. The weld metal contained predominantly acicular ferrite at any a heat input, whereas the main microstructures in the HAZ changed from lath martensite/bainite to upper bainite with the increasing heat input. The evolution of HAZ toughness in relation to microstructural changes can be revealed clearly combined with the impact load curve and fracture morphology, although the results of impact tests do not show an obvious change with heat input because the position of Charpy V notch contains the weld metal, HAZ as well as a part of base metal. As a result, based on the bead-on-plate welding tests, the welding parameter affecting the HAZ toughness can be evaluated rapidly.     

Finite element modelling of the creep deformation of T91 steel weldments at 600°C

Finite element modelling of the creep deformation of T91 steel weldments, welded using the manual metal arc (MMA) and submerged arc (SA) welding processes, was carried out to predict creep curves for both of the weldments under different stresses and compared with the experimental data. The stress and strain redistribution across the length of the transverse-weld specimens has also been predicted. Data of creep tests at 600°C at stresses between 90-130 MPa for the base metal, the MMA and SA weld metals, and the simulated heat-affected zone were used to determine Garofalo's equation for creep strain. Finite element meshes for both of the weldments were constructed after calculating the HAZ locations using Rosenthal's heat flow equation.     

Failure behavior of heat-affected zones within HSLA-100 and HY-100 steel weldments

The deformation and fracture behavior of simulated heat-affected zones (HAZ) within HSLA-100 and HY-100 steel weldments has been studied as a function of stress state using notched and unnotched axisymmetric tensile specimens. For the case of the HSLA-100 steel, the results for fine-grained, as well as coarse-grain HAZ (CGHAZ) material, show that, despite large differences in the deformation behavior when compared to base plate or weld metal, the failure strains are only weakly dependent on the thermal history or microstructure. Ductile microvoid fracture dominates the failure of the HSLA-100 steel with small losses of ductility occurring in the HAZ conditions only at high stress triaxialities. In contrast, the HY-100 steel is susceptible to a large loss of ductility over all of the stress states when subjected to a severe, single-pass simulation of a CGHAZ. The ductility loss is greatest at the high stress triaxiality ratio in which case failure initiation occurs by a combination of localized cleavage and ductile microvoid fracture.     

Effect of Application of Short and Long Holds on Fatigue Life of Modified 9Cr-1Mo Steel Weld Joint

Modified 9Cr-1Mo steel is a heat-treatable steel and hence the microstructure is temperature sensitive. During welding, the weld joint (WJ) is exposed to various temperatures resulting in a complex heterogeneous microstructure across the weld joint, such as the weld metal, heat-affected zone (HAZ) (consisting of coarse-grained HAZ, fine-grained HAZ, and intercritical HAZ), and the unaffected base metal of varying mechanical properties. The overall creep–fatigue interaction (CFI) response of the WJ is hence due to a complex interplay between various factors such as surface oxides and stress relaxation (SR) occurring in each microstructural zone. It has been demonstrated that SR occurring during application of hold in a CFI cycle is an important parameter that controls fatigue life. Creep–fatigue damage in a cavitation-resistant material such as modified 9Cr-1Mo steel base metal is accommodated in the form of microstructural degradation. However, due to the complex heterogeneous microstructure across the weld joint, SR will be different in different microstructural zones. Hence, the damage is accommodated in the form of preferential coarsening of the substructure, cavity formation around the coarsened carbides, and new surface formation such as cracks in the soft heat-affected zone.     

The effects of strain rate and welding current mode on the dynamic impact behavior of plasma-arc-welded 304L stainless steel weldments

This article uses a split-Hopkinson pressure bar to investigate the effects of strain rate in the range of 10³ s⁻¹ to 8 × 10³ s⁻¹ and welding current mode upon the dynamic impact behavior of plasma-arc-welded (PAW) 304L stainless steel (SS) weldments. Stress-strain curves are plotted for different strain rates and welding parameters, and optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques are used to analyze the microstructure and fracture characteristics of the weldments. The results confirm that the strain rate and the welding current mode have a significant influence upon the dynamic impact behavior and microstructure evolution of 304L SS weldments. It is shown that for a constant strain, the flow stress increases with strain rate for both welding current modes, and that the pulsed current (PC) mode results in a higher weldment strength than the continuous current (CC) mode. Weldments fabricated using the PC mode exhibit an improved resistance to thermal softening, a greater strain-rate sensitivity, and a lower activation volume. The OM and SEM observations reveals that an adiabatic shear band dominates the fracture characteristics of both weldment types under impact loading. Microstructural analysis reveals that for both welding current modes, the dislocation density and volume fraction of α′ martensite increase with an increasing strain rate, while the twin formations reduce under the same conditions. Comparing the evolution of the microstructure in the base metal and the fusion zone, it is found that for both welding current modes, a higher dislocation density exists in the fusion zone, and that a larger volume fraction of α′ martensite and a greater twin density are present in the base metal. Furthermore, the dislocation density and volume fraction of α′ martensite is greater in PC weldments than in their CC counterparts. Finally, the present results indicate that the PC welding mode produces a weldment with superior dynamic impact response and improved weldment fracture characteristics.     

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.     

Remaining life assessment issues in high Cr martensitic steels and development of new innovative tools for damage monitoring and integrity assessment

The relatively newer high Cr martensitic steels such as P91 have now been in use in power industry for over twenty years. Over this time, there have been a number of incidents of cracking and failure in components made from P91 steel, both in thick and thin section equipments mainly due to creep damage. The thick section components have been usually failing due to Type IV cracking associated with the weldments while thin section components have been failing due to higher than expected levels of steam oxidation resulting in enhanced metal loss, increase in metal temperature above design, creep cavitation and cracking. However, it has not been possible to detect early stage creep damage/cavitation in high Cr martensitic steels using conventional replication type methods. This is because unlike the low alloy CrMoV steels, spherodisation of microstructure does not occur in high Cr martensitic steels and cavitation clearly visible by traditional methods only appears later in life when the material is about to fail. Thus there has been a need to develop new tools and more sensitive methods for integrity and damage assessment in these steels. European Technology Development (ETD) together with its industrial collaborators from Europe, Japan and North America have been looking at the development of tools and methodologies for early stage damage detection and life prediction as a part of its international multi-client project ‘P91 Integrity’. The tools which have shown successful results are portable Scanning Force Microscopy, laser guided hardness tester and more innovative use of ultrasonic probes for detecting early stage creep damage. This paper discusses the issues involved and describes some of the developments in this project.     

Influence of microstructural variations in the weldment on the high-temperature corrosion of 2.25Cr-1Mo steel

In order to study the influence of microstructural variation on the oxidation of the weldment of 2.25Cr-1Mo steel, regions with different microstructures were identified by optical microscopy. The weld metal, the base metal, and the heat-affected zone (HAZ), as well as the subzones within the HAZ, i.e., the intercritical (ICR), the fine-grain bainite (FGB), and the coarse-grain bainite (CGB) regions were separated from the weldment by precise steps of metallography. Transmission electron microscopic examinations for the identification of the secondary phases in microstructurally different regions and subzones have suggested that M₂₃C₆ and M₇C₃ pre-cipitates form predominantly in the subzones of HAZ, whereas the Mo₂C type of carbide forms exclusively in the weld-metal and base-metal regions of the weldment. However, population and distribution of the secondary phases were different in the three subzones of the HAZ. In order to understand the influence of these microstructural variations on the oxidation behavior, the various regions and subzones were oxidized at 773 and 873 K. The HAZ and its constituents were found to oxidize at much higher rates than the weld metal and the base metal. Relative compositions and morphologies of the scales were compared by scanning electron microscopy with energy-dispersive analyses of X-rays (SEM/EDX), and secondary ion mass spectrometry (SIMS). Scale formed over the weld metal shows a greater tendency for spallation, as suggested by tests monitoring acoustic emission. X-ray diffraction (XRD) patterns of the scales over these specimens were taken. Results of the SEM/EDX, SIMS, and XRD investigations suggest for-mation of inner scales with less Cr(i.e., less protective) over the HAZ than over the weld-metal and the base-metal regions. Variation in the Cr contents of the scales formed over the various regions is proposed to arise from the difference in microstructural features in different regions of the weldments. Formerly with the Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkan, India     

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     

An assessment of creep deformation and fracture behavior of 2.25Cr-1Mo similar and dissimilar weld joints

The evaluation of the creep deformation and fracture behavior of a 2.25Cr-1Mo steel base metal, a 2.25Cr-1Mo/2.25Cr-1Mo similar weld joint, and a 2.25Cr-1Mo/Alloy 800 dissimilar weld joint at 823 K over a stress range of 90 to 250 MPa has been carried out. The specimens for creep testing were taken from single-V weld pads fabricated by a shielded metal arc-welding process using 2.25Cr-1Mo steel (for similar-joint) and INCONEL 182 (for dissimilar-joint) electrodes. The weld pads were subsequently given a postweld heat treatment (PWHT) of 973 K for 1 hour. The microstructure and microhardness of the weld joints were evaluated in the as-welded, postweld heat-treated, and creep-tested conditions. The heat-affected zone (HAZ) of similar weld joint consisted of bainite in the coarse-prior-austenitic-grain (CPAG) region near the fusion line, followed by bainite in the fine-prior-austenitic-grain (FPAG) and intercritical regions merging with the unaffected base metal. In addition to the HAZ structures in the 2.25Cr-1Mo steel, the dissimilar weld joint displayed a definite INCONEL/2.25Cr-1Mo weld interface structure present either as a sharp line or as a diffuse region. A hardness trough was observed in the intercritical region of the HAZ in both weld joints, while a maxima in hardness was seen at the weld interface of the dissimilar weld joint. Both weld joints exhibited significantly lower rupture lives compared to the 2.25Cr-1Mo base metal. The dissimilar weld joint exhibited poor rupture life compared to the similar weld joint, at applied stresses lower than 130 MPa. In both weld joints, the strain distribution across the specimen gage length during creep testing varied significantly. During creep testing, localization of deformation occurred in the intercritical HAZ. In the similar weld joint, at all stress levels investigated, and in the dissimilar weld joint, at stresses ≥150 MPa, the creep failure occurred in the intercritical HAZ. The fracture occurred by transgranular mode with a large number of dimples. At stresses below 150 MPa, the failure in the dissimilar weld joint occurred in the CPAG HAZ near to the weld interface. The failure occurred by extensive intergranular creep cavity formation.     

Type IV Creep Damage Behavior in Gr.91 Steel Welded Joints

Modified 9Cr-1Mo steel (ASME Grade 91 steel) is used as a key structural material for boiler components in ultra-supercritical (USC) thermal power plants at approximately 873 K (600 °C). The creep strength of welded joints of this steel decreases as a result of Type IV creep cracking that forms in the heat-affected zone (HAZ) under long-term use at high temperatures. The current article aims to elucidate the damage processes and microstructural degradations that take place in the HAZ of these welded joints. Long-term creep tests for base metal, simulated HAZ, and welded joints were conducted at 823 K, 873 K, and 923 K (550 °C, 600 °C, and 650 °C). Furthermore, creep tests of thick welded joint specimens were interrupted at several time steps at 873 K (600 °C) and 90 MPa, after which the distribution and evolution of creep damage inside the plates were measured quantitatively. It was found that creep voids are initiated in the early stages (0.2 of life) of creep rupture life, which coalesce to form a crack at a later stage (0.8 of life). In a fine-grained HAZ, creep damage is concentrated chiefly in an area approximately 20 pct below the surface of the plate. The experimental creep damage distributions coincide closely with the computed results obtained by damage mechanics analysis using the creep properties of a simulated fine-grained HAZ. Both the concentration of creep strain and the high multiaxial stress conditions in the fine-grained HAZ influence the distribution of Type IV creep damage.     

Microstructure,Hardness, and Residual Stress of the Dissimilar Metal Weldments of SA508-309L/308L-304L

A dissimilar metal weldment consisting of SA508-309L-308L-304L is widely used in light-water nuclear reactors. These weldments demonstrate dissimilar susceptibility to stress corrosion cracking that are related to the microstructure, properties, and residual stress. In this work, microstructures, hardness, and the residual stress distribution of the dissimilar metal weldments were investigated, with the correlation of increased hardness in the heat-affected zone (HAZ) to the microstructure. 304L HAZ demonstrated similar grain morphology as the base material, and the increase in hardness was primarily attributed to the increased dislocation density. SA508 HAZ demonstrated a change of grain morphology resulting from the different peak temperatures and cooling rates. The increased hardness in the SA508 HAZ was attributed to the refined grain morphology, higher dislocation density, and higher number density of precipitates. A ~ 20–30-μm-wide martensitic zone formed at the fusion boundary of SA508-309L, where Cr-rich carbide precipitates were observed, with the average size and the number density of 44.1 ± 16.9 nm and 1.5 × 10²¹ m⁻³, respectively. Residual stress results demonstrated the largest tensile stress at 309L butter, indicating its high cracking susceptibility.

     

Effect of filler alloys on heat-affected zone cracking in preweld heat-treated IN-738 LC gas-tungsten-arc welds

The effect of filler alloys C-263, RENé-41, IN-718, and FM-92 on heat-affected zone (HAZ) cracking susceptibility of cast IN-738 LC, which is a high-temperature Ni-based superalloy used at temperatures up to 980 °C and is precipitation hardened by the γ′ (Ni₃Al,Ti) phase, by gas-tungsten-arc (GTA) welding was studied. In addition, autogenous welds were also made on the IN-738 parent material. The preweld treatments consisted of the standard solution treatment at 1120 °C for 2 hours followed by air cooling, and a new heat treatment, which was developed to improve the HAZ cracking resistance of IN-738 LC. This heat treatment consisted of solution treating at 1120 °C followed by air cooling then aging at 1025 °C for 16 hours followed by water quenching. Welds were observed to suffer intergranular HAZ cracking, regardless of the filler alloy; however, the autogenous welds were most susceptible to HAZ cracking. In general, the cracking tendency for both heat treatments was maximum for C-263 and RENE-41 fillers and decreased with the use of FM-92 and IN-718 filler alloys. The HAZ cracking was associated mainly with constitutional liquation of γ′ and MC carbides. On some cracks, liquated low melting point containing Zr-carbosulfide and Cr-Mo borides were also observed to be present. The cooling portion of the weld thermal cycle induced precipitation hardening via γ′ phase in the γ matrix of the weld metal. The HAZ cracking increased as the weld metal lattice mismatch between γ′ precipitates and γ matrix of the weld and its hardness (Ti + Al) increased. However, the weld-metal solidus and solidification temperature range, determined by high-temperature differential scanning calorimetry, did not correlate with the HAZ cracking susceptibility. It is suggested that the use of filler alloys with small γ′-γ lattice mismatch and slow age-hardening response would reduce the HAZ cracking in IN-738 LC superalloy welds.