Liquation Microfissuring in the Weld Heat-Affected Zone of an Overaged Precipitation-Hardened Nickel-Base Superalloy

The effect of preweld overaging heat treatment on the microstructural response in the heat-affected zone (HAZ) of a precipitation-hardened nickel-base superalloy INCONEL 738LC subjected to the welding thermal cycle (i.e., rapid) was investigated. The overaging heat treatment resulted in the formation of an interfacial microconstituent containing M₂₃X₆ particles and coarsening of primary and secondary γ′ precipitates. The HAZ microstructures around welds in the overaged alloy were simulated using the Gleeble thermomechanical simulation system. Microstructural examination of simulated HAZs and those present in tungsten inert gas (TIG) welded specimens showed the occurrence of extensive grain boundary liquation involving liquation reaction of the interfacial microconstituents containing M₂₃X₆ particles and MC-type carbides. In addition, the coarsened γ′ precipitate particles present in the overaged alloy persisted well above their solvus temperature to temperatures where they constitutionally liquated and contributed to considerable liquation of grain boundaries, during continuous rapid heating. Intergranular HAZ microfissuring, with resolidified product formed mostly on one side of the microfissures, was observed in welded specimens. This suggested that the HAZ microfissuring generally occurred by decohesion across one of the solid-liquid interfaces during the grain boundary liquation stage of the weld thermal cycle. Correlation of simulated HAZ microstructures with hot ductility properties of the alloy revealed that the temperature at which the alloy exhibited zero ductility during heating was within the temperature range at which grain boundary liquation was observed. The on-cooling ductility of the alloy was significantly damaged by the on-heating liquation reaction, as reflected by the considerably low ductility recovery temperature (DRT). Important characteristics of the intergranular liquid that could influence HAZ microfissuring of the alloy in overaged condition are also discussed.

Improved Resistance to Laser Weld Heat-Affected Zone Microfissuring in a Newly Developed Superalloy HAYNES 282

Gleeble thermomechanical simulation and microstrucutural analyses of laser beam weldability of a newly developed precipitation-hardened nickel-base HAYNES alloy 282 were performed to better understand the fundamental cause of heat-affected zone (HAZ) cracking and how to prevent the cracking problem in the material. Submicron size intergranular M₅B₃ particles are identified for the first time in the present work by transmission electron microscopy, and were found to be the primary cause of HAZ grain boundary liquation cracking in the alloy. Complete dissolution of the liquating M₅B₃ particles by preweld heat treatment exacerbated rather than reduced susceptibility to cracking, which could be attributed to nonequilibrium intergranular segregation of boron atoms, liberated by the complete dissolution of the boride particles, during cooling from heat treatment temperature. Consequently, to reduce the HAZ cracking, a preweld heat treatment that reduces the volume fraction of the M₅B₃ particles while minimizing nonequilibrium grain boundary boron segregation is necessary, and this is possible by heat treating the alloy at 1353?K to 1373 K (1080?°C to 1100 °C). Further improvement in cracking resistance to produce crack-free welds is achieved by subjecting the alloy to thermomechanically induced grain refinement coupled with the preweld heat treatment at 1353 K (1080 °C). A Gleeble hot ductility test showed that formation of the crack-free welds is unexplainable by mere reduction in grain size without considering the effect of grain refinement on intergranular liquid produced by subsolidus liquation of the M₅B₃ borides.     

Grain Boundary Curvature in a Model Ni-Based Superalloy

The local grain boundary (GB) curvature in a model Ni-based superalloy was measured experimentally using Dehoff’s tangent count method. The results show that, in materials containing significant amounts of second-phase particles, the curvature parameter, κ, which relates the mean local curvature to the grain size, can adopt far lower values than have been reported previously. It is also shown that the value of κ is not a constant, as is usually assumed, but instead varies both with the volume fraction of second-phase particles and with the holding time during high-temperature annealing. The lowest values for κ were obtained for high particle volume fractions and long annealing times. Because the local boundary curvature constitutes the driving force for grain growth, these observations could help to explain grain growth phenomena in heavily pinned systems.

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On the Time-Temperature-Transformation Behavior of a New Dual-Superlattice Nickel-Based Superalloy

Recent research has identified compositions of nickel-based superalloys with microstructures containing appreciable and comparable volume fractions of γ′ and γ″ precipitates. In this work, an alloy capable of forming such a dual-superlattice microstructure was subjected to a range of thermal exposures between 873 K and 1173 K (600 °C and 900 °C) for durations of 1 to 1000 hours. The microstructures and nature of the precipitating phases were characterized using synchrotron X-ray diffraction and electron microscopy. These data have enabled the construction of a T-T-T diagram for the precipitating phases. Hardness measurements following each thermal exposure have identified the age-hardening behavior of this alloy and allowed preliminary mechanical properties to be assessed.     

Significant Grain Refinement in the Simulated Heat-Affected Zone (HAZ) of Ferritic Stainless Steels by Alloying with Tungsten

We elucidate here the effect of tungsten on grain refinement in the HAZ of ferritic stainless steels (FSSs) at 1200 °C and 1350 °C. The average simulated HAZ grain size of W-containing FSS (1.5 wt. pct W) was reduced by ~ 50 pct compared to W-free FSS at 1350 °C. The replacement of Mo with W in FSSs enhanced the thermal stability of Laves phase at high temperatures and contributed to fine grain size in the HAZ via pinning effect.

     

Microstructural Characterization of the Simulated Heat-Affected Zone of 9 Pct Ni Steel

Metallurgical and Materials Transactions A - The present work characterized the 9 pct Ni steel heat-affected zone (HAZ). HAZ thermal cycles (peak temperatures—Tp—of...     

The Mechanism of Grain Boundary Serration and Fan-Type Structure Formation in Ni-Based Superalloys

Metallurgical and Materials Transactions A - A model of discontinuous precipitation of γ′ phase at grain boundaries (GBs) in polycrystalline Ni-based superalloys during continuous...     

Microstructure Evolution of a Simulated Coarse-Grained Heat-Affected Zone of T23 Steel During Aging

In this work, simulated CGHAZ of T23 steel was produced via a thermomechanical simulator, and then the CGHAZ specimens were aged at 650 °C for 0 to 240 hours to simulate the microstructure evolution of as-welded CGHAZ during service. Microstructure change and carbide precipitation were observed by OM, SEM, EBSD and TEM + EDS. Carbide precipitation kinetics in T23 steel at 650 °C was calculated for comparison with the experiment results. The hardness change of CGHAZ during aging was detected, and the effect of microstructure evolution on hardness was analyzed. The results showed that the CGHAZ of T23 steel exhibited a mixed microstructure of martensite and bainite with high hardness in as-welded condition. After aging at 650 °C, the microstructure recovered, recrystallization occurred, the dislocation density decreased, and the lath width increased. Consequently, the hardness dropped, the drop depending on the aging time. In the early stage of aging (before 24 hours), the precipitations inside the grain were mainly M3C, M7C3 and a small number of M23C6 carbides, while the precipitation at the grain boundaries was M23C6. The precipitation of M23C6 caused the hardness to drop rapidly. When aged for 24 to 48 hours, MX precipitated inside grains extensively. The precipitation hardening produced by MX could slow down the decline of hardness. As the aging proceeded, carbide precipitated and transformed as follows: M3C → M3C + M7C3 + M23C6 → M3C + M7C3 + M23C6 + MX → M23C6 + MX + M6C. W-rich carbides precipitated in some grain boundaries of CGHAZ during aging, which may be related to the W segregation at those grain boundaries.     

Modeling of Grain Structure and Heat-Affected Zone in Laser Surface Melting Process

A combination of phase-field and cellular automata methods is used to study the effect of initial grain size and laser power density on heat-affected zone (HAZ) formation during laser surface melting. Also, an analytical model is developed to estimate the depth of HAZ as a function of initial grain size and process parameters. Both analytical and numerical results indicate that the size of HAZ, as measured with respect to the changes in the grain structure, is inversely proportional to the initial grain size. They also show how increasing the laser power leads to an increase in the extent of HAZ. The proposed models thus provide a basis for the prediction and control of HAZ in laser surface melting.     

Effect of Rare-Earth Additions on the Texture of Wrought Magnesium Alloys: The Role of Grain Boundary Segregation

Magnesium alloys that contain certain rare-earth (RE) additions are known to have improved formability and this can be partly attributed to the different texture they display after recrystallization. Previous experimental work has identified segregation of RE to grain boundaries and dislocations as being potentially important in producing this change in behavior. In the present paper, two classical models (Langmuir–McClean and Cahn–Lücke–Stüwe) are used to explore the likely effect of RE additions on grain boundary solute concentration and drag. It is demonstrated that a wide range of RE elements are predicted to segregate strongly to grain boundaries due to the large atomic size misfit with magnesium. The maximum level of segregation is produced for elements such as Y or Gd that combine a high misfit and high bulk solubility. Segregated Y is predicted to produce a solute drag pressure on migrating boundaries several orders of magnitude greater than that obtained by Al or Zn additions. It is demonstrated that while this drag is predicted to be insufficient to strongly retard static recrystallization under typical annealing conditions, it is expected to suppress dynamic recrystallization by any mechanism requiring boundary migration.     

Grain Boundary Engineering of a Low Stacking Fault Energy Ni-based Superalloy

The effects of thermo-mechanical processing parameters on the resulting microstructure of an experimental Nickel-based superalloy containing 24 wt pct Co were investigated. Hot compression tests were performed at temperatures ranging from 1293 K to 1373 K (1020 to 1100 °C) and strain rates ranging from 0.0005 to 0.1/s. The mechanically deformed samples were also subject to annealing treatments at sub-solvus 1388 K (1115 °C) and super-solvus 1413 K (1140 °C) temperatures. This investigation sought to quantify and subsequently understand the behavior and evolution of both the grain boundary structure and length fraction of Σ3 twin boundaries in the low stacking fault energy superalloy. Over the range of deformation parameters investigated, the corresponding deformation mechanism map revealed that dynamic recrystallization or dynamic recovery was dominant. These conditions largely promoted post-deformation grain refinement and the formation of annealing twins following annealing. Samples deformed at strain rates of 0.0005 and 0.001/s at 1333 K and 1373 K (1060 °C and 1100 °C) exhibited extensive grain boundary sliding/rotation associated with superplastic flow. Upon annealing, deformation conditions that resulted predominately in superplastic flow were found to provide negligible enhancement of twin boundaries and produced little to no post-deformation grain refinement.     

Use of Friction Stir Processing for Improving Heat-Affected Zone Liquation Cracking Resistance of a Cast Magnesium Alloy AZ91D

Metallurgical and Materials Transactions B - In this work, a cast magnesium alloy AZ91D was friction stir processed. Detailed microstructural studies and Gleeble hot ductility tests were conducted...     

Effects of Carbon Variation on Microstructure Evolution in Weld Heat-Affected Zone of Nb-Ti Microalloyed Steels

We investigated the effects of C concentration variation from 0.028 to 0.058 wt pct on microstructure of the coarse grained heat-affected zone (CGHAZ) of low heat input girth welded Ti-Nb microalloyed steels by using electron microscope and atom probe tomography. It is found that the CGHAZ microstructure exhibits a systematic response to C variation. Increased C raises the temperature for precipitation of NbC. This leads to coarser (Ti, Nb)N-Nb(C, N) but finer delayed strain-induced NbC in the high-C steel than in the low-C steel. Fine strain-induced NbC are ineffective in preventing austenite grain coarsening in CGHAZ due to their fast dissolution upon heating. For a given inter-particle spacing originally determined by (Ti, Nb)N particles, increased epitaxial growth of Nb(C, N) on pre-existing (Ti, Nb)N in the high-C steel results in a smaller austenite grain size of 34 µm in the CGHAZ of the high-C steel than that of 52 µm in the low-C steel. Increased C promotes a microstructure consisting of bainitic lath structure with C Cottrell atmospheres at dislocation debris and martensitic layers of 30 to 100 nm in thickness at inter-lath boundaries in the CGHAZ. Increased C promotes configuration of crystallographic variants belonging to different Bain groups in the neighbors, preferentially twin-related variant pairs within a bainite packet.

     

Formation Mechanism of Stray Grain of Nickel-Based Single-Crystal Superalloy Under a High Magnetic Field During Directional Solidification

Metallurgical and Materials Transactions B - We investigate the formation mechanism of stray grain of nickel-based single-crystal superalloy under a high magnetic field. It was shown that the high...     

Unravelling Microstructure Evolution and Grain Boundary Misorientation in Coarse-Grained Heat-Affected Zone of EH420 Shipbuilding Steel Subject to Varied Welding Heat Inputs

Microstructure evolution and grain boundary misorientation in coarse-grained heat-affected zone of EH420 shipbuilding steel have been investigated at different welding heat inputs. As heat input increases, main constituents transform from lath bainites to granular bainites, and to acicular ferrites as a result of reduced cooling rate. Corresponding fraction of high angle grain boundaries decreases from 28.24 to 20.04 pct as heat input is raised from 50 to 150 kJ/cm but reverses to 29.04 pct at 200 kJ/cm.     

Effect of Magnesium on the Austenite Grain Growth of the Heat-Affected Zone in Low-Carbon High-Strength Steels

To research the effect of Mg on the austenite grain growth of the heat-affected zone (HAZ) in low-carbon high-strength steels, two steels with different Mg contents were prepared using a laboratory vacuum. It was observed through optical microscope (OM) that the HAZ austenite grain size decreased from 385 to 86 μm after Mg treatment. In-situ observation by a confocal scanning laser microscope showed that the HAZ austenite grain size of the steel with Mg treatment could hold fine-grained structure after 1673 K (1400 °C) heating for 300 seconds, which was mainly attributed to the formation of pinning particles after the addition of Mg. Analysis showed that the number of pinning particles was much increased and the mean size was much decreased by the Mg treatment. It also can be found that more particles occurred with the MgO as the nucleating center. The HAZ toughness was improved from 33 to 185 J by refinement of austenite grains in the simulated HAZ due to the Mg treatment. The results presented in this article point to a potential method for improving HAZ toughness of low-carbon high-strength steels.     

On the Effect of Environmental Exposure on Dwell Fatigue Performance of a Fine-Grained Nickel-Based Superalloy

The influence of sulfur contamination on the corrosion-fatigue behavior of a polycrystalline superalloy used in aero-engines is considered. Samples tested under a variety of environmental conditions (including exposures to air, SO_x gas, and salt) are characterized through a suite of high-resolution characterization methods, including transmission electron microscopy (TEM), secondary ion mass spectroscopy (nanoSIMS), and atom probe tomography (APT). The primary effect of sulfur contamination is to accelerate the crack growth rate by altering the failure mechanism. The SIMS and TEM analyses indicate Cr-Ti sulfide particle formation at grain boundaries ahead of and around oxidized cracks. The APT analysis suggests that these particles then oxidize as the crack propagates and are enveloped in chromia. The chromia is surrounded by a continuous layer of alumina within the cracks. All of the sulfur detected was confined within the particles, with no elemental segregation found at grain boundaries.     

Characterization of Microstructures across the Heat-Affected Zone of the Modified 9Cr-1Mo Weld Joint to Understand Its Role in Promoting Type IV Cracking

In the postweld heat-treated (PWHT) fusion welded modified 9Cr-1Mo steel joint, a soft zone was identified at the outer edge of the heat-affected zone (HAZ) of the base metal adjacent to the deposited weld metal. Hardness and tensile tests were performed on the base metal subjected to soaking for 5 minutes at temperatures below Ac₁ to above Ac₃ and tempering at the PWHT condition. These tests indicated that the soft zone in the weld joint corresponds to the intercritical region of HAZ. Creep tests were conducted on the base metal and cross weld joint. At relatively lower stresses and higher test temperatures, the weld joint possessed lower creep rupture life than the base metal, and the difference in creep rupture life increased with the decrease in stress and increase in temperature. Preferential accumulation of creep deformation coupled with extensive creep cavitation in the intercritical region of HAZ led to the premature failure of the weld joint in the intercritical region of the HAZ, commonly known as type IV cracking. The microstructures across the HAZ of the weld joint have been characterized to understand the role of microstructure in promoting type IV cracking. Strength reduction in the intercritical HAZ of the joint resulted from the combined effects of coarsening of dislocation substructures and precipitates. Constrained deformation of the soft intercritical HAZ sandwich between relatively stronger constitutes of the joint induced creep cavitation in the soft zone resulting in premature failure.