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.     

Recrystallization in a directionally solidified cobalt-base superalloy

The recrystallization behavior in a range of annealing temperature from 1020 to 1280 °C in a directionally solidified cobalt-base superalloy was studied. Local recrystallization first appeared at 1040 °C. The recrystallized volume increased rapidly as increasing the annealing temperature. Pinning effect of all carbides (M₂₃C₆, M₇C₃ and MC) was observed and large amount of twin formed at low annealing temperature. The size of the recrystallized grains increased significantly at high annealing temperatures accompanied with the sharp decrease of twin. The effect of annealing temperature and the role of carbide and twin on the development of the recrystallization were discussed.     

Anisotropic viscoplasticity constitutive model for directionally solidified superalloy

Abstract

The macroscopic deformation behaviour of a Ni-based directionally solidified (DS) superalloy was experimentally investigated, and an anisotropic constitutive model of the material was developed. Monotonic and creep tests were performed on uniaxial test specimens machined from DS plates so that the angle between the loading direction and the solidified grain direction varied between 0 and 90°. Tension-torsion creep tests were also conducted to examine the anisotropic behaviour under multiaxial stress conditions. The material exhibited marked anisotropy under elastic and viscous deformation conditions, whereas it showed isotropy under plastic deformation conditions of high strain rates. Then crystal plasticity analyses were carried out to identify slip systems under creep loading conditions, assuming the anisotropic creep behaviour of the DS material. A viscoplastic constitutive model for expressing both the anisotropic elasticity-viscosity and the isotropic plasticity was proposed. The elastic constants were determined using a self-consistent approach, and viscous parameters were modelled by crystal plasticity analyses. The calculation results obtained using the constitutive model were compared with the experimental data to evaluate the validity of the model. It was demonstrated that the constitutive model could satisfactorily describe the anisotropic behaviour under uniaxial and multiaxial stress conditions with a given set of material parameters.     

定向凝固镍基合金DZ444声学特性的各向异性

选取定向凝固镍基高温合金DZ444不同方向片状试样,利用电子背散射衍射等技术表征晶体取向和微观组织,利用脉冲回波技术分析纵波声速和声衰减系数。结果表明:两声学特性呈各向异性,随着试样平面法向与凝固方向之间夹角φ由0°到45°再到90°,纵波声速由5533m/s增大到6595m/s后又降至5634m/s,而声衰减系数逐渐增大,变化约0.19dB/mm;对信号频谱分析发现,表面回波与一次底波的主频差值、主频幅值差值及表观积分反射系数均逐渐增大,这主要是由微观组织和晶体取向差异造成的。     

Cyclic overaging pre-weld heat treatment of Rene 80: effect of solution treatment and end aging temperatures

Abstract

The present work investigated the effect of a cyclic overaging pre-weld heat treatment proposed by Lim (US Patent no. 5 509 980, 1996) on the microstructure, tensile ductility, and weldability of Rene 80 nickel based superalloy (approximate composition 60Ni–14Cr–9.5Co–4Mo–5Ti–3Al– 0.17C–Zr–B, wt-%), with a focus on the role of solution treatment and end aging temperatures. The results showed that maximum ductility corresponds to a microstructure consisting of very coarse and widely spaced γ', with fine γ' not resolvable even at a magnification of × 10 000. This was achieved by cyclic cooling the material from the solution temperature to a temperature in the range 500–750 ° C followed by fast furnace cooling to suppress the precipitation of fine γ'. The solution treatment temperature was found to control the number density of coarse γ', which, in turn, slightly influenced the optimum end aging temperature. After the optimum cyclic overaging heat treatment with a solution temperature of 1080°C and an end aging temperature of 550°C, the material exhibited a tensile type fracture path with a high registered tensile ductility of ~ 30% in elongation. Fractographic studies revealed that the necked γ ligaments were thicker and taller, with an increased frequency of cleaved coarse γ' occurring at different levels in the material. Precision welding tests showed that the weldability of the optimally overaged alloy was significantly improved, owing to both the resultant soft and ductile base metal and the reduced heat affected zone size.     

Microsegregation in directionally solidified dendritic-cellular structure of superalloy CMSX-4

A directionally solidified sample of superalloy CMSX-4 was investigated to show the effect of crystal orientation on the segregation distribution. The solute distribution of alloying elements across a dendritic cell was measured. Due to the preferred crystal growth in <100> orientation the segregation profiles in this direction is much flatter than that in <110> orientation.     

Repair of directionally solidified superalloy GTD-111 by laser-engineered net shaping

GTD-111 directionally solidified superalloy was repaired by the Nd:YAG laser based laser-engineered net shaping (LENS) process. Selection of process parameters are discussed against the formation of deposits and occurrence of defects. Formation of microfissures, porosity, lack of fusion, and stray crystals in the deposit zone was studied. It was identified that formation of stray crystals in the directionally solidified alloy could be due to two types of sources: the shape of the melt pool, and the existence of second-phase particles at the melt pool fusion boundary.     

Surface treatment of DZ4 directionally solidified nickel-based superalloy by high current pulsed electron beam

Surface treatment of DZ4 directionally solidified nickel-based superalloy was carried out by using high current pulsed electron beam (HCPEB) technique. Microstructure changes in modified surface layer were characterized together with their corrosion property. It is found that the treated surface becomes rougher with increasing the number of HCPEB pulse due to the formation of craters occurring preferentially in the interdendritic areas. The thickness of remelted layer increased slightly when more HCPEB pulses applied and reached about 3 μm after 10 HCPEB pulses. The corrosion resistance of modified surfaces exhibited an effective improvement when measured in 0.5 mol/L sulfuric acid solution. The factors that influence the corrosion resistance of DZ4 alloy were discussed by considering the surface purification effect featured by HCPEB surface treatment.     

Creep deformation and rupture behaviour of directionally solidified GTD 111 superalloy

The creep behaviour of directionally solidified (DS) Ni‐base superalloy GTD 111 has been investigated at various temperatures (649 °C to 982 °C) and stresses (124 MPa to 896 MPa). Specimens machined in longitudinal and transverse directions with respect to the grain orientation from three batches of the material were tested. The specimens in the longitudinal direction consistently exhibited higher creep rupture life and creep ductility than specimens from the transverse direction. There were some systematic variations in creep deformation and rupture behaviour among specimens from different batches. Optical and scanning electron microscopy investigations were conducted to understand the creep rupture behaviour. Various deformation and rupture models were evaluated for representing the creep behaviour of the alloy and a neural network model was applied to creep rupture data to assess its predictive capability.     

Low cycle fatigue behavior of a directionally solidified cobalt base superalloy

An investigation has been made of the low cycle fatigue behavior and microstructure evolution of a directionally solidified cobalt-base superalloy at room temperature, 700 and 850°C under the control of different total strain amplitudes. The results show that at room temperature the cyclic hardening of the alloy appears during the first few cycles, and then a long saturation stage begins. At 700°C, the alloy exhibits a pronounced initial hardening, and a secondary hardening after a short saturation. At 850°C, the alloy shows a continuous cyclic hardening until fracture. Examination by TEM indicates that the initial hardening of the alloy at room temperature is caused by the pile-ups of the stacking faults at the stacking fault intersections, while the saturation is related to the formation of the hexagonal close-packed (HCP) zones and twins. The mechanism of initial hardening at 700°C is similar to that at room temperature, while the stress saturation is due to interaction obstacle to stacking-fault becoming weaker, because of thermal activation. The secondary hardening is attributed to the formation of sessile dislocation tangles. The continuous cyclic hardening at 850°C is related to the interaction between the precipitates (M₂₃C₆)and dislocations.     

Elevated temperature fatigue crack growth in directionally solidified GTD-111 superalloy

GTD‐111 is a General Electric proprietary nickel‐base superalloy widely employed in a growing fleet of industrial gas turbines. However, much of the published research to date has focused on creep damage and coating durability, with no available data on fatigue crack growth rate (FCGR). Accordingly, FCGR has been measured at several load ratios and an elevated temperature, 760 °C (1400 °F) corresponding to typical turbine bucket conditions. In order to account for load ratio effect, several different methods for determining Walker model parameters are discussed in terms of methodological consistency and accuracy.     

Fluctuation of J-integral due to grain arrangement of Ni-based directionally solidified superalloy

The effect of local anisotropy due to the arrangement of anisotropic and arbitrary shaped grains of a directionally solidified superalloy on the J-integral is investigated by means of finite element analyses, where the models simulate the grain shape, its orientation, and the crack path of real material. The magnitudes of J_aniso, obtained by a detailed model, where most of grains are simulated, shows a stepwise change against the crack length a. This is mainly caused by the anisotropy of the grain at the crack tip. J_CV, obtained by a simplified model, where only the grains in the vicinity of the crack path are simulated, shows the same stepwise change as J_aniso. It elucidates that the anisotropy in the vicinity of the crack path is the dominant factor for the fluctuation of J_aniso. The magnitude of J_aniso and J_CV, however, are different from each other in detail. A large inhomogeneous area such as a cluster of grains and a large grain has a far-field influence and is mainly responsible for the differences between J_aniso and J_CV.     

On preventing HAZ cracking in laser welded DS Rene 80 superalloy

Abstract

The heat affected zone (HAZ) cracking behaviour in a laser beam directionally solidified (DS) Rene 80 nickel based superalloy subjected to preweld heat treatments was studied. The HAZ cracks in the alloy are grain boundary liquation cracks caused by liquation reaction of both non-equilibrium secondary solidification product, MC carbides and equilibrium solid state reaction product, γ′ precipitates. In contrast to theoretical prediction based a preweld heat treatment that reduced grain boundary liquid film thickness did not result in a lower HAZ cracking, which can be related to concomitant reduction in the ability of the base alloy to relax welding stress. In addition, formation of intergranular M₅B₃ boride particles in preweld alloy appeared to have aided cracking susceptibility by lowering grain boundary liquation temperature and widening the brittle temperature range in the HAZ during cooling. Based on the analysis of the results, application of a new preweld heat treatment that prevents the formation of the intergranular borides and induces moderate base alloy hardness resulted in a nearly crack free HAZ in laser welded DS Rene 80 superalloy.     

Correlation between ultrasonic velocity and solutionising time in Rene 80 superalloy

Abstract

Ultrasonic measurements have been used to characterise the solutionising phenomenon in Rene 80, a Ni based superalloy. Starting material was solutionised at 1204°C for 30–120 min on identical samples. The microstructures of these samples were evaluated by ultrasonic immersion technique, X-ray diffraction and scanning electron microscopy. It was observed that the area fractions and, consequently, the γ′ volume fractions were decreased by increasing the solutionising time. A direct correlation was observed between ultrasonic wave velocity and solutionising time at 1204°C. The ultrasonic compression wave velocity followed a descending pattern similar to that of γ′ area fraction with the increase in solutionising time. The observed variation in ultrasonic velocity has been attributed to the effect of the γ′ dissolution on the elastic constants of the material.     

Effect of pre-exposure on crack initiation life of a directionally solidified Ni-base superalloy

Blades and vanes are just two of several industrial gas turbine (IGT) components often subjected to long periods of elevated temperature before, during, and after high stress operating conditions. In these systems, cyclic loading is induced by repeated start-ups, firings, and shut-down ramps. Combinations of complex thermal and mechanical service conditions in the presence of aggressive reactants facilitate crack initiation via oxide spike formation. In the current study, the effect of pre-exposure on the oxide spiking damage mechanism and crack initiation life is characterized for a representative directionally solidified (DS) Ni-base superalloy, e.g. DS GTD-111. Comparisons of unexposed and pre-exposed samples reveal that 100 h of either creep pre-strain and/or thermal pre-exposure strongly influences the dominant damage mechanism that leads to crack initiation under subsequent fatigue cycling. A mechanistic model for crack initiation is modified to capture the influence of pre-exposure on life.     

Mechanical property anisotropy in superalloy El-929 directionally solidified by an exothermic technique

Directional solidification of the nickel-based superalloy El-929 was carried out by employing the exothermic technique for preparing several 150mm long × 55mm diameter rods. Specimens machined from the blanks cut at 0°, 45°, 75° and 90° to the chill surface were tensile and stress-rupture tested at different temperatures. The air-melted DS alloy, when loaded parallel to the growth direction, shows considerable improvement in stress-rupture life and tensile ductility as compared with the vacuum induction melted, forged and heat-treated alloy. However, these property advantages rapidly degrade with the increasing deviation of the load axis from the growth direction.     

Influence of high-temperature air pre-exposure on mechanical strength of a directionally solidified cobalt-base superalloy

The present work investigated the influence of air pre-exposure at 850°C for 1000 h on the mechanical properties and microstructures of the directionally solidified cobalt-base alloy DZ40M. The results show that the air pre-exposure did not embrittle the alloy, but resulted in a marked strengthening. Microstructural observations and energy dispersive X-ray spectroscopy (EDS) indicates that the air pre-exposure caused a substantial external and internal oxidation, which is assumed to protect the alloy from being oxidized further and make a significant contribution to alloy strengthening, respectively. The immunity of the alloy to air embrittlement is attributed to elimination of transverse grain boundaries by directional solidification.     

Effect of notch on fatigue behaviour of a directionally solidified superalloy at high temperature

The effect of notch types and stress concentration factors (K_t) on low cycle fatigue life and cracking of the DZ125 directionally solidified superalloy has been experimentally investigated. Single‐edge notched specimens with V and U type geometries were tested at 850 °C with stress ratio R = 0.1. High temperature in situ optical method was used to observe crack initiation and short crack propagation. Scanning electron microscope observation of fracture was used to analyse the failure mechanism. The results reveal that fatigue resistance decreases with K_t increasing from 1.76 to 4.35. The ratcheting is found to be affected by both K_t and the nominal stress from the displacement–force curve. In situ observations indicate that the cracking does not occur at the notch apex but at the location where the max principal stress or Hill's stress is the highest. According to the scanning electron microscope observations, the failure of the notched specimens strongly depends on the anisotropy microstructures.     

Effect of microstructure on crack propagation in high-temperature fatigue of directionally solidified Ni-based superalloy

In order to clarify the crack propagation properties of an anisotropic material (Ni‐based directionally solidified superalloy), longitudinally loaded specimens (L‐specimens) and transversely loaded specimens (T‐specimens) with a crack are subjected to high temperature fatigue. The crack propagation rate is reasonably well correlated with the effective stress intensity factor range regardless of the propagation direction (specimens L and T), the stress range and the stress ratio. However, the crack propagation rate shows a notable fluctuation particularly in the T‐specimens. It is at most about five times faster than the average. The fracture surface features can be classified into four types with three transgranular and one intergranular types. In the former, though the crack is along the {100} or {110} planes on a macroscopic scale, it threads through the {111} or {100} planes on a microscopic scale. Crack propagation is notably accelerated in the intergranular region, while deceleration is caused by crack branching.