High Temperature Creep Rupture Research Articles

Effect of Base Metal Misorientation on Single-Crystal Superalloy Transient Liquid-Phase Bonded Joints

In this paper, experiments and theoretical analysis were conducted to elucidate the internal relation between base metal misorientation and the characteristics of DD5 single-crystal superalloy TLP joints. The alloy was bonded at 1513 K for 10 h in vacuum environment. High-temperature creep rupture tests were carried out to determine the joint’s quality. Results revealed that base metal misorientation exerted a remarkable influence on DD5 alloy TLP joints. The joints of base metals with [001] orientation were used in this research. When the misorientation was smaller than 9°-10°, the single crystallization of joints can be realized and joints can achieve excellent mechanical properties. When the misorientation was more than 10°, there would be second-phase particles distributed in grain boundaries, thus resulting in poor mechanical property of joint; moreover, the characteristics of joint did not change with base metal misorientation in this case.

Effects of Corrosion in Supercritical CO2 on the Microstructural Evolution in 800H Alloy

This study investigates corrosion of Fe–Ni-based alloy 800H that were exposed to supercritical CO2 (sCO2), ambient air and argon gas at pressures up to 20 MPa, at 650 and 750 °C for up to 1000 h. This alloy and other comparable metal alloys are expected to be used in sCO2 heat exchanger cycles as proposed in the DOE Advanced Ultra-Supercritical program. Alloy 800H is considered for this application, because it meets the high-temperature strength and creep rupture requirements and is a lower cost alternative to other Ni-based alloys. The oxidation performance and microstructural changes due to exposure in sCO2 have been evaluated and compared with exposures in air and Ar. The 800H alloy showed similar oxide scale thicknesses in sCO2 as in air. A recrystallized zone was observed beneath the oxide formed in air and sCO2. No such zone was observed after exposure to Ar, suggesting this recrystallization was associated with the oxidation process and not simply an effect of surface finishing. A wider recrystallized zone was observed underneath the oxide formed in sCO2 than in air. The effect of air and sCO2 on internal oxidation and carburization was investigated as well, showing that air led to more internal oxidation but less internal carburization than sCO2. It was concluded that the carbon species provided by the sCO2 atmosphere in conjunction with the increased grain boundary density in the recrystallized zone allowed for more ingress of carbon into the base metal, which resulted in a higher densities of carbides beneath the oxide scale.

Effect of film-hole configuration on creep rupture behavior of a second generation nickel-based single crystal superalloys

Thin-walled specimens with different film-hole configurations were used to model a second generation nickel-based single crystal superalloy. The effect of film-hole configuration on the high temperature creep rupture behavior of the superalloys was experimentally studied at 980 °C/300MPa. A creep numerical model based on the crystal plasticity theory was built and implemented into a finite element procedure. A new factor was introduced in this paper to quantitatively characterize the stress multiaxiality. Experimental results show that one- and two-row specimens exhibit longer lives than those without film-holes. However, the creep rupture life decreases with the increase of film-hole rows. Stress distribution of different types of specimens was obtained by finite element analysis (FEA). Numerical results reveal that the existence of film-holes causes stress concentration and changes local stress from uniaxial to multi-axial. A variety of high temperature oxidation products were observed on the surface of the specimens, which could be a detrimental factor contributing to creep rupture. Numerical results were consistent with the fracture position and morphology of specimens of the experiment.

Effect of Holding Time on Microstructure and Properties of Transient Liquid-Phase-Bonded Joints of a Single Crystal Alloy

Experimental investigations have been done to verify the effects of hold time during transient liquid-phase bonding on joint microstructure and mechanical properties of a nickel-based single crystal superalloy. The superalloy was bonded at 1473-1513 K for 0.25-12 h in vacuum environment. A set of parameters, 1513 K for 10 h, was determined as the optimum bonding condition. SEM results revealed that the joint without the completion of isothermal solidification is comprised of four different distinct regions, namely, rapid solidification zone (RSZ), isothermal solidification zone (ISZ), diffusion zone, and base metal. EBSD data indicated that the ISZ across the centerline of the bond has an undifferentiated crystallographic orientation being the same as the base metal. At increasing hold times at 1513 K, RSZ and also borides would disappear and result in an improvement of mechanical properties. Mechanical property tests at elevated temperatures have been done to determine the joints’ quality. High-temperature creep rupture strength (for 100 h at 1373 K) and tensile strength (at 1273 K) of the joints could both attain 90% of those of the base metal.

Microstructure and Property of N5 Single Crystal Superalloy Joints Bonded by Transient Liquid Phase Diffusion Bonding

N5 single crystal was transient liquid phase (TLP) diffusion bonded under different conditions in vacuum environment using Ni-base insert metal. Scanning electron microscopy (SEM) was used to observe the microstructure of the joints. Results show that the typical TLP joint could be divided into rapidly solidified zone (RSZ), isothermally solidified zone (ISZ) and diffusion zone (DZ). The widths of RSZ decreases with the increase of holding time at any established bonding temperature, amount of bulk compounds decrease at the same time. Bonding temperature has a complex influence on TLP joint. High temperature creep rupture tests were performed, results reveal that the sample fractured in the joint interface and the fracture mechanism has been studied.

INFLUENCE OF Pt ON THE CREEP RUPTURE PROPERTIES OF Ni-BASED SINGLE CRYSTAL SUPERALLOY

The Ni- based single crystal superalloys are considered to be the major materials for advanced areo- engine blades. In order to improve the high temperature properties of Ni- based single crystal superalloys,many refractory elements are introduced into this kind of alloys. Recently Pt has been suggested to be the alloying elements of advanced Ni-based single crystal superalloys. However, there are no researches for the effects of Pt on creep rupture properties of advanced single crystal superalloys. In this work, the influence of Pt element on the creep rupture properties of a Re-containing single crystal superalloy was investigated. The high-temperature creep rupture properties of the Pt-containing Ni-based single crystal superalloy at 1100 ℃, 180 MPa and 1000 ℃, 310 MPa were investigated. The deformation microstructure and the morphology of dislocations were studied by SEM and TEM.The results show that the creep rupture life of Pt-containing superalloy decrease slightly at 1100 ℃, 180 MPa and decreased obviously at 1000 ℃, 310 MPa. The fracture models of different alloys are all ductile fracture, and many irregular microviods and microcracks can be observed in the fracture surfaces. After high temperature creep deformation, regular dislocation networks formed at the g/g' interfaces. The differences of creep rupture properties among those alloys are that Pt element may promote the formation of TCP phase, and the interface between the TCP phase and g matrix may be favorite sites of the initiation of microvoids and microcracks.

Microstructure Stability of V and Ta Microalloyed 12%Cr Reduced Activation Ferrite/Martensite Steel during Long-term Aging at 650 °C

In view of developing novel alloys for applications in supercritical water-cooled reactor fuel cladding and in-core components, a 12%Cr reduced activation ferrite/martensite (RAFM) steel with good corrosion resistance and irradiation performance was developed. V and Ta were added to form fine MX type carbonitrides and enhance the high temperature creep rupture strength. Microstructure stability of the steel during long-term aging at 650 °C was studied experimentally combined with the simulation of Thermo-Calc and DICTRA software. The results show that the precipitates in the steel during long-term aging contain M23C6, MX and Laves phase. M23C6 carbides play a major role in the stabilization of the tempered martensite lath structure by exerting a large Zener pinning force as compared with MX and Laves phase. Adding V and Ta in the steel can not only promote MX precipitation, but also refine M23C6 carbides and thus improve the thermal stability of lath/subgrains, which is beneficial to the improvement of high temperature microstructure stability of the 12%Cr RAFM steel.

Recrystallization Process in Fe-Cr-Al Oxide Dispersion-Strengthened Alloy: Microstructural Evolution and Recrystallization Mechanism

Mechanically alloyed iron-base oxide dispersion-strengthened (ODS) alloys are the class of advanced materials for application in heat exchangers tubing in which creep and oxidation resistance are paramount. The yttria dispersion in such alloys improves the high-temperature creep and stress rupture life. The strength is further enhanced by the development of a coarse-grained microstructure during recrystallization. Factors controlling the evolution of this desirable microstructure are explored in this work, focusing specifically on PM 2000. The results presented in terms of orientation imaging, transmission electron microscopy, and scanning electron microscopy indicate that the recrystallization process consists of two different stages. Before the coarse grain takes place, the alloy undergoes an extended recovery process followed by abnormal grain growth. The initial microstructure consisted of subgrains (submicrometer sizes) with a strong 〈110〉∥RD fiber texture (α fiber), which are transformed into coarse grains (mm sizes) with orientations 〈112〉∥RD. The aim of this study is to describe the mechanisms involved in the intermediate stages of recrystallization process from the submicrometer grain size to the abnormal grain size.

High temperature tensile test and creep rupture strength prediction of T92/Super304H dissimilar steel weld joints

After short term tensile test at 823–923 K, microstructure, fractography and fracture mechanism of the T92/Super304H dissimilar steel weld joints were studied. In the high temperature tensile process, the fracture location of all joints is in the T92 base metal; however, the fracture mechanism of these joints changes from a normal mode to a shear mode with the increase of the temperature. Moreover, an equation of the creep rupture strength related to the short term tensile strength of the joints was firstly derived in terms of the Goldenberg model. And then the creep rupture strength of the joints was obtained, = 42·7 MPa.

Study of Creep Strength for P91 Steels after Short Term Overheating above Ac<sub>3</sub> Temperature

Advanced ferritic steels containing 9 wt% Cr are widely used in the construction of supercritical and ultra supercritical boiler components. The microstructure of the as supplied 91 materials consists of a tempered martensite matrix, a fine dispersion of intergranular chromium rich M23C6 precipitates and intragranular carbonitrides MX particles rich in V and Nb. This steel requires post weld heat treatment (PWHT) to produce a tempered microstructure after welding to develop excellent creep strength for high temperature service. Based on past experience, situations may arise whereby the components are subjected to an accidental overshoot in temperature during PWHT. The short excursion to high temperature beyond Ac3 would have resulted in the formation of deleterious phases, for example, soft α-ferrite which has poor creep strength and hard martensite which has a low toughness. In this study, the degraded specimens with soft α ferrite as a result of cooling transformation from 900°C are proven to have a limited creep rupture life where the creep rupture strength dropped remarkably after 1000 hours. As the peak temperature increased to 950°C and 1000°C, hard and brittle martensite was formed on cooling. The creep specimens were found to exhibit better creep strength; most probably the creep behavior was improved by the tempering effect at 600°C during creep tests. Nevertheless, despite the tempering which might have improved the toughness slightly, the high temperature creep rupture stress still had dropped approximately 40%, as compared to the virgin alloys in the range of rupture time from 1,000 hours to 10,000 hours.

High-temperature short-term tensile test and creep rupture strength prediction of the T92/TP347H dissimilar steel weld joints

After short-term tensile test at 848–923K, microstructures, fractographies and fractural mechanisms of the T92/TP347H dissimilar steel weld joints were studied. An equation of the creep rupture strength related to the short-term tensile strength of the joints was firstly derived based on the Goldenberg model [1]. In the high-temperature tensile process, the fracture location of the joints is all in the T92 side fine grained heat affected zone (FGHAZ); however, the fracture mechanism of the joints is changed from a mixed mode (normal plus shear) to a shear mode as the testing temperature risen, which was explained in terms of the stress tri-axiality theory. Moreover, an equation of the creep rupture strength of the weld joints was derived, and then the creep rupture strength of the joints was doped out, σ105873=49.0MPa. It indicates that the joints have a high reliability when used in the USC power units.

Application of kinetic theory to the analysis of high-temperature creep rupture of metals under complex stress (review)

This paper gives an analytical review of the results obtained using the kinetic theory of creep and creep rupture to analyze the creep rupture of metals under complex stress. Special note is made of the outstanding contribution of Soviet scientists L. M. Kachanov and Yu. N. Rabotnov, who introduced the concept of material damage and developed the fundamentals of the kinetic theory. Different versions of this theory are used in studies of Russian and foreign scientists. The possibility of applying the kinetic theory to model the deformation and fracture of metals under creep conditions using scalar, vector, and tensor damage parameters and their combinations is considered.

The problem of preparation of creep rupture strength data during the formulation of regulatory documents for metal-intensive power equipment

The paper addresses the problem of compiling and updating the high-temperature creep rupture strength data for steels and alloys used in heat-and-power engineering, and various aspects of tackling the problem are discussed. Special attention is given to the necessity of processing the available creep rupture strength data by the base diagram method, the generalized and detailed representation of such data for the widest possible temperature–time ranges, and subsequent system analysis.

Role of strain heterogeneity on recrystallisation of oxide dispersion strengthened Fe–Cr–Al alloys for high-temperature applications

Mechanically alloyed iron-based oxide dispersion-strengthened alloys have the potential for application in heat exchangers for biomass power plants, with operating temperature and pressure at entry of the gas turbine working fluid being around 1,100 °C and 15–30 bar, respectively. The yttria dispersion in such alloys improves the high-temperature creep and stress rupture life. The strength is further enhanced by the development of a coarse-grained microstructure during recrystallization. Factors controlling the evolution of this desirable microstructure are explored in this study, focusing specifically on PM 2000. The results can be interpreted if it is assumed that anything which makes the microstructure heterogeneous stimulates recrystallization. The combination of these results with finite element modelling is used to interpret the role of strain heterogeneity on the development of recrystallized grain structure. In this sense, larger strain gradients lead to more refined and more isotropic grain structures.

Effect of ruthenium on high-temperature creep rupture life of a single crystal nickel-based superalloy

► Ru improves the high-temperature creep rupture lives. ► Discussion involves the dislocation movement and γ′ phase evolution. ► The change of γ/γ′ lattice misfit is considered as the key role of Ru. ► The influence of TCP phases on the creep rupture lives could be negligible. The addition of 3 wt% ruthenium (Ru) has been found to improve the creep rupture lives of a single crystal Ni-based superalloy under both conditions of 1100 °C/150 MPa and 1000 °C/310 MPa. Creep curve analysis indicates that the creep mechanisms are different from each condition. The improvement of creep rupture lives by 3 wt% Ru addition is discussed not only from the view of dislocation movement but also the γ′ phase evolution. The change of γ/γ′ lattice misfit in the initial microstructure is believed to be the key role of Ru on the high-temperature creep deformation. The larger negative lattice misfit caused by an addition of 3 wt% Ru induces smaller and more regular γ′ particles in the initial state, as well as denser dislocation networks at the γ/γ′ interface during creep. These two aspects are crucial to the high-temperature creep rupture life. In addition, a little topologically close-packed (TCP) phases are observed after creep rupture in the two experimental alloys.

Degradation Characteristics of Inconel738LC with Thermal Exposure

Gas turbine components operated by hot combustion gas undergo material degradation due to the thermal cycle by daily startup and shutdown. The failure mechanism of the hot gas components is accompanied by degradation in the properties of high temperature strength and creep rupture time. Many hot gas components in gas turbines are made of Ni-based superalloy because of their high temperature performance. In this work, we survey the time and temperature dependent degradation of Ni-based superalloy. We prepared specimens from Inconel738LC that were then exposed at 871~982°C in 1,000~5,000hours. We carried out stress-rupture tests and microstructural investigation.

Effects of High Temperature Deformation and Thermal Exposure on Microstructure and Mechanical Properties of Co-Based Superalloy

The gas turbine components operated in hot combustion gas undergo material degradation due to the thermal cycles. The failure mechanism of the hot gas components would be accompanied by material degradation in the properties of high temperature strength and creep rupture time. Many stationary parts in gas turbine are made of Co-based superalloy because of their high temperature performance. In this work, we survey the time and temperature dependent degradation of Co-based superalloy. We prepared the specimens of Co-based superalloy ECY768 that are exposed at 871 and 982 oC in 1000 ~ 5000 hours. We carried out the mechanical test and microstructural observation.

Effects of High Temperature Exposure on the Microstructure and Properties of ATI 718Plus®

Alloy ATI 718Plus® is a precipitation hardened nickel-based alloy with excellent high temperature tensile and creep rupture strength. The aim of the work described here was to investigate the hardness of the alloy with long term exposure at temperature (650°C, 700°C and 750°C) and to correlate those results with the evolution in the microstructure. At 650°C, the hardness value remains relatively constant through 6,000 hours exposure, whereas at 700°C it remains level for the first 1,000 hours or so but then starts to decline. At 750°C, the hardness decreases rapidly as soon as exposure at high temperature starts. The changes in hardness are related to the overaging of the γ’ phase and the precipitation of d phase resulting from the thermal exposure.

Effect of Withdrawal Rates on Microstructure and Creep Strength of a Single Crystal Superalloy Processed by LMC

A nickel base single crystal (SC) superalloy was directionally solidified using liquid metal cooling (LMC) process at various withdrawal rates. The microstructure was refined as increasing the withdrawal rate from 3 to 12 mm/min. However, higher withdrawal rate of 15 mm/min induced the formation of stray grains. Size and volume fraction of the eutectics were found to decrease with the increasing in withdrawal rate. After solution heat treatment at 1250 degrees C, un-dissolved eutectic was observed in specimens. High temperature creep rupture life was observed to be very sensitive to the fraction of these remaining eutectics. Creep rupture tests at 1000 degrees C/235 MPa showed that refined microstructure and low fraction of the remaining eutectic lead to significant improvement of the rupture life.

Failure analysis of J85 Engine turbine blades

This paper analyzes the cause of the engine damage due to the fracture of the J85 Engine first stage turbine blades. Discoloration due to thermal effect and fine multi-cracks as well as necking at 1/3 of the way down from the tip on the entire first stage turbine blade concave side surface have been noted. Of the first stage blades, nine had fractures 1/3 of the way down from the tip, and at the fractured location more severe necking has been found than blades without fracture. As a result of observing the fine texture of fractured blades and the blades where deformation has occurred, wedge-type intergranular cracks and TCP phase as well as carbide phase have been identified, and γ′ deformation and depletion found around the fracture surface. There were no traces of thermal damage to the first stage vane, no IOD phenomenon has been noted. Analysis of turbine blade deformation and fracture profile and microstructural deformation phenomenon determined that blade fracture was due to high temperature creep rupture.