The effect of silicon on age-hardening and creep of alloy 800 at 650°C

Fifteen casts of commercially produced Alloy 800 (Grade 2 and Alloy 800H) were creep-tested at 650°C for times up to 10 000 h. Many of these displayed a significant increase in room-temperature hardness, attributable to precipitation of γ'[Ni₃(A1,Ti)]. The creep strength (stress for 0.1% strain in 5000 h) of a particular cast was found to be approximately proportional to the hardness change after ageing for 4000–8000 h. (Ageing kinetics were such that hardness was insensitive to ageing times in this range). It was further noted that silicon, as well as titanium and aluminium, could contribute to γ' formation, so that reducing silicon from 0.6 to 0.3 wt% increased matrix solubility for (Ti + A1) from about 0.6 to 0.8 wt%. This has implications for choice of alloy specifications for resistance to creep, creep embrittlement and intergranular stresscorrosion cracking.     

Carburization and tensile behavior of alloy 800 in liquid sodium

The carbon transfer has been analyzed in the Alloy 800/sodium/stainless steel system by determining the carbon-uptake of Alloy 800 foils, which were exposed in liquid sodium of known carburizing potential. p ]Under equilibrium conditions between 650 and 550°C the measured total carbon concentrations in the Alloy 800 labs were found to be roughly related to the carbon activities of the sodium environment by the equation already stated for the $\text{18Cr-}\text{8}\text{10}$ Ni stainless steels, extrapolated to the chemical composition of the Ni-rich austenitic alloy. However the Alloy 800 was not found to undergo any decarburization in low-carbon activity environments. The carbon diffusion kinetics was determined as a function of temperature, it was found to be similar to that reported for the AISI-304 type of steel. p ]The effects of sodium exposure on microstructural and mechanical properties of several Alloy 800 heats were examined at 550°C as a function of the active carbon concentration in the sodium and of the $\text{Ti}\text{C}$ ratio in the alloys.     

Alloy 800: new stress rupture and creep data for pressurized components in high temperature reactors

In an HTR plant high temperature components of Alloy 800 materials are subjected to temperatures from about 550 to 850°C in the long term; higher temperatures may occur in the short term. Thus the creep and stress-rupture parameters govern the design of these components. Since in recent years the scope of experimental data available for Alloy 800 materials has considerably increased, a new evaluation of the creep and creep-rupture properties was performed using a data bank computer. The relationships between the characteristics of the creep and creep-rupture behaviour and the metallurgical parameters were investigated by multilinear regression analyses. On the basis of the results of these analyses and after discussion in material expert committees new material specifications were determined for different types of Alloy 800. They were included into the draft standards DIN 17459 and DIN 17460 under the material standard nos. 1.4958, 1.4958 Rk and 1.4959. Besides the new values for the 1% total plastic strain limit and the creep-rupture strength for the types of Alloy 800 under consideration, isochronous stress-strain relations were derived on the basis of creep curves of reference heats.     

Evaluation of aged Incoloy 800 alloy sensitization to intergranular corrosion by means of double loop electrochemical methods and image analysis

In this paper, the sensitization of aged Incoloy 800 alloy to intergranular corrosion has been systemically investigated by double loop electrochemical potentiokinetic reactivation (DL-EPR) technique in combination with oxalic acid etching test and microstructure observation. The DL-EPR results show that the specimens aged at 650 °C and 700 °C for 4 h were intensely sensitized with I_r:I_a value greater than 30% while there was no sensitization phenomenon for the specimens aged at 800 °C for 4 h. It was also found that the degree of sensitization increased gradually with the aging time in the range of 0-10 h at 650 °C, and I_r:I_a value reached the maximum −46% after an aging time of 10 h. However, further increasing aging time decreased the sensitization due to the healing effect incurred by the diffusion of chromium from adjacent grains to chromium-depleted zones. Comparison between two evaluating techniques (the DL-EPR and oxalic acid etching test) has also been conducted.     

Oxidation and creep failure of alloy 617 foils at high temperature

The microstructure of thermally grown oxides (TGO) and the creep properties of alloy 617 were investigated. Oxidation and creep tests were performed on 100 μm thick foils at 800-1000 °C in air environment, while the thickness of TGO was monitored in situ. According to energy dispersive X-ray (EDX) mapping micrographs observation, superficial dense oxides, chromia (Cr₂O₃), which was thermodynamically unstable at 1000 °C, and discrete internal oxides, alumina (α-Al₂O₃), were found. Consequently, the weight of the foil specimen decreased due to the spalling and volatilization of the Cr₂O₃ oxide layer after an initial weight-gaining. Secondary and tertiary creeps were observed at 800 °C, while the primary, secondary and tertiary creeps were observed at 1000 °C. Dynamic recrystallization occurred at 800 °C and 900 °C, while partial dynamic recrystallization at 1000 °C. The apparent activation energy, Q_app, for the creep deformation was 271 kJ/mol, which was independent of the applied stress.     

Uniaxial creep response of Alloy 800H in impure helium and in low oxygen potential environments for nuclear reactor applications

Studies were conducted on the creep behavior of Alloy 800H in impure helium and in a 1%CO-CO₂ environment. At relatively low applied stresses and at low temperatures, the presence of methane in helium reduced the rupture strain significantly while increasing the rupture life relative to the behavior in pure helium. The degradation in rupture strain is due to the occurrence of cleavage fracture in the He + CH₄ environment; this explanation is also supported by high activation energy (Q = 723 kJ/mol) for creep in He + CH₄. At higher applied stresses and also at higher temperatures, creep-rupture behavior in He and He + CH₄ was similar. Creep response in pure He and in CO-CO₂ follows a dislocation climb-controlled power-law behavior whereas that in He + CH₄ has a different behavior as indicated by the high stress exponent (n = 9.8). The activation energy for creep in pure He was 391 kJ/mol and in CO-CO₂ was 398 kJ/mol, and appeared to be independent of stress in both environments. On the other hand, in He + CH₄, the activation energy (Q = 723 kJ/mol) seems to be dependent on stress.     

Creep and creep rupture of a Zr-6%Sn-1%Mo alloy

Creep and creep rupture of a Zr-6%Sn-1%Mo alloy was investigated in a temperature interval 350 to 550°C using isothermal test and transmission electron microscopy techniques. The heat treatment of the alloy consisted in quenching to martensite and ageing at 550°C. The apparent activation energy of creep increases with temperature and applied stress from values lower to those significantly higher than the expected value of the activation enthalpy of the lattice diffusion to which the self-diffusion of all the three components of the solid solution (forming the matrix of the alloy) contributes. The parameter characterizing applied stress sensitivity of steady state creep rate increases with the stress from values only slightly higher than one to values close to thirty. The apparent activation energy of rupture life is close to that of creep and the stress sensitivity parameter of rupture life is close to that of steady state creep rate. The mechanisms likely to control creep rate under various external conditions are discussed. It is suggested that the mechanisms controlling the creep rate also control the rupture life.     

Estimation of crack growth in the creep range during plastic cyclic loading

Crack growth investigations were performed on the creep-resistant steel 13 CrMo 4 4 in the fatigue and the creep fatigue regime, especially regarding the influence of creep damage on crack growth. To this effect, 2% creep strain was applied to the material at a temperature of 560°C. The crack propagation rate was determined as a function of the specimen shape, temperature, test frequency and hold times. In the case of compact tension (CT-)specimens, creep pretreatment does not affect crack growth. For center-cracked tension (CCT-)specimens, however, the creep pretreatment results in a considerable increase in the crack propagation rate. Hold times of 90 minutes at maximum loading cause an increase in da/dN due to further cavity nucleation. The hold time at which cavity nucleation might occur is evaluated. The dependency on frequency of crack growth may be evaluated by means of a linear superposition of creep and fatigue crack growth. The transition frequency above which pure fatigue crack growth occurs is calculated and the regimes of fatigue, creep and creep—fatigue interaction with environmental influences are characterized.     

High temperature oxidation of cold worked and annealed alloy 800 at low oxygen pressure

The morphology of the oxide formed on Alloy 800 oxidised at a pressure of 10^?5 N/m² oxygen has been studied. Cold worked material was oxidised at temperatures between 870 and 1020 K whilst the effect of vacuum annealing after cold work has been studied at 1020 and 1070 K. The variation in surface composition was followed during oxidation by Auger electron spectroscopy. After oxidation, scanning electron microscopy, electron probe microanalysis and metallographic examinations were carried out. Two modes of oxidation were observed: above and below 1000 K. These are described in terms of diffusion of alloying elements and trace impurities. On the basis of these results an oxidation sequence involving oxide spalling is outlined.     

Numerical simulation of creep damage for low alloy steel welded joint considering as-welding residual stress

A model to calculate the welding temperature and residual stress was built using finite element code ABAQUS, and a subroutine of creep damage was also developed. Based on the coupling of welding residual stress and creep damage, the welding residual stress and creep damage of a tube made of Cr5Mo steel were simulated. This method can obtain the distributions of complex residual stress, creep damage and stress relaxation, which provide a reference for discussing the effect of residual stress on creep damage. The results show that the welding residual stress is very large at initial stage, then it is relaxed in a short time at high temperature. The distribution of creep and damage is mainly decided by the as-welding residual stress. Welding residual stress has a great effect on the creep and damage, which provides a reference for the design and life prediction of high temperature component.     

Effect of thermo-mechanical processing on microstructure and creep properties of the foils of alloy 617

The effect of rolling and annealing on the microstructure and high temperature creep properties of alloy 617 were investigated. Two types of foil specimens with different thickness reductions were prepared by thermo-mechanical processing. Recrystallization and grain growth were readily observed at specimens annealed at 950 and 1100 °C. The uniform coarse grains increase resistance against creep deformation. The grain size effect in creep deformation was dominant up to 900 °C, while dynamic recrystallization effect became dominant at 1000 °C. Dynamic recrystallization was observed in all the creep deformed foils, even though some specimens had already been (statically) recrystallized during annealing. Steady state creep rates decreased with increasing annealing temperature in the less rolled foils. The apparent activation energy Q_app for the creep deformation increased from 271 to 361 kJ/mol as the annealing temperature increased from 950 to 1100 °C.     

Evaluation of the thermal creep behaviors and microstructure of Zircaloy-4 strip

The thermal creep behaviors and the microstructure characteristics for the crept specimens of Zircaloy-4 strip were investigated under a constant load stress in the temperature range of 350-450 °C and a stress range from 110 to 230 MPa. A microstructure evaluation was carried out for the specimens before and after the creep test by using a TEM to understand the correlation between the creep mechanism and the microstructure. A variation of the crystal orientation with the creep deformation was investigated by using the electron back-scattered diffraction (EBSD) analysis. The stress exponent was in the range of 8-10 and the value of the stress exponent was varied by the applied stress. From the analysis of the stress exponent and a TEM microstructural observation of the crept specimens, the creep of Zircaloy-4 strip at a tested condition was mainly controlled by dislocations. From the EBSD results, it was observed that the crystal orientation in the crept sample strained about 10% was not changed as compared to the as-received sample.     

Effect of magnesium on the lead induced corrosion and SCC of alloy 800 in neutral crevice solution at high temperature

Dissolved magnesium species in the feed water reduce the incidence of lead-induced stress corrosion cracking (PbSCC) of Alloy 800. The passivity of material was improved by replacing a part of chlorides in the lead-contaminated chemistry with magnesium chloride, as indicated by: (1) a higher pitting potential; (2) lower passive current densities; (3) a film structure with less defects and more spinel oxides. According to the constant extension rate tensile (CERT) tests conducted in the neutral crevice solutions at 300 °C, lead contamination would reduce the ultimate tensile strength (UTS) and elongation of material. The CERT test results were in agreement with the fracture morphology observations. Magnesium addition significantly reduced the detrimental effect of lead contamination.     

The effect of grain boundary engineering on the oxidation behavior of INCOLOY alloy 800H in supercritical water

Grain boundary engineering (GBE) was applied to INCOLOY alloy 800H by means of thermomechanical processing. The oxidation behavior of GBE-treated alloy 800H exposed in supercritical water (SCW) with 25 ppb dissolved oxygen at 500 °C and 25 MPa was significantly improved as compared to 800H in the annealed condition. Gravimetry, optical microscopy, scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD) were employed in this study to analyze the oxidation behavior of control (annealed) and GBE-treated samples. GBE improves the protective oxidation behavior by enhancing spallation resistance and reducing oxidation rate. Spallation resistance correlates with a reduction in texture of the oxide layers.     

Life prediction techniques for combined creep and fatigue

This paper contains a review of the techniques that are currently available for predicting the life of metallic materials when they fail at high temperatures under the combined action of creep and fatigue. The work concentrates on those theories that are relevant to engineering design without specifically employing fracture mechanics concepts. As a consequence this generally limits the prerequisite data to that of low cycle fatigue under high rate strain cycling conditions and monotonic creep at a given temperature. Some techniques require additional or alternative data on tensile ductility, high cycle fatigue, cyclic plasticity and creep.Each technique is appraised from the results of laboratory experiments. It is shown that life may be predicted quite reliably in one instance but not in another. Some attempt is made to provide explanations and recommendations where these anomalies occur. The range of application is also clarified. Many approaches are limited to strain controlled cycling under uniaxial stress conditions. Only a limited number may further be applied to predict life for cycles in which ratcheting occurs. At present it appears that very few techniques are available to predict the creep-fatigue life of materials under high temperature multiaxial stress states. under high temperature multiaxial stress states.     

A solution procedure for thermo-elastic-plastic and creep problems

An effective solution procedure for finite element thermo-elastic-plastic and creep analysis with temperature-dependent material properties is presented. The material model employed is summarized, the basic iterative equations are developed and the solution procedure is theoretically analyzed and numerically tested for its stability and accuracy properties.     

Effect of hydrogen on the creep behavior of Zr-2.5%Nb alloy at 723 K

Increased plasticity is reported in Ti alloys, stainless steels and Pd by the introduction of hydrogen. The dissolved hydrogen in zirconium and its alloys which have similar properties as those of titanium and its alloys, can modify the creep properties of the matrix. Hydrogen, formed during the corrosion reaction of Zr-2.5%Nb pressure tube, in a CANDU design nuclear reactor, with the coolant can ingress into the metal matrix. This absorbed hydrogen can lead to an unexpected increase in length and in diameter of the pressure tube. In order to evaluate the effect of hydrogen on the creep behavior of the pressure tube material, tensile specimens with longitudinal and transverse orientations were hydrided to 65 and 160 wt ppm and creep tested at 723 K over a stress range. The combined influence of hydrogen and specimen orientations on creep rate is evaluated, and an attempt is made to explain the results using the deformation mechanisms reported in literature.