Heat-to-heat variation in long-term creep strength of some ferritic steels

Heat-to-heat variation in creep life has been investigated for some ferritic steels, mainly 12Cr steels, using long-term creep data in the NIMS Creep Data Sheets. The tempered martensitic plain 12Cr and 12Cr–1Mo–1W–0.3V steels exhibit large heat-to-heat variation in creep life, as shown by about one order of magnitude difference in time to rupture or more between the strongest and weakest heats. On the other hand, low-Cr steels of tempered bainitic 1Cr–1Mo–0.25V, ferritic–pearlitic 2.25Cr–1Mo and ferritic 9Cr–1Mo steels exhibit small heat-to-heat variation in creep life. The heat-to-heat variation in long-term creep strength is correlated with the degradation behaviour at long times, which depends on initial strength and concentrations of Al, nitrogen and Cr. The present results suggest that taking the mechanisms responsible for the heat-to-heat variation in creep life into account, quality of heat resistant steels as well as reliability of remaining life estimation can be further improved.     

Creep strength of high chromium steel with ferrite matrix

Long-term creep strength of material in the low-stress regime below elastic limit is difficult to predict by an extrapolation of short-term creep strength in the high-stress regime above elastic limit. Long-term creep strength of fully annealed ferrite-pearlite microstructure of low alloy Cr–Mo steel is higher than that of martensite and bainite microstructures. It is explained by lower dislocation density of fully annealed microstructure. According to the above concept, creep strength of high chromium steel with ferrite matrix is investigated. Creep rupture life of 15Cr–Mo–W–Co steel with ferrite matrix which is longer than that of ASME Grade 92 steel is obtained at 650 °C by controlling the chemical composition and heat treatment condition.     

Hydrogen environment embrittlement of stable austenitic steels

Seven stable austenitic steels (stable with respect to γ → α′ transformation at room temperature) of different alloy compositions (18Cr–12.5Ni, 18Cr–35Ni, 18Cr–8Ni–6Mn–0.25N, 0.6C–23Mn, 1.3C–12Mn, 1C–31Mn–9Al, 18Cr–19Mn–0.8N) were tensile tested in high-pressure hydrogen atmosphere to assess the role of austenite stability on hydrogen environment embrittlement (HEE). The influence of hydrogen on tensile ductility was small in steels that are believed to have a high initial portion of dislocation cross slip (18Cr–12.5Ni, 18Cr–35Ni, 18Cr–8Ni–6Mn–0.25N), while the effects of hydrogen were significantly greater in steels with other primary deformation modes (planar slip in 18Cr–19Mn–0.8N and 1C–31Mn–9Al or mechanical twinning in 0.6C–23Mn and 1.3C–12Mn) despite comparable austenite stability at the given test conditions. It appears that initial deformation mode is one important parameter controlling susceptibility to HEE and that martensitic transformation is not a sufficient explanation for HEE of austenitic steels.     

Creep damage evaluation of 9Cr–1Mo–V–Nb steel welded joints showing Type IV fracture

By conducting long-term creep rupture tests for 9Cr–1Mo–V–Nb (P91) steel welded joints, creep rupture properties and microstructures were examined. Creep rupture tests were conducted at three temperatures of 823, 873 and 923 K, under applied stresses of 160–230, 80–130, and 40–80 MPa, respectively. The rupture locations were found to shift from the weld metal at the higher stress condition to the fine-grained HAZ adjacent to the base metal at lower stress conditions at 873 and 923 K. The relationship between microstructural changes and crack nucleation site and propagation path was clarified. A remarkable decrease of dislocation density and growth of precipitates of M₂₃C₆ and Laves phase during creep was observed in the vicinity of the fine-grained HAZ adjacent to the base metal for the Type IV fractured welded joint specimen. The stress–strain distribution in the welded joint was investigated by the finite element method (FEM) using creep data of the simulated HAZ specimen. It was found that the observed crack initiation site and crack growth path coincided better with the distribution of the stress triaxiality factor than that of the equivalent creep strain.     

Evaluation of creep damage in heat affected zone of thick welded joint for Mod.9Cr–1Mo steel

Mod.9Cr–1Mo steel has been used for boiler components in ultra-supercritical (USC) thermal power plants. The creep strength of welded joint of this steel decreases due to the formation of Type IV cracking in heat affected zone (HAZ) at higher temperatures. The present paper aims to clarify the damage processes and mechanisms of the welded joint for Mod.9Cr–1Mo steel. Long-term creep tests of base metal, welded joint and simulated fine- grained HAZ were conducted at 550, 600 and 650 °C. Creep tests using thick plate welded joint specimen were interrupted at several time steps, and evolutions and distributions of creep damages were measured quantitatively using laser microscope. It is found that creep voids initiate at early stage of creep life (0.2 of life), the number of creep voids increases until 0.7 of life, and then voids coalesced into the macro crack at the later stage of life (0.8 of life). Creep damages concentrate mostly at a quarter depths of the plate thickness within the fine-grained HAZ of the present welded joint. The experimental creep damage distributions were compared with the computed results by using the FEM analysis. Both creep strain concentration and high stress triaxiality in fine-grained HAZ of welded joint are considered to accelerate the creep void formation and growth.     

Investigation of ratcheting characteristics of modified 9Cr–1Mo steel by using the Chaboche constitutive model

In this paper, an inelastic constitutive equation of the Chaboche model, which can simulate ratcheting behaviour at high temperatures, is investigated for modified 9Cr–1Mo steel. The plastic modulus, which primarily governs the calculation scheme for plasticity, is derived for the Chaboche model and implemented into a computer program with the radial return algorithm to compute the cyclic backstress components. With the cyclic test data of modified 9Cr–1Mo steel, the Chaboche three-decomposed constitutive parameters of the kinematic hardening rule and the isotropic softening rule, which controls the cyclic softening characteristic of modified 9Cr–1Mo steel, are identified by a simple method. By using the identified constitutive parameters, the ratcheting behaviour of modified 9Cr–1Mo steel is simulated to investigate the effects of mean stress levels at a certain stress amplitude and ratcheting parameter's values. In particular, the progressive deformation instability phenomena due to the cyclic softening characteristics of modified 9Cr–1Mo steel are investigated.     

Extrapolation of creep life data for 1Cr–0.5Mo steel

A new approach to analysis of stress rupture data allows rationalization, extrapolation and interpretation of multi-batch creep life measurements reported for ferritic 1Cr–0.5Mo tube steel. Specifically, after normalizing the applied stress through the appropriate UTS value, the property sets at various creep temperatures are superimposed onto a sigmoidal ‘master curve’ using the activation energy for lattice diffusion in the alloy steel matrix (300 kJ mol⁻¹). Despite the considerable batch-to-batch scatter, results from tests lasting less than 30,000 h then allow straightforward prediction of creep lives for stress–temperature conditions causing failure in times up to 150,000 h.     

Mechanical behaviour of HTR materials: Developments in support of defect assessment,structural integrity and lifetime evaluation

Mod 9Cr–1Mo steel (T91) is a candidate material for pressure vessels and for some internal structures of GCR (Gas Cooled Reactors). In order to validate this choice, it is necessary, firstly to verify that it is able to withstand the planned environmental and operating conditions, and secondly to check if it is covered by the existing design codes, concerning its procurement, fabrication, welding, examination methods and mechanical design rules. A large R&D program on mod 9Cr–1Mo steel has been undertaken at CEA in order to characterize the behaviour of this material and of its welded junctions. In this program, the role of the Laboratory for structural Integrity and Standards (LISN) is to develop high temperature defect assessment procedures under fatigue and creep loadings. Concerning the GCR, complementary studies are conducted in order to validate the existing methods (developed for the fast reactors) and to get new experimental data on Mod 9Cr–1Mo steel. Moreover, if the geometry and the loadings of a standard CT specimen allow performing a 2D analysis, the case of industrial loadings appears much more complicated, notably because of surface defects which propagate and present shapes that can be considered as half ellipse. Therefore, in the frame of the defect assessment methods validation, the LISN undertakes both standard tests on CT specimens to determine the propagation laws and bending tests on large plates under high temperature fatigue and creep loadings. These components present an initial semi-elliptical surface notch normal to the loading direction and its initiation and propagation are studied.     

A creep life assessment method for boiler pipes using small punch creep test

The small punch creep (SPC) test is considered as a highly useful method for creep life assessment for high temperature plant components. SPC uses miniature-sized specimens and does not cause any serious sampling damages, and its assessment accuracy is at a high level. However, in applying the SPC test to the residual creep life assessment of the boiler in service, there are some issues to be studied. In order to apply SPC test to the residual creep life assessment of the 2.25Cr–1Mo steel boiler pipe, the relationship between uniaxial creep stress and the SPC test load has been studied.     

Change in contribution of block boundary to macroscopic strength of 10Cr–1Mo–1W–VNbN steel due to creep

Outstanding strength performance of high Cr ferritic steels is attributable to the combined strengthening mechanisms of matrix and various grain boundaries. However, it is by no means easy to separate the contributions of such strengthening factors and quantitatively understand them because of extremely fine and complicated microstructures. In this study, the instrumented indentation test was carried out to clarify the change in contribution of “block” during creep. The materials used in this study were turbine rotor steel (Fe–10Cr–1Mo–1W–VNbN). The indentation test was applied to the as-tempered and creep damaged specimens under the maximum loads ranging from 1 to 1000 mN. The test results revealed that the decrease in contribution of block was the predominant factor controlling the material deterioration, namely, softening at the early stage of the creep life. This decrease in block's contribution was caused by the decrease in resistance of the block boundary to deformation.     

A CDM-based study of fatigue–creep interaction behavior

Under stress control mode, the damage evolution during fatigue, creep and their interaction behavior actually is a ductility exhaustion process in response to cyclic and static creep. Based on the ductility dissipation theory and effective stress concept of continuum damage mechanism (CDM), a new fatigue–creep interaction damage model has been developed in this paper, where the change of the inelastic strain energy density is used to define the damage variable. To assess this damage model, high temperature fatigue–creep interaction experiments have been carried out for 1.25Cr0.5Mo steel under stress control mode employing a trapezium waveform. The damage evolution laws of 1.25Cr0.5Mo steel under various combinatorial conditions with different maximum stresses and stress amplitudes are derived. Results indicate that the damage parameter and damage model presented in this paper are applicable to describe the damage evolution for fatigue–creep interaction.     

Assessment of hydrogen embrittlement on advanced high strength steels revealed by in-situ electrochemical micro-cantilever bending test

In the current study, the hydrogen-induced embrittlement on advanced high strength steels (AHSSs) is evaluated by in-situ electrochemical microcantilever bending (IECB) tests. Microcantilevers of 1200 M and 1400 M steels were bent while hydrogen charged inside a miniaturized electrochemical cell and then compared to bent-cantilevers in the air. The results of bending experiments and post-mortem evaluation of the bent-cantilevers showed that the plastic deformation occurred for the bent-cantilevers in the air. At the same time, the reduction of yield stress and the formation of hydrogen-enhanced cracking happened for the hydrogen-charged cantilevers. The results indicated that the microcracks are initiated and propagated adjacent to the clamped boundaries of the cantilevers, where the stress intensity is topmost. This finding is demonstrated by created step-wise cracks in 1400 M representative bent cantilever. The results show that the hydrogen-enhanced dislocation nucleation and hydrogen-reduced dislocation mobility are responsible for plastic deformation and hydrogen-enhanced cracking behavior.     

Influence of copper as an alloying element on hydrogen environment embrittlement of austenitic stainless steel

A Cu alloyed (18Cr–10Ni–3Cu) and a Cu free (18Cr–12.7Ni) austenitic stainless steel were tensile tested in gaseous hydrogen atmosphere at 20 °C and −50 °C. Depending on the test temperature, the Cu alloyed steel was extremely embrittled whereas the Cu free steel was only slightly embrittled. Austenite stability and inherent deformation mode are two main criteria for the resistance of austenitic stainless steels against hydrogen environment embrittlement. Based on the well known austenite stability criteria, the austenite stability of both steels should be very similar. Interrupted tensile tests show that martensite formation upon plastic deformation was much more severe in the Cu alloyed steel proving that the influence of Cu on austenite stability is overestimated in the empirical stability equations. When tested in high pressure H₂, replacing Ni by Cu resulted in a fundamental change in fracture mode atmosphere, i.e. Ni cannot be replaced by Cu to reduce the costs of SS without compromising the resistance to hydrogen environment embrittlement.     

Study on creep-fatigue evaluation procedures for high chromium steels—Part II: Sensitivity to calculated deformation

High-chromium steels containing 9–12% chromium such as ASME SA-213 Grade 91 or Grade 122 are widely used in conventional and combined cycle fossil power plants and are also regarded as candidate structural materials for future nuclear power plants aiming at operation in the creep range. Evaluation of failure life under creep-fatigue conditions constitutes an important part of assessing the structural integrity of these plants. The author has been conducting a series of creep-fatigue tests for three types of high-chromium steels and the validity of life prediction methods has been evaluated using the measured deformation data. Here an additional exercise was carried out in order to evaluate the adequacy of the total life prediction procedure, including the process of predicting stress and strain. After making comparisons of the test data with various existing equations developed for describing deformation and failure behaviour of Grade 91 and Grade 122, prediction of creep-fatigue life was attempted using deformation behaviour analytically estimated by these equations. Many calculations revealed that failure lives predicted by the time fraction approach showed a strong dependency on stress relaxation behaviour, whereas those based on the modified ductility exhaustion method showed a much smaller sensitivity, and therefore some error or uncertainty in stress and strain would be tolerated.     

Identification of inelastic material parameters for modified 9Cr–1Mo steel applicable to the plastic and viscoplastic constitutive equations

In this paper, the material parameters of plastic and viscoplastic constitutive equations for modified 9Cr–1Mo steel are developed for various isothermal conditions to support inelastic analysis for a sodium-cooled fast reactor. To do this, the material parameters related with the elastoplastic behaviour are identified with uniaxial cyclic test data by performing computer simulations, which use the combined Chaboche model including the kinematic hardening rule and the isotropic softening rule. The viscous parameters are identified from uniaxial stress relaxation test data through computer simulations with the pre-determined elastoplastic material parameters. Sensitivity studies are performed for the material parameters to investigate cyclic inelastic behaviour and stress relaxation during a hold time. From the comparison between the tests and the simulations, it is expected that the identified material parameters of the plastic and viscoplastic constitutive equations can accurately express the material characteristics of modified 9Cr–1Mo steel sufficiently well to be used for inelastic analysis.     

Strengthening versus softening mechanisms by hydrogen addition in β-Ti40 alloy

The effects of hydrogenation (≈0–0.9 wt.%) on the flow stress behavior and microstructural evolution of Ti40 (Ti–25V–15Cr–0.2Si) alloys during hot deformation are investigated. Isothermal hot compression tests are performed in the temperature range of 1023–1223 K with a strain rate of 0.01 s⁻¹. The stress–strain curves of the Ti40-xH alloys are recorded and the relationship between steady-state stress and hydrogen content at different temperatures is determined. With an increase of hydrogen content, the steady-state flow stress initially increases, and then decreases before increasing again at higher hydrogen contents. This behavior implies that hydrogen exerts a strengthening effect at low and high concentrations and a softening effect at intermediate concentrations. The strengthening mechanism is explained in terms of solid solution strengthening of hydrogen, fine grain strengthening caused by hydrogen induced DRX (dynamic recrystallization) and precipitation strengthening of silicide. The softening mechanism is explained in terms of DRX, grain strength softening caused by grain growth and a continuous reticular precipitation of silicide.     

Thermal creep properties of alloy D9 stainless steel and 316 stainless steel fuel clad tubes

Uniaxial thermal creep rupture properties of 20% cold worked alloy D9 stainless steel (alloy D9 SS) fuel clad tubes for fast breeder reactors have been evaluated at 973 K in the stress range 125–250 MPa. The rupture lives were in the range 90–8100 h. The results are compared with the properties of 20% cold worked type 316 stainless steel (316 SS) clad tubes. Alloy D9 SS were found to have higher creep rupture strengths, lower creep rates and lower rupture ductility than 316 SS. The deformation and damage processes were related through Monkman Grant relationship and modified Monkman Grant relationship. The creep damage tolerance parameter indicates that creep fracture takes place by intergranular cavitation. Precipitation of titanium carbides in the matrix and chromium carbides on the grain boundaries, dislocation substructure and twins were observed in transmission electron microscopic investigations of alloy D9 SS. The improvement in strength is attributed to the precipitation of fine titanium carbides in the matrix which prevents the recovery and recrystallisation of the cold worked microstructure.     

High-temperature tensile and creep properties of a ferritic stainless steel for interconnect in solid oxide fuel cell

The purpose of this study is to investigate the high-temperature mechanical properties of a ferritic stainless steel (Crofer 22 APU) for use as an interconnect material in planar solid oxide fuel cells (pSOFCs). Tensile properties of the Crofer 22 APU steel are evaluated at temperatures of 25-800 °C. Creep properties are evaluated by constant-load tests at 650-800 °C. Several creep lifetime models are applied to correlate the creep rupture time with applied stress or minimum creep rate. Experimental results show the variation of yield strength with temperature can be described by a sigmoidal curve for different deformation mechanisms. The creep stress exponent, n, has a value of 5 or 6, indicating a power-law creep mechanism involving dislocation motion. The apparent activation energy for such a power-law creep mechanism is estimated as 393 kJ mol⁻¹ through some thermally activated relations. Creep rupture time of the Crofer 22 APU steel can be described by a Monkman-Grant relation with a time exponent, m = 1.11. The relation between creep rupture time and normalized stress is well fitted by a universal simple power law for all of the given testing temperatures. Larson-Miller relationship is also applied and shows good results in correlating the creep rupture time with applied stress and temperature for the Crofer 22 APU steel. Fractographic and microstructural observations indicate most of the creep cavities are nucleated along grain boundaries and a greater amount of cavities are formed under high stresses.     

Molybdenum effect on oxidation resistance and electric conduction of ferritic stainless steel for SOFC interconnect

The effect of molybdenum addition on the oxidation, electric property and Cr evaporation of Fe–22Cr–0.5Mn ferritic stainless steel is investigated in terms of mass gain, area specific resistance and Cr evaporation rate. Addition of 0.1–2 wt% Mo reduces the oxidation rate and especially area specific resistance of Fe–22Cr–0.5Mn steel. Mo addition of these contents increases the activation energy and suppresses the inward diffusion of oxygen, which indicates the defect chemistry of oxide scale is altered. This results in the increase of oxidation resistance and electric conductivity. When more than 4 wt% Mo is added, the oxidation rate increases after 300 h of oxidation at 800 °C in ambient air. The evaporation of volatile Mo species reduces the stability of protective chromia, so that rapid growing Fe-rich spinel is formed after 300 h of oxidation. The evaporation rate of Cr is similar in all alloys.     

Creep damage prediction of the steam pipelines with high temperature and high pressure

Creep is the significant factor that caused failure of steam pipelines with high temperature and high pressure in the period of long-term service. In this paper, the creep tests were performed at serviced temperature of 520 °C for 1.25Cr–0.5Mo pipe material, and the creep and fracture constants were obtained by fitting the creep test data. Based on the modified Karchanov–Rabotnov constitutive equation, compiled the user subroutine computing the damage of the pipe element or 3D solid element, the creep damage prediction was carried out by finite element methods using ABAQUS codes for the steam pipelines with high temperature and high pressure, which serviced in a petro-chemical plant, the damage distribution and maximum damage location of the pipelines were obtained, which is testified by metallographic examination result. Furthermore, the local creep damage analysis of a tapered pipe serviced for 100,000 h was also carried out because tapered pipes used in the main steam pipeline is one of weakness in the piping system. Damage distribution and evolution in the analyzed tapered pipe were obtained. The location with the maximum damage value was determined, which is coincident with cracking position of the actual tapered pipe.