Creep fatigue crack growth in 0·5Cr–Mo–V steel

Abstract

An investigation is presented of crack growth in a normalised and tempered 0·5Cr–Mo–V steel under cyclic displacement controlled loading conditions at 565–600°C. A transition from fatigue to creep dominated behaviour was observed as the duration of the tensile dwell period in the cycle was increased. This change was a result of a progressive increase in the extent of crack tip grain boundary damage accumulation which, in the long dwell tests, was sufficient to give rise to crack extension directly. Time dependent crack propagation rates during the dwells of the long dwell tests were found to approach those determined for static load conditions. No evidence was found for a significant creep-fatigue interaction and it appears that overall crack growth rates are determined by crack tip oxidation and damage accumulation processes.

MST/756     

Data-driven prediction of the probability of creep–fatigue crack initiation in 316H stainless steel

Stainless steel components in advanced gas-cooled reactors (AGRs) are susceptible to creep–fatigue cracking at high temperatures. Quantifying the probability of creep–fatigue crack initiation requires probabilistic numerical simulations; these are complex and computationally intensive. Here, we present a data-driven approach to develop fast probabilistic surrogate models of creep–fatigue crack initiation in 316H stainless steel. We perform a set of Monte Carlo simulations based on the R5V2/3 high temperature assessment procedure and determine the sensitivity of the probability of crack initiation to loads and operating conditions. The data are used to train different supervised machine learning models considering Bayesian hyperparameter optimization. We discuss the relative performance of such models and show that a gradient tree boosting algorithm results in surrogate models with the highest accuracy.     

Effect of inadequate heat treatment on creep strength of 12Cr–Mo–V steel

Abstract

Solution treatment of 12Cr–Mo–V steels below the specified temperature range leads to the development of spheroidized microstructures with dramatically reduced creep resistance. This is known to have resulted in the premature service failures of superheater tubing. Compositional and mechanical property checks currently specified in the relevant standards may not be sufficient to reveal deficiencies. Steels with Cr–Ni equivalents at the uppermost extreme of the range possible within the compositional limits of the tube steel standards show enhanced susceptibility to both δ– and α–ferrite formation. The significance of this is discussed in relation to creep strength, with particular reference to the differences between the effects of α- and δ-ferrite.

MST/147     

In-plane and out-of-plane constraint effects on creep crack growth rate in Cr–Mo–V steel for wide range of C*

Abstract

In this work, the stress dependent creep ductility and strain rate model have been implemented in a ductility exhaustion based damage model and the creep crack growth (CCG) rates of a Cr–Mo–V steel in compact tension (C(T)) and middle tension (M(T)) specimens with different thicknesses and crack depths have been simulated over a wide range of C*. The effects of in-plane and out-of-plane constraints on CCG rates are examined. The results show that the in-plane and out-of-plane constraint effects on CCG rate are more pronounced for the high constraint specimen geometry (C(T)), while such effects are less significant for low constraint specimen geometry (M(T)). The constraint effects on CCG rates mainly occur in low and transition C* regions and the CCG rate increases with increasing in-plane and out-of-plane constraints. There exists interaction between in-plane and out-of-plane constraint in terms of their effects on CCG rate. The higher in-plane constraint strengthens the out-of-plane constraint effect on CCG rate and higher out-of-plane constraint also strengthens the in-plane constraint effect on CCG rate. The constraint effects on creep crack growth behaviour for a wide range of C* mainly arise from the interaction of crack-tip stress states and stress dependent creep ductility of the steel in different C* levels.     

Effect of niobium on creep and creep crack growth of cast Ni–Cr austenitic steel

Abstract

An investigation of the effect of Nb on creep properties and creep crack growth rate in a 25Cr–35Ni–0·4C (wt-%) cast steel at 871 and 950°C was carried out. Tensile tests were also carried out at room temperature, 871, and 950°C. The tensile strength and elongation increased with an increase in Nb content at high temperatures. There existed an optimum Nb content for the creep properties and creep crack growth rate. Creep crack growth is controlled by creep deformation.

MST/1222     

Quantitative observations of precipitation in 2.25Cr–1Mo steel exposed to different creep conditions in a power station

Samples of 2.25Cr–1Mo steel in the form of tubes have been examined in the new condition and after extended service in a power station. The differences in microstructure in terms of precipitates have been characterized using a TEM; the various precipitates present have been identified individually by using extraction replicas and energy dispersive spectroscopy analysis. The area fraction of each type of precipitate was calculated. The results have shown that the quantity of M₇C₃ precipitated within grains as well as on grain boundaries decreased markedly with the time in service while the quantity of M₆C increased in both regions. It was observed that precipitates of Mo₂C and (CrMo)₂CN, detected after aging respectively on the samples with the shorter and the longer aging times, were present only within grains. The procedure used was not accurate for identification and quantification of small VC and (VNb)C precipitates. The results of the relative quantity of M₇C₃ and M₆C, as a function of time in service, particularly on grain boundaries, showed a well defined interdependence. These results can be used as a complementary analysis for creep damage and remaining life estimation for this class of steel operating at high temperature under steady state conditions.     

Carbide stabilities in a Cr–Mo–V bolting steel

Abstract

An electron metallographic examination has been performed on samples of a 1Cr–1Mo–0·75V (Ti,B) bolting steel (Durehete 1055) as part of a long term creep programme. The apparently anomalous carbide composition results have been rationalised by the use of pseudoternary phase diagrams. This novel method of presenting the alloy carbide composition data enables the changes accompanying service exposure to be described. It is obvious that the conventional thermodynamic order of stabilities does not obtain over the entire range of temperatures experienced by this alloy.

MST/914     

Effect of W–Mo balance on long-term creep life of 9Cr steel

The effect of tungsten–molybdenum (W–Mo) balance on creep life has been investigated for five heats of martensitic 9Cr steel with 1.5 % Mo equivalent (= 1/2W + Mo) at 600, 650 and 700°C. The combination of W and Mo concentrations in the present steel is 3W–0Mo, 2.8W–0.1Mo, 2.4W–0.3Mo, 1.8W–0.6Mo and 0W–1.5Mo. The time to rupture t_r exhibits a monotonous increase with increasing the W–Mo balance parameter 1/2W/(1/2W + Mo), namely, with increasing W concentration and concomitantly with decreasing Mo. The increase in t_r with increasing 1/2W/(1/2W + Mo) becomes less significant at long times. The precipitation of Fe₂(W,Mo) Laves phase takes place preferentially at prior austenite grain boundaries during creep, which enhances the grain boundary (GB) precipitation hardening. The amount of Laves phase increases with increasing 1/2W/(1/2W + Mo). The coarsening of Laves phase takes place at long times during creep, which reduces the GB precipitation hardening.     

Zone-wise investigation of creep behaviour 9Cr–1Mo steel weld joints

Distinct regions such as weld metal, heat-affected zone (HAZ) and base metal of P9 steel weld joints fabricated by various welding processes were investigated using impression creep testing. Smaller prior austenitic grain size, lower density of precipitates and dislocations resulted in faster recovery and higher creep rate of HAZ in comparison to the weld and base metal. Compared to base metal, shielded metal arc weld (SMAW) and activated tungsten inert gas (A-TIG) weld of the P9 steel weld joints exhibited better resistance to creep and displayed higher activation energy due to their coarser prior austenite grain size. A-TIG HAZ exhibited superior creep properties compared to the SMAW and TIG HAZ due to the presence of higher number density of precipitates.     

Effect of microstructure on appearance of near-threshold fatigue fracture in Cr–Mo–V steel

Near-threshold fatigue crack growth behavior in 25Cr2NiMo1V steel with different microstructures was investigated by utilizing the load-shedding technique at ambient temperature. Crack surface morphology was observed by SEM with special emphases on the incidence of intergranular fracture and the influence on crack growth rates. Results show that the maximum intergranularity occurs at the ΔK corresponding to the cyclic plastic zone size being equivalent to the prior austenitic grain size. Two types of crack growth mode were observed in the near-threshold regime, i.e., the crystallographic mode of crack growth and the striation mode of crack advance. The incidence of faceted fracture was mainly rationalized by comparing the cyclic plastic zone size with the grain size. It is concluded that, in the crystallographic mode, lower crack growth rates in samples with higher heat treatment temperatures are caused by a greater degree of roughness-induced crack closure (RICC), faceted fracture induced crack closure (FFICC), and oxide-induced crack closure (OICC). The faceted fracture shows negligible influence on crack growth rates when cracks grow in a striation controlled mode.     

Creep cavity nucleation and growth in 12Cr–Mo–V steel

Abstract

Quantitative measurements of the number of cavities per unit area, mean cavity size, and cavitated area fraction are carried out on creep tested specimens of a 12Cr–Mo–V martensitic steel. Both interrupted and ruptured specimens are included. Cavities initiate at an early stage of tests. The number of cavities, mean cavity size, and cavitated area fraction exhibit a continued increase with increasing strain and time. At rupture, the number of cavities is about 30 000 mm^?2, and the mean cavity size has a maximum value of about 2·5 μm.The cavitated area fraction is approximately 0·1% at 1% strain, 0·5% at 2% strain, and 10% at rupture. Cavity initiation and growth are analysed regarding temperature, stress, strain, and strain rate. A constrained cavity growth model is discussed and compared with the experimental results.

MST/3097     

Cyclic creep behaviour under tension–tension loading cycles with hold time of modified 9Cr–1Mo steel

Abstract

Cyclic creep behaviour of modified 9Cr–1Mo steel was investigated by a series of cyclic creep (CC) tests at 600°C, which were performed under controlled tension–tension loading cycles with the magnitude of stress ranges in a constant stress ratio (R?=?0·1). Hold time was applied for a 10 min hold at the maximum stress (σ_max) and minimum stress (σ_min). The CC properties were compared with the static creep (SC) using Norton’s power law, Larson–Miller plot, and Monkman–Grant relation, and the microstructure was examined. For the test conditions employed in the present investigation, retardation in the CC behaviour in terms of a lower creep rate and longer rupture time compared to those in the SC was obtained. The retardation was ascribed to the effects associated with anelastic recovery during the 10 min hold time at the minimum load of the cyclic loading. The creep rupture ductility decreased with a general decrease in stress, and there was no difference in the creep ductility between the CC and SC. The steel displayed a transgranular fracture characterised by the presence of dimples resulting from micro-void coalescence. Carbide precipitation was more coarsened with increasing in exposure time in the CC tests.     

Degradation in tensile and creep properties of 2.25Cr–1Mo steel by long term service in plant

Abstract

Degradation in tensile and creep properties has been investigated for 2·25Cr–1Mo steel, after long term service at 577°C for 1·9 × 10⁵ h. Creep tests were carried out at 550–690°C for up to about 10 000 h for the long term serviced material. The results are compared with those for virgin material tested for up to 100 000 h. The creep rupture time is shorter but creep ductility is larger for the long term serviced material than for the virgin material at high stress and short time conditions. The difference between the two materials becomes decreased with decreasing stress and increasing time. Microstructure evolution during long term service causes a softening and promotes dynamic recovery or recrystallisation during subsequent creep, which accelerates the onset of acceleration creep. This results in a higher minimum creep rate and a shorter rupture time for the long term serviced material than for the virgin material. The deviation from Monkman–Grant relationship is correlated with a decrease in total elongation.     

Prediction of the temperature dependence of fracture toughness of 12Cr–2Ni–Mo refractory steel

We study the influence of temperature and the size of the specimens on the characteristics of static crack resistance of 12Cr–2Ni–Mo refractory steel. It is shown that, in the temperature range 20–450°C, the increase in the thickness of specimens leads to an insignificant increase in fracture toughness obtained along a 5% secant line according to the standards of evaluation of the characteristics of crack resistance. The evaluation of the characteristics of crack resistance of 12Cr–2Ni–Mo steel with regard for the scale effect according to an earlier developed numerical-experimental model reveals the existence of satisfactory agreement with the experimental data in the entire investigated temperature range. Translated from Problemy Prochnosti, No. 4, pp. 78–88, July–August, 2009.     

A generalisation of the Wilshire–Scharning methodology to creep life prediction with application to 1Cr–1Mo–0.25 V rotor steel

Wilshire–Scharning have recently developed a new methodology that has been demonstrated to deliver accurate longer term creep life predictions, and so offers the prospect of cost-effective acquisition of long-term creep design data. This methodology differs from existing approaches to creep life prediction by normalising the applied stress through the appropriate tensile strength. This article develops a generalisation of this Wilshire–Scharning model that has the potential to increase the predictive accuracy of this methodology—which will be so essential if it is to be adopted as a way of economising on the acquisition of creep design data. When applied to 1Cr–1Mo–0.25 V steel, it was found that this generalisation reduced the average error in prediction from 2.5 under the Wilshire–Scharning specification to 1.8 years when extrapolating from 5,000 out to over 100,000 h. Further, over this time scale the generalised model produces a mean absolute percentage error of 28%. This compares to 47% obtained using the traditional 4Θ projection technique and 26% using a modification of this methodology as recently proposed by Evans.     

Interpretation of creep behaviour of a 9Cr–Mo–Nb–V–N (T91) steel using threshold stress concept

Abstract

The creep behaviour and the microstructural evolution of a 9Cr–Mo–Nb–V (T91) steel were extensively evaluated by means of short term constant load creep tests and TEM analysis. Statistical analysis of the microstructural data revealed that the precipitated phases M₂₃ C₆ (where M is a metal, mainly Cr or Fe) and MX (where M is Nb or V, and X is C and/or N) were subject to coarsening during creep exposure. The coarsening law and its dependence on applied stress were identified, and the model was used to predict the magnitude of the Orowan stress at the time corresponding to the minimum creep rate. The minimum creep rate dependence on applied stress at 873 K was described by incorporating the threshold stress concept in a power law with stress exponent n = 5. In the resulting phenomenological model, the strengthening effect of the dispersed phases was thus expressed by a threshold stress proportional to the Orowan stress.     

Prediction of deformation and failure of modified 9Cr–1Mo steel under creep-fatigue interaction

Abstract

Plant components often undergo loadings which have aspects of both fatigue and creep by experiencing repeated start-up/shut-downs between steady-state operation at high temperature. In such a case, structural materials show inelastic deformation different from that observed under pure fatigue or pure creep conditions. This finding prompted the development of “unified” constitutive models in which cyclic and creep deformations are treated in a unified way. The author’s group has been developing such a unified model for modified 9Cr–1Mo steel which is widely used in ultrasupercritical fossil power plants. Although the latest model has been demonstrated to be capable of describing deformation behaviour under various loading conditions, including cyclic and steady-state loadings, the model still has room for improvement, such as the need to represent accelerated deformation in the tertiary creep regime. In this study, an attempt is made to improve the capability of the model from various perspectives. A failure prediction model which can deal with various failure modes is also formulated. The deformation and failure lives predicted using these models show reasonable agreement with the results of various load sequence tests obtained on modified 9Cr–1Mo steel.     

Prediction of grain growth of coarse-grained austenite in Nb–V–Ti microalloyed steel

Abstract

Grain growth of coarse-grained austenite in Nb–V–Ti microalloyed steel during equalisation was predicted by extending the previous investigation. The prediction worked with initial austenite grain size distribution instead of average grain size. An improved model taking into account the holding time was used in the prediction. The result showed that only part of initial austenite grains grow at each equalisation temperatures, but the grain size of growing grains is expanded to a wider range with increasing equalisation temperature, which indicates that grain size distribution should be considered when grain growth of coarse-grained austenite is evaluated. The predicted austenite grain size distribution is close fit to the measured one and further work is expected to improve the quality of model prediction.