Creep ductility of 1CrMoV rotor steel

Abstract

The creep ductility of 1CrMoV steels at a given temperature is high at high stresses responsible for high strain rates and ductile rupture, and low at lower stresses responsible for low strain rates and constrained cavity growth at grain boundaries. The magnitudes of ductility in the high and low stress regimes and the time to, and strain rate at, the transition between the two is determined by the chemical composition and the adopted quality heat treatment procedure of the steel. The basis for a material pedigree function forming part of a creep ductility model for 1CrMoV rotor steel is presented. Low ductility 1CrMoV steels are expected to be extremely notch sensitive. While this appears to be true for medium to high strength heats of the alloy, it is not necessarily the case for lower strength heats. There is not a simple inverse relationship between creep ductility and creep strength. Increasingly, creep-fatigue lifetime predictions for high temperature 1CrMoV power plant components subjected to thermo-mechanical transients employ a creep ductility exhaustion methodology to determine the creep damage accumulated per cycle. A creep ductility model of the type developed in the paper is suitable for forming the basis of such an approach.     

Effects of Stress Level and Stress State on Creep Ductility:Evaluation of Different Models

The last few decades have witnessed an increasing emphasis on the development of strain-based approach for predicting the creep life or damage of components operating at elevated temperatures. Creep ductility, as a key parameter in this approach, may vary with a number of factors including strain rate,state of stress, operating temperature, material microstructure, etc. The present paper, however, is focused on reviewing the state-of-the-art understanding of the effects of stress level and stress state on the creep ductility. Mechanisms involving the void growth and coalescence are presented to describe the role of stress level in the variation of uniaxial creep ductility. The prediction capacity of existing empirical ductility models is also assessed in light of uniaxial test data. On the other hand, a vast body of multiaxial creep test data, collected from open literature, is utilized to examine the influence of the state of stress on the creep ductility. Then, a variety of multiaxial ductility factor models are introduced and evaluated with the available experimental data. Finally, a brief discussion on the dependence of creep ductility on the stress triaxiality and Lode parameter, predicted by numerical methods, is provided.     

An efficient fatigue and creep‐fatigue life prediction method by using the hysteresis energy density rate concept

Fatigue damage, time‐dependent creep damage and their interaction are considered as the main failure mechanisms for many high temperature structural components. A generalized methodology for predicting both the high temperature low cycle fatigue (HTLCF) and creep‐fatigue lives by using the hysteresis energy density rate (HEDR) and fatigue damage stress concepts was proposed. Experimental data for HTLCF and creep‐fatigue in Alloy 617, Haynes 230 and P92 steel were respectively collected to validate the method. A better prediction capacity and most of the data points that fall within a 1.5 scatter band were obtained compared with the traditional energy‐based method, time fraction rule and ductility exhaustion model. Moreover, a creep‐fatigue damage diagram was also constructed by using the proposed approach.     

Research on representative stress and fracture ductility of P92 steel under multiaxial creep

Creep experiments on both plain and notched specimens were conducted at 650 °C over a stress range of 120–185 MPa. The notch strengthening effect was found to exhibit in notched specimens. By using stress components at the skeletal point, several expressions of representative stress were compared to validate their effectiveness in predicting creep rupture lives of P92 steel under multiaxial stress states. The results showed that Hayhurst representative stress was more suitable for life predictions of P92 steel. In the mean time, the relationship between the fracture ductility and multiaxiality was presented to investigate the influence of the multiaxial stress states on creep rupture behavior of P92 steel. A more reasonable prediction model was proposed, and the validity of the model was verified by experimental data.     

Cumulative damage model based on equivalent fatigue under multiaxial thermomechanical random loading

A new creep–fatigue damage cumulative model is proposed under multiaxial thermomechanical random loading, in which the damage at high temperature can be divided into the pure fatigue damage and the equivalent fatigue damage from creep. During the damage accumulation process, the elementary percentage of the equivalent fatigue damage increment is proportional to that of the creep damage increment, and the creep damage is converted to the equivalent fatigue damage. Moreover, combined with a multiaxial cyclic counting method, a life prediction method is developed based on the proposed creep–fatigue damage cumulative model. In the developed life prediction method, the effects of nonproportional hardening on the fatigue and creep damages are considered, and the influence of mean stress on damage is also taken into account. The thermomechanical fatigue experimental data for thin‐walled tubular specimen of superalloy GH4169 under multiaxial constant amplitude and variable amplitude loadings were used to verify the proposed model. The results showed that the proposed method can obtain satisfactory life prediction results.     

Effects of creep ductility and notch constraint on creep fracture behavior in notched bar specimens

The creep rupture life of U-type notched specimens and smooth specimens has been calculated based on the ductility exhaustion damage model using stress-dependent creep ductility. Effects of creep ductility and notch constraint on creep fracture behaviour in notched bar specimens have been investigated. The results show that the U-type notch exhibits notch strengthening effect under a wide range of stress level and notch constraint condition (notch acuity) for creep ductile materials. The lower equivalent stress in notched specimens plays main role for reducing creep damage and increasing rupture life. The rupture life of notched specimens of creep brittle materials (with lower creep ductility) decreases with the increase in stress level and notch constraint. With increasing creep ductility and decreasing notch constraint, the degree of the notch strengthening effect increases. In creep life designs and assessments of high-temperature components containing notches, the material creep ductility, notch constraint and stress levels need to be fully considered.     

Deformation,diffusion and ductility during creep – continuous void nucleation and creep-fatigue damage

Abstract

It is shown that the assumption of unit (negative) slope in the well known Monkman–Grant plot of time to failure against minimum creep rate is too restrictive. By acknowledging observed slopes in the range 0.8–1, a ductility–strain-rate relation is deduced where ductility decreases with reducing strain rate. This in turn has implications for the ductility exhaustion method as applied during stress relaxation in the dwell period of low cycle fatigue tests of austenitic steels at elevated temperature. The simple method is used to calculate the cyclic creep damage in typical tests on austenitic steels in the region 550–650 °C and is compared to other calculations as employed in the R5 high temperature assessment procedure. The assumption of a uniform nucleation rate of grain boundary voids with creep strain goes some way to predicting the slope of the ductility–strain-rate relation. Both the ‘unconstrained’ and ‘constrained’ (lower shelf) regions of void growth are discussed.     

Effect of stress dependent creep ductility on creep crack growth behaviour of steels for wide range of C*

Abstract

In this work, the effect of stress dependent creep ductility on the creep crack growth (CCG) behaviour of steels has been investigated by finite element simulations based on ductility exhaustion damage model. The relationship between the transition region of creep ductility and the transition behaviour of CCG rate on da/dt-C* curves has been examined and the CCG life assessments of components and CCG resistance of materials for a wide range of C* were discussed. The results show that with increasing the transition region size of creep ductility, the transition C* region size on da/dt-C* curves increases. With moving transition region position of creep ductility to high stress region (increasing transition stress levels), the transition C* region on the da/dt-C* curves also moves to high C* region. Decreasing transition stress levels and transition region sizes of creep ductility and increasing the lower shelf and upper shelf creep ductility values can improve the CCG resistance of materials. If the extrapolation CCG rate data from the high C* region or from the transition C* region are used in life assessments of the components at low C* region, the non-conservative or excessive conservative results may be produced. Therefore, the CCG rate data should be obtained for a wide range of C* by long term laboratory tests or numerical predictions using the stress dependent creep ductility and model.     

High temperature low cycle fatigue

Fatigue at high temperature is a complex phenomenon as it is influenced by a number of time-dependent processes which become important at elevated temperatures. These processes include creep, oxidation, phase instabilities and dynamic strain ageing (DSA), acting either independently or synergistically influence fatigue behaviour, often lowering the fatigue life. Current design approaches employ linear summation of fatigue and creep damage with suitable factors on permissible damage to take care of uncertainties in interaction between cyclic and time-dependent processes. It is, therefore, important to develop a deeper understanding of the processes that occur during high temperature fatigue so that realistic life predictions could be made. Results on the high temperature fatigue behaviour of austenitic stainless steels, ferritic steels and nickel base alloys are presented here. The important mechanisms of interaction of high temperature time-dependent processes with fatigue under various conditions are discussed in detail. Emphasis is placed on cyclic stress response, fatigue life, deformation substructure and fracture behaviour. This is followed by a review of important life prediction techniques under combined creep-fatigue loading conditions. Life prediction techniques considered here include linear damage summation, strain range partitioning, ductility exhaustion approach, frequency modified and frequency separation methods, techniques based on hysteresis energy and damage rate models, and methods based on crack-cavitation interation models.     

The creep and fracture behaviour of tempered martensitic steels

Creep strength enhanced ferritic steels contain 9 to 12% Cr and were developed to exhibit excellent high temperature properties. These should be achieved when the microstructure exhibits a tempered martensitic matrix containing a substructure with a high dislocation density and a uniform dispersion of fine, second phase precipitates. It is interesting to note that when properly processed the typical alloy compositions for these steels provide reasonable strength but can exhibit brittle creep behaviour. The levels of ductility required in engineering applications necessitate proper control of composition (including trace elements), steel making and processing and all heat treatments. The properties needed for modern design methods can only be obtained using validated procedures for both uniaxial and multiaxial testing and documentation to establish the mechanisms controlling deformation and fracture for relevant stress states.     

A temperature dependent creep damage model for polycrystalline ice

We present a continuum damage model for the temperature dependent creep response of polycrystalline ice under a multiaxial state of stress, suited for ice in polar regions. The proposed model is based on a thermo-viscoelastic constitutive law for ice creep and a local orthotropic damage accumulation law for tension, compression and shear loadings. Orthotropic damage is represented by a symmetric second-order damage tensor and its effect on creep is incorporated through the effective stress concept. The unknown model parameters are first calibrated using published experimental data from constant uniaxial stress tests and then predictions are made for constant strain rate and multiaxial loadings. The predicted results are in good agreement with both experimental and numerical results in the literature illustrating the viability of the proposed model. The model is mainly intended for studying the failure mechanisms of polar ice at low deformation rates with depth varying temperature profiles.     

Prediction of creep crack initiation in Cr–Mo–V steel specimens with different geometries

By using stress dependent creep ductility and strain rate model in a ductility exhaustion based damage model, the creep crack initiation (CCI) behaviour in Cr–Mo–V steel specimens with different geometries and dimensions (different constraints) over a wide range of C^* has been predicted by finite element simulations. The predicted creep crack initiation time agree well with the existing experimental data. In low and transition C^* regions, the constraint induced by specimen geometries and dimensions has obvious influence on CCI time. With increasing constraint level of specimens, the CCI time decreases due to the increase of stress triaxiality ahead of crack tip. Different CCI trends and constraint effects on CCI behaviour in a wide range of C^* result from the interaction of crack-tip stress state and stress dependent creep ductility of the steel. It is suggested that in CCI life assessments of high temperature components, the long-term CCI time data at low C^* region should be obtained and used, and the constraint effects need to be considered by using constraint dependent CCI data.     

Beurteilung von mehrachsigen Spannungs-Dehnungszuständen im Langzeitkriechbereich auf der Basis von Festigkeitshypothesen

Assessment of multiaxial stress states in the long-term creep range on basis of the strength hypotheses The validity of strength hypotheses to describe the multiaxial stress states in the long-term creep range was examined by means of creep tests on specimens of 1Cr1Mo-steel at 550°C. It was found out that the v. Mises hypothesis is suitable to describe the creep deformation behaviour in the range, where no irreversible creep demage could be observed. The creep damage generally occurs at the end of secondary creep range reaching the minimum creep rate. In the presence of creep demage and to evaluate creep rupture a combined strength hypothesis should be used. By means of the combination of different stresses not only the deformation but also the damage in the microstructure could be taken into account. There is a good coincidence between the predicted and the metallographic determined damage, so that this hypothesis is also generally suitable to estimate the remanent life.     

The importance of multiaxial stress in creep deformation and rupture

This paper investigates the importance of multiaxial stress states by considering several distinct testing techniques used in assessing both creep deformation and creep damage accumulation. The requirements of testing programmes to determine the necessary data are discussed in respect of sensitivity and interdependence of the principal and hydrostatic stress ratios.     

Creep failure simulations of 316H at 550 °C: Part I – A method and validation

This paper proposes a method to simulate creep failure using finite element damage analysis. The creep damage model is based on the creep ductility exhaustion concept, and incremental damage is defined by the ratio of incremental creep strain and multi-axial creep ductility. A simple linear damage summation rule is applied and, when accumulated damage becomes unity, element stresses are reduced to zero to simulate progressive crack growth. For validation, simulated results are compared with experimental data for a compact tension specimen of 316H at 550 °C. Effects of the mesh size and scatter in uniaxial ductility are also investigated.     

Creep ductility considerations for high energy components manufactured from creep strength enhanced steels

Abstract

It is well established that the tendency for low ductility ‘creep brittle’ fracture behaviour in tempered martensitic steels is linked to the formation and growth of micro voids or ‘cavities’. Details of the contributions of all factors affecting damage development are still under investigation. However, it is known that for tempered martensitic steels voids often initiate over most of the creep life. Nucleation has been recorded on both prior austenite grain boundaries and at other micro structural features such as lath boundaries. The number of voids formed, and the fracture behaviour observed, depend on the type of creep strength enhanced ferritic (CSEF) steel and specific details of fabrication and heat treatment. In Grade 91 steel, void nucleation is sensitive to metallurgical factors such as composition and steel making practices. Key indicators of susceptibility to creep cavitation also include the levels of trace elements present and the presence of hard non-metallic inclusions. In Grade 92 steel, creep void formation has been linked to boron nitrides and other inclusions. These inclusions are present when there has been insufficient control of composition and heat treatment. Metallurgical factors linked to whether a particle will nucleate a void include the nature of the inclusion/matrix interface, the shape and size and the location of the inclusions within the microstructure. This paper describes the results of critical uniaxial and multiaxial testing for CSEF steels and compares data from nominally the same steels which have different metallurgical susceptibilities to void formation.