Creep Deformation Behavior Research Articles

Modeling Creep Deformation and Damage Behavior of Tempered Martensitic Steel in the Framework of Additive Creep Rate Formulation

Additive creep rate model has been developed to predict creep strain-time behavior of materials important to engineering creep design of components for high temperature applications. The model has two additive formulations: the first one is related to sine hyperbolic rate equation describing primary and secondary creep deformation based on the evolution of internal stress with strain/time, and the second defines the tertiary creep rate as a function of tertiary creep strain. In order to describe creep data accurately, tertiary creep rate relation based on MPC-Omega methodology has been appropriately modified. The applicability of the model has been demonstrated for tempered martensitic plain 9Cr-1Mo steel for different applied stresses at 873 K. Based on the observations, a power law relationship between internal stress and applied stress has been established for the steel. Further, a higher creep damage accumulation with increasing life fraction has been observed at low stresses than those obtained at high stresses.

Interrelationship between Creep Deformation and Damage for Advanced Creep-Resistant Steels

The components used in power plants generally operate at elevated and/or high temperature and are subjected to internal pressure. Under such conditions creep is of a great concern and there is an urgent demand for methods which can be used to predict the creep life. In this work, using our earlier published creep data for advanced creep-resistant T23 and P92 steels, the interrelationship between creep deformation and damage have been analysed by linking them to the identified acting mechanisms, in terms of empirical formulas for the fracture time assessment. The validity and the applicability of various formulas are examined with the objective to gain insight into the creep deformation and fracture behaviour of the steels under investigation.

On the comparison of interrupted and continuous creep behaviour of nanocrystalline copper: A molecular dynamics approach

Molecular dynamics (MD) analysis of continuous creep (CC) and interrupted creep (IC) behaviour of nano crystalline (NC) Cu having grain size ∼6 nm has been carried out employing Embedded atom potential. The creep deformation intensifies considerably during IC tests compared to CC process. Higher steady-state strain rates are also observed for IC tests as opposed to CC test. Partial strain recovery is also perceived during cooling and subsequent heating at the end of each cycle of IC test. Dislocation nucleation mechanism has a significant role on the accelerated creep behaviour in the IC tests of the NC Cu. Shockley partial dislocations play a crucial role for such enhanced creep deformation behaviour.

Influence of dwell time on the creep–fatigue behavior of a directionally solidified Ni-based superalloy DZ445 at 850 °C

Total strain-controlled creep-fatigue tests were performed for a directionally solidified Ni-based superalloy at 850 °C in air. Dwell times of 0, 1, 2, 3, 5, and 8 min were introduced at the tensile peak for each cycle. The results demonstrated that the fatigue life firstly decreased and then slightly increased with increasing dwell time. The fatigue life showed a minimum at a dwell time of 3 min. The initial softening rate was faster than the subsequent softening rate when dwell times of 1 and 2 min were used compared to the case with no dwell time. A compressive mean stress was produced by stress relaxation in the tensile dwell period. The reduction in the fatigue life was caused by a mixture of fatigue and creep deformation behaviors. However, the initial softening rate increased and a small amount of subsequent hardening occurred when 5 and 8 min of dwell times were applied. The higher compressive mean stress and a slight increase in the fatigue life for these samples of 5 and 8 min dwell times were attributed to a mixture of creep and plastic deformation. The change in the fatigue life with increasing dwell time was explained on the basis of the mechanical response, deformation mechanism, and fracture mechanism of the DZ445 superalloy.

Microstructure and creep behaviour of P92 steel after HPT

The influence of the application of severe plastic deformation on microstructure changes and creep deformation behaviour was investigated in P92 creep-resistant pipe steel. P92 steel in its as-received state, with a microstructure typical of tempered martensite, was subjected to high-pressure torsion at room temperature. An investigation of the microstructure using an electron backscatter diffraction technique revealed that severe plastic deformation led to the subsequent formation of an ultrafine-grained microstructure, containing {110} fibre texture and randomly misoriented boundaries. The microstructure characteristics and hardness measurements exhibited saturated values at an equivalent strain of higher than about 20. The creep behaviour of the ultrafine-grained specimen produced in the saturated region exhibited a minimum creep rate that was about two orders of magnitude faster, and a larger creep ductility, in comparison with the as-received state. Creep exposure at 873 K with a time to fracture of about 3 h led to a slight grain coarsening and to the formation of Laves phase.

Effect of grain boundary complexions on the deformation behavior of Ni bicrystal during bending creep.

The dependence of creep deformation behavior of nickel bicrystal specimens on grain boundary (GB) complexion was investigated by performing a simulated bending creep test using molecular dynamics methods. Strain burst phenomena were observed during the low temperature [500K, i.e., <0.3 * melting point of nickel (Tm)] bending creep process. Atomic strain and dislocation analyses showed that the time of occurrence of strain burst depends on how easily GB migration happens in bicrystal specimens. Specimens with kite monolayer segregation GB complexion were found to be stable at low temperature (500K), whereas specimens with split-kite GB complexion were stable at a comparatively higher temperature (900K). In case of further elevated creep temperatures, e.g., 1100K and 1300K, split-kite GB complexion becomes unstable and leads to early failure of the specimen at those temperatures. Additionally, it was observed that split-kite bilayer segregation and normal kite GB complexions exhibit localized increases in elastic modulus during bending creep process, occurring at temperatures of 1100K and 1300K, respectively, due to the formation of interpenetrating icosahedral clusters. Graphical abstract Representative creep curves during bending creep deformation of various grain boundary complexions at 900 K.

Assessment of Creep Deformation, Damage, and Rupture Life of 304HCu Austenitic Stainless Steel Under Multiaxial State of Stress

The role of the multiaxial state of stress on creep deformation and rupture behavior of 304HCu austenitic stainless steel was assessed by performing creep rupture tests on both smooth and notched specimens of the steel. The multiaxial state of stress was introduced by incorporating circumferential U-notches of different root radii ranging from 0.25 to 5.00 mm on the smooth specimens of the steel. Creep tests were carried out at 973 K over the stress range of 140 to 220 MPa. In the presence of notch, the creep rupture strength of the steel was found to increase with the associated decrease in rupture ductility. Over the investigated stress range and notch sharpness, the strengthening was found to increase drastically with notch sharpness and tended toward saturation. The fractographic studies revealed the mixed mode of failure consisting of transgranular dimples and intergranular creep cavitation for shallow notches, whereas the failure was predominantly intergranular for relatively sharper notches. Detailed finite element analysis of stress distribution across the notch throat plane on creep exposure was carried out to assess the creep failure of the material in the presence of notch. The reduction in von-Mises stress across the notch throat plane, which was greater for sharper notches, increased the creep rupture strength of the material. The variation in fracture behavior of the material in the presence of notch was elucidated based on the von-Mises, maximum principal, and hydrostatic stresses. Electron backscatter diffraction analysis of creep strain distribution across the notch revealed localized creep straining at the notch root for sharper notches. A master curve for predicting creep rupture life under the multiaxial state of stress was generated considering the representative stress having contributions from both the von-Mises and principal stress components of the stress field in the notch throat plane. Rupture ductility was also predicted based on the multiaxial state of stress.

Transient creep behavior of a novel tempered martensite ferritic steel G115

The transient creep deformation behavior and dislocation evolution in G115 steels were investigated in the temperature range 625–675 °C for applied stresses of 120–220 MPa. The transient creep curves showed that creep rate decreased with increases in time and creep strain. In this study, a novel method was proposed to determine the transient creep strain and the transient creep time. The strain decreased with increase in applied stress between 625 °C and 675 °C. A phenomenological constitutive equation was used to characterize the transient creep curves of G115 steels. The results showed that the constitutive equation for the G115 steel had a high precision. An internal stress was introduced to study the interactions of dislocations and precipitates, and a mechanism-based equation for transient creep in G115 steels was derived. The dependences of normalized strain and transient creep time on applied stress and temperature were analyzed in detail to better understand the transient creep deformation mechanism. It was concluded that dislocation annihilation at the grain boundaries was the dominant rate-controlling mechanism in the transient creep deformation of G115 steels. It was observed that the dislocation structures became more complex and no obvious textural features occurred after the transient creep deformation. The calculated dislocation density decreased at 650 °C relative to the initial value, which was mainly attributed to the annihilation processes occurring at the grain boundaries and the subsequent formation of subgrain boundaries.

Effect of microscopic damage propagation on the creep deformation behavior of SiC fiber reinforced SiC matrix composite

Effect of microscopic damage propagation on the creep deformation behavior of SiC fiber reinforced SiC matrix composite

Effect of Water content Condition on Creep Deformation Behavior of Catheter generated under Two Stage Step Stress for Tension and Torsion

Effect of Water content Condition on Creep Deformation Behavior of Catheter generated under Two Stage Step Stress for Tension and Torsion

Creep deformation behavior of SiC fiber reinforced SiC matrix composites at elevated temperature

Creep deformation behavior of SiC fiber reinforced SiC matrix composites at elevated temperature

Creep behavior of porous La0.6Sr0.4Co0.2Fe0.8O3-δ substrate material for oxygen separation application

Advanced oxygen transport membrane designs consist of a thin functional layer supported by a porous substrate material that carries mechanical loads. Creep deformation behavior is to be assessed to warrant a long-term reliable operation at elevated temperatures. Aiming towards an asymmetric composite, the current study reports and compares the creep behavior of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) perovskite porous substrate material with different porosity and pore structures in air for a temperature range of 800–1000 °C. A porosity and pore structure independent average stress exponent and activation energy are derived from the deformation data, both being representative for the LSCF material. To investigate the structural stability of the dense layer in an asymmetric membrane, sandwich samples of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) with porous substrate and dense layers on both side were tested by three-point bending with respect to creep rupture behavior of the dense layer. Creep rupture cracks were observed in the tensile surface of BSCF, but not in the case of LSCF.

Theoretical and experimental analysis on the small punch creep deformation of Incoloy800H

Abstract The small punch creep deformation behavior estimation of Incoloy800H was performed theoretically and experimentally in this study. The results indicate that three creep stages of the small punch test are similar to that of the conventional creep tests. Considering the deformation characteristics and geometry form, an obvious initial elastic-plastic deformation is presented and the primary creep stage is proven to be nonnegligible in the small punch test. Both the three-point bending model and the modified geometric model were proposed. It indicates that creep strain is proportional to small punch deflection in the primary and steady creep stages. Meanwhile, membrane stretching model was modified and a constant stress phase during deformation was found. The results indicate that small punch character parameters are capable to replace traditional creep data in the creep curve analysis of Incoloy800H. Thus, the creep curves with three deformation phases were analyzed and predicted by both of the modified Norton-Bailey model and modified theat model. Finally, the primary, steady and tertiary creep stages were described, and the creep life prediction of Incoloy800H was realized.

Creep properties, creep deformation behavior, and microstructural evolution of 9Cr-3W-3Co-1CuVNbB martensite ferritic steel

Creep deformation behavior and microstructure evolution of G115 steel were systematically investigated for temperatures of 625–675°C under uniaxial tensile stress of 120–220MPa. The relationship between minimum creep rate and applied stress followed the Bird–Mukherjee–Dorn (BMD) equation. The modified BMD equation was proposed using threshold stress to elucidate the actual creep deformation mechanism. The values of the threshold stress were determined to be 177.8, 91.4 and 87.6MPa at 625, 650, and 675°C, respectively. The true creep activation energy and the true stress exponent were 275.76kJ/mol and 6, respectively. Thus, the dominant creep deformation mechanism was identified as dislocation climb. Three types of precipitates can be revealed after creep deformation: W-rich Laves, Nb-rich MX, and Cu-rich phases. The creep damage of G115 steel after creep deformation was characterized by martensite cracks and martensite fractures owing to the hardness and brittleness of the lath martensite structure. Further, a dense array of deep and equiaxed dimples appeared in the central region of fracture surfaces under the tested creep conditions. Ductile fracturing was the main fracture mechanism during creep deformation.

Creep deformation and rupture behaviour of service exposed P91 weld and base steel measured by miniature tensile creep testing

Early cracking had occurred at a weld joint of a service-exposed P91 steel hot reheat pipe branch. In this study, a series of miniature tensile creep tests were conducted for both the base and weld materials extracted from the retired P91 pipe weld. Tests were conducted at various stress levels at 569 °C to measure the creep properties and investigate the failure mechanisms. Microstructural analyses were also carried out for post-test specimens. The measured creep strengths were represented in terms of the Larson-Miller parameter, and compared to literature data. Most of the data fell within the 95% error band of grade 91 steel. However, the weld metal exhibited higher creep rupture strengths. It showed creep brittle fracture, and base metal showed creep ductile behaviour. The weld metal sometimes had non-uniform microstructures, yielding a seriously tempered zones with minor tempered martensites, thereby causing early creep ductile failure of the weld.

The creep deformation and fracture behaviors of nickel-base superalloy M951G at 900 °C

The creep behaviors of M951G alloy were carried out under stress ranging from 240MPa to 400MPa at 900°C, and the corresponding deformation mechanisms and fracture behaviors after rupture had been investigated by various techniques. Results showed that both the deformation mechanisms and fracture behaviors were dependent on the applied stress. According to the transmission electron microscope (TEM) observations, the dominant deformation mechanism changed from a combined process of slip and climb of dislocations in matrix channel to shearing of dislocations in γ′ precipitates and cross-slip of dislocations in matrix channel with the applied stress increasing. Fracture behaviors of M951G alloy were characterized using optical microscope (OM) and scanning electron microscope (SEM), which changed from intergranular to transgranular with the increase of applied stress. At low applied stress, M951G alloy was failure in the form of intergranular owing to coalescence of micropores along the grain boundaries. However, at higher applied stress the microcracks initiated at broken carbides in the grain interior, and finally resulted in transgranular fracture. Additionally, creep strain rate also played a key role in determining the transition of creep fracture modes by effect the corresponding temperature of equal strength for grain boundary and grain interior. The values of apparent stress exponent at low and high stress regions were calculated to be 5.14 and 11.13 respectively, which was due to the change of deformation mechanisms and fracture modes with the increase of applied stress.

Applicability of improved Dyson–McLean approach to creep deformation behaviour of tempered martensitic P9 steel

Modelling creep deformation of tempered martensitic P9 steel in quenched and tempered, and simulated post weld heat treatment conditions has been performed in the framework of improved Dyson–McLean approach for wide range of stresses at 873 K. In this approach, kinetic creep law coupled with the set of first-order differential equations representing the evolution of microstructural internal-state-variables with strain/time has been employed to describe creep deformation behaviour of tempered martensitic steel. The optimised material constants associated with the model such as dislocation storage parameter (Kd) and rate constants associated with the precipitate coarsening (Kp) and solute depletion (Ks) reflect the influence of two different heat treatments on creep characteristics examined in the present investigation. At all test conditions, good agreement between the predicted and experimental creep strain/strain rate-time data at 873 K has been observed. Further, good correlations have been obtained between the experimental and predicted steady-state creep rates and time to reach the specified strain levels for both the heat treatment conditions.

Creep deformation and fracture behaviour of modified 9Cr-1Mo steel in flowing liquid sodium environment

The creep deformation and rupture behaviour of modified 9Cr-1Mo ferritic steel in flowing liquid sodium has been studied at 873K over a stress range of 130–160MPa and compared with those tested in air environment. In the flowing sodium environment, the steel exhibited better creep deformation and rupture strength than those in air environment. The onset of tertiary stage of creep deformation has delayed in sodium environment over that in air environment, which led to the decrease in minimum creep rate. Also the rate of tertiary creep strain accumulation was lower in sodium environment than that in air, leading to the increase in rupture life. The steel failed in ductile dimple fracture mode in both the testing environments. The steel exhibits relatively higher creep damage tolerance in sodium environment than in the air environment. Minimal oxidation was observed in the specimens creep tested in sodium environment. The near absence of oxidation of the steel in sodium environment delayed on onset of tertiary stage of creep deformation and decrease the rate of tertiary creep strain accumulation, leading to the increase in creep deformation and rupture strength.

On the microtwinning mechanism in a single crystal superalloy

The contribution of a microtwinning mechanism to the creep deformation behaviour of single crystal superalloy MD2 is studied. Microtwinning is prevalent for uniaxial loading along 〈011〉 at 800°C for the stress range 625 to 675 MPa and 825°C for 625 MPa. Using quantitative stereology, the twin fraction and twin thickness are estimated; this allows the accumulated creep strain to be recovered, in turn supporting the role of the microtwinning mode in conferring deformation. Atom probe tomography confirms the segregation of Cr and Co at the twin/parent interface, consistent with the lowering of the stacking fault energy needed to support twin lengthening and thickening. A model for diffusion-controlled growth of twins is proposed and it is used to recover the measured creep strain rate. The work provides the basis for a thermo-mechanical constitutive model of deformation consistent with the microtwinning mechanism.

Precipitation behaviour during creep in 10%Cr steel for turbine rotors

To reveal the precipitation behaviour in relation to creep deformation, microstructural investigation on the specimens creep-ruptured and creep-interrupted at various time under 600°C and 120 MPa was conducted in a 10%Cr steel forging. M23C6 carbides, Cr-rich M2X and V-rich VX carbonitrides are detected on the heat-treated specimen. Most of the precipitates are M23C6 carbides and the recovery of the martensitic lath microstructure during creep are suppressed by them. There is a good relationship between the carbides growth and the creep deformation behaviour: the beginning of the significant coarsening corresponds to the transition from the primary to acceleration creep. The creep deformation is not affected by the precipitation of Laves phase and VX carbonitrides present around the grain boundaries.This paper is part of a thematic issue on the 9th International Charles Parsons Turbine and Generator Conference. All papers have been revised and extended before publication in Materials Science and Technology.