Creep Life Research Articles

Creep behavior, microstructure evolution and fracture mechanism of a novel martensite heat resistance steel G115 affected by prior cold deformation

In this paper, the influences of prior cold deformation on creep behavior, microstructure evolution, and fracture mechanism of the novel G115 steel were systematically investigated. It was found that the creep rupture life of the G115 steel could be significantly prolonged by moderate deformation reduction, which was a synergetic effect of deformation-induced dislocations and dispersed strengthening particles. The optimal deformation reduction was approximately 20%, at which the precipitation of Laves phases could be obviously promoted, and then restricted the coarsening of M23C6 carbides due to the competitive growth relationship. Besides, more nano-sized Cu-rich particles precipitated within lath interior, and obstructed the free dislocation motions. As a feedback, the dislocation recovery and the subgrain formation could be impeded, thus extending the creep rupture life. With the further increase of deformation reduction to 30% or higher, the coarsening of precipitates and the recovery of dislocations were intense, which could facilitate the formation of subgrains and then shorten the creep life. For the G115 steel with a deformation reduction within 30%, the creep damage was identified as microstructural degradation with limited contribution from necking. And, for the steel with a deformation reduction of 45%, the creep damage was cavity growth controlled by power law and diffusion creep.

Design of a novel creep testing machine to investigate the creep life history of austenitic steel foil at elevated temperature

AbstractA novel creep testing machine was designed to test austenitic steel foil AISI 347 under the elevated temperature of 700°C. The creep test was repeated in five stress levels from low‐stress 55 MPa to severe applied stress 220 MPa for 0.25 mm thickness samples. The machine was equipped with a shield inert gas injection system, and tests were repeated in a nonoxidative area. The creep rupture behavior of the foil is well represented using the modified three‐parameter Theta Projection Concept model. Each model parameter is best represented as a function of the applied stress. Comparison between the results of creep life in different mediums shows that the oxidation effect was the dominant phenomenon in the creep process. Moreover, avoiding oxidation affects the steady‐state stage growth, and afterward, the creep life was increased by more than 40%. Post analyses of ruptured creep samples compare the creep cavitation at triple grain boundaries in a different environment.

Application of a modified hyperbolic sine creep rate equation to correlate uniaxial creep data vs. stress and temperature for creep resistant low chrome steel alloys

ABSTRACT Modified hyperbolic sine minimum creep rate equations were used to correlate uniaxial minimum creep strain rate data of 1/2Cr-1/2Mo-1/4 V and Grade 22 steel alloys and to correlate creep life data of Grade 22 steel when coupled with the Monkman-Grant equation. Hyperbolic sine equations were selected because of their physical basis in dislocation creep and numerical stability. Both equations provided very good fits to minimum strain rate data over a wide range of temperatures and stresses. Reasonable correlations of a large Grade 22 creep life data set were obtained with goodness of fit nearly equivalent to those obtained with the Larson-Miller and Wilshire equations, and with more conservative predictions of long-term creep strength. However, all these equations provided a poor fit to the entire data set when only short-term data (tr < 5,000 h) were correlated, and they predicted lower long-term creep rupture strengths vs. the full data set correlations.

Performance model of an additively manufactured micro-pin array solar thermal central receiver

Concentrated solar power (CSP) with thermal storage is a renewable approach for meeting baseload and peak load electric power. An additivelymanufactured (AM) supercritical carbon di-oxide microscale pin array receiver, MPAR, is introduced as an option for a gaseous receiver for CSP. A numerical model is developed to compare the performance of the additively manufactured pin array receiver (AM2PAR) and a microlaminated pin array receiver (µLPAR). Thermofluidic experimental data from an AM heat sink are used to validate correlations used in the model. When compared to traditional microlamination approach, which consists of etching and bonding, AM enables longer pin array lengths, thus reducing the complexity and mass of the header network, without compromising the thermal efficiency or peak surface temperature. Furthermore, the impact of non-uniform heat flux on an AM2PAR central receiver is characterized. To improve the creep life of the AM2PAR, a design with tunable pin array height in accordance to the flux on the module is introduced. Operational adjustments required to ensure the creep life of the receiver meets a 30-year lifetime for CSP are discussed.

Effect of TiB, TiC and Y2O3 on tensile properties and creep behavior at 650 ℃ of titanium matrix composites

Two kinds of (TiB + TiC + Y2O3)/α-Ti composites with different TiB and TiC contents and the corresponding matrix alloy were prepared by induction skull melting. The results showed that the matrix alloy possessed a typical widmanstatten structure while the composites displayed a basket-weave structure characterized by α lamellae with different orientations. Obvious TiB/TiC structures were observed with the increase of reinforcements, which were very stable in the tensile and creep processes at high temperature. The ultimate tensile strength at 650 ℃ increased from 589 MPa to 730 MPa and 812 MPa with the increase of reinforcements. The enhancement in strength of the composites was elucidated on the load transfer strengthening of the reinforcements. The creep properties of the composites at 650 ℃ have been significantly improved due to the impediment of reinforcements and silicides to dislocation movement. The strengthening effect increased with the increase of reinforcements and stress. At 650 ℃/290 MPa, compared with the matrix alloy, the creep life of composite with highest reinforcements content was increased by 931% and the steady-state creep rate was reduced by 93%. According to the different stress levels, the composites exhibited different creep behaviors. At 230 MPa, the sub-grains were obvious and the number of actuated dislocations is small. At 290 MPa, the dislocation pile-up was serious and the obviously coarsened silicides lost their pinning effect.

Evaluation of Creep Life Prediction Models for Low Alloy Creep Resistance Steel Grade 22 (2.25Cr-1Mo) using Short-term Creep Test

Traditional Power Law equations and modern creep equations were evaluated to estimate long term creep life of Grade 22 material. Evaluation of models made on a short-term database for predicting its capacity of precise long-term creep life. Linear trend line curve fitting method used for extrapolation of data for long-term creep life. Open SourceNRIM creep rupture data for Grade 22tube in annealed/tempered condition and plates in quenched/tempered plates used in this evaluation. This evaluation is helpful for power plant industries for selecting an economically viable and precise model.

Empirical relationships between hardness, replica and strain and their roles in health monitoring based life assessment for aged power plant materials

In this paper, for the first time, a holistic empirical lifing approach, which accommodates the on-site information of replica, hardness and strain, is established based on a large amount of outage inspection data on ageing high temperature parent ½Cr½Mo¼V (CrMoV) material and has been used to illustrate how such routinely collected inspection data can be better utilized to provide the plant operator with predictions of residual creep life. The model differentiates between long term and persistent thermal softening behaviour revealed by change in hardness over time and short-term creep cavitation that accelerates material damage. Importantly the models developed are designed to be used iteratively with surface replica and hardness data available from an outage inspection. The study shows that the availability of more data will enable further refinements, but more importantly it emphasises the importance of systematically capturing this data and processing at the time of inspection to forecast residual life and then updating and tuning the model periodically at future inspections. The capture of strain data from pipe diametral measurements is also a routine outage activity and this data is included in the case study to demonstrate the capabilities in the residual life forecast by the new methods.

Novel Tube Design for Superheater Heat Exchanger Enabled Via Additive Manufacturing

Abstract Superheater tubes are critical boiler components that operate at relatively higher temperatures and pressure. Amongst the primary concerns for these tubes is the deposition of ash particles on the tube surface, leading to the reduced thickness of the tube due to material corrosion, consequently causing early creep failure of the component. In this research, a novel tube design has been proposed which resembles a teardrop or ogive shape to reduce the drag and concurrently improve the creep life of the superheater tubes. To administer the practicality of novel tubes, metal additive manufacturing (AM), for instance, laser-powder bed fusion (L-PBF), has been proposed. These unconventional designs were assessed and compared with the baseline circular tube design for mechanical design requirements (hoop stress and creep life) and the particle and flue gas flow characteristics around the differently shaped tubes. A thermomechanical finite element (FE) analysis was performed for hoop stress calculations. This study also emphasizes on effect of circumferential thermal variation on hoop stress distribution in tubes. Therefore, a detailed two-dimensional (2D) thermal simulation has been performed to report the circumferential thermal variation on the tube. A computational fluid dynamics (CFD) analysis coupled with particle tracing was performed for gas flow visualization and particle tracing around the proposed shapes and baseline circular-shaped tube design. The Schlieren optic setup was built and leveraged for qualitative validation of the proposed design. The complete design methodology established in the paper shows teardrop-shaped tubes better in terms of drag and creep life in contrast to the circular-shaped tube.

Creep Life Prediction of 10CrMo9-10 Steel by Larson-Miller Model.

Creep is defined as the permanent deformation of materials under the effect of sustained stress and elevated temperature for long periods of time, which can essentially lead to fracture. Due to very time-consuming and expensive testing requirements, existing experimental creep data are often analyzed using derived engineering parameters and models to predict and find the correlations between creep life (time to rupture), temperature and stress. The objective of this study was to analyze and compare different numerical algorithms by using the Larson–Miller parameter ( extrapolation model. Calculations were performed using the classical equation where different values of parameter were selected, as well as using a modified equation in which parameter was stress dependent . The impact of two different approaches of extrapolation and correlation functions (linear and polynomial) applied to fit the model was also investigated. A detailed analysis was performed to choose the best extrapolation fit function and error tolerance. The numerical algorithm implemented was validated through creep rupture testing performed on 10CrMo9–10 steel at 600 °C (873 K) and 80 MPa. Creep model behavior analysis proved that different values of can significantly change the estimated time to rupture. An excellent response of the model was obtained by considering polynomial dependency when parameter was assumed to be 18, especially for the temperature range from 773 to 873 K. Promising results were also achieved when parameter was taken as stress-dependent, but only for linear fitting, which requires further analysis. However, at validation stage it turned out that only the linear extrapolation function and taken as a constant value provided adequate time-to-rupture prediction. In the case of = 18, estimated time was slightly overestimated (~8%) and for = 20 it was underestimated by 27%. In all other cases error largely exceeded 50%.

The Coupling Effect of Hierarchical Structures and MN‐Type Precipitates on the Creep Resistance for the High N Heat‐Resistant Martensitic Steel

Microstructure of the high nitrogen heat-resistant martensitic-steel (HNHM) crept using 140-170 MPa loading stresses at 923K, is investigated in correlation with creep lifetime and creep rupture strength. Upon creep deformation, both recrystallized grains and the pre-existed hierarchical structures grow via SIBM induced the degradation of strengthening. For pre-existed hierarchical structures in HNHM, martensitic packets and blocks are more inclined to form at higher stresses (170 MPa), causing the degradation of strength, while laths retain easily at lower stresses (140 MPa-155 MPa), retarding the creep damage. Additionally, under all stresses and upon 923K, the highly stabilized MN precipitates not only pin the hierarchical structural boundaries effectively, but also act as the core for the nucleation of Laves, realizing strengthening. The findings elucidate the coupling contribution of hierarchical structures and MN-type precipitates on the creep strengthening for HNHMs, challenging the common viewpoint that the coupling coarsening of lath and precipitates dominates the degradation of strengthening, and thus, deepens the understanding of the creep resistance of the HNHM and provides the first insights into obtaining reliable predicted creep life of the novel HNHMs. This article is protected by copyright. All rights reserved.

Creep anisotropy of a 3rd generation nickel-base single crystal superalloy in the vicinity of [001] orientation

The effect of orientation on the creep behavior of a 3rd generation Ni-base single crystal superalloy with orientation deviated from [001] direction 10°∼20° was studied at 1100 °C/150 MPa and 850 °C/650 MPa. It is shown that the creep anisotropy almost disappears at 1100 °C/150 MPa for samples within 20° of [001]. Rafts with waving structures are formed in all samples which eliminate the various γ/γʹ microstructures caused by orientation deviation and hinder the matrix dislocation movement in similar ways. While the creep properties are highly anisotropic at 850 °C/650 MPa. The creep lives of samples close to [001]-[011] boundary are much longer than those of samples close to [001]−[1‾11] boundary, which is mainly caused by the activated slip systems at primary creep stage. Multiple <112>{111} slips are more likely to be activated in samples close to [001]-[011] boundary, and single <112>{111} slip system dominates the deformation of samples close to [001]−[1‾11] boundary. The phenomenon is rationalized in view of the nucleation and propagation of <112>{111} slip systems by means of the nominal resolved shear stress (RSS) analysis.

Creep behavior and life prediction of P91 heat-resistant steel using modified Wilshire model

The reliable prediction of long-term creep life is of great importance in order to ensure the service safety of high-temperature components. In this work, the novel Wilshire-Cano-Stewart (WCS) relationships were introduced to model the creep behavior and predict the long-term creep life of P91 steel based on the short-term creep data. The accelerated creep experiments were firstly carried out at the temperature range of 580–620 °C, while the stresses varied from 135 to 200 MPa. According to the analyses of normalized-mean-squared error (NMSE) and mean logarithmic error (MLE), it demonstrated that the WCS model can express all the obtained creep data very well due to the average relative errors lower than 10%. Meanwhile, these values are much lower than those of traditional Time-Temperature-Parameter (TTP) models. In addition, the WCS model can provide the reasonable estimates of 200,000 h creep life with accelerated creep measurements lasting up to only 2000 h.

Bolt loading effects on the structural integrity assessment of defects in industrial components

Applied loads in bolted geometries of safety critical components can vary with time and operating conditions. Structural integrity and remaining life assessments of such components in aging industrial plants must consider the resultant changes in damage accumulation rates and acceptable defect sizes. Two case studies are presented that demonstrate the effect of bolt pre-load on creep and fatigue lives as well as on the acceptability assessments of defects. In the first case the sensitivity of creep damage accumulation and crack propagation rates to bolt pre-load in high temperature flanged connections are considered. Predicted results were found to compare well with actual damage quantified on a high pressure turbine loop pipe flange connection. It was shown that decreased pre-loads, in this case, leads to an increase the allowable safe defect size during assembly at room temperature. In contrast to this the second case study of corrosion fatigue cracking in a boiler water circulating pump illustrates that an increase in bolt pre-load leads to an increase in fatigue initiation life, a decrease in fatigue crack propagation rate and an increase in the acceptable defect size. Strain gauge measurements of bolt and casing strain, which correlated well with finite element calculations, indicated the necessity for close control of bolt pre-load during assembly to ensure specified levels are attained. In both cases metallurgical analysis and structural integrity assessments of cracked and excavated geometries were conducted which enabled limited continued operation of the components after which repairs and/or replacements will be implemented.

Research on creep damage model of high alumina bricks

In order to understand the mechanism of lining failure caused by creep, it is significant to evaluate the three creep stages of lining materials. In this paper, the three stages creep curves of 3 MPa, 3.25 MPa and 3.5 MPa for high alumina bricks at 1350 °C were obtained by high temperature creep testing equipment. Then the K-R model, the Sinh model and the Liu-Murakami model combined with the Garofalo formula are used to fit the measured creep curves. The results show that all the three models are suitable for the description of the creep behavior and the creep life prediction of high alumina bricks, and the Sinh-Garofalo model has the best fitting effect. Therefore, the Sinh-Garofalo model is selected for finite element modeling. The reliability of the model is verified by comparing the simulated and analytical values. Finally, by comparing the model with/without considering the creep damage, it is found that the creep damage model can be used to characterize the creep damage evolution process and predict the life of structures, which can be further used in the failure study of furnace lining.

Short-term creep strain localization in 12%Cr turbine steel under biaxial stress states at 600 °C via digital image correlation

FV566 martensite steel is an important material for turbine components whose extreme operating conditions potentially give rise to the creep deformation. This paper investigates the short-term creep strain localization of this steel under biaxial stress states at 600 °C. Short-term creep tests utilizing digital image correlation (DIC) were performed on different geometries of specimens with various applied stresses to investigate the strain and damage localization. Using the macroscopic creep data obtained from uniaxial specimens, finite element (FE) models were developed to evaluate stress redistribution and damage evolution. Contrast to the uniaxial state, strain heterogeneities at biaxial states were very similar under different stress levels and the notch tends to extend the creep life. The ‘crack’ growth rate was low during the steady-state creep and increases significantly at the tertiary stage creep, especially close to failure time. Further microstructure characterization involving SEM and EBSD had shown that the formation, growth and coalescence of martensite cracking and microcavities in near fracture frontier area during inelastic deformation are the main creep damage mechanism. By linking DIC, FE and microstructure results, the short-term creep fracture mechanism under biaxial stress states was thoroughly discussed.

Measurement of local creep life of laser cladding coatings by small punch creep test

Examining the creep behavior of materials locally provides important information about material performance. In this study, IN625 powder was layered on GTD-111 superalloy by the laser cladding (LC) method. After LC, the creep behavior of the clad zone (CZ), substrate, and heat affected zone (HAZ) was investigated by the small punch creep test (SPCT) method. The results showed that the CZ has the highest creep life. This was attributed to the widespread presence of equiaxed grains, high volume fraction and fineness of the γ′ phase as well as NbC carbide particles. The HAZ region, on the other hand, had the shortest creep life due to the dissolution of γ′ particles during LC. γ′ phase in the substrate and γ″ phase in CZ, the most important obstacles to dislocations movement during creep were identified, among which γ″ particles were superior to γ′ particles in the substrate due to their high volume fraction and fineness. The results of micro-hardness also confirmed this.

Influence of Post-Bond Heat Treatment on Microstructure and Creep Behavior of the Brazed Single-Crystal Nickel Superalloy.

Post-bond heat treatment (PBHT) is an effective way to improve the bonding quality of a brazed joint. Herein, brazing of a nickel-based single crystal superalloy is carried out using a Ni-Cr-Co-B-Si-Al-Ti-W-Mo filler alloy, and the microstructure and creep property of the brazed joint are systematically investigated using scanning electron microscopy (SEM), Thermo-Calc software, an electron probe micro-analyzer (EPMA), X-ray diffractometer, confocal scanning laser microscope (CSLM), and transmission electron microscopy (TEM). The results reveal that the as-prepared joint only consists of an isothermally solidified zone (ISZ) and an athermally solidified zone (ASZ), where the cubic γ′ phase is observed in the ISZ, and skeleton-like M3B2, γ + γ′ eutectic and reticular G phases are observed in the ASZ. Furthermore, the γ + γ′ eutectic and G phases disappear and the M3B2 alters from a skeleton-like to block-like shape in the ASZ after PBHT. Meanwhile, some lath-like M3B2 phases are precipitated at the edge of the ISZ and several M3B2 phases are precipitated in the base metal, forming a new zone in the brazed joint, namely at the diffusion affected zone (DAZ). Owing to the removal of low melting point eutectics from the as-prepared joint, the creep life also increases from 188 h to 243 h after PBHT. The current work provides a method for the optimization of brazed joints based on the Ni-based single crystal superalloy.

Effect of phosphorus segregation on carbides and creep damage of Ni–Fe-based GH984G alloy

The effect of phosphorus segregation on carbides and creep damage of Ni–Fe based GH984G alloy, for the application of advanced ultra-super critical (A-USC) power plant, were investigated under the creep condition of 750 °C and 200 MPa. The results indicate that the primary precipitates of GH984G alloy are MC, M23C6 and γ′ after standard heat treatment. P-doping has no effect on the grain size, γ′ size and morphology. Nevertheless, P atoms segregate at the grain boundaries and around the MC carbides, resulting in a change in the morphology of M23C6 carbides at grain boundary from film-like to granular and a decrease in the density and area percentage of MC carbides. The creep behavior of GH984G alloy at 750 °C/200 MPa changes significantly after P doping and the creep properties (creep life and elongation) are optimized with the increasing P content. The damage feature is microcrack at grain boundaries for P-free alloy, while the creep cavities in P-doped alloy. The fracture mode changes from intergranular fracture to transgranular fracture with the increasing P doping for GH984G alloy after creep at 750 °C/200 MPa. The optimizing of creep properties and transformation of fracture mode of GH984G alloys after P doping are mainly attributed to increase of grain boundary cohesion and the change of carbides caused by the P segregation behaviors during creep process.

Towards creep property improvement of selective laser melted Ni-based superalloy IN738LC

• SLM IN738LC superalloy was treated with unconventional two-step heating schemes. • The 950 °C first step was found the most effective in redistributing the carbides. • Carbide redistribution significantly reduced grain boundary pinning and promoted grain growth. • Larger grain size led to 56% to 339% increments in creep life depending on creep strength. Nickel-based superalloy IN738LC produced by selective laser melting (SLM) exhibits inferior high-temperature creep properties than its cast counterparts due to relatively smaller grain size, particularly for the plane normal to the building direction. This work studied effects of post heating strategy on the microstructure and especially the grain size to improve the high temperature creep resistance. The as-built microstructure exhibited a fine grain size and large quantities of M C carbides that could effectively hinder grain growth. It was found that unconventional two-step heat treatments could lead to substantial grain growth, and the effect is particularly prominent at a specific temperature. The ease of grain growth was explained after classifying the microstructural evolution (boundary carbide transformation) during each heating step and related to the reduced grain boundary pinning force from M C carbides. Creep tests validated the effect of the new heat treatment scheme on the SLM-processed IN738LC at 850 °C. An extended creep fracture life (1.5 to 4 times improvement) and lower secondary creep rates were achieved with samples subjected to the newly optimized two-step heat treatment. The complete creep curves are also firstly presented for SLM-IN738LC, confirming the effectiveness of grain growth and highlighting the importance of dedicated heat treatment for SLM superalloys.

An investigation on creep life assessment of welded steam pipeline intersection using classical and progressive damage analyses

An investigation on creep life assessment of welded steam pipeline intersection using classical and progressive damage analyses