Development of an Advanced Creep Model for Type 316 Stainless Steel

Methods for predicting stress relaxation behaviour rely on either the integration of a forward creep equation derived from constant load creep data or a stress relaxation equation specifically derived from data measured during dwell periods in either monotonic or cyclic loading. In general, these methods do not explicitly address the effects of prior loading history on the observed relaxation behaviour. In the current work, forward creep data have been acquired to allow identification of a stress-temperature dependent function that represents the observed correlation between measured values of primary creep strain and the initial plastic loading strain. The prediction of stress relaxation from forward creep equations has been extended to investigate the effects of plasticity on creep deformation through the inclusion of a back stress component in the creep deformation equations. The work is based on British Energy’s evolving experimental database for Type 316H stainless steel which includes data from ongoing low stress and stress change creep tests. These tests are currently being conducted to improve the understanding of plasticity-creep interaction effects. Improved methods for predicting stress relaxation in Type 316H stainless steel based on these experimental data are assessed against their ability to adequately describe the influence of prior loading history on primary creep and stress relaxation behaviour.

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Development of the RCC-MR Creep Deformation Model for the Prediction of Creep and Stress Relaxation in Type 321 Stainless Steel

Volume 6A: Materials and Fabrication ◽

10.1115/pvp2014-28917 ◽

2014 ◽

Cited By ~ 2

Author(s):

A. J. Moffat ◽

J. P. Douglas ◽

M. White ◽

M. W. Spindler ◽

C. Austin ◽

...

Keyword(s):

Stainless Steel ◽

Stress Relaxation ◽

Creep Strain ◽

Creep Deformation ◽

Constant Load ◽

Fatigue Tests ◽

Creep Model ◽

Deformation Model ◽

Relaxation Data ◽

Creep And Stress Relaxation

In this paper a creep deformation model has been developed for Type 321 stainless steel which has been based on a modified version of the creep model that is used in the French fast reactor design code RCC-MR. The model has been evaluated using: 1) constant load creep data covering the temperature range from 550°C to 650°C and 2) constant displacement, stress relaxation data obtained from creep-fatigue tests at 650°C. Samples in the heat-treatment conditions of solution-treated, aged, and simulated ‘heat affected zone’ have been assessed. The standard RCC-MR model was fitted to the constant load data and provided good predictions of forward creep. However, when this model was used to predict stress relaxation it was observed that the model significantly over predicted creep strain rates and therefore the level of stress drop during each cycle. During constant load tests the stress remains relatively constant (noting that true stress does increase a small amount prior to rupture). However, in relaxation tests the stress varies significantly over the dwell. Due to the poor predictions of stress relaxation it was hypothesised that the fitted model did not capture the stress dependence of creep appropriately. The RCC-MR model was therefore modified to include a primary and secondary threshold stress term that is a function of the accumulated creep strain. This work indicates that the RCC-MR model, modified to include threshold stresses, can be used to provide good predictions of both forward creep and stress relaxation in Type 321 stainless steel. Further work is required to validate this model on stress relaxation data at additional temperatures and lower start of dwell stresses.

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Model Correlations for Investigating Creep Deformation and Stress Relaxation in Structures

Journal of Mechanical Engineering Science ◽

10.1243/jmes_jour_1965_007_011_02 ◽

1965 ◽

Vol 7 (1) ◽

pp. 57-66 ◽

Cited By ~ 1

Author(s):

C. O. Frederick

Keyword(s):

Stress Relaxation ◽

Differential Equations ◽

Strain Hardening ◽

Creep Deformation ◽

Constant Stress ◽

Creep Model ◽

Stress Index ◽

Creep Data ◽

Elastic Strains

This paper establishes a range of possible creep model correlations based on uniaxial constant stress creep data and variable stress creep laws. It is shown that the model behaves as an analogue which can be used to solve the differential equations governing the deformation of the prototype. The most general correlations hold for time-hardening materials where creep data can be flitted by a stress index. The least general correlations hold for strain-hardening materials whose data can only be fitted to a function of stress. Elastic strains and variable loads are included in the analysis.

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Development of a Constitutive Backstress Model for the Prediction of Creep and Stress Relaxation in Gas Turbine Materials

Volume 10B: Structures and Dynamics ◽

10.1115/gt2020-16191 ◽

2020 ◽

Author(s):

Andrew Moffat ◽

Richard Green ◽

Calum Ferguson ◽

Brent Scaletta

Keyword(s):

Stress Relaxation ◽

Creep Deformation ◽

Gas Turbines ◽

Operating Conditions ◽

Creep Model ◽

Effect Of Temperature ◽

Deformation Model ◽

Corrosion Resistant ◽

Single Temperature ◽

Wide Range

Abstract There is a drive towards a broader range of fuels in industrial gas turbines, with higher levels of sulphur and potentially hydrogen. Due to these harsher environments, there is also a drive for corrosion resistant alloys and coatings. A number of key corrosion resistant superalloys, which are being employed to cope with these evolving conditions, exhibit primary creep. It is therefore imperative that fundamental material models, such as those for creep deformation, are developed to ensure they can accurately predict the material response to evolving operating conditions. The requirements for a creep model are complex. The model must be able to: predict forward creep deformation in regions dominated by primary loads (such as pressure); predict stress relaxation in regions dominated by secondary loads (such as differential thermal expansion); predict the effects of different creep hardening mechanisms. It is also clear that there is an interaction between fatigue and creep. With flexible operation, this interaction can be significant and should be catered for in lifing methods. A model that has the potential to account for the effect of plasticity on creep, and creep on plasticity is therefore desirable. In previous work the authors presented the concept for a backstress model to predict creep strain rates in superalloys. This model was fitted to a limited dataset at a single temperature. The approach was validated using simple creep-dwell tests at the same temperature. This paper expands on the previous work in several ways: 1) The creep model has been fitted over a wide range of temperatures. Including the effect of temperature in complex creep models presents a number of difficulties in model fitting and these are explored. 2) The model was fitted to constant load (forward creep) and constant strain (stress relaxation) tests since any creep model should be able to predict both forms of creep deformation. However, these are often considered separately due to the difficulty of fitting models to two different datasets. 3) The creep deformation model was validated on stress change tests to ensure the creep deformation response can cope with changes in response variables. 4) The approach was validated using creep-fatigue tests to show that the creep deformation model, in addition to our established fatigue models, can predict damage in materials under complex loading.

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Relaxation of Compression Springs at High Temperatures

Journal of Basic Engineering ◽

10.1115/1.3425089 ◽

1970 ◽

Vol 92 (3) ◽

pp. 627-632

Author(s):

M. J. Siegel ◽

D. P. Athans

Keyword(s):

Stainless Steel ◽

Steady State ◽

Stress Relaxation ◽

Operating Temperature ◽

Transient Creep ◽

Tensile Creep ◽

Creep Data ◽

Limited Life ◽

Compression Springs ◽

Design Stress

An analysis is developed to determine the relaxation of cylindrical compression springs at temperatures where creep is predominantly steady state and the effect of transient creep and anelastic strain is small. Springs which operate under these conditions must be designed for limited life. Equations are derived which predict the relaxation of springs directly from tensile creep data for various materials. Using creep data for 18-8 stainless steel and Inconel-X, families of design curves are presented which give the time-temperature initial-stress relationships for various stress-relaxation ratios. These curves are useful in selecting an initial design stress for a specific operating temperature.

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Creep Deformation Characteristics of Austenitic Stainless Steel Foils

Key Engineering Materials ◽

10.4028/www.scientific.net/kem.462-463.906 ◽

2011 ◽

Vol 462-463 ◽

pp. 906-911 ◽

Cited By ~ 2

Author(s):

Hassan Osman ◽

Mohd Nasir Tamin

Keyword(s):

Stainless Steel ◽

Austenitic Stainless Steel ◽

Creep Deformation ◽

Primary Creep ◽

Composition Analysis ◽

Creep Life ◽

Concept Model ◽

Foil Specimen ◽

Stainless Steel Foils ◽

Steel Foils

Creep deformation process of austenitic stainless steel foil with thickness 0.25 mm was investigated. The foil specimen was creep tested at 750oC, 54 MPa to establish baseline behavior for its extended use as primary surface recuperator in advanced microturbine. The creep curve of the foil shows that the primary creep stage is brief and creep life is dominated by tertiary creep deformation. The curve is well represented by the modified theta-projection concept model with hardening and softening terms. Morphology of fractured foil surface reveals intergranular fracture with shallow network of faceted voids. The formation of w-type creep cavities is significant, as revealed by microstructure of ruptured specimen. Composition analysis indicates the formation of carbides, namely, Cr23C6, NbC and Fe3Nb3C.

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A Modified Theta Projection Creep Model for a Nickel-Based Super-Alloy

Volume 7A: Structures and Dynamics ◽

10.1115/gt2013-94805 ◽

2013 ◽

Cited By ~ 1

Author(s):

W. David Day ◽

Ali P. Gordon

Keyword(s):

Creep Rate ◽

Creep Deformation ◽

Primary Creep ◽

Tertiary Creep ◽

Creep Model ◽

Creep Tests ◽

Creep Equation ◽

Physical Constraints ◽

Primary Creep Strain ◽

Super Alloy

Accurate prediction of creep deformation is critical to assuring the mechanical integrity of heavy-duty, industrial gas turbine (IGT) hardware. The classical description of the creep deformation curve consists of a brief primary, followed by a longer secondary, and then a brief tertiary creep phase. An examination of creep tests at four temperatures for a proprietary, nickel-based, equiaxed, super-alloy revealed many occasions where there is no clear transition from secondary to tertiary creep. This paper presents a new creep model for a Nickel-based super-alloy, with some similarities to the Theta Projection (TP) creep model by Evans and all [1]. The alternative creep equation presented here was developed using meaningful parameters, or θ’s, such as: the primary creep strain, time at primary creep strain, minimum (or secondary) creep rate, and time that tertiary creep begins. By plotting the first and second derivative of creep, the authors were able to develop a creep equation that accurately matches tests. This creep equation is identical to the primary creep portion of the theta projection model, but has a modified second term. An additional term is included to simulate tertiary creep. An overall scaling factor is used to satisfy physical constraints and ensure solution stability. The new model allows a constant creep rate phase to be maintained, captures tertiary creep, and satisfies physical constraints. The coefficients of the creep equations were developed using results from 27 creep tests performed at 4 temperatures. An automated routine was developed to directly fit the θ coefficients for each phase, resulting in a close overall fit for the material. The resultant constitutive creep model can be applied to components which are subjected to a wide range of temperatures and stresses. Useful information is provided to designers in the form of time to secondary and tertiary creep for a given stress and temperature. More accurate creep predictions allow PSM to improve the structural integrity of its turbine blades and vanes.

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Stress Relaxation Continuum Damage Constitutive Equations for Relaxation Performance Prediction

Advanced Materials Research ◽

10.4028/www.scientific.net/amr.455-456.1434 ◽

2012 ◽

Vol 455-456 ◽

pp. 1434-1437

Author(s):

Jin Quan Guo ◽

Wei Zhang ◽

Xiao Hong Sun

Keyword(s):

Stress Relaxation ◽

Constitutive Equations ◽

Damage Mechanics ◽

Constitutive Models ◽

Continuum Damage Mechanics ◽

Creep Model ◽

Continuum Damage ◽

Relaxation Equation ◽

Creep Tests ◽

Analysis Technique

Stress relaxation constitutive equations based on Continuum Damage Mechanics, Kachanov-Robatnov creep model, and stress relaxation equation has been developed by analyzing stress relaxation damage mechanisms and considering the relationship that stress relaxation is creep at various stresses. And, the constitutive differential equations were integrated to predict stress relaxation performance by using numerical analysis technique. In order to validate the approach, the predicted results are compared to the experimental results of uni-axial isothermal stress relaxation tests conducted on 1Cr10NiMoW2VNbN steel with the same temperature of creep tests. Good agreement between results of relaxation tests and the predicted results indicates that the developed constitutive models can be used in the relaxation behavior evaluation of high temperature materials.

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The stress relaxation and creep behaviour of a manganese-stabilized austenitic stainless steel

The Journal of Strain Analysis for Engineering Design ◽

10.1243/03093247jsa476 ◽

2009 ◽

Vol 44 (3) ◽

pp. 201-209 ◽

Cited By ~ 4

Author(s):

A Pagliarello ◽

J Beddoes

Keyword(s):

Stainless Steel ◽

Austenitic Stainless Steel ◽

Stress Relaxation ◽

Creep Strain ◽

Creep Behaviour ◽

Constant Load ◽

Relaxation Behaviour ◽

Creep Tests ◽

Stress Strain ◽

The Difference

The stress relaxation behaviour of 21–4N, a manganese-stabilized austenitic stainless steel, is investigated in terms of the metallurgical state, the application of multiple strain levels during ‘stepped’ stress relaxation testing at 700 °C, the strain level during isostrain stress relaxation tests at 538 °C and 700 °C, and the correspondence with results from constant-load creep tests. The results indicate that for isostrain stress relaxation tests the stress relaxation rate is similar for strains that span both elastic and plastic strain levels. A transition in the stress relaxation behaviour occurs at a stress level approximately equivalent to the tensile stress–strain proportional limit; below this transition the stress–strain rate relationship, or the time predicted for 1 per cent creep strain, obeys a creep power law type of equation. Stress relaxation testing successfully delineates the difference between the creep resistances of two different metallurgical conditions with similar tensile properties using fewer specimens and requiring less time. The time to 1 per cent creep strain determined from the analysis of stress relaxation results is always less than the actual time to 1 per cent creep strain during constant-load creep tests.

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