David W. J. Tanner

Determination of creep and damage properties for P92 at 675 °C:

In order to predict the service life of components that operate at high temperatures, such as steam carrying pipes in conventional power plants, the material creep behaviour needs to be determined. There are little creep data available on grade P92 (9Cr2W) steel (a potential successor to P91) as it is a relatively new material; therefore a testing programme has been undertaken. This paper presents the results of uniaxial and notched bar creep tests on P92 parent material (PM) and P92 weld metal (WM) at 675 °C. The PM had higher failure times and lower minimum creep strain rates for tests in the same stress range (80–100 MPa) as the WM, but the PM and WM values tend to converge at high stress, with a significant difference between the failure times as applied stress decreases. The notch strengthening effect was found to decrease as the applied stress decreased. Processing of the test data including the calculation of the minimum creep strain rates has been performed to determine the material constants required for Norton’s steady state creep and both the Kachanov and the Liu and Murakami creep damage models. Material constant sets for creep of P92 PM and WM at 675 °C, including a parameter to describe the effect of a multiaxial stress state, have been obtained that give a good fit to the test data. Validation was achieved using finite element analysis.

Combined butt joint welding and post weld heat treatment simulation using SYSWELD and ABAQUS

Due to increases in computational power, the finite-element (FE) method is now widely used to predict the thermal, material, and mechanical effects of welding. Welding simulations can provide important information about component distortions and residual stresses. This facilitates a reduction in lead-time and cost associated with the process planning. Moreover, welding is generally performed in combination with other manufacturing processes such as stress relief heat treatment. This article presents a simple method where welding and post weld heat treatment operations are combined using an uncoupled plasticity—creep model. Two FE codes, welding-function-specific SYSWELD and the general-purpose FE package, ABAQUS, were used to perform butt joint welding and post weld heat treatment simulations of two Inconel 718 plates. The predicted results obtained from the two FE codes, such as thermal histories, residual stresses, nodal displacements, and stress relaxation, are compared. Based on the results presented, some useful benchmarking comments on the use of the two FE codes for welding simulation are given.

High temperature life prediction of a welded IN718 component

Life predictions from a case study of a welded feature in a generic spoke structure, determined using three-dimensional quasi-static elastic-plastic and creep finite element analyses, are demonstrated. The complete structure consists of multiple Inconel 718 (IN718) TIG-welded features; the welds exhibit noticeably depleted mechanical properties so a multimaterial analysis is necessary for accurate predictions. The effect the welds have on the life is investigated for both constant (creep) and cyclic loading (creep-fatigue) conditions at 620°C. Creep damage and Smith, Watson and Topper (SWT) strain parameter lifing methods are used, based upon material properties determined using uniaxial test data. The lower fatigue properties of the welded IN718 material at high temperature had a negative effect on the fatigue life of the structure. The effect of the weld on the life under constant loading at high temperature was found to be more difficult to evaluate due to significant stress relaxation.

Creep damage modelling of P92 pipe weld at 675°C

Abstract Creep data from ferritic steel P92 parent material, weld metal, cross-weld and heat affected zone specimens tested at 675°C are used to establish material behaviour models necessary for describing the creep behaviour of a P92 pipe with a circumferential weld. The method for determining the material properties for the heat affected zone of a P92 weld, in a single state variable creep damage constitutive model, using the data from cross-weld and impression creep tests, is described. The material properties for the parent material and weld metal regions have been already been published. Creep continuum damage finite element analyses are subsequently performed for a P92 pipe containing a circumferential weld, subjected to internal pressure and end loading, in order to assess the high temperature integrity and failure behaviour of a typical power plant component, from which the lifetime and potential failure modes can be predicted.

Creep crack incubation in P92 steel at 650°C in the presence of structural elastic follow-up

It is customary to study the creep deformation of materials at high temperatures and the incubation and growth of creep cracks using constant load test machines. However, this highly idealised loading condition does not accurately reflect the practical circumstances that occur when operating high temperature plants. Real loading conditions often lie between load and displacement control and correspond to situations where there is elastic follow-up, with low values relating to near displacement control and high values near to load control.This paper explains a series of experiments where pre-cracked martensitic P92 steel compact tension specimens are loaded and tested for different values of structural elastic follow-up, ranging from constant load to near fixed displacement. It is found that the degree of elastic follow-up significantly changes the time taken for creep crack incubation. This is a consequence of the relaxation of the load applied to the specimens. Elastic-plastic creep finite element simulations are used to reveal the underlying mechanical behaviour of the specimens. The simulations were confined to 2D analyses for plane stress and plane strain conditions. It is observed, that, irrespective of the initial loading and boundary conditions, the predicted mechanical response for plane stress and plane strain lies either side of the experimental results.Copyright

A Review of Current Finite Element Models for Irradiation Creep and Failure of Nuclear Graphite

Finite element models currently used for predicting irradiation creep and failure of nuclear graphite are presented. The underlying material properties required for these analyses and the relevant property changes due to irradiation are reviewed, followed by a description of the generation of constitutive equations. Continuum damage mechanics-based approaches for predicting the cracking and failure of graphite components are assessed. The computational implementation of the different models is briefly discussed and some examples from published literature are highlighted. To conclude, areas are identified that shall be addressed in order to develop a unified multi-scale continuum damage material behaviour model for graphite as part of the ongoing EPSRC ‘Fundamentals of current and future uses of nuclear graphite’ project.

A comparison of three weld consumables for P92 under creep

Abstract To encourage realistic power plant weld heat-affected zone failures within reasonable test times in laboratory creep testing, a welding consumable for P92 steel with the best creep performance was sought, to avoid failures in the weld metal itself. Therefore, a short term creep test programme was undertaken to evaluate weld pads manufactured using three commercially available consumables for P92, the results of which are presented in this paper. The effects of weld consumable composition are discussed both in general and with specific reference to the behaviour observed. The best overall and the poorest performing weld consumables were identified by composition. Microstructural investigations revealed the presence of precipitate free zones, that more readily formed using the poorest performing consumable as the result of retained δ–ferrite, since its deposited chemical composition led to the lowest value of δ–ferrite formation temperature. A consumable not based on P92 but on the stronger steel alloy FB2 did not perform as well as expected, confirming previous suggestions that the compositional factors which make a parent material creep-strong do not apply on a one-to-one basis to weld metals.

Prediction of Fatigue Crack Initiation and Growth During Thermal Cycling

Conventional approaches to assess fatigue under combined thermal and mechanical loading often utilize a fatigue design curve. In this paper models based on the physics and mechanics for the initiation and growth of fatigue cracks in stainless steel are first explained. The models are based on experimental evidence gathered for the initiation and growth of small cracks created during strain controlled laboratory tests. This evidence is then linked with data for the growth of large fatigue cracks in stainless steel. In the paper these models are coupled with finite element analyses to explore the fatigue initiation and growth of cracks in stainless steel pipes subjected to thermal cycling. It is assumed that the material behaviour is elastic-perfectly plastic, rate independent and fatigue occurs in air.The stress and strain fields for pipes subjected to a range of thermal loading conditions are explored. The fields are shown to be sensitive to parameters such as the Biot and Fourier numbers that include pipe dimensions, physical properties, dwell time and thermal conditions. Of particular interest is the temperature range and dwell time during thermal loading. Finite element analyses are then used to determine the stress and strain ranges created by thermal loading and these ranges are used in the crack initiation and growth models to estimate fatigue life.Copyright

Creep Behaviour of P92 and P92 Welds at 675°C

The results of an accelerated creep test programme on the leading new build high temperature power plant pipework ferritic steel P92 and two P92 welds are presented.Tests were performed at 675°C, which is above the service operating temperature range recommended for P92, but allowed for more realistic operating stresses to be used. Comparison with similar tests of P92 at lower temperatures has shown that testing at 675°C produces the same general creep behavior, and can therefore be used for component life assessments in the service operating temperature range.Axially loaded parent P92 material uniaxial round bar and notched bar specimens, given a heat treatment equivalent to that given post-welding, are compared with weld metal specimens and cross-weld specimens extracted from welds made using both a similar P92 consumable and a dissimilar IN625 nickel alloy consumable. Both welds exhibited typical premature ferritic steel weld failure at the heat-affected zone (HAZ) and parent material interface, known as Type IV.The creep crack growth behavior of parent material, weld metal and the HAZ was studied using compact tension specimens. Impression testing was performed to determine the relative creep behavior of the HAZ. The HAZ was found to have the highest minimum creep strain rates and creep crack growth rates, indicating the relative weakness of this region.Copyright

Creep and low cycle fatigue characterisation of IN718 TIG butt-welds at 620°C

The testing setups, results and analysis of constant load creep and low-cycle high temperature fatigue tests of tungsten inert gas (TIG) butt-welded, thin-section INCONEL 718 (IN718) specimens are presented. The main objectives were to determine the effect the welds have on failure time and analyse any differences in their failure behaviour. It was found that although welded IN718 may exhibit comparatively little loss of tensile strength, its ductility and creep and high temperature fatigue properties are severely compromised due to its changed microstructure.