Ondrej Muránsky

Creep Deformation and Microstructural Examination of a Prior Thermally Exposed Nickel Base Superalloy

When exposed for long time at elevated temperatures of 430°C and 650°C the nickelbase superalloy EI 698 VD can experience a significant decrease in the creep. The cause of the creep degradation of nickel-base superalloy has never been unequivocally explained, but it has generally attributed to the microstructural instability at prolonged high temperature exposure. The creep lifetime decrease was more expressive as time of thermal exposure was extended up to 52 000 hours. In this article, the creep life data, generated on prior long thermally exposed wrought nickelbase superalloy were related to the microstructural changes observed using SEM and TEM analysing techniques. Qualitative and quantitative structure analyses provided supporting evidence concerning changes associated with grain boundary carbide precipitation and with volume fraction changes of gamma prime the exposure time prolonged. In order to clarify the role of gamma prime distribution modification in thermally exposed superalloy on creep degradation the SANS (Small Angle Neutron Scattering) diffraction experiment was employed in the characterization of the gamma prime morphology and the size distribution with respect to the period of thermal exposure.

Analysis of Residual Stresses in Three-Pass Slot Weld (NeT-TG4): Finite Element Modelling and Neutron Diffraction

In this paper finite element simulation and neutron diffraction measurements are employed to assess the residual stresses in an AISI 316LN austenitic stainless steel plate containing a three-pass finite length weld in a machined slot. This work forms a part of the Task Group 4 (TG4) measurement and simulation round robin being undertaken by the NeT European network. Finite element (FE) simulations were carried out using the FEAT-WMT and ABAQUS commercial finite element packages. The welding process was modelled both using a 3D moving-heat source (MHS) which is spatially correct but computationally expensive and using a simplified “block-dumped” methodology (BD) in which an entire weld bead is deposited simultaneously. This does not reproduce the effects of welding torch movement, but is computationally less expensive. The mechanical analyses used combined isotropic-kinematic material constitutive models with a two-stage annealing functionality to remove plastic strain accumulated at high temperatures. The finite element predictions of weld residual stress are compared with neutron diffraction measurements obtained on the KOWARI diffractometer at the OPAL reactor at ANSTO.Copyright

The Impact of Key Simulation Variables on Predicted Residual Stresses in Pressuriser Nozzle Dissimilar Metal Weld Mock-Ups: Part 2—Comparison of Simulation and Measurements

British Energy (BE) has funded a large work programme to assess the possible impact of primary water stress corrosion cracking on dissimilar metal welds in the primary circuit of the Sizewell ‘B’ pressurised water reactor. This effort has included the design and manufacture of representative pressuriser safety/relief valve (SRV) nozzle welds both with and without a full structural weld overlay, multiple residual stress measurements on both mock-ups using the deep hole and incremental deep hole methods, and a number of finite element weld residual stress simulations of both the mock-ups and equivalent plant welds. Three organisations have performed simulations of the safety/relief valve nozzle configuration: Westinghouse, Engineering Mechanics Corporation of Columbus (EMC2 ) and the Australian Nuclear Science and Technology Organisation (ANSTO). The simulations employ different welding heat input idealisations, make different assumptions about manufacturing history, and use a variety of different material constitutive models, ranging from simple bilinear kinematic hardening to a full mixed isotropic-kinematic formulation. The availability of both high quality measurements from well characterised mock-ups, and a large matrix of simulations, offers the opportunity for a “mini-round-robin” examining both the accuracy and key solution variables of dissimilar metal weld finite element simulations. This paper is one of a series at this conference that examine various aspects of the BE work programme. It draws together residual stress measurement results and the results of all three simulation campaigns (described in detail in other papers at this conference) to examine the impact of manufacturing history, thermal modelling assumptions, material constitutive models and other key solution variables on the accuracy of residual stress predictions in this dissimilar metal weld geometry.Copyright

In Situ Neutron Diffraction Analysis of Phase Transformation Kinetics in TRIP Steel

The precise characterization of the multiphase microstructure of low alloyed TRIP steels is of great importance for the interpretation and optimisation of their mechanical properties. In-situ neutron diffraction experiment was employed for monitoring of conditioned austenite transformation to ferrite, and also for retained austenite stability evaluation during subsequent mechanical loading. The progress in austenite decomposition to ferrite is monitored at different transformation temperatures. The relevant information on the course of transformation is extracted from neutron diffraction spectra. The integrated intensities of austenite and ferrite neutron diffraction profiles over the time of transformation are then assumed as a measure of the volume fractions of both phases in dependence on transformation temperature. Useful information was also obtained on retained austenite stability in TRIP steel during mechanical testing. The in-situ neutron diffraction experiments were conducted at two different diffractometers to assess the reliability of neutron diffraction technique in monitoring the transformation of retained austenite during room temperature tensile test. In both experiments the neutron investigation was focused on the volume fraction quantification of retained austenite as well as on internal stresses rising in structure phases due to retained austenite transformation.

The Role of Plasticity Theory on the Predicted Residual Stress Field of Weld Structures

Constitutive plasticity theory is commonly applied to the numerical analysis of welds in one of three ways: using an isotropic hardening model, a kinematic hardening model, or a mixed isotropic-kinematic hardening model. The choice of model is not entirely dependent on its numerical accuracy, however, as a lack of empirical data will often necessitate the use of a specific approach. The present paper seeks to identify the accuracy of each formalism through direct comparison of the predicted and actual post-weld residual stress field developed in a three-pass 316LN stainless steel slot weldment. From these comparisons, it is clear that while the isotropic hardening model tends to noticeably over-predict and the kinematic hardening model slightly under-predict the residual post-weld stress field, the results using a mixed hardening model are quantitatively accurate. Even though the kinematic hardening model generally provides more accurate results when compared to an isotropic hardening formalism, the latter might be a more appealing choice to engineers requiring a conservative design regarding weld residual stress.

Round robin prediction of residual stresses in the edge-welded beam R6 validation benchmark problem

A simple austenitic steel beam specimen with a single autogenous weld bead laid along its top edge is an ideal training problem for novice weld modellers. This geometry may be analysed using either 3D or 2D FE models and employing either block-dumped or moving heat source techniques, with a variety of material constitutive models. This paper describes a residual stress simulation round robin performed on the validation benchmark problem of this type recently added to the R6 defect assessment procedure. A variety of solution strategies of increasing complexity are applied to the problem and the results compared with accuracy targets specified in the validation benchmark. It is found that the greatest solution accuracy is obtained using a 3D moving heat source, and mixed isotropic-kinematic material hardening behaviour.Copyright

Optimised modelling of weld metal constitutive behaviour in the NeT TG4 international weld simulation and measurement benchmark

The NeT TG4 benchmark specimen consists of a three pass type 316L TIG slot weld in a AISI type 316L plate. Phase one of the finite element simulation round robin performed on TG4 by the NeT network made the assumption that the weld metal exhibits the same mechanical behaviour as parent material. A comprehensive series of material characterisation tests on weld metal has now been completed, and these have allowed the derivation of a number of mixed isotropic-kinematic material models specifically for weld metal. The derived models have been used to improve the predicted stresses in the TG4 benchmark specimen. This paper first reviews the weld metal materials testing programme, and then discusses the optimum material hardening model fitting strategy to use for austenitic weld metal. The derived material models are tested by using them to predict residual stresses in the TG4 benchmark, and validating the predictions against the extensive database of measured residual stresses, and distortions.Copyright

The Impact of Key Simulation Variables on Predicted Residual Stresses in Pressuriser Nozzle Dissimilar Metal Weld Mock-Ups: Part 1—Simulation

British Energy (BE) has funded a large work programme to assess the possible impact of primary water stress corrosion cracking on dissimilar metal welds in the primary circuit of the Sizewell ‘B’ pressurised water reactor. This effort has included the design and manufacture of representative pressuriser safety/relief valve nozzle welds both with and without a full structural weld overlay, multiple residual stress measurements on both mock-ups using the deep hole and incremental deep hole methods, and a number of finite element weld residual stress simulations of both the mock-ups and equivalent plant welds. Three organisations have performed simulations of the safety/relief valve nozzle configuration: Westinghouse, Engineering Mechanics Corporation of Columbus (EMC2 ) and the Australian Nuclear Science and Technology Organisation (ANSTO). The simulations employ different welding heat input idealisations, make different assumptions about manufacturing history, and use a variety of different material constitutive models, ranging from simple bilinear kinematic hardening to a full mixed isotropic-kinematic formulation. The availability of both high quality measurements from well characterised mock-ups, and a large matrix of simulations, offers the opportunity for a “mini-round-robin” examining both the accuracy and key solution variables of dissimilar metal weld finite element simulations. This paper is one of a series at this conference that examine various aspects of the BE work programme. It describes the detailed finite element simulation of the mock-ups performed by BE and ANSTO. This makes use of the extensive mock-up manufacturing records to perform a detailed pass-bypass simulation of the entire manufacturing process from initial nozzle buttering through to completion of the safe end to pipe weld. The thermal simulation makes use of a dedicated welding heat source modelling tool to derive Gaussian volumetric heat source parameters from the welding records, and the mechanical simulation employs isotropic, kinematic and mixed isotropic-kinematic material constitutive models. Additional sensitivity studies examine sensitivity to manufacturing history and physical properties such as expansion coefficient mismatch.Copyright

In Situ Neutron Diffraction during Thermo-Mechanically Controlled Process in Low Alloy Steels

A challenge was made to examine the micro-structural evolution during thermomechanically controlled processing (TMCP) by in situ neutron diffraction. Since the neutron beam is too weak to achieve a time-division measurement to follow a rapid transformation in alow carbon steel, 2%Mn was added to make the austenite to ferrite transformation slower. Round bar specimens were heated up to 900°C with an electrical resistance method, then cooled down to 700°C, and compressed by 25% followed by step-by-step cooling. During the step-by-step cooling, neutron diffraction profiles were obtained and the volume fraction of ferrite, phase stresses and FWHM were analyzed. Using a similar TMCP simulator, specimens were quenched into water at several stages of the heat schedule to freeze the corresponding microstructures, which were observed with OM and SEM. As results, the ferrite volume fraction determined by neutron diffraction on cooling agrees well with that by microscopy. It is found that the austenite deformation and/or Nb addition accelerate the ferrite transformation to result in finer grain size.

Measured Biaxial Residual Stress Maps in a Stainless Steel Weld

Here, this paper describes a sequence of residual stress measurements made to determine a two-dimensional map of biaxial residual stress in a stainless steel weld. A long stainless steel (316L) plate with an eight-pass groove weld (308L filler) was used. The biaxial stress measurements follow a recently developed approach, comprising a combination of contour method and slitting measurements, with a computation to determine the effects of out-of-plane stress on a thin slice. The measured longitudinal stress is highly tensile in the weld- and heat-affected zone, with a maximum around 450 MPa, and compressive stress toward the transverse edges around ₋250 MPa. The total transverse stress has a banded profile in the weld with highly tensile stress at the bottom of the plate (y = 0) of 400 MPa, rapidly changing to compressive stress (at y = 5 mm) of ₋200 MPa, then tensile stress at the weld root (y = 17 mm) and in the weld around 200 MPa, followed by compressive stress at the top of the weld at around ₋150 MPa. Finally, the results of the biaxial map compare well with the results of neutron diffraction measurements and output from a computational weld simulation.