Philip Bendeich

Laser cladding repair of turbine blades in power plants: from research to commercialisation

AbstractReliable and efficient power generation is a major global issue due to both political and environmental concerns. Nevertheless, many critical components, particularly the blades of the low pressure (LP) side of power generating steam turbines, are subject to failure due to severe erosion at the leading edges. Since taking machines offline for maintenance and removal of damaged blade for repair is extremely expensive, increasing the service life of these critical components offers significant economic and political benefits. Conventional techniques to increase service life include brazing of an erosion shield at the leading edge of the turbine blade, open arc hardfacing, and cladding with erosion resistant materials using gas tungsten, manual metal or plasma transferred arc welding. The authors have been investigating since 2001 the use of laser cladding technology to deposit a high quality and erosion resistant protection shield on the leading edge of LP blades. The project has demonstrated the fe...

Optimisation of Mixed Hardening Material Constitutive Models for Weld Residual Stress Simulation Using the NeT Task Group 1 Single Bead on Plate Benchmark Problem

A single weld bead deposited on a flat plate is a deceptively simple problem that is in practice a challenge for both measurement and prediction of weld residual stresses. Task Group 1 of the NeT collaborative network has examined this problem in an exten

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

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

Sensitivity of predicted weld residual stresses in the NeT Task Group 1 single bead on plate benchmark problem to finite element mesh design and heat source characteristics

A single weld bead deposited on a flat plate is a deceptively simple problem that is, in practice, a significant challenge for both measurement and prediction of weld residual stresses. Task Group 1 of the NeT collaborative network has examined this probl

Predicting Solid-State Phase Transformations during Welding of Ferritic Steels

The current work presents the numerical analysis of solid-state transformation kinetics relating to conventional welding of ferritic steels, with the aim of predicting the constituent phases in both the fusion zone and the heat affected zone (HAZ) of the weldment. The analysis begins with predictions of isothermal transformation kinetics using thermodynamic principles, such that the chemical composition of the parent metal is the sole user-defined input. The data is then converted to anisothermal transformation kinetics using the Scheil-Avrami additive rule, including the effects of peak temperature and austenite grain growth. Subroutines developed for the Abaqus finite element package use the semi-empirical approach described to predict phase transformations in SA508 Gr.3 Cl.1 steel. To study the effect of the cooling rates and the ability of the current model to predict the final microstructure, two weld samples were subjected to autogenous beam TIG welds under a fast (TG5-F, 5.00 mm/s) and slow (TG5-S, 1.25 mm/s) torch speed. Model validation is carried out by direct comparison with microstructural observations and hardness measurements (via nanoindentation) of the fusion and heat affected zones in both welds. Excellent agreement between the measured and predicted hardness has been found for both weld samples. Additionally, it is shown that the correct identification of the partial austenisation region is a crucial input parameter.

The Impact of Axi-Symmetric Boundary Conditions on Predicted Residual Stress and Shrinkage in a PWR Nozzle Dissimilar Metal Weld

Simulation of a dissimilar metal weld (DMW) in a pressurised water reactor (PWR) nozzle was performed to predict both axial distortion and hoop residual stresses in the weld. For this work a computationally efficient axi-symmetric finite element (FE) simulation was carried out rather than a full 3D analysis. Due to the 2-dimensional nature of the analysis it was necessary to examine the effect of structural restraint during welding of the main dissimilar metal weld (DMW). Traditionally this type of analysis is set up to allow one end of the structure, in this case the safe-end forging, to be unrestrained in the axial direction during welding. In reality axial expansion and subsequent contraction of deposited weld metal at the current torch position is restrained by solidified material both ahead and behind the torch. Thus the conventional axi-symmetric analysis is under-restrained in the axial direction at least during the early weld passes. The significance of this was examined by repeating the current simulation with the safe-end forging fixed to limit expansion during the heat up cycle. Contraction was however, allowed during cooling cycle. This modified boundary control method provided a significantly improved prediction of the axial distortion across the weld as well as improved prediction of through wall axial and hoop residual stresses.Copyright

Predicting post-weld residual stresses in ferritic steel weldments

The implementation of a semi-empirical solid-state phase transformation subroutine in the ABAQUS finite element package is presented to predict the influence of transformation strain on the post-weld residual stress field in ferritic steels. The phase transformation subroutine has been outlined in a previous study (PVP2011-57426), where it was proven accurate in predicting the phase compositions in the fusion and heat affected zone (HAZ) of an autogenous TIG beam weld in SA508 Gr.3 Cl.1 steel.While previous work focused on predictions of the steady-state material response using a 2D thermal model, the present analyses are 3D and capture the varying phase composition at weld start- and stop-ends. Predicted cooling rates at either end of the specimen are significantly higher, leading to a variation in the predicted microstructure along the weld line.To better understand the structural changes that occur in ferritic steels during a conventional welding process, a representative model of SA508 Gr.3 Cl.1 steel is discussed. The contribution of thermal, metallurgical, and transformation-induced plastic strain is highlighted in this example, providing insight to the key simulation variables necessary for accurate weld models of ferritic steels. Preliminary coupled thermo-mechanical analyses are presented that compare predicted residual stress distributions with those measured in SA508 Gr.3 Cl.1 beam welds via neutron diffraction; good agreement is observed.© 2012 ASME

Accounting for Phase Transformations During Welding of Ferritic Steels

The numerical application of solid-state phase transformation kinetics relating to conventional welding of ferritic steels is presented. The inclusion of such kinetics in weld models is shown to be necessary for capturing the post-weld residual stress field. To this end, a comparison of two approaches is outlined: a semi-empirical approach that uses thermodynamic transformation kinetics to predict phase morphology; and a fully empirical approach that directly links local material temperature to the present constituent phase(s). The semi-empirical analysis begins with prediction of TTT diagrams using thermodynamic principles for ferritic steels. The data is then converted to CCT diagrams using the Scheil-Avrami additive rule, including austenite grain growth kinetics. This information is used to predict the phases present under varying peak temperatures and cooling rates. In the fully empirical approach, dilatometric experiments of steel samples are performed during heating to simulate expected welding conditions. The constitutive response of the sample is then used as input for the subsequent numerical weld analyses. Input derived from each technique is transferred into weld models developed using the Abaqus finite element package. Model validation is carried out by direct comparison with neutron diffraction residual stress measurements on two beams of SA508 Gr.3 Cl.1 steel subjected to autogenous beam TIG welds under varying torch speeds, heat input and preheat conditions.© 2011 ASME