A.H. Yaghi

A Comparison Between Measured and Modeled Residual Stresses in a Circumferentially Butt-Welded P91 Steel Pipe

Residual macrostresses in a multipass circumferentially butt-welded P91 ferritic steel pipe have been determined numerically and experimentally. The welded joint in a pipe with an outer diameter of 290 mm and a wall thickness of 55 mm is typical of power generation plant components. An axisymmetric thermomechanical finite element model has been used to predict the resulting residual hoop and axial stresses in the welded pipe. The effects of the austenite to martensite phase transformation have been incorporated into the simulation. Residual stresses have been measured using the X-ray diffraction technique along the outer surface of the pipe and using the deep-hole drilling technique through the wall thickness at the center of the weld. Good correlation has been demonstrated between the residual hoop and the axial stresses obtained numerically and experimentally. The paper demonstrates the importance of using a mixed experimental and numerical approach to determine accurately the residual macrostress distribution in welded components.

Finite element simulation of welded P91 steel pipe undergoing post-weld heat treatment

Abstract Residual stresses induced by fusion welding of P91 steel pipes in power generation plants can be detrimental to their integrity and mechanical performance in service. Post-weld heat treatment (PWHT) of the pipes substantially reduces the magnitude of residual stresses, mitigating their ill effects. In this paper, the residual stress field of a typically welded P91 steel pipe, reproduced from a previous publication by the authors, is subsequently subjected to PWHT for 3 h at a temperature of 760°C. A finite element model has been generated to simulate the PWHT of the pipe. Stress relaxation testing has been conducted on P91 steel uniaxial tensile specimens to obtain material property data necessary for PWHT modelling. The simulation method and the influence of PWHT on residual stresses are described.

Use of the reference stress method in estimating the life of pipe bends under creep conditions

Abstract The effect that the change of ovality of pipe bends, due to creep under internal pressure, has on the stationary-state stress state has been investigated using a reference stress approach. Attention has been focused on practical pipe bend geometries and loading conditions in power plant applications. Prediction of failure times based on stationary-state stresses, for cases in which ovality changes are neglected and in which ovality is included, indicate that the change of ovality can be very significant in some practical situations. Failure times obtained using a creep continuum damage mechanics approach have also been compared with those obtained using the stationary-state predictions with ovality changes included. These results indicate that even sophisticated damage mechanics analyses are inadequate unless the effects of the changing ovality which occurs are taken into account.

Current capabilities of the thermo-mechanical modelling of welding processes

Some fundamental principles and advanced applications of thermo-mechanical modelling techniques used for industrial welding processes are described. The paper covers a range of modelling procedures comprising welding simulation and residual stress analysis of multi-pass, thick-walled ferritic steel pipes; welding simulation and distortion analysis of nickel-based superalloy thin plates; and inertia friction welding of dual alloy drive shafts. A number of pertinent mechanical and metallurgical concepts are discussed, including material behaviour models and material properties, microstructural evolution, solid state phase transformation and post-weld heat treatment. The paper emphasises the general methodologies of thermal modelling procedures and their role in the holistic process of the life assessment and design improvement of power plant piping systems and aeroengine casings and shaft components, operating under high temperature creep and fatigue service conditions.

Thermo-mechanical modelling of P91 steel weld microstructure and residual stresses in power plant pipework

A circumferentially multi-pass butt-welded P91 steel pipe, typically used for high-temperature applications in power plants, has been numerically analysed to determine residual stresses, induced by the process of welding, as well as microstructural regions in the weld, caused by thermal cycles. The finite element (FE) method has been applied to simulate residual stresses generated in the weld region and heat affected zone (HAZ). The axisymmetric FE simulation incorporates solidstate phase transformation (SSPT) by allowing for volumetric changes in steel and associated changes in yield stress due to austenitic and martensitic transformations. The thermal cycles during welding cause different microstructural regions to emerge in the vicinity of the weld metal. Columnar and equiaxed microstructural zones have been numerically modelled in the weld region of the pipe. The predicted FE microstructural regions have been corroborated by columnar and equiaxed zones that have been mapped out on a cross-sectional macro-image of the weld.

Measuring and modelling residual stresses in a butt-welded P91 steel pipe

Residual stresses in a circumferentially butt-welded steel pipe have been measured and numerically predicted. The pipe, containing the circumferential weld, has an outer diameter of 290mm and a wall thickness of 55mm, typical of components in power generation plants. Residual stresses have been measured using the X-ray diffraction and deep-hole drilling techniques. Good correlation has been demonstrated with the predictions of an axisymmetric thermo-mechanical finite element model. The paper demonstrates that a mixed experimental and numerical approach is useful for determining the residual stress distribution in welded joints.

A holistic approach to structural integrity of high temperature welds in power plants

Assessment of the structural integrity of welded components in power plant steels, operating at elevated temperature, requires a multi-disciplinary approach, involving materials science, experimental techniques, computational modelling and engineering design. This paper briefly introduces some aspects concerning the development of a holistic process for high temperature performance assessment of power plant welds. This covers a range of subjects including microstructure characterization, welding process modelling, experimental testing, formulation of constitutive material behaviour models and life assessment methods for welds. Requirements for future improvement and development are outlined.