P91 Steel Pipe Research Articles

Finite element calculation of contour integral parameters for cracked P91 pipe weld

This paper reports the results of elastic–creep finite element (FE) analyses of a P91 steel pipe weld with two external ‘type IV’ circumferential cracks, subjected to internal pressure and end (system) load using creep properties obtained at 650°C. Numerical contour integral calculations have been performed to obtain both transient and stationary high temperature fracture mechanics parameters. A mesh sensitivity analysis was performed in order to ensure the accuracy of FE analyses in the transient creep stage. The effects of load magnitude, the material mismatch near the crack surfaces and the crack depth on the stationary creep C* contour integrals have been investigated, and corresponding analytical correlations are presented.

Finite element simulation of residual stresses induced by the dissimilar welding of a P92 steel pipe with weld metal IN625

Residual stresses induced by the fusion arc-welding of steel pipe joints in power generation plants are a concern to the industry. Residual stresses are induced by the process of welding due to the extreme nature of thermal cycles during the process. Welding is essential in the construction of high-grade steel pipelines, used as a conduit for steam at high temperature and pressure. The integrity and endurance of the welded pipes are necessary for the safe operation in power plants, which may be compromised by the presence of residual stresses. The finite element (FE) method is an effective tool for the prediction of residual stresses in such components, as long as the material behaviour can be accurately modelled. This paper reports the FE simulation of residual stresses, due to the arc-welding of a P92 steel pipe mainly using a nickel-based alloy (IN625) as a dissimilar weld material. The structural analysis part of the FE method of determining the residual stress field in the welded pipe is described and the results presented and discussed. Two user-defined subroutines have been used in the FE structural analysis to simulate the way the different phases of steel evolve during welding, including their differing plastic and hardening behaviour, derived from uniaxial tensile material testing carried out over a wide range of temperature. Thermal-expansion, including the effects of solid-state phase transformations in P92, has also been numerically modelled in the two subroutines, one of which prescribes two phases of P92 steel (tempered martensite and austenite) while the other assumes three phases (tempered martensite, austenite and untempered martensite).

Influence of heat input and post-weld heat treatment on boiler steel P91 (9Cr–1Mo–V–Nb) weld joints Part 1 – Microstructure

AbstractSteel P91 is known for its excellent high temperature properties. The achievement of optimum weld metal properties for steel P91 within the course of its extensive applications in power plants has however often caused concern. In the present work, three thick pipes of P91 steel were welded using three different levels of heat inputs within the range of 1·15–3·5 kJ mm−1. A circumferentially multipass butt welded P91 steel pipe, typically used for high temperature applications in power plants was selected for this investigation. The achievements of optimum weld metal properties, which are closely linked to microstructure, are known to cause concern in such weldments. Two types of heat treatments were employed, subcritical post-weld heat treatment and normalising/tempering treatment. The microstructure was evaluated by optical, scanning electron microscope, X-ray diffraction and magnetic permeability. The results have shown a great influence of heat input and heat treatment on the microstructure. Mar...

Residual stress measurements in a ferritic steel/In625 superalloy dissimilar metal weldment using neutron diffraction and deep-hole drilling

This paper reports the use of non-invasive and semi-invasive techniques to measure the residual stresses in a large dissimilar weldment. This took the form of a butt weld between two sections of a P92 steel pipe, joined using an In625 welding consumable. Residual stress measurements have been carried out on the 30 mm thick welded pipe using the deep-hole drilling technique to characterise the through wall section residual stress distribution for the weld metal, HAZ and parent material. In addition, neutron diffraction measurements have been carried out within the weld zone. Diffraction patterns presented a high intensity and sharp peaks for the base P92 steel material. However measurements in the weld superalloy material were proven problematic as very weak diffraction patterns were observed. A thorough examination of the weld material suggested that the likely cause of this phenomenon was texture in the weld material created during the solidification phase of the welding procedure. This paper discusses the challenges in the execution and interpretation of the neutron diffraction results and demonstrates that realistic measurements of residual stresses can be achieved, in complex dissimilar metal weldments.

Analysis of creep crack growth behavior of P92 steel welded joint by experiment and numerical simulation

High temperature creep crack growth tests were carried out on standard compact specimens machined from the welded joint of ASME P92 steel pipe. The creep crack growth behaviors of the distinct sub-regions of welded joint were investigated to clarify the ability of creep crack resistance under high temperature. In addition, good correlations between creep crack growth rate and C⁎ for different microzones of welded joint were obtained. The tested results revealed that the sub-regions of welded joint exhibited different creep crack behaviors and at the same value of C* the highest creep crack growth rate occurred in the fine grained heat affected zone (FGHAZ) which was known as Type IV cracking zone. Furthermore, finite element method analyses coupled with continuum damage mechanics were conducted to predict the creep crack growth behavior and to study the effect of multistress state on crack propagation.

Mapping Multiple Components of the Residual Stress Tensor in a Large P91 Steel Pipe Girth Weld Using a Single Contour Cut

The contour method is applied in an innovative manner to measure the distribution of hoop residual stress in a large martensitic-ferritic steel pipe containing a multi-pass girth weld. First, a novel one-step wire electro-discharge machining cut is conducted to divide the pipe lengthways into two halves. The deformation of the cut halves is then measured and analysed in a way that simultaneously gives maps of hoop stress across the wall thickness on both sides of the pipe and automatically accounts for through-thickness hoop bending effects and how they may vary along the pipe. Finally the contour method results are combined with X-ray diffraction residual stress measurements using the principle of superposition to determine the distribution of the axial and radial residual stresses in the pipe. It is thereby demonstrated how the distribution of three direct components of the residual stress tensor in a welded pipe can be readily determined using a “hybrid” contour measurement approach.

Finite element thermal analysis of the fusion welding of a P92 steel pipe

Abstract. Fusion welding is common in steel pipeline construction in fossil-fuel power generation plants. Steel pipes in service carry steam at high temperature and pressure, undergoing creep during years of service; their integrity is critical for the safe operation of a plant. The high-grade martensitic P92 steel is suitable for plant pipes for its enhanced creep strength. P92 steel pipes are usually joined together with a similar weld metal. Martensitic pipes are sometimes joined to austenitic steel pipes using nickel based weld consumables. Welding involves severe thermal cycles, inducing residual stresses in the welded structure, which, without post weld heat treatment (PWHT), can be detrimental to the integrity of the pipes. Welding residual stresses can be numerically simulated by applying the finite element (FE) method in Abaqus. The simulation consists of a thermal analysis, determining the temperature history of the FE model, followed by a sequentially-coupled structural analysis, predicting residual stresses from the temperature history. In this paper, the FE thermal analysis of the arc welding of a typical P92 pipe is presented. The two parts of the P92 steel pipe are joined together using a dissimilar material, made of Inconel weld consumables, producing a multi-pass butt weld from 36 circumferential weld beads. Following the generation of the FE model, the FE mesh is controlled using Model Change in Abaqus to activate the weld elements for each bead at a time corresponding to weld deposition. The thermal analysis is simulated by applying a distributed heat flux to the model, the accuracy of which is judged by considering the fusion zones in both the parent pipe as well as the deposited weld metal. For realistic fusion zones, the heat flux must be prescribed in the deposited weld pass and also the adjacent pipe elements. The FE thermal results are validated by comparing experimental temperatures measured by five thermocouples on the pipe outside surface with the FE temperature history at corresponding nodal points.

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

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.

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.

Thermomechanical modelling of weld microstructure and residual stresses in P91 steel pipe

AbstractA multipass circumferentially 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), which are then validated by published experimental data. The axisymmetric FE simulation incorporates solid state phase transformation by allowing for volumetric changes and associated changes in yield stress and hardening behaviour due to austenitic and martensitic transformations. The thermal cycles during welding cause different microstructural regions to emerge within the weld metal and HAZ. 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...

Measuring and modelling residual stresses in butt welded P91 steel pipe including effects of phase transformations

AbstractResidual 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 290 mm and a wall thickness of 55 mm, typical of components in power generation plants. An axisymmetric thermomechanical finite element (FE) simulation has been performed to obtain the residual stress field induced by the fusion welding of the pipe, taking solid state phase transformation effects into account and using temperature dependent material property data. Residual stresses have been measured using the X-ray diffraction and deep hole drilling techniques. Good correlation has been demonstrated with the predictions of the FE model. The paper demonstrates that a mixed experimental and numerical approach is useful for determining the residual stress distribution in welded joints.

Finite element simulation of welding and residual stresses in a P91 steel pipe incorporating solid-state phase transformation and post-weld heat treatment

The finite element (FE) method has been applied to simulate residual axial and hoop stresses generated in the weld region and heat-affected zone of an axisymmetric 50-bead circumferentially butt-welded P91 steel pipe, with an outer diameter of 145 mm and wall thickness of 50 mm. The FE simulation consists of a thermal analysis and a sequentially coupled structural analysis. Solid-state phase transformation (SSPT), which is characteristic of P91 steel during welding thermal cycles, has been modelled in the FE analysis by allowing for volumetric changes in steel and associated changes in yield stress due to austenitic and martensitic transformations. Phase transformation plasticity has also been taken into account. The effects of post-weld heat treatment (PWHT) have been investigated, including those of heat treatment holding time. Residual axial and hoop stresses have been depicted through the pipe wall thickness as well as along the outer surface of the pipe. The results indicate the importance of including SSPT in the simulation of residual stresses during the welding of P91 steel as well as the significance of PWHT on stress relaxation.

Numerical simulation of P91 pipe welding including the effects of solid-state phase transformation on residual stresses

The methodology of numerically simulating residual stresses in a welded P91 pipe section is described. The finite element (FE) method has been applied to simulate residual axial and hoop stresses generated in the weld region and heat affected zone (HAZ) of an axisymmetric 50-bead circumferentially butt-welded P91 steel pipe, with outer diameter of 145 mm and wall thickness of 50mm. The FE simulation consists of a thermal analysis which is followed by a sequentially-coupled structural analysis. Solid-state phase transformation (SSPT), which is characteristic of P91 steel during welding thermal cycles, has been modelled in the FE analysis by allowing for volumetric changes in steel and associated changes in yield stress due to austenitic and martensitic transformations. Phase transformation plasticity has also been taken into account. Preheat and interpass temperature control has been included in the modelling process. Thermally-obtained temperature contours indicate the size of the weld region, parent metal penetration, and HAZ. Residual axial and hoop stresses have been depicted through the pipe wall thickness as well as along the outer surface of the pipe. The results indicate the importance of including SSPT in the simulation of stresses during the welding of P91 steel.