P91 Steel Pipe Research Articles

Effect of Structural Induced Stress on Creep of P92 Steel Pipe to Elbow Welds

Pipe to elbow welds are usually identified as the weakest parts in the pipeline system of ultra-supercritical boilers due to the structural induced stress arising from internal steam pressure, and the constraint of supports and hangers. The finite element (FE) method has been applied to investigate the effect of structural induced stress on creep evolution in pipe to elbow welds. The results show that compressive axial structural induced stress can significantly increase the creep strain near the pipe’s outer surface. In contrast, the creep strain near the pipe’s inner surface is clearly accelerated by tensile axial structural induced stress. Compared with free deformation conditions in the pipe ends, when subject to a compressive axial structural induced stress under −30 MPa, the equivalent creep strain in the fine-grained heat affected zone (FGHAZ) at the 12:00 position on the outer surface increases by about 13.7 times. In the case of a 30 MPa tensile axial structural induced stress, the equivalent creep strain increases by about 83.3% in the FGHAZ at the 12:00 position on the inner surface. The maximum creep strain of the pipe to elbow weld in the ultra-supercritical boiler after creep for 5000 h is 1.9% and located at the 10:30 position in the FGHAZ on the pipe’s outer surface, which makes it the weakest part of the welded joint. The location of a crack in a pipe to elbow weld after running for 20,000 h is in agreement with the simulation results.

The Effects of Various Conditions of Short-Term Rejuvenation Heat Treatment on Room-Temperature Mechanical Properties of Thermally Aged P92 Boiler Steel.

In this work, the effects of various conditions of short-term rejuvenation heat treatment on room-temperature mechanical properties of long-term aged P92 boiler steel were investigated. Normalized and tempered P92 steel pipe was thermally exposed at 600 °C for time durations up to 5000 h in order to simulate high-temperature material degradation, as also occurring in service conditions. Thus, thermally embrittled material states of P92 steel were prepared, showing tempered martensitic microstructures with coarsened secondary phase precipitates of Cr23C6-based carbides and Fe2W-based Laves phase. Compared with the initial normalized and tempered material condition, thermally aged materials exhibited a slight decrease in strength properties (i.e., yield stress and ultimate tensile strength) and deformation properties (i.e., total elongation and reduction of area). The hardness values were almost unaffected, whereas the impact toughness values showed a steep decrease after long-term ageing. An idea for designing the rejuvenation heat treatments for restoration of impact toughness was based on tuning the material properties by short-term annealing effects at various selected temperatures somewhat above the long-term ageing temperature of P92 material. Specifically, the proposed heat treatments were performed within the temperature range between 680 °C and 740 °C, employing variable heating up and cooling down conditions. It was revealed that short-term annealing at 740 °C for 1 h with subsequent rapid cooling into water represents the most efficient rejuvenation heat treatment procedure of thermally aged P92 steel for full restoration of impact toughness up to original values of normalized and tempered material state. Microstructural observations clearly indicated partial dissolution of the Laves phase precipitates to be the crucial phenomenon that played a key role in restoring the impact toughness.

Influence of filler metal on residual stress in multi-pass repair welding of thick P91 steel pipe

Based on the thermal-elastic-plastic theory, a three-dimensional finite element (FE) model for residual stress in multi-layer and multi-pass repair welding of P91 steel pipe with wall thickness of 60 mm is established, which considers the sequential coupling among thermal field and microstructure and mechanical properties. The arc heat input is described as a double-ellipsoid heat source, and the solid-state phase transformation (SSPT) concerning austenite and martensite during welding of P91 steel is modeled through allowing for the changes of material volume and yield stress as well as phase transformation plasticity in the finite element analysis. Using the established model, the residual stresses in repair welding of P91 steel pipe with similar and dissimilar filler metals are calculated through SYSWELD software, which are also compared with experimental results to validate the accuracy of the numerical model. The distribution features of residual stresses under the two conditions are comparatively investigated, and the influence of different filler metals on them is also analyzed. When using similar filler metal, the stress components and equivalent stress have strong fluctuations on the weld surface due to martensitic transformation, and there exists relatively wide compressive stress region on the weld surface. At the fusion zone (FZ) near the weld surface, a serious stress concentration appears. The tensile stress peak can reach 813 MPa, close to the yield stress of untempered martensite. While using Ni-based alloy as filler metal, the peak values of tensile stress components and equivalent stresses in the HAZ of weldment surface are higher than those for the former owing to lack of relaxation by phase transformation. In the interior of weldment, the region with high stress in the FZ is much smaller, and the welding stress in the HAZ is also lower compared with those for the former.

Comparison of measured and modelled residual stresses in a welded P91 steel pipe undergoing post weld heat treatment

The process of fusion arc welding of steel pipes in power generation plants induces residual stresses which may be detrimental to the integrity and endurance of plant pipelines. P91 is high-grade steel used in the construction of pipelines carrying hot steam at high pressure, conditions which cause creep during service. Welded P91 pipes are usually subjected to post-weld heat treatment (PWHT) to mitigate the magnitude of residual stresses and temper the material, hence improving its resistance to creep. In this paper, the finite element (FE) method of modelling residual stresses due to PWHT in a circumferentially butt-welded P91 pipe is presented. The PWHT hold temperature is 760 °C. The paper describes the X-Ray Diffraction (XRD) and Deep-Hole Drilling (DHD) experimental techniques and how they are applied to measure residual stresses in the welded P91 pipe after PWHT. The material property data, necessary for the FE simulation of PWHT, has been obtained from stress-relaxation tests on P91 uniaxial tensile specimens at 760 °C. Good agreements have been achieved between the results of the FE method and the two sets of experimentally-measured residual stresses.

Effect of Internal Air Flow on Local Postweld Heat Treatment for Large Diameter ASME P92 Steel-Welded Pipes

Abstract Local postweld heat treatment (PWHT) is usually employed in field fabrication of large-sized ASME SA-335 Grade P92 steel pipes. Internal air flow in pipes that arise from field fabrication can result in considerable convection losses on the inside surface of the pipe when the pipe is not strictly sealed off. Welding and local PWHT experiments of a large diameter P92 steel pipe were conducted both with and without internal air flow, and temperature field of both sides of the pipe was measured. The conjugate heat transfer between the pipe and the internal air is simulated using computational fluid dynamics (CFD) method. The effect of internal air flow on temperature field was further investigated. Results indicate that temperature gradient along through-thickness direction and axial direction during local PWHT is significantly increased due to internal air flow. The increasing rate of temperature difference between inner and outer surface at weld centerline to internal air velocity is about 14.5 °C/(m s−1). The maximum temperature is no longer located at the weld centerline, which will lead to a risk of overheating. The temperature drop is severer in the air inlet side than air outlet side at same distance from weld centerline. For local PWHT to be successful, the internal air flow should be strictly limited during local PWHT; otherwise, the width of heated band (HB) should be extended.

Application of the Barkhausen effect probe with adjustable magnetic field direction for stress state determination in the P91 steel pipe

Abstract The paper presents the results of application of a novel Barkhausen effect (BE) probe with adjustable magnetizing field direction for the stress level evaluation in ferromagnetic materials. The investigated sample was in a form of a pipe, made of P91 steel that was anisotropic due to the production process. The measurements were performed before and after welding, revealing the influence of welding process on the residual stress distribution. As was observed, the process introduced high tensile stresses in the normal to the weld direction (which can be interpreted as a decrease of strongly compressive residual stresses present in martensitic steels). In addition to that, the paper presents investigations of the measurement set performance corroborating its applicability for Barkhausen effect signal measurements in the magnetically anisotropic materials. The signals obtained during manual rotation of the probe (typical method of BE measurements) are very similar to those recorded during automatic field axis rotation.

Development of Creep Damage in Similar Weld Joints of P92 Steel Pipe

The microstructure and creep behaviour of the welded joints of P92 steel pipe were investigated in order to determine the influence of orbital heat welding technology on the creep resistance. Creep specimens were machined from the welded joints. Tensile creep tests of welded joints were performed at 873 K using different stresses. The microstructure of tested specimens was investigated by scanning electron microscope Tescan equipped with an electron-back scatter diffraction. The creep results showed that the creep fracture strain of the welded joints decreases with decreasing value of applied stress. Microstructure investigation showed that fracture behaviour of welded joints is influenced by an enhanced cavity formation at grain boundaries in the heat-affected zone causing lower fracture ductility.

Microstructure and transverse shrinkage stress analysis in GTA welds of P91 steel pipe

In a steam power plant, several components such as boiler tube, condenser and steam lines are made of high creep resistant steel. The P91 steel pipes are generally used in steam power plant because of high creep strength at service temperature of approximately 600oC. In the present research work, the study about shrinkage stresses and their distribution in the four quadrants of P91 pipe weld of 11 mm thickness is reported. The conventional-V and narrow-groove welds were prepared by using the gas tungsten arc welding (GTAW) process. Welding current, arc voltage, groove design, and straining length were the main parameters that affect the transverse shrinkage stresses. In the present research work, the effect of groove design on transverse shrinkage stresses has been evaluated. It also describes the effect of the number of passes on shrinkage. It is concluded that, for a given heat input, the narrow groove pipe weld joint exhibits comparatively less transverse shrinkage stress. Scanning electron microscope (SEM) with field emission gun and optical microscope has been used to characterize the weld fusion zone and HAZs of P91 pipe weldments.

Creep properties in similar weld joint of a thick-walled P92 steel pipe

The creep behaviour of a welded joint of a real thick-walled P92 steel pipe was investigated at 600 and 650°C and a stress ranging of 60–250MPa using uniaxial tensile creep tests. Creep smooth cross-weld specimens were machined from different positions of the weld representing the welding sequence. Following analysis of creep data, all creep tests were performed in the region of power-law creep. The rate-controlling creep deformation mechanism is most probably climb of intergranular mobile dislocations for both the weld and base P92 steel. The creep resistance of the weld joint was lower than that of the base metal. No substantial differences were found in the minimum creep, the time to fracture and creep ductility for creep specimens taken from different locations of the weld. The fracture morphology of the crept welded specimens was found to be a mixture of transgranular ductile and intergranular mode. The fracture locations were found to shift from the weld metal in the higher stress region to the heat affected zone in the lower stress region. Many cavities formed at grain boundaries during creep. It was considered that the coarse precipitates of Laves phase acted as the preferential nucleation sites for cavities.

Effect of Groove Design and Post-Weld Heat Treatment on Microstructure and Mechanical Properties of P91 Steel Weld

The martensitic creep-resistant steel designated as ASTM A335 for plate and as P91 for pipe is primarily used for high-temperature and high-pressure applications in steam power plants due to its excellent high-temperature properties such as high creep strength, high thermal conductivity, low thermal expansion, and so on. However, in the case of welded joints of such steels, the presence of an inter-critical heat-affected zone (IC-HAZ) can cause the joint to have lower creep strength than the base metal. In the present study, the effect of post-welding heat treatment (PWHT) and weld groove designs on the overall microstructure and mechanical properties of P91 steel pipe welds produced by the gas tungsten arc welding process was studied. Various regions of welded joints were characterized in detail for hardness and metallographic and tensile properties. Sub-size tensile samples were also tested to evaluate the mechanical properties of the weld metal and heat-affected zone (HAZ) with respect to PWHT. After PWHT, a homogenous microstructure was observed in the HAZ and tensile test fracture samples revealed shifting of the fracture location from the IC-HAZ to the fine-grained heat-affected zone. Before PWHT, the conventional V-grooved welded joints exhibited higher tensile strength compared to the narrow-grooved joints. However, after PWHT, both narrow- and V-grooved joints exhibited similar strength. Fractography of the samples indicates the presence of carbide precipitates such as Cr23C6, VC, and NbC on the fracture surface.

Finite Element Creep Damage Analyses and Life Prediction of P91 Pipe Containing Local Wall Thinning Defect

AbstractCreep continuum damage finite element (FE) analyses were performed for P91 steel pipe containing local wall thinning (LWT) defect subjected to monotonic internal pressure, monotonic bending moment and combined internal pressure and bending moment by orthogonal experimental design method. The creep damage lives of pipe containing LWT defect under different load conditions were obtained. Then, the creep damage life formulas were regressed based on the creep damage life results from FE method. At the same time a skeletal point rupture stress was found and used for life prediction which was compared with creep damage lives obtained by continuum damage analyses. From the results, the failure lives of pipe containing LWT defect can be obtained accurately by using skeletal point rupture stress method. Finally, the influence of LWT defect geometry was analysed, which indicated that relative defect depth was the most significant factor for creep damage lives of pipe containing LWT defect.

Laves Phase Formation and Its Effect on Mechanical Properties in P91 Steel

Effect of Laves phase formation on mechanical properties in a pressurized T-junction of P91 steel pipe at 849 K for 58,000 h with 25.65 MPa vapor pressure was studied. Thermodynamic calculations had been performed by using the software Thermo-Calc to study the phase at equilibrium state. Counter plot of von Mises stress in the pipe during service life was calculated by finite element analysis to study the effect of the operated stress distribution on the evolution of Laves phase. The change in the microstructure and mechanical properties in the sites with different stress was also studied. The results indicated that the formation of Laves phase in P91 steel was a thermodynamically possible process due to enrichment of Mo and depletion of C adjacent to M 23C6 particles or along martensite lath and packet boundaries. The formation of Laves phase had a detrimental influence on the mechanical properties in P91 steel. The mean size of Laves phase would be significantly increased with increasing operated stress, leading to a reduction in tensile properties and impact energy. In particular, crack initiation energy and crack growth energy during impact test rapidly decreased with increasing the mean size and volume fraction of Laves phase.

The effect of hot bending and thermal ageing on creep and microstructure evolution in thick-walled P92 steel pipe

In this work, the creep behaviour of an actual thick-walled P92 steel pipe bent (90°) using local induction heating was investigated at 600 and 650°C using uniaxial tension creep tests. Creep specimens were machined from the bend at different positions, namely at the intrados and extrados areas and neutral position and from the straight part of the pipe. Creep tests were followed by metallographic and fractographic analyses of the crept specimens to explain the observed creep behaviour. In order to accelerate the microstructural changes and thus to simulate long-term service conditions, isothermal ageing at 650°C for 10,000h was applied to selected creep specimens before creep exposures. The results of the creep tests performed on specimens extracted from the intrados and extrados of the thick-walled bent P92 steel pipe fall into the scatter band of the base P92 steel average line. No substantial differences were found at different positions of the pipe in the minimum creep rate, the time to fracture and creep fracture strain. However, significant detrimental effects on the creep resistance of the pipe were found after long-term static thermal ageing due to the microstructural instability of the material. The large Laves phase particles, which coarsened during ageing and creep testing, served as preferential sites for cavity nucleation leading to an accelerated tertiary stage of creep and/or premature creep fracture. Activation analysis of the creep data leads to the conclusions, that the creep tests were undertaken in the region of the power-law creep regime and the rate controlling creep deformation mechanism is most probably the climb of the intragranular dislocations.

Microstructure in Unbent and As-Hot Bent Segments of Thick-Walled P92 Steel Pipe

The microstructure of P92 steel pipe processed by hot bending was evaluated in order to characterize the influence of hot bending on microstructure changes in extrados and intrados of bend. The straight pipes, with an outer diameter 350 mm and wall thickness of 39 mm (Productos Tubulares, s.a.u. Spain), were normalized at 1050°C/60min/air and tempered at 740°C/140min/air.The thick-walled P92 pipe was hot bended at 920-960 °C using bending rate 7 mm/min. The bends were normalized at 1050°C/60min/air and tempered at 775°C/140min/air after hot bending. The selected specimens, machined from different locations of pipe, were additionally thermal ageing at 650 °C to simulate microstructure degradation typical for long-term service conditions.Microstructure was investigated by scanning electron microscope Tescan Lyra 3 equipped with NordlysNano EBSD detector operating at an accelerating voltage of 20 kV with specimen tilted at 70° and by transmission el. microscope JEM-2100F.Microstructure investigation revealed that different processing history influenced distribution and size of secondary phases and caused changes in interboundary spacing in straight pipe and bends. It was found that microstructure changes induced during hot bending and subsequent post-bending heat treatment caused deterioration in creep properties of bends in comparison with those of straight pipe.

Characterisation and quantification of cavities in 9Cr martensitic steel for power plants

This work focuses on the characterisation of cavities evolution in a P91 steel pipe in three conditions: as received, and after creep at 650°C and 60 MPa for 7000 and 9000 h. A field emission gun scanning electron microscope (FEG-SEM) equipped with focused ion beam (FIB) gun, a conventional scanning electron microscope (SEM) and a light optical microscope (LOM) have been employed for the investigation. This study reveals two types of cavities: the pre-existing cavities, which are rare in this type of heat resistant steels, with a mean diameter of 2·56 μm and the cavities produced during creep with diameters smaller than 0·6 μm. Lath boundaries and precipitates are found to be preferential sites for cavity nucleation. Furthermore, the number density and volume fraction of these small cavities are calculated from 2D measurements and compared to 3D results obtained by FIB serial sectioning.

Ultimate creep load and safety assessment of P91 steel pipe with local wall thinning at high temperature

Safety assessment of pipe with local wall thinning (LWT) at high temperature is highly important in power and nuclear engineering. Based on strain criterion and isochronous stress–strain curves at high temperature, this paper studied the ultimate creep load of pipe with LWT under creep condition by orthogonal experimental design and finite element numerical simulation. Firstly, the influence of LWT geometries was analyzed, which indicated that relative defect depth was the most significant factor to ultimate creep load. Secondly, the ultimate creep load formulas were regressed based on the ultimate creep loads of pipe with LWT at different creep times. And safety assessment curves for the pipe with LWT at high temperature by different strain criteria were proposed. Finally, the engineering method of allowable load of pipe with LWT by different strain criteria was constructed. The research results try to improve the safety assessment and structure integrity analysis of pipe with LWT at high temperature.

Crack growth behaviour of P92 steel under creep and creep–fatigue conditions

High temperature creep and creep–fatigue crack growth tests were carried out on standard compact specimens machined from ASME P92 steel pipe. The effects of various loading conditions on crack growth behaviours were investigated. Crack initiation time was found to decrease with the increasing initial stress intensity factor under creep condition and further to decrease by the introduction of fatigue condition. For creep test, the crack growth rate can be well characterised by the facture mechanics parameter C*. For creep–fatigue test, the crack growth behaviour is dominated by the cycle dependent fatigue process when the hold time is shorter, but it becomes dominated by the time dependent creep process when the hold time becomes longer.

Experimental and Numerical Investigation of Heated Band Width for Local Post Weld Heat Treatment of ASME P92 Steel Pipe

Local post weld heat treatment (local PWHT) is usually carried out when it is impractical to place the entire component in a furnace or oven, which is an effective way to relax residual stress due to welding. However, there are various international codes or standards that define different criteria for local PWHT and it may bring confusion in engineering applications. In the present study, welding and local PWHT experiments on ASME SA-335 Grade P92 large-diameter pipes were conducted under field conditions. In order to simulate the temperature field distribution of welded joints during the process of local PWHT, a thermal tracking program has been successfully developed using the ansys parametric design language (APDL) and the numerical results agree well with experimental data. Furthermore, a series of pipe models were developed using the finite element method (FEM) and through repeated calculations, optimized numerical values for each pipe's heated band (HB) width and gradient control band (GCB) width were calculated. Through numerical analysis, recommended estimation of heated band width for local PWHT of P92 large-diameter pipes is obtained, which ensures the minimum temperature throughout the soak band.

Influence of Roll Rotational Speed on inside Bore and Lamination Defects in Rotary Piercing Large Diameter Heavy Wall P92 Steel Pipe Process

To investigate the influence of different roll rotational speeds on tendency of inside bore and lamination defects of large diameter heavy wall seamless P92 (9Cr-0.5Mo-1.8WVNb) steel pipe pierced by two-high rotary piercing process, with the aid of commercial FE code MSC. SuperForm, 3D thermo-mechanical coupled simulations were presented. The damage field of the rolled piece during rotary piercing process, and the tendency of forming inside bore and lamination defects were analyzed using Oyane ductile fracture criterion. The results show that When roll rotational speed increases from 7rpm to 11rpm, the maximal damage characteristic value decreases from 0.4279 to 0.3340 in center area of rolled piece in front of the plug, but which increases from 0.3780 to 0.4858 in a certain depth zone (15~25mm) adjacent to the internal surface of the rolled piece contacting with the front end of the plug. With the increase of roll rotational speed, the tendency of forming inside bore defects of rolled piece reduces, but the tendency of forming lamination defects increases. Therefore, there exists a critical roll rotational speed to make the tendency of both inside bore and lamination defects reduce. This study provides scientific basis for formulating reasonable rotational speed schedule to prevent or reduce inner overlap and lamination defects.

Finite element simulation of welding residual stresses in martensitic steel pipes

Finite element (FE) simulations of the welding of two high grade steel pipes are described. The first is a P91 steel pipe welded with a similar P91 weld consumable, and the second is a P92 steel pipe welded with dissimilar nickel–chromium based weld consumables. Both welds are multipass circumferential butt welds, having 73 weld beads in the P91 pipe and 36 beads in the P92 pipe. Since the pipes and welds are symmetric around their axes, the FE simulations are axisymmetric, allowing high FE mesh refinement and residual stress prediction accuracy. The FE simulations of the welding of the P91 and P92 pipes comprise thermal and sequentially coupled structural analyses. The thermal analyses model the heat evolution produced by the welding arc, determining the temperature history throughout the FE models. Structural analyses use the computed temperature history as input data to predict the residual stress fields throughout the models. Post-weld heat treatment (PWHT) of both pipes has also been numerically simulated by assuming that the FE models obey the Norton creep law during the hold time period at 760°C. The residual stresses presented here have all been validated by corresponding experimental measurements. Before PWHT, it has been found that, at certain locations in the weld region and heat affected zone (HAZ) in the pipes, tensile hoop and axial residual stresses approach the tensile strength of the material, presenting a high risk of failure. It has also been found that PWHT substantially reduces the magnitude of residual stresses by varying degrees depending on the material.