P91 Steel Research Articles

Microstructure and creep rupture strength of 2 kinds of P91 steels with abnormally low hardness

P91 samples were prepared by means of 2 kinds of heat treatments which involved low normalizing cooling rate &tempering temperature exceeding Ac1 followed by low cooling rate respectively. The correlation between low hardness, microstructure and creep rupture strength was discussed. Results showed that there was a clear microstructure disparity between samples enduring different heat treatments, even though low hardness emerged in both samples. The sample undergoing low normalizing cooling rate featured ferrite structure with coarse grains and continuously distributed precipitates at grain boundaries after normalizing, and intermittently distributed precipitates at grain boundaries after tempering. The sample witnessing tempering temperature exceeding Ac1 followed by low cooling rate represented precipitates inside grains and the morphology of lath martensite in the majority of areas, whereas other areas were characterized by fine blocks of ferrite. The creep rupture strength of the sample experiencing low normalizing cooling rate was higher than the one experiencing tempering temperature exceeding Ac1 followed by low cooling rate, although hardness of the former was lower than that of the latter. For P91 steel, hardness and creep rupture strength did not show a positive correlation, because the grain size and distribution of precipitate also mattered.

Investigation of the High-Temperature Low-Cycle fatigue failure characteristics of P91 steel weld joints and their fatigue strength reduction factors under various load control regimes

Low cycle fatigue tests are conducted on the base metal, weld metal, a welded joint under the condition that the strain control region is the joint (WJWJ) and a welded joint under the condition that the strain control region is the base metal (WJBM) of P91 steel at 500℃ using different total strain ranges. The cyclic stress response is highest for the weld metal, followed by the base metal, and is relatively similar for WJWJ and WJBM. At 2.0 % total strain range conditions, the materials generally exhibit non-Masing behavior and dynamic strain hardening aging. At all total strain ranges, WJWJ failed in the heat-affected zone, and WJBM broke in the base metal. For the difference in fatigue life between base metal and welded joints, the fatigue strength reduction factor of P91 steel welded joints at 500 °C is given as a suggested value. The fatigue behavior of P91 steel welded joints is verified based on a hybrid hardening model and a modified damage model. This study presents a theoretical framework for analyzing the fatigue strength reduction factors of weldments and comparatively analyzes the effects of different geometrical features.

Investigation of creep performance degradation mechanism of service exposed P92 steel and microstructural characterization of FGHAZ

Investigation of creep performance degradation mechanism of service exposed P92 steel and microstructural characterization of FGHAZ

Creep behaviour and microstructure evolution in Super 304H–P92 heterogeneous welds

ABSTRACT The paper deals with results of long-term stress rupture tests on “cross-weld“ specimens made of Super 304 H – P92 heterogeneous welds. Stress rupture tests were carried out at temperatures of 625°C and 650°C up to 20,000 hours to rupture. Creep rupture strength and WSF values of Super 304 H – P92 welds for 10,000 hours at both 625°C and 650°C were calculated. The preferential location of failure was the intercritical part of the heat affected zone in P92 steel. Hardness of both base materials did not show significant changes during creep exposure at both 625°C and 650°C. However, gradual hardening of the weld metal occurred. Metallographic studies were performed in individual parts of heterogeneous welds. A special attention was paid to precipitation reactions in both base materials and heat affected zones. A residual content of vanadium in Super 304 H steel affected precipitation processes. Vanadium partly substituted niobium in the modified Z-phase ((V,Nb)CrN).

Correlation Between Microstructure and Corrosion Behavior of the Dissimilar Joints of Incoloy 800HT and P91 Steel Using GTAW Process

Correlation Between Microstructure and Corrosion Behavior of the Dissimilar Joints of Incoloy 800HT and P91 Steel Using GTAW Process

Microstructure and mechanical behaviour study of the dissimilar weldment of ‘IN82 buttered’ P92 steel and AISI 304L steel for ultra super critical power plants

This study examines the influence of the ERNiCr-3 (Inconel 82) butter layer on the microstructure and mechanical feature of the dissimilar weld joint (DWJ) of ferritic grade P92 and austenitic grade AISI 304 L steel, manufactured using gas tungsten arc welding (GTAW) method with IN82 filler. The butter layer aims to prevent the formation of a hard martensitic layer adjacent to the fusion line by mitigating the carbon migration from the P92 steel to the welding pool. The optical microscope, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and electron probe microanalyzer (EPMA) were used to examine the microstructures of the buttered welded plate. The mechanical characteristics of the joints were assessed using the standard and sub-sized tensile and Charpy tests. The flat and round specimens underwent the tensile test to determine their relative tensile properties at room temperature and high temperature. The high-temperature tensile properties were assessed at 600 °C and 650 °C at 1 mm/min extension rates. Along the weldments, hardness profilometry was also performed. Microstructural analysis validated that the application of a buttering layer successfully eliminated the extensive martensitic layer on the P92 side. Nevertheless, in the vicinity of the interface between the buttering layer and P92, there was evidence of a narrow unmixed zone and macrosegregation. SEM/EDS and EPMA studies confirmed that the predominant phases identified in IN82 butter and weld metal are NbC and TiC. The butter layer, overall, improved elongation/ductility by 25 % without a noticeable reduction in tensile strength. During tensile testing, the specimen fractured from the IN82 butter layer, revealing the presence of dimples and voids on the fracture surface, along with the TiC/NbC phases, as confirmed by SEM analysis. Tensile specimens subjected to high-temperature testing exhibited failure at the base materials of AISI 304 L (tensile strength=404 MPa) and P92 (307 MPa), specifically at 550 °C and 650 °C, respectively, rather than at the buttering layer or weld metal. The results of the tensile tests met the minimum criteria specified for Ultra-Supercritical (USC) boilers. More importantly, the IN82 butter layer successfully serves as a protective barrier between the weld pool and the P92 HAZ. As a result, the dual hardness peaks on either side of the welding pool are eliminated. After adding the buttering layer, the peak hardness in the coarse-grained heat-affected zone (HAZ) of P92, which ranged from 450 to 500 HV in the unbuttered sample, dropped by 200 MPa. The relationships between microstructural characteristics and mechanical properties have been thoroughly examined and discussed.

Development of a P91 uniaxial creep model for a wide stress range with an artificial neural network

ABSTRACT A uniaxial creep model that describes creep over a wide stress range was developed for P91 steel using an artificial neural network (ANN). The training dataset was based on measurements from uniaxial creep tests and information derived from a combination of the logistic creep strain prediction and the Wilshire models. The ANN model reproduces the training dataset with high accuracy (R 2 = 0.975; RMSE (Root Mean Square Error) = 0.19). The model can be easily implemented in finite element analysis (FEA) codes since it provides an analytical expression of the true creep rate as a function of temperature, true stress and true creep strain. In FEA simulations under the same conditions as the training dataset, the model provides times to rupture and minimum creep rates very close to those in the training dataset. The model can be adapted for heats with different properties from the average behaviour of the training dataset by means of a stress-scaling factor.

Microstructural characterization of simulated and actual constituent regions of P91 steel weldment

In the present study simulated and the actual microstructures of the coarse grain, fine grain and intercritical heat-affected zones of P91 steel weldment are characterized using electron backscattered diffraction (EBSD) technique. Both the simulated and actual heat-affected zone (HAZ) microstructures showed profound substructural recovery after post-weld heat treatment/tempering. The subgrain size distributions between the simulated and the actual HAZs are comparable. The actual HAZ microstructures exhibited greater low angle grain boundaries and local strain distributions compared to the simulated HAZ microstructures due to the complex microstructural geometry of the actual weldment. The similarity in the variations of grain size, precipitate size and hardness of the simulated and actual HAZs validates the simulation of HAZ microstructures through furnace heat treatment.

Numerical predictions of low cyclic fatigue and creep-fatigue behavior of P92 steel under non-isothermal-mechanical loading

Low-Cycle Fatigue (LCF) and creep-fatigue-interaction (CFI) behavior of a ferritic-martensitic stainless steel P92 is studied under strain-controlled non-isothermal -mechanical fatigue loading in the temperature range of 300°C∼620°C and 500°C∼620°C, respectively. The strain-controlled tests with and without the tensile and compression holding times are simulated for different mechanical strains amplitudes. An improved Unified Viscoplastic constitutive Model (UVM) is proposed based on the framework of modified Chaboche non-linear kinematic hardening rule. The improved UVM incorporates strain rate sensitivity, temperature rate dependence, static recovery associated with plastic strain, mean stress evolution and cyclic softening of strain range dependence, and so on. Furthermore, the expressions of the implicit stress integration algorithm and the consistent tangential modulus are deduced on the basis of radial return mapping and backward Euler integration methods, respectively, and the improved UVM is implemented into the ABAQUS software using the user material subroutine UMAT. The simulation results of the UVM correlate well with the experimental data.

Steel grades 91 and 92 microstructure and precipitate evolution atlas and life assessment tool

ABSTRACT P91 and P92 steels are widely used in power plants and petrochemical industry for long-term service components. Due to high resistance to creep, fatigue and corrosion, the use of grade 91 and grade 92 steels allows ultrasupercritical conditions (600 °C, 300 bar). To achieve this goal the alloy design was based on the following metallurgical concepts: stable tempered martensitic microstructure, precipitation strengthening through M23C6 (Cr, Fe, Mo or W carbides) and fine MX (V, Nb carbides), solution hardening through elements as Mo or Mo/W and high Cr content. This study is focused on the microstructure evolution of grade 91 and grade 92 steels under aging and creep conditions. Three sets of laboratory-aged specimens heated in oven at 550°C, 600°C and 650°C were examined. Furthermore, the influence of stress on the microstructure was evaluated. The microstructures were characterized by several means of investigations and the results were compared to literature.

Effect of PWHT on metallurgical and mechanical characterization of dissimilar welded joint of P91 and P92 steels

P91 and P92 steels are widely used in nuclear and thermal power plant’s high-temperature (650–700 °C) operating components. The welding procedure, base plate composition, filler material composition, and post-weld heat treatment (PWHT) significantly affect the metallurgical characterization of weldments at room temperature. This work focuses on joining P91 and P92 steel by the TIG welding with filler ER90S-B9. The study analyzes the materialization of coarse-grained HAZ (CGHAZ), inter-critical HAZ (ICHAZ), and fine grained HAZ (FGHAZ) under welded and PWHT. PWHT was conducted at 840 °C for 2, 4 and 6h. An optical microscopic (OM) scanning electron microscope (SEM) was employed to characterize the welded and PWHT samples. It was perceived that the tensile strength of PWHT increased as the heat treatment times increased from 2 to 6 h, while the hardness value decreased accordingly. The maximum tensile strength (594.71 MPa) was observed in PWHT_6 h, while the minimum tensile strength (537.52 MPa) was perceived in the as-welded sample. The PWHT_6h reveals the minimum hardness values (271.12 HV) at the fusion zone among all the conditions. The WFZ consisted of an untampered lath martensitic grain structure, demonstrating an average hardness value of 314.15 HV in the PWHT_2h condition. The hardness value in CGHAZ samples for as-welded, PWHT_2h, PWHT_4h, and PWHT_6h was observed at 456.85, 436.81, 368.63, and 327.41 HV, respectively.

Identification of creep parameters from multi‐step relaxation tests on miniaturised specimen

AbstractThe small punch test (SPT) is a miniaturised method for thermo‐mechanical materials testing. The combination of simple specimen geometry and low required specimen volume is ideal for minimally invasive characterisation and monitoring of the current material condition in technological systems, including nuclear plants.The aim of the present study is to identify creep material properties of the chromium steel P91 (X10CrMoVNb9‐1) via small punch testing. It is examined to which extent a single relaxation test is suitable to replace several creep tests at different force levels. The inverse parameter identification method utilises a non‐linear optimisation approach in order to match the results of simulation and experiment. The simulation is performed using finite element analysis (FEA) and the considered material law aims to represent the high‐temperature behaviour of steels. To this end, a modified Liu–Murakami model is specifically used along with linear elasticity and non‐linear isotropic plasticity. We propose a strategy, that can be used to separately identify the Norton‐parameters from multi‐step relaxation tests performed on one specimen. Thereby, the applied strain (displacement, if utilising SPT) is kept constant after initial loading, which enables stress relaxation within the specimen. In case of the SPT, this behaviour yields a decreasing reaction force which approximately approaches a constant value. During the repeated loading phases, specimen failure may occur, due to further plastification of the specimen. Loading and relaxation cycles are repeated until specimen failure.The identified parameters are validated via small punch creep experiments at different force levels. It will be shown that good agreement is observed when comparing the sets of identified parameters.

Hot workability behaviour of two P92 creep resistant steels: Constitutive analysis

This article reports the flow stress behaviour of two P92 steels at a temperature range of 850–1000°C and a strain rate of 0.1–10 s−1 using the Gleeble® 3500 thermomechanical simulator. A physically-based constitutive model was used to analyse the effects of deformation conditions on the flow stress behaviour during deformation. This model incorporates the influence in the variation of Young’s modulus and the self-diffusion coefficient as affected by temperature. The study developed constitutive equations that predict the flow stress behaviour of the two steels investigated. From the constitutive analysis of the results, the stress exponent n was: 9.8 (steel A) and 10.3 (steel B). The model used the self-diffusion activation energy of steel. The statistical parameters: correlation coefficient of 0.99 (for steel A and B), the absolute average relative error of 2.18% (steel A) and 2.20% (steel B) quantified the applicability of the model. The quantification results show that the constitutive equations developed have high accuracy in predicting the workability of the two P92 steels. The study has shown that this method is applicable in predicting the metal flow pattern of two P92 steels in the metalworking processes.

A novel pathway for joining power plant dissimilar steels by diffusion bonding in as-machined surface condition

Surface preparation, which includes polishing and cleaning, has become a crucial stage in diffusion bonding for creating excellent quality durable joints. Polishing is a time-consuming and labor-intensive task that reduces process productivity. To increase process productivity, the joints must be established under as-machined surface conditions. Pressure impulses were used to conduct diffusion bonding of P92 steel and 316L austenitic stainless steel under as-machined surface conditions. At 1000 °C, 30 min bonding time and 20 MPa Maximum impulse pressure, tensile strength and elongation of the joint was found to be 628.5 MPa and 38.6 % respectively.

Effect of cyclic waveforms on cavitation propensity of P91 steel weld joint at 873 K

Effect of cyclic waveforms on cavitation propensity of P91 steel weld joint at 873 K

Study on dislocation density, microstructure, and mechanical properties of P92 steel for different heat treatment conditions

AbstractThe impact of various heat treatment procedures on microstructure, dislocation density, hardness, tensile characteristics, and impact toughness of P92 steel was examined in the current experiment. The martensitic microstructure and average microhardness of 463 HV 0.2±8 HV 0.2 of the normalized steel were prevalent. A tempering procedure was carried out at 760 °C for a range of 2 hours to 6 hours. Additionally, an X‐ray diffraction examination was carried out, and the results were used to determine the dislocation density. The normalized sample was characterized by a high dislocation density. The dislocation density was decreased by tempering of normalized samples. With an increase in tempering time, the effect of the treatment coarsened the grains, precipitates, and decreased the area fraction of precipitates. After tempering, MX, M23C6, and M7C3 types precipitates were found to have precipitated, according to energy dispersive spectroscopy and x‐ray diffraction research. The ideal tempering period was determined to be 4 hours at a tempering temperature of 760 °C based on the microstructure and mechanical characteristics. Steel that was tempered at 760 °C for 4 hours had a yield strength of 472 MPa, an ultimate tensile strength of 668.02 MPa, and an elongation of 26.05 %, respectively.

Deformation behavior, microstructure evolution, and rupture mechanism of the novel G115 steel welded joint during creep

To promote the wide application of the G115 steel in ultra-supercritical (USC) power plants, creep deformation, microstructure evolution and rupture mechanism of the G115 welded joint were systematically investigated. The fabricated G115 welded joint exhibits superior creep strength to the P92 or MARBN steels. The deformation behaviors are analyzed by the Norton's power law, proving that the deformation mechanism is closely associated with the stress levels. During short-term creep under higher stress, the weld metal (WM) and heat-affected zone (HAZ) exhibit outstanding microstructure stability, and the rupture occurs within the base metal (BM) due to the lath fracture. While, during long-term creep under lower stress, the brittle Type IV cracking occurs within the inter-critical heat-affected zone (ICHAZ) or fine-grained heat-affected zone (FGHAZ) due to the formed micro-voids along boundaries, obviously deteriorating the creep strength. The weld strength reduction factors are calculated under different stresses, and prove that synergistic action of back stress and dislocation motion can enhance the creep resistance of the G115 welded joint. Additionally, the allowable service stress of G115 weldments should be designed according to the creep strength of the welded joints under expected performance life.

High-Temperature Tensile Behaviour of GTAW Joints of P92 Steel and Alloy 617 for Two Different Fillers.

This study explores the high-temperature (HT) tensile rupture characteristics of a dissimilar gas-tungsten-arc-welded (GTAW) joint between P92 steel and Alloy 617, fabricated using ER62S-B9 and ERNiCrCoMo-1 fillers. The high-temperature tensile tests were performed at elevated temperatures of 550 °C and 650 °C. An optical microscope (OM) and a field emission scanning electron microscope (FESEM) were utilized to characterize the joint. The high-temperature test results indicated that the specimen failed at the P92 base metal/intercritical heat-affected zone (ICHAZ) rather than the weld metal for the ERNiCrCoMo-1(IN617) filler. This finding confirmed the suitability of the joint for use in the Indian advanced ultra-supercritical (A-USC) program. The fracture surface morphology and presence of precipitates were analysed using an SEM equipped with energy dispersive spectroscopy (EDS). The appearance of the dimples and voids confirmed that both welded fillers underwent ductile-dominant fracture. EDS analysis revealed the presence of Cr-rich M23C6 phases, which was confirmed on the fracture surface of the ER62S-B9 weld (P92-weld). The hardness plot was analysed both in the as-welded condition and after the fracture.

Precipitates change of P92 steel under 3.5 MeV Fe13+ ion irradiation at 400 °C

9–12% Cr ferritic/martensitic steels are promising candidate materials for applications in fourth generation nuclear reactors. 9%Cr ferritic/martensitic steel P92 was normalized at 1050 °C for 0.5 h and subsequently tempered at 765 °C for 1 h. The precipitates of the normalized-and-tempered steel after 3.5 MeV Fe13+ ion irradiation at 400 °C to 0.66 dpa were studied using transmission electron microscopes. After irradiation, the number of precipitates increased, especially for the number of large-sized precipitates, and there was an increase in the overall size of precipitates. Pre-existing Cr-rich M23C6, Nb-rich MX and V-rich MX precipitates in the normalized-and-tempered steel still existed in the irradiated steel, and the M23C6 precipitates remained almost unchanged in their composition. Irradiation-induced Nb-rich M6C5 carbide with a base-centered monoclinic structure, V-rich M2X (V2C type) carbonitride with a simple orthorhombic crystal structure, and Fe71Cr26(Mo,W)3 (σ-FeCr type) intermetallic compound with a simple teragonal crystal structure were identified in the irradiated steel. The formation of these irradiation-induced phases is also discussed.

Creep-Fatigue Life Property of P91 Welded Piping Subjected to Bending and Torsional Moments at High Temperature

Abstract In recent years, the role of thermal power plants has shifted from providing a baseload to providing supplemental supply to compensate for fluctuations in the output of renewable energy sources. Thus, the operation of these plants involves frequent startup and shutdown cycles, which lead to extensive damage caused by creep and fatigue interactions. In addition, the piping utilized in thermal plants is subjected to a combined stress state composed of bending and torsional moments. In this study, a high-temperature fatigue testing machine capable of generating such a bending-torsional loading was developed. Creep-fatigue tests were conducted on P91 steel piping with weldment. The results clarified that the creep-fatigue life was reduced by the superposition of the torsional and bending moments and that it was further reduced by a holding load. It was shown that the creep-fatigue life of piping with weldment can be estimated accurately using the equivalent bending moment, which is composed of the torsional and bending moments. It was also confirmed that crack occurred in the heat-affected zone (HAZ) of the welded part, which has been often observed in actual thermal power equipment. From the finite element analysis, it was identified that cracking was initiated in the HAZ due to the accumulation of creep strain and increase in the hydrostatic pressure component during a holding load.