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Steels 9-12% Cr, having the high creep rupture strength are advocated for the modern low polluting thermal power plants. In the present investigation, the P91 pipe weldments have been characterized for microstructural responses in as-welded, post-weld heat treatment (PWHT) and ageing conditions. The PWHT of welded samples were carried out at 760 °C for time of 2 h and ageing at 760 °C for 720 h and 1440 h, respectively. The effect of time has been studied on precipitates size, distribution of precipitates and grain sizes present in various zones of P91 steel weldments. The impact toughness and hardness variation of heat affected zone (HAZ) have also been studied in as-welded condition as well as at different heat treatment condition. A significant change was observed in grain size and precipitates size after each heat treatment condition. The maximum impact toughness of HAZ was obtained after PWHT at 760 °C for 2 h. The main phase observed in weld fusion zone in as-welded, PWHT and ageing conditions were M23C6, MX, M7C3, Fe-rich M3C and M2C. The unwanted Z-phase (NbCrN) was also noticed in weld fusion zone after ageing of 1440 h. © 2017, The Korean Institute of Metals and Materials and Springer Science+Business Media Dordrecht.

Department of Mechanical Engineering

World University root node

Metals and Materials International

Characterization of microstructure of HAZs in as-welded and service condition of P91 pipe weldments

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. © 2017, Korean Society of Steel Construction and Springer-Verlag GmbH Germany.

International Journal of Steel Structures

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

The work presented in this study was performed with the intent to characterize the microstructure evolution for short term creep exposure of cast-forged P91 steel. The short-term creep test was performed at temperature range of 620–650 °C and stresses ranging from 120 to 200 MPa. To characterize the sample after creep exposure, field emission scanning electron microscopy (FESEM) with energy dispersive X-ray spectroscopy (EDS), optical microscope and micro-hardness testing were utilized. Creep tests were performed on round creep specimens. For low temperature service condition, longer creep life was obtained. The fracture surface of creep ruptured specimen were characterized by using the FESEM. The transgranular fracture mode was noticed in all the tests condition. The creep rupture life was found to be decreased with increase in applied stress. The maximum rupture life was measured about to be 3329.28 h for the sample exposed at 620 °C for 120 MPa. A negligible microstructural change was measured in gripping area compared to the gauge area (necking area) of crept sample. The laves phase formation was also noticed along the grain boundaries for creep exposure life of 3329.28 h. © 2017 Elsevier B.V.

Materials Science and Engineering A

Microstructure-based assessment of creep rupture behaviour of cast-forged P91 steel

The microstructural and mechanical properties of creep enhanced ferritic (CSEF) steels are affected by various parameters, solutionizing temperature is one of them. In the present investigation, the effect of normalizing temperature on the microstructural and mechanical properties of cast and forged (C&F) P92 steel were carried out. Grade P92 steel is considered as a candidate material for Thermal and Nuclear power plants at temperatures of up to 650 °C. In this study, C&F P92 steel was subjected to various normalizing temperatures (from 950 °C to 1150 °C). For microscopic characterization, Optical microscope and Field emission scanning electron microscope (SEM) were used. The grain size, precipitate size, area fraction of precipitates and Cr/Fe were calculated from micrographs. The normalized specimens were tested for tensile strength, hardness, and toughness. Considering observation for the optimum combination of strength, ductility, and toughness, the normalizing at 1000 °C and tempering at 760 °C has been suggested for C&F P92 steel. © 2017 Elsevier B.V.

Characterization and evaluation of mechanical properties of CSEF P92 steel for varying normalizing temperature

The new generation super critical thermal power plants are required to operate at enhanced thermal efficiency of over 50% to reduce the fuel consumption and environmental pollution. Creep strength-enhanced ferritic steels, commonly known as Cr-Mo alloys such as P91 (X10CrMoVNb 9-1) are such material of choice for the next generation power plants. The operating requirement of these next generation power plants is that steam temperature of around 650 °C is maintained. For such high-temperature application, creep strength of material is the primary consideration together with adequate weld heat-affected zone (HAZ) toughness. Present work deals with the effect of high service temperature on impact toughness of P91 (X10CrMoVNb 9-1) base material, weld fusion zone, and HAZ. The impact toughness of HAZ for conventional weld groove design and narrow weld groove design has been evaluated experimentally in as-welded and at different post-weld heat treatment conditions. Fractography of the impact toughness specimens of base metal, weld fusion zone, and HAZ was carried out using scanning electron microscope. The effects of heat treatment schemes on the percentage of element present at the fracture surface were also studied. © 2016, ASM International.

Journal of Materials Engineering and Performance

Effect of Heat Treatment on Microstructure and Hot Impact Toughness of Various Zones of P91 Welded Pipes

Hydrogen-assisted cracking (HAC) is a serious issue in P91 steel welds because of unwanted martensitic metallurgical-transformation in the heat affected zone (HAZ). In the present research, different electrode conditions have been employed during welding of P91 steel to study their effects on HAC. The HAC susceptibility of cast and forged (C&F) P91 steel welds produced by shielded metal arc welding (SMAW) process has been evaluated through implant test with respect to the different level of diffusible hydrogen content in the deposited metal. The weld implant test results such as applied stress vs time to failure, the lower critical stress (LCS), embrittlement index (EI) and normalized critical stress ratio (NCSR) were evaluated with respect to the different level of hydrogen content in the deposited metal. The microstructure of coarse grain heat affected zone (CGHAZ) and weld metal in different condition have been characterized by using scanning electron microscope (SEM). The fracture morphology of implant specimen was characterized by using the SEM. The paper also describes the effect of hydrocarbon contamination (oil or grease) in low-hydrogen basic SMAW electrode for P91 steel. It can be concluded that P91 steel welded by the electrode having higher contamination (high hydrogen level) is more susceptible to HAC. The LCS value was observed to be decreasing with the increase in the diffusible hydrogen content in deposited metal and minimum value was measured to be 180 MPa for 12.54 ml of diffusible hydrogen per 100 g of deposited metal. © 2016 Hydrogen Energy Publications LLC

International Journal of Hydrogen Energy

Hydrogen induced cold cracking of creep resistant ferritic P91 steel for different diffusible hydrogen levels in deposited metal

Present paper describes the microstructural and mechanical properties of creep strength enhanced Cr–Mo P91 steel subjected to a regime of tempering time 720–3000 h. Study on the effect of varying tempering time on microstructure revealed increase in size of precipitates, amount of precipitates and grain size. It was also observed that compared to particle size, effect of tempering time was more pronounced on the grain size. The various phases that formed in the tempered P91 steel were analyzed by X-ray diffraction. Yield strength and tensile strength were observed to be decreasing with increase in tempering time. Increase in duration of tempering time led to decrease in impact toughness of P91 specimens possibly due to cracking of secondary phase particles during straining and decohesion at precipitates and matrix interface. The tempering time also observed to be affecting the hardness of P91 steel but less pronounced compared to the yield strength and tensile strength. Tensile fracture and impact toughness fracture surfaces were studied by using scanning electron microscope. The fracture surface of tempered P91 tensile specimens indicated increased dimples and presence of fewer cleavage facets. The fracture surface of P91impact toughness specimens exhibited both inter lath-decohesion and ductile dimple tearing. © 2016, The Indian Institute of Metals - IIM.

Transactions of the Indian Institute of Metals

Effect of Long-term Ageing on the Microstructure and Mechanical Properties of Creep Strength Enhanced Ferritic P91 Steel

Mechanical properties of creep enhanced ferritic (CSEF) steels is affected by various parameters, the solutionizing temperature is one of them. The present work demonstrates the effect of solutionizing temperature on microstructure and mechanical properties of CSEF P91 steel. Optical metallography (OM) and Scanning electron microscopy (SEM) were carried out to study the microstructure of P91 steel in different heat treatment conditions. In order to determine the precipitates present in microstructure; X-ray analysis was performed. Moreover, the influence of solutionizing temperature on the mechanical properties (strength, hardness and impact toughness) has also been studied. © 2016 Elsevier B.V.

Effect of normalizing temperature on microstructural stability and mechanical properties of creep strength enhanced ferritic P91 steel

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. © 2016, ASM International.

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

The present study describes how the effect of direction of welding is useful to minimize distortion in submerged arc welded double-sided fillet joints. In present work, angular distortion of 12 mm thick double-sided fillet welds was measured for same direction and reverse direction welding. It has been observed that the maximum magnitude of the angular distortion for reverse direction fillet welding is lower. Thermal history in horizontal and vertical plate of fillet welds was also measured for both same direction and reverse direction welding. After welding, the fillet welds were sectioned, polished and etched to measure the effect of direction of welding on weld bead geometry. To predict the temperature distribution and thermal history, a finite element (FE) elasto-plastic thermo-mechanical model has been developed for submerged arc welded double-sided fillet welds by using moving distributed heat source. The three-dimensional FE thermal model was verified by comparing peak temperature obtained with experimental values. The model results matched fairly well with experimental results with a variation of 6 percent for sample 1 and 7.5 percent for sample 2 for the maximum values of distortion, and a variation of 2-8 percent for peak temperatures in horizontal plate and 9 percent in web plate. © 2016, Korean Society of Steel Construction and Springer-Verlag Berlin Heidelberg.

On the prediction of effect of direction of welding on bead geometry and residual deformation of double-sided fillet welds

To achieve high thermal efficiency, modern day thermal power plants operate at higher operating temperature and pressure which necessitates use of steels with high creep rupture strength such as modified 9Cr-1Mo steels. In the present study, the evolution of phases in modified 9Cr-1Mo P91 steel and their effects on microstructural stability and mechanical properties have been studied for specimens that were subjected to different thermal heat treatment conditions. The main focus has been to study the effect of heat treatment temperature ranging from 623 K to 1033 K (350-760 °C) on P91 steel. Further, the effect of furnace cooling, water quenching, tempering at 1273 K (1000 °C) and austenitizing on the mechanical properties and microstructure has been studied. The techniques used for material characterization were scanning electron microscopy (SEM), optical microscopy (OM) and X-ray diffraction. For low tempering temperature, i.e. 623 K (350 °C), M23C6, M3C, M7C3, and MX precipitates have been observed with high yield strength (YS), tensile strength (UTS), hardness and low toughness. In the high temperature range, 923-1033 K (650-760 °C), fine MX, M7C3, M23C6, M2X, and M3C precipitates have been observed with low YS, UTS, hardness and high toughness. The steel tempered at 1033 K (760 °C) was observed to be having best combination of YS, UTS, hardness, toughness and ductility. © 2016 Elsevier B.V.

Evolution of phases in P91 steel in various heat treatment conditions and their effect on microstructure stability and mechanical properties

The blind hole drilling technique, utilized for measuring residual stress by using strain gauge rosette, is associated with plastic deformation of material that is removed from the hole and also with stress concentration effect of multi point cutting tool. Further, the plasticity effect is also induced in presence of high applied stress which may lead to local yielding of the hole boundary. The present paper describes the plasticity and stress concentration effects of blind hole drilling technique on the calculation of residual stresses for SS 304L and modified Cr-Mo Grade 91 alloys. For uniaxial loading, the responses of strain gauge rosettes were investigated experimentally through tensile testing and for biaxial loading through numerical modeling. Correlations were developed to directly calculate the corrected values of residual stresses from the experimental strain data obtained through the strain gauge rosette and from stress values as per ASTM E837-13. © 2015 Elsevier Ltd. All rights reserved.