P91 Weld Research Articles

Microstructure-Linked Mechanical Properties of P91-Incoloy 800HT Dissimilar Metal Welds

Dissimilar metal welds (DMWs) between P91 steel and Incoloy 800HT® using shielded metal arc welding (SMAW) with ENiCrCoMo-1 filler electrode were characterized in detail. The impact of multiple weld passes on microstructure in the heat-affected zone (HAZ) and weld fusion zone (WFZ) was thoroughly investigated. Postweld heat treatment (PWHT) impacts on mechanical characteristics and microstructure were also observed. The DMW was observed with heavy C, Cr, and Mo microsegregation in the WFZ, which deteriorated mechanical strength. The microstructural changes directly affected the mechanical behavior of the DMW. The ultimate tensile strength of the weld fusion zone was low at 576 MPa compared to the base metals’ strength. The tensile specimens failed in the IN 800HT base metal and WFZ. The WFZ had significant microsegregation near the solidified grain boundaries (SGBs). The metallurgical heterogeneity in the WFZ caused the formation of localized stress zones, which directly affected the mechanical behavior of the WFZ. The carbide particles influenced the microhardness in the WFZ and P91 HAZ regions. The PWHT successfully homogenized the microhardness in the WFZ and P91 HAZ regions. The impact toughness decreased to 64 J from 82 J after PWHT due to precipitation at SGBs and its coarsening. The SMAW process involves multiple thermal cycles during welding. The deposition of filler metal in multiple welding passes created uncontrolled variations in the solidification behavior of the WFZ. Microsegregation had a detrimental impact on the microstructure and mechanical properties of the P91 and IN 800HT dissimilar metal welds.

A comparative study of creep crack growth behaviour amongst the dissimilar P91-IN182-SS316LN and similar P91 weld joints

ABSTRACT Towards ensuring the structural integrity of the high-temperature components, it is essential to evaluate the creep crack growth (CCG) behaviour, especially for the weaker creep regions of the components. In this study, the CCG behaviour was investigated in the modified 9Cr-1Mo (P91) and Inconel (IN182) buttered layer interface regions of dissimilar metal weld (DMW) joints of P91 and type 316LN SS at 798 K and 898 K. For a quantitative assessment, the CCG rates were compared with the heat-affected zone (HAZ) of similar metal P91 welds (P91HAZ). The CCG rate was characterised using the creep fracture C* parameter as per ASTM E1457–17. The CCG rate in the P91/IN182 interface region was found to be higher than the P91HAZ, viz. 3.5 times and 4 times at 798 K and 898 K, respectively. This could be attributed to the propensity for creep cavitation in the interface region of the DMW joint.

Hydrogen effect on mechanical properties and cracking of creep-resistant 9% Cr P92 steel and P91 weld metal

Martensitic 9% Cr steels like P91 and P92 can show an increased susceptibility to delayed hydrogen-assisted cracking. The focus of this study was the microstructure and heat treatment effect on the mechanical properties of P92 base material and P91 multi-layer weld metal in both as-welded and post weld heat treated (PWHT) condition. Tensile tests with hydrogen-free reference samples and electrochemically hydrogen charged samples were carried out; the mechanical properties were assessed and supported by detailed fractographic analysis. Finally, a hydrogen and microstructure-dependent fracture criterion is established. All investigated microstructures showed a hydrogen-influenced degradation of the mechanical properties compared to the hydrogen-free reference samples. The as-welded martensitic P91 weld metal had the highest degree of degradation in the presence of hydrogen. The P91 PWHT weld metal and the P92 base material had comparable properties. From that point of view, a significantly increased risk for hydrogen-assisted cold cracking during welding fabrication of P91 weld joints must be considered before any heat treatment is conducted.

Improving mechanical properties of P91 and IN617 dissimilar weld joints for advanced ultra super critical power plants by buttering technique

• Enhancement of mechanical properties using buttering layer for dissimilar weld. • Buttering helps in achieving favorable ductility for dissimilar welds. • With buttering creep rupture life of dissimilar welds has increased. • Buttered dissimilar welds can be implemented for very high-temperature boilers. To improve the mechanical properties of ferritic steel and nickel superalloy, while maintaining economic symmetry, this investigation is performed on fabricating dissimilar metal welds for advanced ultra-supercritical (AUSC) boilers, with a buttering technique. The present work compares a dissimilar weld of IN617 alloy and P91 steel with and without buttering by multipass gas tungsten arc welding for mechanical properties variations, creep strength at elevated temperature, and microstructural evolution. Thin layers of buttering material are applied at the ferritic end, followed by the closure weld consisting of welding a second member to a butter layered component. Further, significant improvement in mechanical properties (tensile strength, toughness, and hardness) and creep rupture life (though the failure remains ductile characterized by dimples variations) 2 to 3 times longer has been found with buttering in contrast with no buttering.

Characterization of the cyclic deformation behavior of simulated HAZs and other constituent microstructural regions of P91 steel weldment

Low cycle fatigue tests were conducted on theweld metal, coarse grain heat-affected zone (CGHAZ), fine grain heat-affected zone (FGHAZ), inter-critical heat-affected zone (ICHAZ), base metaland the actual weldmentof P91 steelat 550 °C using different strain amplitudes.The P91 weldment and its constituent microstructural regions exhibited typical continuous cyclic softening behavior. In the HAZs, the FGHAZ exhibited better fatigue behavior. The CGHAZ and ICHAZ showed higher fatigue lives at the lower and higher strain amplitudes, respectively. An empirical relationship was established for predicting the fatigue life of the weldment using weighted factors of all the constituent regions.

Thickness and microstructure effect on hydrogen diffusion in creep-resistant 9% Cr P92 steel and P91 weld metal

Martensitic 9% Cr steels like P91 and P92 show susceptibility to delayed hydrogen assisted cracking depending on their microstructure. In that connection, effective hydrogen diffusion coefficients are used to assess the possible time-delay. Limited data on room temperature diffusion coefficients reported in literature vary widely by several orders of magnitude (mostly attributed to variation in microstructure). Especially P91 weld metal diffusion coefficients are rare so far. For that reason, electrochemical permeation experiments had been conducted using P92 base metal and P91 weld metal (in as-welded and heat-treated condition) with different thicknesses. From the results obtained, diffusion coefficients were calculated using to different methods, time-lag, and inflection point. Results show that, despite microstructural effects, the sample thickness must be considered as it influences the calculated diffusion coefficients. Finally, the comparison of calculated and measured hydrogen concentrations (determined by carrier gas hot extraction) enables the identification of realistic diffusion coefficients.

Experimental investigation of creep crack growth behavior in the heat affected zone of boron added modified 9Cr–1Mo steel weld

Abstract This study aims to quantify the improvement in the creep crack growth (CCG) resistance in the heat-affected zone (HAZ) of boron added modified 9Cr–1Mo (P91B) steel weld at 898 K. The crack growth rates in the HAZ of P91B weld have been compared with the crack growth rates in HAZ of without boron P91 steel weld. The creep crack growth rates are estimated as per ASTM E1457 using 7 mm thick compact tension specimens with a notch in the HAZ and test duration 248h-11,000 h. The CCG rate ( a ˙ ) vs. C* correlation, commonly expressed as a ˙ = D ( C ∗ ) ϕ , has been established. In comparison, the crack growth rate in the HAZ of P91 weld is found to be threefold as compared to HAZ of P91B weld; with the difference in CCG rate increasing towards a higher C* range. Creep damage assessment study has been done in the vicinity of creep crack in HAZ of a P91B weld specimen tested for 11,000 h. The extent of cavitation, cavities link-up, and micro-crack formation in HAZ of P91B weld is found to be relatively lower than HAZ of P91 weld. Most of the cavities appear to be discrete in nature. This testifies improvement in the creep crack growth resistance in the HAZ of P91B weld due to controlled boron addition of about 100 ppm to modified 9Cr–1Mo steel.

Enhancing Heat Treatment Conditions of Joints in Grade P91 Steel: Looking for More Sustainable Solutions

Grade P91 is a relatively new class of steel, which has received special attention from designers because it presents extremely interesting characteristics for specific applications. This steel exhibits ideal properties for demanding applications, especially involving high temperature and pressure, being employed in facilities such as power plants and other equipment, such as heat exchangers. P91 welds usually need heat treatments, which are already parameterized in the codes. However, standardized treatments are time-consuming and harmful to the environment, as they massively consume energy. Some attempts have been made in the past to reduce the time and energy spent on these treatments. This work aims to extend this study, now presenting better solutions than those obtained previously. This work presents four new conditions for the heat treatment of joints carried out on P91 steel, with a view to reducing processing time, reducing energy consumption, and an even better balance between mechanical strength and elongation after failure. Heat treatment conditions were established in which there was a loss of about 14% in Ultimate Tensile Strength (UTS), but in which a gain of about 50% in elongation was obtained, compared to welding without any treatment, but also with 10% losses in the UTS and 30% gains in elongation when compared to the solution recommended as more correct in the codes, saving a lot of time and energy in the treatment process. Thus, these solutions may be adopted in the future with gains in terms of productivity and economic and environmental sustainability.

Analysis of Cracking Causes of P91 Welding Seam of High Pressure Steam Guide Pipe of Supercritical Unit

A 600MW supercritical unit in a thermal power plant has been in operation for 63,000 hours, and a P91 weld leak occurred between the high-pressure steam guide pipe and the high-pressure governor valve. After macroscopic morphology analysis, alloy composition analysis, metallographic structure observation, and microhardness test analysis, it is found that the cracking of the base metal and weld on the high-pressure speed control valve is caused by structural stress and welding stress, which leads to cracking due to creep. The crack originated in the fine-grained area of the heat-affected zone, and it has been operating in a high-temperature operating environment for a long time. The P91 weld on the high-pressure steam guide tube produces creep holes in the fine-grained heat-affected zone, which gradually grow to form creep cracks, and eventually lead to cracks and leakage. Therefore, the failure of pipeline cracking is caused by the joint action of tensile stress and type IV crack.

Microstructural studies of P91 and SS304L dissimilar welding

P91 and 9Cr1Mo bimetallic amalgamation are extensively used in power generation and nuclear reactor projects. Various characteristics like weldability, microstructure analysis of welded, HAZ and base and mechanical strength of dissimilar joining P91 and SS304L-16L welds using Gas Tungsten Arc Welding Processes (GTAW) were explored in this work. In the gas tungsten arc welding process, two fillers, 309L-16L and ERNiCr-3, were used. Any weldment has its own structure–property, and the relations between them have been studied. Microstructural improvements and mechanical characterization, including hardness rating, tensile strength andimpact strengthwere evaluated through experimentation. The ERNiCr − 3 filler had a stronger weld than the SS309L filler, but the weld zone was where the failure occurred. To analyse the structure–property relationships, additional optical microscopy and SEM/EDAX results were compared on the weldments. Fabricators and dissimilar weldments used in power plants and nuclear reactors will benefit from these tests.

Role of Ni-based filler on charpy impact toughness of the P91 welds joint

The aim of the present work is to investigate the role of Ni-based filler on Charpy impact toughness (CIT) of the P91 welds joint. The Gas Tungsten Arc Weld (GTAW) joint of creep strength enhanced ferritic/martensitic P91 steel was produced by using the dissimilar Ni-based IN617 and IN718 filler. The welds joint subjected to varying post-weld heat treatment (PWHT) in the temperature range of 650–810 °C. Post weld normalizing based tempering (PW 4) was also carried out at a normalizing temperature of 1040 °C.The weld metal shows the cellular dendrites and equiaxed dendrites for IN617 filler along with the segregation of the secondary phase particles at inter-dendritic areas and boundaries. For IN718 filler, columnar and cellular dendrites with well-defined primary and secondary arm were observed along with segregation of the particles at inter-dendritic areas and boundaries. PW 4 provides the uniform distribution of the particles inside the austenitic matrix of weld metal for both filler condition. The CIT test was conducted at room temperature for each condition of the filler and PWHT.The CIT value of the WFZ was measured maximum for as-welded condition and showed a minute change after the PWHT. However, a significant decrease in CIT was measured after the PW 4 for both the filler.

Role of Mo in the nucleation of intermetallic laves phase

The present research work investigates the effect of the re-austenitizing based post-welded tempering (PWNT) treatment on microstructure evolution and fracture characteristics of the P91 welds joint under creep exposure condition. The major problem associated with the P91 welded joint is the failure from the soft inter-critical heat affected zone (ICHAZ) during elevated creep service conditions. The failure is associated with the altering of the microstructure of the P91 steel as a result of the welding heat, which leads to the formation of ICHAZ. To investigate the creep performance of the welded joint, creep tests have been conducted at 620 °C for applied stress of 200 MPa (PWNT 1) and 150 MPa (PWNT 2). The crept specimen was carried out in detail by using the scanning electron microscope (SEM). The PWNT have observed a significant effect on fracture characteristic, and it shifted from the soft ICHAZ to over-tempered base zone. The inter-metallic Laves phase was observed in each region of the weldments for specimen exposed at 620 °C for applied stress of 150 MPa (PWNT 2).

Stress Distributions in P91 Martensitic Steel and in AISI 316LN Steel Welds for Gen IV Nuclear Applications

Neutron diffraction has been used to investigate the stress field in two different welds developed for nuclear applications, namely those obtained by tungsten inert gas welding of P91 martensitic steel (Cr 9, Mn 6, Mo 1, C 0.1, and Fe bal wt %) and hybrid (laser beam and gas metal arc) welding of AISI316LN austenitic steel (17.8 Cr, 12.3 Ni, 1.7 Mn, 2.4 Mo, 0.3 Si, and Fe bal wt %). The sizes of the investigated samples were 100 × 50 × 12 mm for the P91 weld and 220 × 160 × 15 mm for the 316LN weld; unstrained references were prepared for both welds. The neutron diffraction measurements have been carried out utilizing the E3 diffractometer at the BER II reactor in Berlin, with a gauge volume of 2 × 2 × 2 mm3. Lines perpendicular to the weld direction were scanned at different depths inside the material and at different distances from the weld centerline, including the heat affected zone and the weld centerline. Strain and stress values were determined in the three principal directions. In the TIG P91 weld, the stresses are almost completely relieved after a post weld heat treatment for 2 h at 760°C. In the laser beam and gas metal arc 316LN weld not submitted to post weld heat treatment, nearly balancing longitudinal and transverse stress components as high as 300–350 MPa are found within a range of approximately 3 mm around the centerline of weld.

Role of dissimilar IN617 nickel alloy consumable on microstructural and mechanical behavior of P91 welds joint

This paper investigates the metallurgical behavior and mechanical properties of the P91 steel welds joint. The joint of heat-resistant P91 steel has been welded by the gas tungsten arc welding (GTAW) process using the dissimilar Inconel grade 617 filler. The P91 welds joints have been subjected to varying heat treatment regimes in the temperature range of 650–810 °C for 2 h. The normalizing-based tempering was also performed for the welded joint. The weld fusion zone (WFZ) with austenitic structure and heat-affected zones (HAZs) with martensitic structure was characterized using the optical microscope and scanning electron microscope (SEM). The detailed characterization of the weld metal and HAZ interface has also been performed for as-welded and post-weld heat treatment (PWHT) conditions. For mechanical properties of the welds joint, tensile testing and hardness testing were performed. The relationship between mechanical behavior and microstructure of the welded joint has been evaluated for as-welded and heat treatment conditions. The microstructure studies revealed the formation of an unmixed zone (UZ) close to the fusion line, and it was characterized as peninsula and island. The WFZ showed the complete austenitic mode of the solidification and revealed the austenitic structure with cellular and equiaxed grains in the center of the weld metal. The columnar and cellular dendrites were seen near the boat fusion line, i.e., interface of the weld metal and HAZ. The soft δ ferrite patches were observed near the fusion line in the area of HAZ and remain undissolved up to tempering temperature of 810 °C (PW 3). The dissolution of the ferrite patches was noticed for PW 4. The maximum and minimum tensile strength of the welds joint was measured 731 MPa and 502 MPa for PW 3 and PW 2, respectively. A uniform hardness variation in the transverse direction of the welded joint was observed for PW 3 and PW 4 conditions. The optimum combination of strength and ductility was obtained for the PW 3 condition.

A microstructural and mechanical behavior study of heterogeneous P91 welded joint

The current work has been performed to characterize the heterogeneous P91 welded joint. The gas tungsten arc welded (GTAW) P91 welds joint has been prepared using the dissimilar Inconel 617 filler. The P91 welded plates have been subjected to post-weld normalizing and tempering (PWNT) heat treatments for varying normalizing temperature in the range of 950–1150 °C. To homogenize the microstructure across the welded joint, tempering has been performed at 760 °C for 2 h for each normalized conditions. The tensile and microhardness tests have been conducted to determine the mechanical properties of the welded joint in as-welded and heat-treated conditions. The microstructure characterization has been performed for the weld fusion zone (WFZ) and efforts have been made to develop a relation between microstructure evolutions and mechanical behavior of the welded joint. Microstructure analysis was carried out with optical and field scanning electron microscope (FESEM). The optimum combination of strength and ductility was obtained for the sample heat-treated at 1050 °C for 40 min (normalized)+760 °C/2 h (tempering). The lower strength was obtained for the Inconel 617 filler as compared to P91 filler, while ductility obtained for Inconel 617 filler was higher than that of P91 filler.

A comparative study on the microstructure and mechanical properties of fusion welded 9 Cr-1 Mo steel

Modified 9 Cr–1 Mo is an ideal candidate preferred for high temperature applications in power plants. In this study, three P91 welds of 12 mm thickness are produced using Gas Tungsten Arc Welding (GTAW), Cold Metal Transfer (CMT) and Pulsed-CMT (P-CMT) processes. The impact toughness is evaluated in the as-welded as well as in the PWHT (760 °C-2 h) condition. It is found that P-CMT process resulted in the highest toughness in both the conditions. The heat input was maximum in the case of GTAW (800 J/mm) welds and minimum in CMT (330 J/mm). Even though P-CMT (380 J/mm) has heat input slightly above CMT, the former is compensated by higher welding speeds. Electron microscopy analysis revealed the presence of fine M23C6 and MX precipitates. Among the welds, P-CMT had minimal amount of the precipitates whereas GTAW welds had the maximum. The dimple morphology observed in the ruptured surface significantly varied and P-CMT process had finer dimples indicating finer weld microstructures and enhanced mechanical properties.

Hydrogen diffusion in creep-resistant 9% Cr P91 multi-layer weld metal

Welded components of P91 9% Cr steel demand for careful welding fabrication with necessary post weld heat treatment (PWHT). Before the PWHT, a hydrogen removal heat treatment is necessary for avoidance of hydrogen assisted cracking (HAC). In this context, the microstructure and temperature-dependent hydrogen diffusion is important, and reliable diffusion coefficients of P91 weld metal are rare. For that reason, the diffusion behavior of P91 multi-layer weld metal was investigated for as-welded (AW) and PWHT condition by electrochemical permeation experiments at room temperature and carrier gas hot extraction (CGHE) from 100 to 400 °C. Hydrogen diffusion coefficients were calculated, and the corresponding hydrogen concentration was measured. It was ascertained that both heat treatment conditions show significant differences. At room temperature the AW condition showed significant hydrogen trapping expressed by to seven times lower diffusion coefficients. A preferred diffusion direction was found in perpendicular direction expressed by high permeability. The CGHE experiments revealed lower diffusion coefficients for the AW condition up to 400 °C. In this context, a hydrogen concentration of approximately 21 ml/100 g was still trapped at 100 °C. For that reason, a certain HAC susceptibility of as-welded P91 weld metal cannot be excluded, and hydrogen removal should be done before PWHT.

The Characterization of Soft Fine‐Grained Heat‐Affected Zones in P91 Weldments under Creep Exposure Conditions

The martensitic 10CrMoVNb 91 (P91) steel welded joint is subjected to reaustenitizing‐based post‐weld heating, referred to as post‐weld normalizing and tempering (PWNT). The various zones of P91 weldments in PWNT treatment and after creep exposure at 620 °C for 150 MPa are studied. The evolution of intermetallic Laves phase in the various zones of P91 weldments is observed after the creep rupture tests. The evolution of Laves phase is characterized using field‐emission scanning electron microscopy (FESEM) with energy‐dispersive X‐ray spectroscopy.

Effect of shielding gas composition and activating flux on the weld bead morphology of the P91 ferritic/martensitic steel

In the current work, efforts have been made to study the effects of the activating flux and the shielding gas composition on the Tungsten Inert Gas (TIG) welding of SA 387 Gr 91 CL.2 steel (P91 steel). Multicomponent flux comprising of TiO2, SiO2, NiO, CuO, and CeO2 was used as an activating flux. Pure argon (Ar) and argon with nitrogen (Ar + N2) were used as shielding gases. The relation between weld bead morphology and the welding conditions were emphasized with the help of welding arc column profile and arc voltage. Stereomicroscope, Optical microscope and Field emission scanning electron microscope (FE-SEM) equipped with energy dispersive spectroscopy (EDS) were used for macrostructure and microstructure analysis of the P91 weldments. The results suggested that the depth of penetration increases and bead width decreases in presence of the activating flux under Ar shielding. However, depth of penetration and bead width both increased by under Ar + N2 shielding. Activating flux was found insignificant to reduce the bead width during welding under Ar + N2 shielding. The welding arc inconstancy was found responsible for weld bead characteristics obtained during welding under Ar + N2 shielding.

Effects of Multiple Post Weld Heat Treatments on Microstructure and Precipitate of Fine Grained Heat Affected Zone of P91 Weld

To understand the potential impact of multiple post weld heat treatments (PWHTs) on service life of P91 weld, the multiple PWHTs are designed and carried out to explore their influences on microstructure and precipitate of fine grained heat affected zone (FGHAZ) of the P91 weld. The results show that the multiple PWHTs cause the recovery of martensite matrix (specifically, the elimination of lath‐like structure and the decrease of dislocation's density) and the evolution of M23C6 carbide (specifically, the coarsening of M23C6 carbide and the change of its distribution). The recovery of martensite matrix and the coarsening of M23C6 carbide are undesirable in terms of the creep resistance of the FGHAZ. Besides, the change trend of hardness is consistent with that of M23C6 carbide density, indicating that the dispersion hardening introduced by the M23C6 carbide plays a dominant role in hardening effect.