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1

Study on narrow gap welding of martensitic grade P92 and austenitic grade AISI 304L SS steel for ultra-supercritical power plant application

Dak Gaurav, Khanna Navneet, Pandey Chandan

Archives of Civil and Mechanical Engineering

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2023

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Vol. 23, no. 1

art. no. e14, 2023

EN

The present work investigated the microstructural feature, mechanical properties, and residual stress variation for the dissimilar welded joints (DWJs) of P92 and AISI 304L steel. The multi-pass DWJs were attempted for narrow gap geometry using the tungsten inert gas (TIG) welding process employing the ERNiCrMo-3 filler metal. The martensitic microstructure produced in the P92 HAZ region after welding is brittle due to quenched martensite and the dissolution of precipitates. Thus, the post-weld heat treatment (PWHT) known as tempering was carried out at 760 °C for a period of 2 h to get tempered martensitic microstructure and re-precipitation of dissolved precipitates. The radiographic examination and macrostructure analysis showed defect-free P92/304L SS DWJs. The weld metal showed the complete austenitic microstructure with a Ni weight percentage of 36%. However, segregation of the alloying elements along with the inter-dendritic areas and variation in grain growth during solidification was observed. There is columnar grain morphology at interface, cellular, and equiaxed in the center. The major segregation along the inter-dendritic areas was observed for Nb, Mo, Ti, and Cr that led to the formation of the carbides of type Mo6C, TiC, and NbC, which was confirmed from the energy dispersive spectroscopy (EDS) analysis. From the tensile test result, 304L SS base metal (BM) was inferred as the weakest region in P92/304L SS DWJs. The ultimate tensile strength (UTS) of the as-weld joint was about 626 MPa, along with fracture location in 304L SS base metal. The Charpy impact test results showed that the region with relatively poor impact toughness was austenitic ERNiCrMo-3 filler weld (57 J) which might be due to the segregation of the Nb and Mo along the inter-dendritic areas. However, the impact toughness of the ERNiCrMo-3 filler weld met the minimum requirement of 47J (EN ISO 3580:2017). The micro-hardness result showed that in the as-welded condition, the coarse grain heat affected zone (CGHAZ) has the highest micro-hardness value (340 HV) due to the high weight percentage of Cr and N resulting from the dissolution of M23C6 precipitates followed by the fine grain heat affected zone (FGHAZ, 270 HV), and the inter-critical heat affected zone (ICHAZ, 205 HV). After PWHT, the hardness value was decreased below the maximum allowable value of 265 HV due to the tempering of the martensite. The residual stresses developed in the case of the narrow groove design were less due to the less quantity of weld metal available for volumetric contraction in the case of the narrow groove geometry. The tensile stress was dominant in the weld fusion zone due to the volumetric contraction of the weld metal, while compressive stress was dominant in P92 HAZ because of the martensitic phase transformation.

2

Structure-property relationships and corrosion behavior of laser‑welded X‑70/UNS S32750 dissimilar joint

Maurya Anup Kumar, Pandey Shailesh M., Chhibber Rahul, Pandey Chandan

Archives of Civil and Mechanical Engineering

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2023

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Vol. 23, no. 2

art. no. e81, 2023

EN

This research aims to study the microstructure characteristics, mechanical properties, and corrosion behaviors of the dissimilar autogenous laser beam welded joint of pipeline steel (X-70) and super duplex stainless steel (sDSS 2507). Pipelines for the transmission of oil and gas and risers for offshore oil and gas drilling require this dissimilar joint. A dissimilar joint must maintain its properties and be defect-free under such challenging operating conditions. The microstructure of the interface, weld zone and heat-affected zone (HAZ) were all investigated thoroughly using optical microscopy (OM) and scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS). This dissimilar joint had signifcant microstructure anomalies in the weld and interfaces. Microstructure inhomogeneity’s effect on welded joint mechanical properties, including microhardness, tensile and impact strength, was also studied. The linear potentiodynamic polarisation test in neutral 3.5 wt.% NaCl solution was used to study this weldment’s corrosion behavior. The corroded surfaces were examined using an OM and SEM for the surface morphology investigation of corroded specimens. The macro-optical investigation has revealed full penetrations in the weld without any inclusions or porosities. The interface between the sDSS 2507 weld zone and the X-70 coarse grain heat-affected zone (CGHAZ) indicated a peak hardness of 418 Hv0.5. With an average of 345 Hv0.5, the WZ’s hardness variation was reported to be in the 298-420 Hv0.5 range. The hardness of the X-70/sDSS 2507 weld interface was assessed to be greater than that of the other region of weldments. An untempered martensitic region in WM and the CGHAZ of X-70, and the presence of M-A components are credited with the increase in hardness. The welded joint achieved reasonably excellent strength and ductility and met the marine and offshore standards requirements. The base metals and weldment for X-70 and sDSS 2507 have respective ultimate tensile strengths (UTS) of 610±6 MPa, 995±8 MPa, and 675±10 MPa. The tensile findings revealed that the fracture location for weldment was evident in the X-70 base metal, ensuring that the weld metal was of adequate strength for the laser-weld joints. It was observed that the weldment’s WM had the lowest impact strength. The Charpy impact toughness of the weld metal, however, was higher than both the ASME standard (>41 J) and the EN 1599:1997 standards (>47 J). The sDSS 2507 BM (310±4 J) clearly outperforms the weld zones (185±3 J) and X-70 base metal (295±2 J) in terms of impact strength. The electrochemical corrosion test shows the corrosion potential, and the weld zone's corrosion rate is between sDSS 25,070 (- 260±1.3 mV, 0187±0.002 mm/year) and X-70 base metal (- 454±1.8 mV, 0.321±0.017 mm/year). Additionally, the surface morphologies and the electrochemical measurements matched significantly.

3

Kumar Amit, Pandey Shailesh M., Pandey Chandan

Archives of Civil and Mechanical Engineering

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2023

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Vol. 23, no. 1

art. no. e54, 2023

EN

A dissimilar joint of martensitic grade P92 and Ni-based Inconel 617 (IN617) alloy are employed commonly in advanced ultra-supercritical (AUSC) units to fabricate thick section components such as steam pipes and turbine rotors. This study investigated the weldability of the IN617 alloy and P92 steel dissimilar welds. Ni-based superalloy ERNiCrMo-3 filler was used to attempt the dissimilar joining for conventional V groove and narrow groove design by employing the gas tungsten arc welding (GTAW) process. The weld metal for the capping pass, backing pass, and near the interface showed the columnar and cellular grains while equiaxed grains are observed corresponding to root and filling passes. The energy dispersive spectroscopy and Electron probe micro-analyzer (EMPA) study confirmed the segregation of the Nb and Mo particles in inter-dendritic spaces and resulted in the formation of the Nb-rich NbC and laves phases and Mo-rich phases. The EDS line map and EPMA study of the P92 interface showed a sharp increase in Cr, Mo, and Ni concentration and a steep decrease in Fe concentration as moving from P92 base metal to weld metal. A negligible diffusion from filler weld to IN617 or vice versa across the IN617 interface was detected. The microhardness gradient along the weldments indicated a sharp rise in hardness value near the P92 fusion boundary due to the formation of the unmixed zone of lower hardness and the P92 coarse-grained heat-affected zone of higher hardness. The weld metal hardness results showed a great variation with an average hardness value for V groove and narrow groove welds of 227 and 262 HV, respectively. The mechanical tests were conducted at the ambient temperatures and data obtained for the weldments were compared with the base metals. The room temperature tensile tests showed the failure from the region of the P92 BM or the interface of P92 BM/weld metal, with joint strength of 646 ± 6 MPa and 747 ± 4 MPa in AW conditions for V groove and narrow groove, respectively. The Charpy impact test (CIT) also showed the variation in impact toughness along the weldments, and the ERNiCrMo-3 filler weld was identified as the weakest region of the welded joint in terms of impact toughness for both the groove designs. The residual stress variation along the thickness of the weld plate was measured using the deep hole drilling (DHT) methods, and the results indicated the peak magnitude of the residual stress for the V groove welded joint. The test results indicated that welded joint produced using ERNiCrMo-3 filler was safe for AUSC power plants' boiler applications for both the groove weld while optimum mechanical properties were measured for narrow groove weld.

4

Analysis of mechanical and microstructural characteristics of plunger-assisted ECAP strengthened Ti-6Al-4V alloy sheets

Sahoo Partha Sarathi, Meher Arabinda, Mahapatra Manas Mohan, Vundavilli Pandu Ranga, Pandey Chandan

Archives of Civil and Mechanical Engineering

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2023

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Vol. 23, no. 3

art. no. e194, 2023

EN

Often known as the workhorse among titanium alloys, Ti-6Al-4 V has been useful in the aerospace and biomedical sectors. For further enhancement of the mechanical characteristics of Ti-6Al-4 V alloy, its sheets procured for the present study have been subjected to equal channel angular pressing (ECAP) using a die setup having a channel angle of 120° and corner angle of 10° at its forming temperature of 650 °C followed by appropriate annealing treatments. Microstructural analysis post the hot-ECAP process has demonstrated ultrafine grain (UFG) refinement because of this severe plastic deformation technique of ECAP. Phase analysis has further substantiated the reduction of β-phase in the alloy as a controlling factor in improving the mechanical properties. As a result, the room temperature hardness and tensile strength have improved by 10% and 15%, respectively, due to a drastic reduction in grain size from ~ 906 nm to ~ 359 nm, which is in line with the well-established Hall–Petch equation. Basic finite element modeling has been studied as concerned with the sustainability and feasibility of the die setup to withstand the heavy metal forming forces involved in the ECAP of Ti-6Al-4 V. This success in processing Ti-6Al-4 V by a single pass of an ECAP using channel angle of 120° and corner angle of 10° under a controlled equivalent strain further opens doors for incorporating additional steps and criteria to achieve even higher grain refinement and strength enhancements thereby catering to the needs for manufacturing the assault vehicles and bioimplants.

5

Study on effect of weld groove geometry on mechanical behavior and residual stresses variation in dissimilar welds of P92/SS304L steel for USC boilers

Dak Gaurav, Pandey Chandan

Archives of Civil and Mechanical Engineering

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2022

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Vol. 22, no. 3

art. no. e140

EN

This research article reports the correlation between microstructure, mechanical properties, and residual stresses of dissimilar weld joints (DWJs) between P92 martensitic steel and 304L austenitic stainless steel (ASS). The groove geometry plays a vital role in DWJs. Thus the effect of groove geometry on mechanical and microstructural properties was also investigated. The V-shape and narrow shape groove profile were implemented for P92/304L SS DWJs. The microstructural characteristic, tensile strength, micro-hardness, Charpy impact toughness and residual stresses were evaluated for both the groove geometry in as-weld (AW) and post-weld heat treatment (PWHT) (760 °C, 2 h) state. Microstructural observations performed using an optical microscope (OM), and scanning electron microscope (SEM) showed that high temperature during the weld thermal cycle leads to the formation of the coarse grain heat-affected zone (CGHAZ), fine grain HAZ (FGHAZ), and inter-critical HAZ (ICHAZ) across the P92 HAZ. The ERNiFeCr-2 (Inconel 718) welding consumable wire (filler rod) of diameter 2.4 mm was used for this investigation. The ERNiFeCr-2 weld fusion zone showed a fully austenitic microstructure with the formation of the secondary phases due to the solidification segregation. The EDS and SEM area mapping results indicated that the secondary phases in the inter-dendritic region contain a higher amount of the Mo and Nb than the matrix region. The ultimate tensile strength (UTS) of the as-weld and PWHT tensile specimen of the P92/304L SS DWJs was 630 and 621 MPa, respectively, for V-groove geometry specimens and 620 and 629 MPa, respectively, for narrow groove geometry specimens. The tensile fracture was experienced at the interface between weld metal and 304L base metal, and the UTS value of DWJs was very close to the UTS of the 304L SS. The abrupt variation in the micro-hardness value of the CGHAZ (456HV0.5), FGHAZ (375HV0.5), and ICHAZ (221HV0.5) was noticed in the as-weld state due to their distinguish microstructure characteristics. After PWHT, the micro-hardness value of the CGHAZ (255HV0.5), FGHAZ (236HV0.5), and ICHAZ (207HV0.5) was below the maximum allowable value of 265HV0.5 for P92 material because of the tempering of the martensite. The Charpy impact test indicated that the ERNiFeCr-2 weld fusion zone has a low toughness value of 33 J (AW) and 25 J (PWHT) for V-groove design and 35 J (AW) and 28 J (PWHT) for narrow groove design than that of the P92 and 304L parent metal. The impact toughness of the ERNiFeCr-2 filler weld was below the minimum requirement of 47 J (EN ISO 3580:2017). The tensile residual stresses were generated in the weld fusion zone due to the volumetric contraction during the solidification. The residual stresses developed in the case of the narrow groove design were less than that for the V-groove design due to the less quantity of weld metal available for volumetric contraction in the case of the narrow groove geometry. From comparing mechanical and microstructural properties obtained for V-groove and narrow groove geometry DWJs, it was found that narrow groove design reduces the overall heat affected zone span, and it requires less welding time and less heat input.

6

Effect of filler metal composition on microstructural and mechanical characterization of dissimilar welded joint of nitronic steel and super duplex stainless steel

Maurya Anup Kumar, Pandey Chandan, Chhibber Rahul

Archives of Civil and Mechanical Engineering

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2022

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Vol. 22, no. 2

art. no. e90, 1--28

EN

The present paper experimentally investigates the effect of filler metal on the mechanical behavior, solidification, and microstructure of the super duplex stainless steel (sDSS2507) and nitronic steel (N50) dissimilar welded joint. This dissimilar joint is primarily applicable in the subsea control unit for high-pressure tubing and coupler assembly. For this investigation, the gas tungsten arc welding process (GTAW) employed the super duplex filler ER2594 and carbon steel grade ER70S-2 filler. The weld's structural integrity has been assessed to compare both the fillers through multiple investigations on the joint. The microstructure characterization of the base metal and as-welded specimen was carried out using an optical microscope (OM) and scanning electron microscope (SEM). Super duplex filler ER2594 weld solidified in primary ferritic mode with precipitation of several reformed austenite in the ferrite matrix, whereas ER70S-2 filler weld had long marten site laths embedded in ferrite matrix. The microstructural study reported the presence of microsegregation and Type II boundary formation. The type-II boundary is detected close to the fusion boundary at the N50 and the sDSS 2507 side of the ER70S-2 weldment. The Vickers microhardness test, Charpy impact test, and the tensile test were performed to obtain the mechanical properties of this joint. The microhardness investigation of the weld zone of ER2594 and ER70S-2 shows the average hardness of 287.34±10 Hv0.5 and 372.36±10 Hv0.5, respectively. The peak hardness of 410 Hv0.5 was observed in the weld zone of ER70S-2. The formation of large marten site laths in the ferrite matrix in the weld zone leads to higher hardness in ER70S-2 filler compared to the precipitation of softer reformed austenite in the ER2594 fller. The average impact toughness result of ER2594 and ER70S-2 is 165±5 J and 110±8 J, respectively. The Charpy impact trials showed the ductile fracture mode by employing ER2594 filler, while ER70S-2 showed the mixed fracture mode (ductile-brittle). The weldment tensile strength of filler ER2594 and ER70S-2 is 897 MPa and 873 MPa, respectively. The tensile test results indicate the ductile fracture mode for both fillers, and the failures were detected in sDSS2507.

7

Autogenous laser-welded dissimilar joint of ferritic/martensitic P92 steel and Inconel 617 alloy: mechanism, microstructure, and mechanical properties

Kumar Amit, Pandey Chandan

Archives of Civil and Mechanical Engineering

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2022

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Vol. 22, no. 1

art. no. e39, 2022

EN

Dissimilar autogenous welding of ferritic/martensitic P92 steel and Inconel 617 alloy was conducted using the laser beam-welding process. The microstructure evolution such as element segregation, grain size, and precipitate morphology was investigated in different areas of the welded joint for as-welded (AW) and post-weld heat treatment (PWHT) condition by using optical microscopy (OM) and Field Emission Scanning Electron Microscopy (FE-SEM). The tensile strength at room temperature, microhardness variation, and Charpy V impact toughness was evaluated for the welded joint to study the effect of laser beam welding on mechanical properties. Finally, the fractured impact-tested specimen was characterized by FE-SEM. Microstructure observation showed the microstructure heterogeneity across the welded joint. A typical martensitic lath structure devoid of carbide precipitates was observed in P92 HAZ, while Inconel 617 HAZ exhibited the fine-equiaxed austenite grains of average size 24 ± 7 µm with Ti-enriched Ti(C, N) and Mo-enriched carbide precipitates. A columnar and cellular structure consisting of the alloying element segregation in the inter-dendritic areas was observed for the weld metal (WM) near the interface, while the interior WM showed the cellular and equiaxed dendrites along with particles of type NBC, TiC, and M23C6. Fairly good strength and ductility were obtained for the welded joint, and it qualified the ultra-supercritical (USC) boiler requirement. The WM was witnessed as the weakest part of the welded joint for impact strength. However, the impact strength of the WM (61 ± 2 J) was measured higher than the ASME standard value (> 41 J) and EN ISO 3580:2017 standard value (> 47 J) in the AW joint. After the PWHT, a drastic reduction in impact strength of WM (48 ± 3 J) was measured, and it was very close to EN ISO 3580:2017 recommended value of 47 J. The HAZ of P92 and Inconel 617 showed good impact strength for both AW and PWHT joints. The austenitic microstructure in WM exhibited the hardness of 150 ± 8 HV0.5, which was lower than the hardness of P92 and Inconel 617 base metal. The hardness in P92 HAZ was estimated higher than other regions of the weldments (WM and Inconel 617 HAZ). PWHT resulted in a drastic reduction in hardness of P92 HAZ, while WM (154 ± 7 HV0.5) and Inconel 617 HAZ showed a minute change in hardness value. The tensile strength of welded joints for the transverse tensile-tested specimen was also measured higher than the base metal, and fracture was observed in weaker parent metal, i.e., P92 steel for both AW and PWHT joint.

8

A comparative study of two nuclear steel grades welded joints

Sirohi Sachin, Pandey Chandan, Goyal Amit

Archives of Metallurgy and Materials

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2020

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Vol. 65, iss. 2

583--593

EN

In present work, two nuclear grade steel (P91, P92) are joined using the arc welding process. The welded joints were subjected to the heat treatment in order to restore the mechanical properties and overcome the heterogeneity across the joints. The weldments were studied for microstructure evolution and mechanical behavior under different condition of heat treatment. The variation in mechanical behavior obtained for the welded joints were tried to relate the microstructural evolution. After the normalizing based heat treatment, homogeneity with negligible δ ferrite across the welded joints was observed.

9

Kumar Sanjeev, Pandey Chandan, Goyal Amit

Archives of Civil and Mechanical Engineering

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2020

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Vol. 20, no. 3

506--532

EN

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.

10

Softening mechanism of P91 steel weldments using heat treatments

Pandey Chandan, Mahapatra Manas Mohan, Kumar Pradeep, Daniel F., Adhithan B.

Archives of Civil and Mechanical Engineering

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2019

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Vol. 19, no. 2

297--310

EN

The tungsten inert gas welded P91 steel welded joints were subjected to the two different type of heat treatments including the postweld direct tempering (PWDT) and re-austenitizing based tempering (PWNT) treatment. The microstructure of weld fusion and heat affected zone (HAZ) were characterized in different heat treatment conditions using optical microscope and scanning electron microscope. For as-welded joint, a great heterogeneity was observed in microstructure and mechanical properties across the weldments. The Charpy toughness of the as-welded joint was measured much lower than the minimum recommended value of 47 J and it was measured 8 ± 5 J. The PWHTs have found a beneficial effect in decreasing the microstructure heterogeneity across the welded joint and improving the mechanical properties. The PWDT resulted in a drastic improvement in the Charpy impact toughness of the welded joint and it was measured 59 ± 5 J which was higher than the minimum required value of 47 J but still inferior than the base metal. The δ ferrite still remained in overlap zone of the weld fusion zone. The PWNT treatment resulted in homogeneous microstructure and hardness variation across the welded joint in transverse direction and Charpy impact toughness (149 ± 6 J) exceeded than that achieved in base metal.