Research Publications
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Item Softening mechanism of P91 steel weldments using heat treatments(2019) Pandey C.; Mahapatra M.M.; Kumar P.; Daniel F.; Adhithan B.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 ? 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. � 2018 Politechnika Wroc?awskaItem Effect of post weld heat treatments on microstructure evolution and type IV cracking behavior of the P91 steel welds joint(2019) Pandey C.; Mahapatra M.M.; Kumar P.; Kumar S.; Sirohi S.The creep tests were conducted on multi-pass shielded metal arc welded P91 steel weld joint at creep exposure temperature of 620 �C for an applied stress of 150 and 200 MPa in as-welded, and post weld heat treatments (PWHTs) state. The PWHTs involved the tempering at 760 �C/2 h/air cooling (referred as PWHT) and re-austenitizing at 1040 �C/60 min/air cooling + tempering at 760 �C/2 h/air cooling (refereed as PWNT). Results indicate a great influence of heat treatment conditions on the creep rupture life, fracture location and microstructure evolution of the P91 welded joint. As-welded and PWHT specimens showed the Type IV mode of failure and the fracture occurred in soft fine-grained heat affected zone/inter-critical heat affected zone (FGHAZ/IC-HAZ). In PWNT specimens, a shift in fracture location was observed from FGHAZ/ICHAZ to over tempered base zone. The degree of heterogeneity in terms of the microstructure and mechanical properties across the welded joints before the creep exposure have observed a great influence on the creep rupture life. As compared to other heat treated specimen, PWNT specimen exhibited higher creep rupture life at the lower applied stress of 150 MPa and creep exposure temperature of 620 �C due to lower degree of heterogeneity across the welded joints. However, Laves phase formation (Fe2W/Fe2Mo) was noticed in different zones (weld zone + heat affected zones) of the crept specimen for PWNT condition exposed at 620 �C for 150 MPa. � 2018 Elsevier B.V.Item Role of Heat Treatment on Grain Refinement and Microhardness of 9Cr�1Mo�V�Nb Steel(2019) Pandey C.; Sirohi S.; Mahapatra M.M.; Kumar P.; Bansal K.K.Effect of �double austenitizing� (DN) on microstructure evolution and mechanical properties of martensitic 9Cr�1Mo�V�Nb (P91) steel were studied and compared with the �conventional normalizing� (CN) process. In CN treatment, P91 steel is normalized at 1050��C for 1�h, finally air cooled. In DN treatment, the first stage of normalizing was performed at 1050��C/1�h/air cool. The second stage of normalizing was performed in the temperature range of 950�1150��C for 1�h (950��C-DN1, 1050��C-DN2, 1150��C-DN3), followed by water quenching. The grain size was measured 42 and 35��m for CN and DN1 treatment, respectively. The double normalizing (DN) produced complete martensitic microstructure as a result of complete dissolution of precipitates. In DN-based heat treatment, optimized microstructure and mechanical properties were obtained for the sample that normalized at 950��C, followed by water quenching. � 2019, ASM International.Item Effect of strain rate and notch geometry on tensile properties and fracture mechanism of creep strength enhanced ferritic P91 steel(2018) Pandey C.; Mahapatra M.M.; Kumar P.; Saini N.Creep strength enhanced ferritic (CSEF) P91 steel were subjected to room temperature tensile test for quasi-static (less than 10?1/s) strain rate by using the Instron Vertical Tensile Testing Machine. Effect of different type of notch geometry, notch depth and angle on mechanical properties were also considered for different strain rate. In quasi-static rates, the P91 steel showed a positive strain rate sensitivity. On the basis of tensile data, fracture toughness of P91 steel was also calculated numerically. For 1 mm notch depth (constant strain rate), notch strength and fracture toughness were found to be increased with increase in notch angle from 45� to 60� while the maximum value attained in U-type notch. Notch angle and notch depth has found a minute effect on P91 steel strength and fracture toughness. The fracture surface morphology was studied by field emission scanning electron microscopy (FESEM). � 2017 Elsevier B.V.Item Homogenization of P91 weldments using varying normalizing and tempering treatment(2018) Pandey C.; Mahapatra M.M.; Kumar P.; Saini N.Creep strength enhanced ferritic/martensitic P91 steel is considered as a candidate material for the reactor pressure vessels and reactor internals of Very High Temperature Reactor (VHTR). Heterogeneous microstructure formation across the P91 weldments lead to premature Type IV cracking and makes the weldability of P91 steel a serious issue. The present research work describes the effect of normalizing and tempering (N&T) treatment on microstructure evolution in various zones of gas tungsten arc welded (GTAW) P91 pipe weldments. For N&T treatment, P91 pipe weldments were subjected to various normalizing (950�1150 �C) and tempering (730�800 �C) temperature. The effect of varying heat treatment on tensile properties and hardness of P91 pipe weldments were studied for V-groove and narrow-groove weld designs. The effect of increase in normalizing temperature (fixed tempering temperature) resulted in increase in strength and hardness, while increase in tempering temperature (fixed normalizing temperature) resulted in the decrease in strength and hardness of P91 steel weldments. The better combination of strength, ductility and microstructure were obtained for the maximum normalizing temperature of 1050 �C and tempering temperature of 760 �C. � 2017 Elsevier B.V.Item Effect of post weld heat treatments on fracture frontier and type IV cracking nature of the crept P91 welded sample(2018) Pandey C.; Mahapatra M.M.; Kumar P.The research work explores the microstructure evolution and creeps rupture behavior of the shielded metal arc welded joint of reduced activation ferritic/martensitic P91 steel plate in the as-welded and different states of post weld heat treatment (PWHT). Two types of heat treatments were employed including (i) post weld heat treatment (PWHT) at 760 �C for 2 h and finally air cooled and (ii) re-austenitizing at 1040 �C for 60 min and air-cooled and tempered at 760 �C for 2 h, followed by air cooling (PWNT). The P91 steel plates were received in the cast and forged condition having fully tempered lath martensite with lath blocks, packet boundaries and prior austenite grain boundaries (PAGBs). In this research, microstructural evolution at fracture frontier of crept P91 weld sample, creep rupture life and effect of creep exposure time on microstructure evolution in the fine-grained heat affected zone (FGHAZ) were studied. The results show a dominating effect of PWNT treatment on fracture behavior and creep rupture life of the P91 welded joints. PWNT treatment results in negligible hardness variation across the transverse section of the weld joint. The creep rupture life of PWNT specimen was 594.895% higher as compared to that of PWHT specimen and 142.7% higher than that of the as-welded specimen for the similar condition of creep exposure (620 �C/150 MPa). In PWHT condition, the most common type IV cracking was observed. However, it was eliminated in PWNT specimens and fracture occurred form the base zone. The fracture frontier of crept PWNT specimen showed a higher amount of creep cavities that can be attributed to the formation of Laves phase. � 2018 Elsevier B.V.Item Grain refinement of P91 steel using double austenitization treatment(2018) Pandey C.; Mahapatra M.M.; Kumar P.; Saini N.; Thakre J.G.; Kumar P.The effect of conventional normalizing and tempering (CNT) and double austenitization-based normalizing and tempering (DNT) heat treatments on the microstructure evolution and mechanical properties of creep strength-enhanced ferritic P91 steel were studied. CNT treatment was performed at 1,050�C and air cooled for 1 h and also at 760�C and air cooled for 2 h. In double austenitization- based normalizing heat treatment, steel was austenitized at 1,050�C and air cooled for 1 h and then normalized in a temperature range of 950�C -1,150�C for 1 h followed by water quenching. After the double normalizing, tempering is performed at 760�C for 2 h. A DNT treatment resulted in homogeneous microstructure formation that led to improved mechanical properties as compared to CNT treatment. The incomplete dissolution of precipitates in single-stage tempering resulted in incomplete martensitic structure formation. � 2018 by ASTM International.Item A comparative study of ductile-brittle transition behavior and fractography of P91 and P92 steel(2017) Saini N.; Pandey C.; Mahapatra M.M.; Narang H.K.; Mulik R.S.; Kumar P.The modified 9Cr-1Mo (P91) and 9Cr- 0.5Mo- 1.8W (P92) steel used in fast breeder reactor is exposed to irradiation during service which severely affects the dynamic fracture resistance by increasing the ductile to brittle transition temperature (DBTT). Thus, even at room temperature, the steel can become brittle and prone to cracking. In the present investigation, to elucidate the influence of low temperature on the DBTT, Charpy toughness test was performed on creep strength enhanced ferritic P91 and P92 steel. Lower DBTT was observed for the P92 steel as compared to P91 steel. To find the mode of fracture, the fractured Charpy toughness specimens were investigated using a field electron scanning electron microscope (FESEM). The fracture surface revealed the brittle mode of fracture at a lower temperature for both the steels while the mixed mode of fracture was noticed at room temperature and above. � 2017 Elsevier Ltd