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Kazuhiro Kimura - One of the best experts on this subject based on the ideXlab platform.
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the formation and dissolution of residual δ ferrite in asme Grade 91 Steel plates
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2014Co-Authors: Satoru Kobayashi, Kota Sawada, Hideaki Kushima, Toru Hara, Kazuhiro KimuraAbstract:Abstract This paper investigated the relationship between creep strength and the existence of residual δ ferrite in the initial microstructure in three heats of Grade 91 Steel plates and examined the origin, the transformation process and the dissolution of the δ ferrite in the Steel plates. The creep strength of the Steel specimens was lower when the δ ferrite was present in the initial microstructure. The δ ferrite showed a disk shape and a bumpy interface, and had MX carbonitride particle arrays inside and coarse M 23 C 6 carbide particles on the bumpy interface. Chromium and molybdenum were found to segregate in/around the δ ferrite disk. The residual δ ferrite dissolved completely after an additional normalizing heat treatment at 1050 °C for 30 min. The results obtained strongly indicate that the presence of δ ferrite is due to insufficient normalizing heat treatment time to remove segregation during solidification. The δ ferrite was also found to grow through a diffusional transformation during cooling from the normalizing heat treatment.
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influence of chemical composition and heat treatment on long term creep strength of Grade 91 Steel
Procedia Engineering, 2013Co-Authors: Kazuhiro Kimura, Kota Sawada, Hideaki Kushima, Yoshiaki TodaAbstract:Abstract Long-term creep strength of ASTM/ASME Grade 91 Steels was investigated. Two heats of Grade 91 Steels indicated lower creep rupture strength than the other four heats from short-term to long-term, and presence of delta ferrite phase was observed. In the short-term, no difference in creep rupture strength was observed among four heats of Grade 91 Steels, however, the large heat-to-heat variation of creep rupture strength was observed in the long-term at 600 °C. The higher nickel containing heat indicates lower creep rupture strength in the long-term at 600 °C, although nickel concentration was 0.28mass% in maximum. Homogeneously recovered subgrain structure was observed on the specimens creep ruptured after about 80,000 h at 600 °C for both high nickel low strength heat and low nickel high strength one. Only a small number of fine MX carbonitride particles with a large number of coarse Z-phase were observed on the creep ruptured specimen of high nickel low strength heat, in contrast to low nickel high strength heat in which many MX particles were still observed and Z- phase formation was not pronounced. The difference in stability of fine MX carbonitride particles during creep exposure at the elevated temperatures is a cause of heat-to-heat variation of long-term creep strength of the Steels. Decrease in phase transformation temperature of Ac1 with increase in nickel content may reduce stability of the precipitates at the elevated temperatures. Nickel content should be reduced in order to suppress a large drop in long-term creep strength of Grade 91 Steel.
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creep rupture ductility of creep strength enhanced ferritic Steels
Journal of Pressure Vessel Technology-transactions of The Asme, 2012Co-Authors: Kazuhiro Kimura, Kota Sawada, Hideaki KushimaAbstract:Creep rupture strength and ductility of creep strength enhanced ferritic Steels of Grades 23, 91, 92, and 122 was investigated with particular emphasis on remarkable drop in the long-term. Large difference in creep rupture strength and ductility was observed on three heats of Grade 23 Steels. Remarkable drop of creep rupture strength in the long-term of T91 was comparable to those of Grades 92 and 122. Remarkable drop in creep rupture ductility in a stress regime below 50% of 0.2% offset yield stress was observed on Grade T23 Steel, however, that of Grade P23 Steel did not indicate any degradation of creep rupture ductility. Higher creep rupture ductility of Grade P23 Steel was considered to be caused by its lower creep strength than that of T23 Steels. Creep rupture ductility of Grades 92 and 122 Steels indicated rapid and drastic decrease with decrease in stress at 50% of 0.2% offset yield stress. Stress dependence of creep rupture ductility of Grades 92 and 122 Steels was well described by a ratio of stress to 0.2% offset yield stress, regardless of temperature. On the other hand, large drop in creep rupture ductility of Grade 91 Steel was observed only in the very low-stress regime at 650 °C. Alloying elements including impurities and changes in precipitates may influence on creep rupture ductility, however, remarkable drop in ductility of the Steels cannot be explained by chemical composition and precipitates. High ductility in the high-stress regime above 50% of 0.2% offset yield stress should be provided by easy plastic deformation, and it has been concluded that a remarkable drop in ductility in the low-stress regime is derived from a concentration of creep deformation into a tiny recovered region formed at the vicinity of grain boundary.
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creep rupture ductility of creep strength enhanced ferritic Steels
ASME 2010 Pressure Vessels and Piping Conference: Volume 6 Parts A and B, 2010Co-Authors: Kazuhiro Kimura, Kota Sawada, Hideaki KushimaAbstract:Creep rupture strength and ductility of Creep Strength Enhanced Ferritic Steels of Grades 23, 91, 92 and 122 was investigated with particular emphasis on remarkable drop in the long-term. Large difference in creep rupture strength and ductility was observed on three heats of Grade 23 Steels. Remarkable drop of creep rupture strength in the long-term of T91 was comparable to those of Grades 92 and 122. Remarkable drop in creep rupture ductility in a stress regime below 50% of 0.2% offset yield stress was observed on Grade T23 Steel, however, that of Grade P23 Steel did not indicate any degradation of creep rupture ductility. Higher creep rupture ductility of Grade P23 Steel was considered to be caused by its lower creep strength than that of T23 Steels. Creep rupture ductility of Grades 92 and 122 Steels indicated rapid and drastic decrease with decrease in stress at 50% of 0.2% offset yield stress. Stress dependence of creep rupture ductility of Grades 92 and 122 Steels was well described by a ratio of stress to 0.2% offset yield stress, regardless of temperature. On the other hand, large drop in creep rupture ductility of Grade 91 Steel was observed only in the very low stress regime at 650°C. Alloying elements including impurities and changes in precipitates may influence on creep rupture ductility, however, remarkable drop in ductility of the Steels cannot be explained by chemical composition and precipitates. High ductility in the high stress regime above 50% of 0.2% offset yield stress should be provided by easy plastic deformation, and it has been concluded that a remarkable drop in ductility in the low stress regime is derived from a concentration of creep deformation into a tiny recovered region formed at the vicinity of grain boundary.Copyright © 2010 by ASME
Jonathan Parker - One of the best experts on this subject based on the ideXlab platform.
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use of a stepped preparation to improve the damage tolerance of Grade 91 Steel welds
Materials at High Temperatures, 2021Co-Authors: Jonathan Parker, J A SiefertAbstract:Damage equivalent to that seen over very long times in Grade 91 Steel welded components was successfully formed in laboratory, feature sized samples tested under controlled conditions. This paper ...
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creep damage in long term Grade 91 Steel component tests
Materials at High Temperatures, 2020Co-Authors: Jonathan Parker, J A SiefertAbstract:This paper describes specific results from a unique study which involves design, manufacture and internal pressure testing of two Grade 91 Steel vessels. The vessel results which are the focus of t...
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assessment and quantification of damage in the Grade 91 Steel partially transformed zone
2019Co-Authors: J A Siefert, Jonathan Parker, Ryan Maclachlan, Xin Xie Yang, Rachel C. ThomsonAbstract:Damage in the Grade 91 Steel partially transformed zone has historically been associated with many different types of features in the microstructure. Susceptible features responsible for the nucleation of damage are frequently cited to include particles such as laves phase, coarse M23C6, inclusions, nitrides, or interactions between creep strong and creep week grains, grain boundaries and potentially other sources. Few studies have attempted to link these observations on a statistically relevant basis using 2D or 3D microscopy techniques. In this study, 2D assessment using scanning electron microscopy (SEM) and quantification techniques such as energy dispersive x-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) are utilized in combination with 3D serial sectioning of large volumes using plasma focused ion beam milling (P-FIB) and simultaneous EDS to evaluate an interrupted cross-weld creep test. The cross-weld creep test was performed using a feature-type geometry, in parent material susceptible to the evolution of creep damage and interrupted at an estimated life fraction of 70%. The findings from these evaluations provide perspective on the features in the microstructure responsible for the nucleation and subsequent growth of the observed damage.
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Cross-weld creep performance in Grade 91 Steel: Macro-based assessment
Welding Journal, 2019Co-Authors: John A. Siefert, Jonathan Parker, Rachel C. ThomsonAbstract:Meaningful characterization of the microstructure in metallurgically complex Steels is complicated by the diversity of thermal cycles experienced by multipass fusion welds. To overcome the problems of relevant documentation, it is necessary to balance information from macro-, micro-, and nano-evaluation with appropriate analysis. This paper presents details regarding recommended approaches that optimize this characterization. Initially, specific procedures relevant to macroanalysis, including hardness mapping and calculation of the peak temperature through the width of the heat-affected zone (HAZ), are described. Then, assessment of the distribution of creep damage in feature-type, cross-weld creep tests using laser microscopy is detailed. Using these methods, the extent of damage through the HAZ was compared to the local reduction in the HAZ hardness and to the calculated peak temperatures in the HAZ. The implications of these findings are discussed with respect to damage, deformation, and sample geometry.
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Metallurgical and Stress State Factors Which Affect the Creep and Fracture Behavior of 9% Cr Steels
Hindawi Limited, 2018Co-Authors: Jonathan Parker, John SiefertAbstract:EPRI-supported research has identified critical material information regarding the factors affecting the performance of creep strength-enhanced ferritic Steels, in general, and Grade 91 Steel, in particular. EPRI recommendations emphasize that a five-point, integrated strategy should be used for the effective life management of components fabricated from tempered martensitic Steels. This integration promotes a balanced use of resources which, when properly focused, reduces uncertainty regarding creep and fracture behavior. Tighter control of processes from Steel making, Steel processing, and heat treatment ensures that alloys with deficient properties never enter service. One cornerstone of this proactive approach is the definition of ‘Metallurgical Risk’ which links the presence of inclusions and trace elements to the susceptibility for creep damage. The improved confidence in the high temperature performance of CSEF Steel components promotes reliability, increases efficiency, and minimizes the risk of component fracture
Yiyu Wang - One of the best experts on this subject based on the ideXlab platform.
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characteristics of premature creep failure in over tempered base metal of Grade 91 Steel weldment
International Journal of Pressure Vessels and Piping, 2021Co-Authors: Yiyu Wang, Wei Zhang, Yanli Wang, Zhili FengAbstract:Abstract In this work, characteristics of premature creep failure in the over-tempered base metal (OT-BM) of a Grade 91 Steel weldment are investigated with specially designed creep experiments and advanced microstructure characterization. In situ digital image correlation (DIC) strain measurements reveal that local creep strain as high as 100% accumulated in the OT-BM, compared with only 10% nominal strain over the gauge length. The creep strain rate in the OT-BM is identical to that in the intercritical heat-affected zone and fine-grained heat-affected zone at the secondary creep stage and early tertiary stage, but faster at the late tertiary stage. Microstructural analysis shows that highly recovered microstructure in the OT-BM leads to the lowest hardness, the largest grain size, the lowest fraction of coincidence site lattice boundaries, and the lowest localized strain energy. Dislocation creep and transgranular creep fracture are the dominant deformation and fracture mechanisms in the OT-BM under the current creep testing condition of a low temperature (550 °C) and a high stress (215 MPa).
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experimental evaluation of localized creep deformation in Grade 91 Steel weldments
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2021Co-Authors: Yiyu Wang, Wei Zhang, Yanli Wang, Yong Chae Lim, Zhili FengAbstract:Abstract Spatially resolved measurement of localized creep deformation in heterogeneous creep resistant Steel weldments is crucial but challenging for lifetime assessments of critical steam components in power plants. In this work, experimental approaches were established to quantitatively evaluate commonly observed localized creep deformation in multi-pass Grade 91 Steel weldments. An in-situ digital image correlation (DIC) system was utilized with a creep testing frame to monitor and measure both full-field strain and localized strain accumulation across the weldments during long-term creep testing at elevated temperatures. The in situ DIC method measured not only the creep deformation behavior of the weld metal, heat affected zone (HAZ), and base metal, but also creep strain evolution for each sub-region within the HAZ itself, including coarse-grained HAZ, fine-grained HAZ (FGHAZ), and intercritical HAZ (ICHAZ). The DIC results revealed that local creep strain in the ICHAZ reached up to 90% strain before final rupture, whereas nominal creep strain measured by the standard extensometer of the tested cross-weld specimen was below 10%, indicative of Type IV cracking of the Grade 91 weld. Microstructural analyses revealed that the faster creep degradation/deformation in the HAZ was caused mainly by accelerated matrix grain recrystallization/growth and a reduced pinning effect from the segregated and coarsened precipitates in the FGHAZ and ICHAZ. The ultimate creep rupture occurred in the ICHAZ owing to its lowest creep resistance induced by the largest recrystallized grain size, the lowest fraction of coincidence site lattice, and the lowest local strain energies/dislocation densities.
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microstructure and mechanical properties of intercritically treated Grade 91 Steel
Materials, 2020Co-Authors: Yiyu Wang, Wei Zhang, Yanli Wang, Yong Chae Lim, Zhili FengAbstract:Premature creep failures at the intercritical heat affected zone (ICHAZ) of creep-resistant Steel weldments have been frequently reported. However, the creep degradation mechanism of different microstructure constituents in ICHAZ is complicated and needs further clarification. In this work, Grade 91 Steel was intercritically heat-treated at a temperature (860 °C) between the critical temperatures AC1 and AC3, and a correlation between microstructure and mechanical properties of the heat-treated specimen was built. The effects of austenitization and tempering resulting from the intercritical treatment (IT) differentiated the local strain energies between the two microstructure constituents: newly transformed martensite (NTM) and over-tempered martensite (OTM). The formation of NTM grains led to a hardness increase from 247 HV0.5 in the base metal to 332 HV0.5 in the IT specimen. The ultimate tensile strength (UTS) increased from 739 MPa in the base metal to 1054 MPa in the IT specimen. Extensive growth of the OTM grains and rapid recovery of NTM grains took place simultaneously in the IT specimen during a typical tempering at 760 °C. These microstructure degradations led to a lowered hardness of 178 HV0.5, a reduced UTS of 596 MPa, and a poor creep resistance with a minimum creep strain rate of 0.49 %/h at 650 °C in an IT + tempering (ITT) specimen.
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insight into type iv cracking in Grade 91 Steel weldments
Materials & Design, 2020Co-Authors: Yiyu Wang, Rangasayee KannanAbstract:Abstract In this work, an insight into the premature Type IV cracking was undertaken to clarify its mechanisms in Grade 91 Steel pipe weldments. High-resolution microscopy observations of the as-welded heat-affected zone (HAZ) reveal that the commonly recognized fine-grained region susceptible of cracking on the edge of HAZ belongs to the inter-critical HAZ (ICHAZ), rather than the fine-grained HAZ (FGHAZ). Instrumented indentation tests uncover that the ICHAZ is the weakest region across the weld, exhibiting the largest displacement and the lowest hardness in three thermal stages. Localized deformation of matrix grains and high stress triaxiality in the ICHAZ promoted nucleation of creep cavities along grain boundaries. This localized deformation was induced by the creep strength mismatch of matrix grains with different Cr concentrations. Cavity-free regions exhibit a relatively homogenous Cr distribution, whereas, an inhomogeneous Cr distribution is observed in the cavity-containing regions. It is believed that this local Cr inhomogeneity in the ICHAZ is caused by the partial dissolution of Cr-rich M23C6 carbides and an insufficient homogenization during rapid welding thermal cycles.
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Correlation Between Intercritical Heat-Affected Zone and Type IV Creep Damage Zone in Grade 91 Steel
Metallurgical and Materials Transactions A, 2018Co-Authors: Yiyu Wang, Rangasayee KannanAbstract:A soft zone in Cr-Mo Steel weldments has been reported to accompany the infamous Type IV cracking, the highly localized creep damage in the heat-affected zone of creep-resistant Steels. However, the microstructural features and formation mechanism of this soft zone are not well understood. In this study, using microhardness profiling and microstructural verification, the initial soft zone in the as-welded condition was identified to be located in the intercritical heat-affected zone of P91 Steel weldments. It has a mixed structure, consisting of Cr-rich re-austenitized prior austenite grains and fine Cr-depleted, tempered martensite grains retained from the base metal. The presence of these further-tempered retained grains, originating from the base metal, is directly responsible for the hardness reduction of the identified soft zone in the as-welded condition. The identified soft zone exhibits a high location consistency at three thermal stages. Local chemistry analysis and thermodynamic calculation show that the lower chromium concentrations inside these retained grains thermodynamically decrease their potentials for austenitic transformation during welding. Heterogeneous grain growth is observed in the soft zone during postweld heat treatment. The mismatch of strengths between the weak Cr-depleted grains and strong Cr-rich grains enhances the creep damage. Local deformation of the weaker Cr-depleted grains accelerates the formation of creep cavities.
Hideaki Kushima - One of the best experts on this subject based on the ideXlab platform.
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the formation and dissolution of residual δ ferrite in asme Grade 91 Steel plates
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2014Co-Authors: Satoru Kobayashi, Kota Sawada, Hideaki Kushima, Toru Hara, Kazuhiro KimuraAbstract:Abstract This paper investigated the relationship between creep strength and the existence of residual δ ferrite in the initial microstructure in three heats of Grade 91 Steel plates and examined the origin, the transformation process and the dissolution of the δ ferrite in the Steel plates. The creep strength of the Steel specimens was lower when the δ ferrite was present in the initial microstructure. The δ ferrite showed a disk shape and a bumpy interface, and had MX carbonitride particle arrays inside and coarse M 23 C 6 carbide particles on the bumpy interface. Chromium and molybdenum were found to segregate in/around the δ ferrite disk. The residual δ ferrite dissolved completely after an additional normalizing heat treatment at 1050 °C for 30 min. The results obtained strongly indicate that the presence of δ ferrite is due to insufficient normalizing heat treatment time to remove segregation during solidification. The δ ferrite was also found to grow through a diffusional transformation during cooling from the normalizing heat treatment.
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influence of chemical composition and heat treatment on long term creep strength of Grade 91 Steel
Procedia Engineering, 2013Co-Authors: Kazuhiro Kimura, Kota Sawada, Hideaki Kushima, Yoshiaki TodaAbstract:Abstract Long-term creep strength of ASTM/ASME Grade 91 Steels was investigated. Two heats of Grade 91 Steels indicated lower creep rupture strength than the other four heats from short-term to long-term, and presence of delta ferrite phase was observed. In the short-term, no difference in creep rupture strength was observed among four heats of Grade 91 Steels, however, the large heat-to-heat variation of creep rupture strength was observed in the long-term at 600 °C. The higher nickel containing heat indicates lower creep rupture strength in the long-term at 600 °C, although nickel concentration was 0.28mass% in maximum. Homogeneously recovered subgrain structure was observed on the specimens creep ruptured after about 80,000 h at 600 °C for both high nickel low strength heat and low nickel high strength one. Only a small number of fine MX carbonitride particles with a large number of coarse Z-phase were observed on the creep ruptured specimen of high nickel low strength heat, in contrast to low nickel high strength heat in which many MX particles were still observed and Z- phase formation was not pronounced. The difference in stability of fine MX carbonitride particles during creep exposure at the elevated temperatures is a cause of heat-to-heat variation of long-term creep strength of the Steels. Decrease in phase transformation temperature of Ac1 with increase in nickel content may reduce stability of the precipitates at the elevated temperatures. Nickel content should be reduced in order to suppress a large drop in long-term creep strength of Grade 91 Steel.
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creep rupture ductility of creep strength enhanced ferritic Steels
Journal of Pressure Vessel Technology-transactions of The Asme, 2012Co-Authors: Kazuhiro Kimura, Kota Sawada, Hideaki KushimaAbstract:Creep rupture strength and ductility of creep strength enhanced ferritic Steels of Grades 23, 91, 92, and 122 was investigated with particular emphasis on remarkable drop in the long-term. Large difference in creep rupture strength and ductility was observed on three heats of Grade 23 Steels. Remarkable drop of creep rupture strength in the long-term of T91 was comparable to those of Grades 92 and 122. Remarkable drop in creep rupture ductility in a stress regime below 50% of 0.2% offset yield stress was observed on Grade T23 Steel, however, that of Grade P23 Steel did not indicate any degradation of creep rupture ductility. Higher creep rupture ductility of Grade P23 Steel was considered to be caused by its lower creep strength than that of T23 Steels. Creep rupture ductility of Grades 92 and 122 Steels indicated rapid and drastic decrease with decrease in stress at 50% of 0.2% offset yield stress. Stress dependence of creep rupture ductility of Grades 92 and 122 Steels was well described by a ratio of stress to 0.2% offset yield stress, regardless of temperature. On the other hand, large drop in creep rupture ductility of Grade 91 Steel was observed only in the very low-stress regime at 650 °C. Alloying elements including impurities and changes in precipitates may influence on creep rupture ductility, however, remarkable drop in ductility of the Steels cannot be explained by chemical composition and precipitates. High ductility in the high-stress regime above 50% of 0.2% offset yield stress should be provided by easy plastic deformation, and it has been concluded that a remarkable drop in ductility in the low-stress regime is derived from a concentration of creep deformation into a tiny recovered region formed at the vicinity of grain boundary.
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creep rupture ductility of creep strength enhanced ferritic Steels
ASME 2010 Pressure Vessels and Piping Conference: Volume 6 Parts A and B, 2010Co-Authors: Kazuhiro Kimura, Kota Sawada, Hideaki KushimaAbstract:Creep rupture strength and ductility of Creep Strength Enhanced Ferritic Steels of Grades 23, 91, 92 and 122 was investigated with particular emphasis on remarkable drop in the long-term. Large difference in creep rupture strength and ductility was observed on three heats of Grade 23 Steels. Remarkable drop of creep rupture strength in the long-term of T91 was comparable to those of Grades 92 and 122. Remarkable drop in creep rupture ductility in a stress regime below 50% of 0.2% offset yield stress was observed on Grade T23 Steel, however, that of Grade P23 Steel did not indicate any degradation of creep rupture ductility. Higher creep rupture ductility of Grade P23 Steel was considered to be caused by its lower creep strength than that of T23 Steels. Creep rupture ductility of Grades 92 and 122 Steels indicated rapid and drastic decrease with decrease in stress at 50% of 0.2% offset yield stress. Stress dependence of creep rupture ductility of Grades 92 and 122 Steels was well described by a ratio of stress to 0.2% offset yield stress, regardless of temperature. On the other hand, large drop in creep rupture ductility of Grade 91 Steel was observed only in the very low stress regime at 650°C. Alloying elements including impurities and changes in precipitates may influence on creep rupture ductility, however, remarkable drop in ductility of the Steels cannot be explained by chemical composition and precipitates. High ductility in the high stress regime above 50% of 0.2% offset yield stress should be provided by easy plastic deformation, and it has been concluded that a remarkable drop in ductility in the low stress regime is derived from a concentration of creep deformation into a tiny recovered region formed at the vicinity of grain boundary.Copyright © 2010 by ASME
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microstructural stability and long term creep strength of Grade 91 Steel
Energy Materials, 2009Co-Authors: K Kimura, Kota Sawada, Hideaki Kushima, Yoshiaki TodaAbstract:Abstract Long term creep strength of ASTM Grade 91 Steels was investigated in conjunction with microstructural stability during creep exposure. In the short term, no difference in creep rupture strength was observed among four heats of Grade 91 Steels; however, the large heat to heat variation of creep rupture strength was observed in the long term at 600°C. Good correspondence between long term creep rupture strength and nickel content was observed, and the long term creep strength of Grade 91 Steel decreased with an increase in nickel content, although the nickel concentration was 0·28 mass-% in maximum. Only a small number of fine MX carbonitride particles with a large number of coarse Z phase were observed on the creep ruptured specimen of high nickel low strength heat. Although the influence of nickel on the precipitation sequence during creep exposure is not yet clearly understood, the nickel content should be reduced in order to suppress a large drop in long term creep strength of Grade 91 ...
Rangasayee Kannan - One of the best experts on this subject based on the ideXlab platform.
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insight into type iv cracking in Grade 91 Steel weldments
Materials & Design, 2020Co-Authors: Yiyu Wang, Rangasayee KannanAbstract:Abstract In this work, an insight into the premature Type IV cracking was undertaken to clarify its mechanisms in Grade 91 Steel pipe weldments. High-resolution microscopy observations of the as-welded heat-affected zone (HAZ) reveal that the commonly recognized fine-grained region susceptible of cracking on the edge of HAZ belongs to the inter-critical HAZ (ICHAZ), rather than the fine-grained HAZ (FGHAZ). Instrumented indentation tests uncover that the ICHAZ is the weakest region across the weld, exhibiting the largest displacement and the lowest hardness in three thermal stages. Localized deformation of matrix grains and high stress triaxiality in the ICHAZ promoted nucleation of creep cavities along grain boundaries. This localized deformation was induced by the creep strength mismatch of matrix grains with different Cr concentrations. Cavity-free regions exhibit a relatively homogenous Cr distribution, whereas, an inhomogeneous Cr distribution is observed in the cavity-containing regions. It is believed that this local Cr inhomogeneity in the ICHAZ is caused by the partial dissolution of Cr-rich M23C6 carbides and an insufficient homogenization during rapid welding thermal cycles.
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Correlation Between Intercritical Heat-Affected Zone and Type IV Creep Damage Zone in Grade 91 Steel
Metallurgical and Materials Transactions A, 2018Co-Authors: Yiyu Wang, Rangasayee KannanAbstract:A soft zone in Cr-Mo Steel weldments has been reported to accompany the infamous Type IV cracking, the highly localized creep damage in the heat-affected zone of creep-resistant Steels. However, the microstructural features and formation mechanism of this soft zone are not well understood. In this study, using microhardness profiling and microstructural verification, the initial soft zone in the as-welded condition was identified to be located in the intercritical heat-affected zone of P91 Steel weldments. It has a mixed structure, consisting of Cr-rich re-austenitized prior austenite grains and fine Cr-depleted, tempered martensite grains retained from the base metal. The presence of these further-tempered retained grains, originating from the base metal, is directly responsible for the hardness reduction of the identified soft zone in the as-welded condition. The identified soft zone exhibits a high location consistency at three thermal stages. Local chemistry analysis and thermodynamic calculation show that the lower chromium concentrations inside these retained grains thermodynamically decrease their potentials for austenitic transformation during welding. Heterogeneous grain growth is observed in the soft zone during postweld heat treatment. The mismatch of strengths between the weak Cr-depleted grains and strong Cr-rich grains enhances the creep damage. Local deformation of the weaker Cr-depleted grains accelerates the formation of creep cavities.
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transition from type iv to type i cracking in heat treated Grade 91 Steel weldments
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2018Co-Authors: Yiyu Wang, Rangasayee KannanAbstract:Abstract Increasing the post-weld heat-treating temperature from 600 °C to 840 °C causes a transition of creep rupture locations from the Type IV cracking in the intercritical heat-affected zone to the Type I cracking in the fusion zone in the Grade 91 Steel weldments. The specimens following a heat-treatment at temperatures below the A1 temperature of the base metal failed in the Type IV cracking mode after high-temperature creep tests at 650 °C. The creep-damaged region associated with the Type IV cracking mode is consistently located in a softened zone, which is identified as the intercritical heat-affected zone. In the specimens heat-treated at temperatures close to or above the A1 temperature of the base metal, the fusion zone enters the intercritical temperature between its A1 and A3 temperatures. The fusion zone experienced a partial austenitization to become a mixture of retained ferrite and new martensite, which is believed the most creep-susceptible. The same microstructure, a mixture of retained soft ferrite and new hard martensite, caused nucleation and growth of cavities in the both Type IV and Type I damaged regions. Therefore, it is suggested that the same creep mechanism may have contributed to both cracking modes in Grade 91 Steel.
