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This research was performed to study the effect of surface roughness on the fatigue limit in martensitic stainless steel (STS 410). After heat treatment of STS 410 steel, mechanical property tests and rotary bending fatigue tests were performed by varying the surface roughness conditions of A-polishing, A-grinding, and A-#60 of STS 410 steel, respectively. The fatigue fracture surface was observed and analyzed using SEM and EDS. The fatigue limit of STS 410 steel decreased by 0.98% from 509 to 504 MPa when the surface roughness increased about 3 times from 0.226 to 0.664 μm. However, when it increased about 9 times from 0.226 to 2.053 μm, it showed a significant decrease of 7.66% from 509 to 470 MPa. The ratio of fatigue limit to tensile strength (fatigue ratio) of STS 410 steel decreased from 54.9 to 50.5% as the surface roughness increased from 0.226 to 2.053 μm. Beach marks, a typical shape of a fatigue fracture surface, were observed on the fracture surface near the start point of fatigue failure, and brittle (Fe, Cr, Mn, Si) based non-metallic inclusions that promote crack propagation and reduce fatigue limit existed. [ABSTRACT FROM AUTHOR]

In this study, using a C(T) specimen for 12Cr steel used for a steam turbine rotor, crack growth tests under creep-fatigue conditions with and without overloading were conducted and the fundamental behaviours of deformation and crack growth were investigated experimentally. As a result, load line displacement was found to be suppressed under overload condition regardless of temperature and stress holding time. Crack initiation life under high temperature and low stress condition with overloading was delayed as compared with no overload condition. However, under low temperature and high stress condition with short stress holding time, crack initiation was accelerated remarkably. Furthermore, although crack growth life under creep-fatigue conditions with overloading was found to be shortened as compared with no overloading, the effect of overloading on crack growth life was decreased with an increase in stress holding time. [ABSTRACT FROM AUTHOR]

The role of BN, AlN and MnS particles on the degradation in creep life and rupture ductility has been investigated for 9 to 12Cr martensitic steels and 1Cr bainitic steel mainly at 550 to 650 oC. The BN particles form in Gr.92 and Gr.122 during normalising at around 1100 oC. The BN particles have nothing to do with the degradation in creep life. The AlN particles precipitate in the high-Al heats of 12Cr-1Mo-1 W-0.3 V steel during creep, reducing dissolved nitrogen and fine nitrides beneficial for the creep strength and degrading the creep life. The MnS particles have nothing to do with the degradation in creep life of 1Cr-1Mo-0.25 V steel. The BN, AlN and MnS particles are responsible for the degradation in reduction of area of the steels by accelerating the formation of creep voids at interfaces between the particles and alloy matrix. [ABSTRACT FROM AUTHOR]

The reliability of welded structure can be evaluated through welding residual stress analysis and fatigue strength assessment. In this study, welding residual stresses of multi-pass dissimilar material welded joint between alloy 617 and 12Cr steel were analyzed numerically and experimentally. Fatigue strength was then assessed in the air. Based on results of welding residual stress analysis and fatigue strength assessment, a fatigue design method considering welding residual stress was investigated. Welding residual stresses at the weld of dissimilar welded joints distributed complicatedly on longitudinal and transverse directions, showing differences but a very similar distribution tendency between numerical and experimental results. Numerical and experimental peak values of welding residual stresses at HAZ of the weld on the 12Cr steel side were predicted to be 333 MPa and 282 MPa HAZ, respectively. The fatigue limit of dissimilar material welded joint between alloy 617 and 12Cr steel was assessed to be 306.8 MPa, which was 40% of tensile strength (767 MPa) of dissimilar material welded joint. However, the stress range including welding residual stress was assessed to be 206.9 MPa, which was 14% lower than that calculated by including the effect of residual stresses. [ABSTRACT FROM AUTHOR]

The reliability of welded structure can be evaluated through welding residual stress analysis and fatigue strength assessment. In this study, welding residual stresses of multi-pass dissimilar material welded joint between alloy 617 and 12Cr steel were analyzed numerically and experimentally. Fatigue strength was then assessed in the air. Based on results of welding residual stress analysis and fatigue strength assessment, a fatigue design method considering welding residual stress was investigated. Welding residual stresses at the weld of dissimilar welded joints distributed complicatedly on longitudinal and transverse directions, showing differences but a very similar distribution tendency between numerical and experimental results. Numerical and experimental peak values of welding residual stresses at HAZ of the weld on the 12Cr steel side were predicted to be 333 MPa and 282 MPa HAZ, respectively. The fatigue limit of dissimilar material welded joint between alloy 617 and 12Cr steel was assessed to be 306.8 MPa, which was 40% of tensile strength (767 MPa) of dissimilar material welded joint. However, the stress range including welding residual stress was assessed to be 206.9 MPa, which was 14% lower than that calculated by including the effect of residual stresses.

• Different changes of fracture locations under various holding times is revealed. • Creep cracking type shifts from BM, II + BM, Ⅱ + Ⅳ to II + III creep cracking. • Dislocation density and orientation contribute build the fracture mechanisms. • Modified LM model is employed to predicting the life of 12Cr joints. The mechanism for creep rupture of 12%Cr /alloy 4715 welded joints was investigated by employing traditional uniaxial tensile creep in this study. The creep tests were maintained at 580 °C with creep loading stresses gradually decreasing from 300 to 240 MPa, and the time to failure was tested. The rupture location shifts from base metal (BM) to BM + fusion line (FL) then to FL + fine grain zone + coarse grain zone (CGZ), and finally to FL + CGZ as holding time increases. Meanwhile, a softening phenomenon is also observed in the heat-affected zone (HAZ) in crept specimens for a more extended holding time, where voids, carbides, and dislocation density are further discussed. The carbides are identified as Cr and Mo-rich M 23 C 6 type, which forms mainly at grain boundaries during creep testing. In HAZ, M 23 C 6 gradually increases and coarsens, providing nucleation sites for creep voids. Finally, the Larson–Miller parameter method is employed to provide a theoretical reference for predicting use cases that satisfy the industry service condition. [ABSTRACT FROM AUTHOR]

The most effective method to reduce CO2 gas emission from the steam power plant is to improve its performance by elevating the steam temperature to more than 700 °C. For this, it is necessary to develop applicable materials at high temperatures. Ni-based Alloy 617 and 12Cr steel are used in steam power plants, due to their remarkable mechanical properties, high corrosion resistance, and creep strength. However, since Alloy 617 and 12Cr steel have different chemical compositions and thermal and mechanical properties, it is necessary to develop dissimilar material welding technologies. Moreover, in order to guarantee the reliability of dissimilar material welded structures, the assessment of mechanical and metallurgical properties, fatigue strength, fracture mechanical analysis, and welding residual stress analysis should be conducted on dissimilar material welded joints. In this study, first, multi-pass dissimilar material welding between Alloy 617 and 12Cr steel was performed under optimum welding conditions. Next, mechanical properties were assessed, including the static tensile strength, hardness distribution, and microstructural analysis of a dissimilar material welded joint. The results indicated that the yield strength and tensile strength of the dissimilar metal welded joint were higher than those of the Alloy 617 base metal, and lower than those of the 12Cr steel base metal. The hardness distribution of the 12Cr steel side was higher than that of Alloy 617 and the dissimilar material weld metal zone. It was observed that the microstructure of Alloy 617 HAZ was irregular austenite grain, while that of 12Cr steel HAZ was collapsed martensite grain, due to repeatable heat input during multi-pass welding. [ABSTRACT FROM AUTHOR]

The most effective method to reduce CO2 gas emission from the steam power plant is to improve its performance by elevating the steam temperature to more than 700 °C. For this, it is necessary to develop applicable materials at high temperatures. Ni-based Alloy 617 and 12Cr steel are used in steam power plants, due to their remarkable mechanical properties, high corrosion resistance, and creep strength. However, since Alloy 617 and 12Cr steel have different chemical compositions and thermal and mechanical properties, it is necessary to develop dissimilar material welding technologies. Moreover, in order to guarantee the reliability of dissimilar material welded structures, the assessment of mechanical and metallurgical properties, fatigue strength, fracture mechanical analysis, and welding residual stress analysis should be conducted on dissimilar material welded joints. In this study, first, multi-pass dissimilar material welding between Alloy 617 and 12Cr steel was performed under optimum welding conditions. Next, mechanical properties were assessed, including the static tensile strength, hardness distribution, and microstructural analysis of a dissimilar material welded joint. The results indicated that the yield strength and tensile strength of the dissimilar metal welded joint were higher than those of the Alloy 617 base metal, and lower than those of the 12Cr steel base metal. The hardness distribution of the 12Cr steel side was higher than that of Alloy 617 and the dissimilar material weld metal zone. It was observed that the microstructure of Alloy 617 HAZ was irregular austenite grain, while that of 12Cr steel HAZ was collapsed martensite grain, due to repeatable heat input during multi-pass welding.

• Crack growth tests under creep and initial overload creep conditions were conducted for 12Cr steel. • The initial overload was found to be suppressed the crack initiation. • The effect of initial overload does not significantly affect the crack growth rate. • The dual value behavior of C* parameter was dominated by crack initiation. 12Cr steel is used as one of the materials for steam turbine rotor. Steam turbine rotor is an important component and high reliability is required. In actual turbine rotor which has central hole, thermal stress induced by inflow of steam is generated at start-up. At this time, an overload greater than the load level during steady-state operation is exerted on the stress concentration area near the central hole. Therefore, it is important to evaluate the effect of overload on the creep crack initiation and growth behavior. In this study, using a C(T) specimen for 12Cr steel used for steam turbine rotor, crack growth tests under creep and initial overload creep conditions were conducted and behaviors of creep deformation, crack initiation and growth were investigated. In addition, these results were evaluated by various fracture mechanics parameters. As a result, it was found that the initial overload suppressed creep deformation and delayed creep crack initiation. Especially, this tendency was more pronounced at the low temperature and high stress condition. However, the initial overload does not significantly affect the creep crack growth rate. [ABSTRACT FROM AUTHOR]