In flexible operation with increased number of startup, shutdown, and load fluctuations, thermal fatigue damage is exacerbated along with existing creep damage in power plant pipe and pressure vessels. Recently, cracks were found in the start-up vent pipe branching from the reheat steam pipe within a heat recovery steam generator(HRSG) of J-class gas turbine, occurring in the P92 base material and repair welds. This pipe has been used at the power plant for about 10 years. Microstructural analysis of the cross-section indicated that the cracks were primarily due to thermal fatigue, growing within the grains without changing direction along the grain boundaries. To identify the damage mechanism and evaluate the remaining life, temperature and strain monitoring were taken from the damaged piping during startup and normal operation.

The microstructure of MoSiBTiC alloys is very complex, with three to four constituent phases and characteristic structures such as fine precipitates and lamellar structures. To perform the microstructural analysis efficiently, image segmentation was first performed for each phase of the microstructural images. Utilizing the Trainable Weka Segmentation method based on machine learning, the required segmentation time was dramatically reduced. Furthermore, by pre-adjusting the contrast of the images, the segmentation could be performed accurately for gray phases with different shades of gray. In addition, the U-Net method, based on deep learning, could perform highly accurate segmentation of characteristic microstructures consisting of multiple phases. The correlations between microstructural features and hardness were investigated using the segmented images in this study. The findings revealed that the volume fraction of each phase and the number of TiC clusters within the field of view significantly influenced hardness. This suggests that the hardness of MoSiBTiC alloys may be controlled by controlling the amount of TiC precipitates.

This study conducted creep tests, microstructural, and hardness analyses on SA213T23-TP347H dissimilar weld joints of long-term serviced coal-fired boiler final superheater tube. The welded joint (SA213 T23-TP347H) of the superheater tube, after approximately 105,000 hours of service, was sampled for creep life assessment and maintenance planning. Creep tests were conducted at 600°C under three stress conditions: 100, 140, and 160MPa. Most cracks were observed in the heat-affected zone of T23, and compared to unused tubes, the creep life consumption rate was approximately 90%. All dissimilar weld joints used welding rods similar in chemical composition to T23, and significant hardness reduction occurred in the flame-affected zone.

The incore instrumentation system of a pressurized water reactor (PWR) facilitates neutron flux mapping and temperature measurements at specific core locations. A guide conduit, extending from the seal table to the lower reactor pressure vessel head, guides and protects each incore guide thimble between the table and the lower reactor vessel head. Each flux thimble houses a detector and drive cable. Once filled with reactor coolant, the conduit becomes an extension of the reactor coolant pressure boundary. This paper reports the examination results of cracking detected in a TP304 stainless steel guide conduit adjacent to a fillet weld at the upper surface of a TP304 seal table. The cracking resulted in reactor coolant leakage that was detected by the presence of boric acid deposits on the exterior of the conduit and table. Failure analysis including dimensional measurements, chemical analysis, stereomicroscopy, metallography, and scanning electron microscopy showed that extensive cracking of the conduit and seal table material occurred due to stress corrosion cracking (SCC). Assessment showed that chlorine-containing deposits were present on the exterior of the conduit and on the surfaces of the seal table and were due to the design and operation of HVAC systems at the coastal plant. Stainless steels are susceptible to SCC in environments with elevated temperatures, chloride contents, and increased tensile stress – particularly in non-post weld heat treated (PWHT) weld regions and the heat affected zone (HAZ). This was the apparent primary cause of the failure. However, chloride-induced SCC of such materials typically results in transgranular crack propagation, whereas the observed cracks were indicative of intergranular stress corrosion cracking (IGSCC). Microstructural analysis showed that the observed cracks initiated in sensitized areas of material adjacent to the weld. Sensitization of the material caused chromium depletion from adjacent areas and increased susceptibility of the depleted areas to IGSCC. In this case, the most probable source of sensitization was related to welding and the long-term growth of grain boundary carbides nucleated during welding. This was considered a contributing cause to the failure.

Tenaris' High Oxidation Resistance (THOR) 115, or T115, is a creep strength-enhanced ferritic (CSEF) steel introduced in the past decade. It is widely used in constructing high-efficiency power plants and heat recovery steam generators (HRSGs) due to its superior steam oxidation resistance and long-term microstructural stability, making it a viable alternative to stainless steels at elevated steam temperatures. The creep damage tolerance of T115 has been recently validated under ASME BPVC CC 3048 guidelines, which address safety concerns related to creep damage in boiler components. Testing confirmed T115's consistent creep damage-tolerant behavior, with cross-weld creep behavior reassessed through extensive metallographic examination of specimens from a 1.5-inch thick pipe girth weld, providing insights into creep damage distribution and hardness, and its relative performance compared to Grade 91 CSEF steel.

The time-dependent behavior of 9Cr creep strength enhanced ferritic (CSEF) steels has long fixated on the creep life recorded in uniaxial constant load creep tests. This focus is a consequence of the need to develop stress allowable values for use in the design by formulae approach of rules for new construction. The use of simple Design by Formula rules is justified in part by the assumption that the alloys used will invariably demonstrate high creep ductility. There appears to be little awareness regarding the implication(s) that creep ductility has on structural performance when mechanical or metallurgical notches (e.g., welds) are present in the component design or fabricated component. This reduced awareness regarding the role of ductility is largely because low alloy CrMo steels used for very many years typically were creep ductile. This paper focuses on the structural response from selected tests that have been commissioned or executed by EPRI over the last decade. The results of these tests demonstrate unambiguously the importance that creep ductility has on long-term, time-dependent behavior. The metallurgical findings from the selected tests are the focus of the Part II paper. The association of performance with notch geometry, weld strength, and other potential contributing factors will be highlighted with a primary objective of informing the reader of the variability, and heat-specific behavior that is observed among this class of alloys widely used in modern thermal fleet components and systems.

According to ASME Case N-888-3, Similar and Dissimilar Metal Welding Using Ambient Temperature SMAW or Machine GTAW Temper Bead Technique, a 48 hr waiting period before conducting the final nondestructive examination (NDE) is required when ferritic filler weld metal is used. The purpose of the 48 hr hold is to confirm the absence of hydrogen-induced cracking in the temper bead heat-affected zone. In previous research, the effect of post-weld heat treatment (PWHT) and temper bead welding (TBW) on the hydrogen-induced cracking (HIC) susceptibility in the coarse-grained heat-affected zone (CGHAZ) in welds of SA-508, P-No. 3 Group 3, pressure vessel steel was investigated using the Delayed Hydrogen Cracking Test (DHCT). In that previous study, the Gleeble thermomechanical simulator was used to generate six CGHAZ microstructural conditions: as-welded (AW), PWHT, and AW with single a TBW reheat at 675, 700, 725, and 735°C. Hydrogen was introduced to the specimen through cathodic charging under in situ constant tensile stress. The HIC susceptibility for these microstructures was ranked by the DHCT at a diffusible hydrogen level significantly exceeding typical GTAW and SMAW processes. The work described in this paper investigates the susceptibility to HIC of these same CGHAZ microstructures with DHCT at variable current densities, further ranking each condition. Test results were analyzed by fracture surface examination of failed tests, and cross-section microstructural analysis under a scanning electron microscope (SEM). Future steps include evaluating critical hydrogen content levels using gas chromatography for each condition. The results from this study will be used to consider potential elimination of the NDE hold time requirement in Case N-888-3 when ferritic weld metal is used.

The time-dependent behavior of 9Cr creep strength enhanced ferritic (CSEF) steels has long fixated on the creep life recorded in uniaxial constant load creep tests. This focus is a consequence of the need to develop stress allowable values for use in the design by formulae approach of rules for new construction. The use of these simple rules is justified in part by the assumption that the alloys used will invariably demonstrate high creep ductility. There appears to be little awareness regarding the implication(s) that creep ductility has on structural performance when mechanical or metallurgical notches (e.g., welds) are present in the component design or fabricated component. This reduced awareness regarding the role of ductility is largely because low alloy CrMo steels used for very many years typically were creep ductile. This paper focuses on the structural response from selected tests that have been commissioned or executed by EPRI over the last decade. The results of these tests demonstrate unambiguously the importance that creep ductility has on long-term, time-dependent behavior. This is the second part of a two-part paper; Part I reviewed the selected tests and discussed them from a mechanical perspective. The association of performance with specific microstructural features is briefly reviewed in this paper and the remaining gaps are highlighted for consideration among the international community.

Creep strength enhanced ferritic (CSEF) steels have shown the potential for creep failure in the weld metal, heat affected zone (HAZ) or fusion line. Details for this behavior have been frequently linked to metallurgical risk factors present in each of these locations which may drive the evolution of damage and subsequent failure. This work is focused on three weld samples fabricated from a commercially sourced Grade 92 steel pipe section. These weld samples were extracted from the same welded section but were reported to exhibit failure in different time frames and failure locations (i.e., HAZ of parent, fusion-line, and weld metal). The only variables that contribute to this observed behavior are the post weld heat treatment (PWHT) cycle and the applied stress (all tests performed at 650 °C). In this work detailed microstructural analysis was undertaken to precisely define the locations of creep damage accumulation and relate them to microstructural features. As part of this an automated inclusion mapping process was developed to quantify the characteristics of the BN particles and other inclusions in the parent material of the samples. It was found that BN particles were only found in the sample that had been subjected to the subcritical PWHT, not those that had received a re-normalizing heat treatment. Such micron sized inclusions are a known potential nucleation site for creep cavities, and this is consistent with the observed failure location in the HAZ of the parent in the sample where these were present. In the absence of BN inclusions, the next most susceptible region to creep cavitation is the weld metal. This has an intrinsically high density of sub-micron sized spherical weld inclusions and this is where most of the creep damage was located, in all the renormalized samples.

Extensive research and development has been undertaken in the UK on MarBN steels. These were first proposed by Professor Fujio Abe from NIMS in Japan. Within the UK, progress has been made towards commercialisation of MarBN-type steel through a series of Government funded industrial collaborative projects (IMPACT, IMPEL, INMAP and IMPULSE). As part of the IMPACT project, which was led by Uniper Technologies, boiler tubes were manufactured from the MarBN steel developed within the project, IBN1, and installed on the reheater drums of Units 2 and 3 of Ratcliffe-on-Soar Power Station. The trial tubes were constructed with small sections of Grade 91 tubing on either side of the IBN1 to allow direct comparison after the service exposure. This is the world’s first use of a MarBN steel on a full-scale operational power plant. In September 2018 the first tube was removed having accumulated 11,727 hours operation and 397 starts. This paper reports microstructural and oxidation analysis, that has been undertaken by Loughborough University as part of IMPULSE project, and outlines future work to be carried out.

A newly developed ferritic heat-resistant steel; 9Cr-3W-3Co-Nd-B steel has higher creep rupture strength both in the base metal and welded joints than the conventional high-Cr ferritic heat-resistant steels. The creep rupture strengths of 9Cr-3W-3Co-Nd-B steel welded joints were below the lower limit of the base metal in long-term creep stage more than 20,000 hours. The creep rupture position was heat-affected zone (HAZ) from 1.0 to 1.5 mm apart from the fusion line on the welded joint specimen ruptured at 34,966 hours. The equiaxed subgrains and coarsened precipitates were observed in HAZ of the ruptured specimen. In order to clarify the creep fracture mechanism of the welded joints, the microstructures of HAZ were simulated by heat cycle of weld, then observed by EBSD analysis. Fine austenite grains formed along the prior austenite grain boundaries in the material heated just above A C3 transformation temperature, however there were no fine grains such as conventional steel welded joints. The prior austenite grain boundaries were unclear in the material heated at 1050 °C. The creep rupture life of the material heated at just above A C3 transformation temperature exceeded the lower limit of base metal and there was no remarkable degradation, although it was shorter than the other simulated materials. It is, therefore, concluded that the creep fracture of 9Cr-3W-3Co-Nd-B steel welded joint in long-term stage occurred at HAZ heated at from just above A C3 transformation temperature to 1050 °C. It is speculated that the fine austenite grains formed along the prior austenite grain boundaries and inhomogeneous microstructures cause the coarsening precipitates and recovery of lath structure during long-term creep deformation.

A new ferritic steel branded as Thor 115 has been developed to enhance high-temperature resistance. The steel design combines an improved oxidation resistance with long-term microstructural stability. The new alloy was extensively tested to assess the high-temperature time- dependent mechanical behavior (creep). The main strengthening mechanism is precipitation hardening by finely dispersed carbide (M 23 C 6 ) and nitride phases (MX). Information on the evolution of secondary phases and time-temperature-precipitation behavior of the alloy, essential to ensure long-term stability, was obtained by scanning transmission electron microscopy with energy dispersive spectroscopy, and by X-ray powder diffraction on specimens aged up to 50,000 hours. The material behavior was also tested in service conditions, to validate the laboratory results: Thor 115 tubing was installed in a HRSG power plant, directly exposed to turbine flue gasses. Tubing samples were progressively extracted, analyzed and compared with laboratory specimens in similar condition. This research shows the performance of Thor 115 regarding steam oxidation and microstructure evolution up to 25,000 exposure hours in the field. So far, no oxide microstructure difference is found between the laboratory and on field tubing: in both cases, the oxide structure is magnetite/hematite and Cr-spinel layers and the oxide thickness values lay within the same scatter band. The evolution of precipitates in the new alloy confirms the retention of the strengthening by secondary phases, even after long-term exposure at high temperature. The deleterious conversion of nitrides into Z phase is shown to be in line with, or even slower than that of the comparable ASME grade 91 steel.

The newly developed 12%Cr steel Super VM12 is characterized by excellent creep rupture strength properties (better than Grade 92) and enhanced steam oxidation resistance of 12%Cr steels such as VM12-SHC. Balanced properties profile of the new steel development in comparison to the existing well-established steels such as Grade 91 and Grade 92, opens opportunities for its application as construction material for components in existing or future high-efficiency power plants. In this study the oxidation behavior of typical 9%Cr steels was compared with the new steel development. The oxide scale morphologies and compositions of different oxide layers as function of temperature and exposure time in steam-containing atmospheres were characterized using light optical metallography, Scanning Electron Microscopy (SEM). Creep testing has been carried out in the temperature range between 525°C and 700°C. Selected creep specimens were investigated using the Transmission Electron Microscopy and the Atom Probe Tomography techniques.

Inconel 740H is one of the most promising candidate Ni-base superalloys for the main steam pipe of 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants. After processing and welding in manufacturing plant in solution-annealed state, large components was commonly suggested to have an extra aging treatment at 800 °C for 16 h, in order to obtain homogeneous γ′ precipitates. In this present work, creep tests and microstructure analyses were conducted on Inconel 740H pipe specimens under two different heat treatments to verify the necessity of aging process. Here we show that aging treatment has limited effect on the creep rupture life of Inconel 740H pipe. Both in grain interiors and along grain boundaries, crept specimens under two different heat treatments have the same precipitates. But the shape and distribution of γ′ in solution annealed sample is not as regular as the aged ones. Our results provide the underlying insight that aging treatment is not so necessary for the straight pipes if the on-site condition was hard to control. But for both groups of specimens, a small amount of h particles and some banded like M 23 C 6 were emerged during creep, which would be harmful to mechanical properties for the long run.

In order to establish a induction bending technique for Ni-based alloy HR6W large pipe, induction bending test was conducted on HR6W, which is a piping candidate material of 700°C class Advanced Ultra-Super Critical. In this study, a tensile bending test in which tensile strain was applied and a compression bending test in which compression strain was applied to the extrados side of the pipe bending part. As the results of these two types of induction bending tests, it was confirmed that a predetermined design shape could be satisfied in both bending tests. In addition, the wall thickness of the pipe was equal to or greater than that of the straight pipe section in compression bending. Therefore, if compression bending is used, it is considered unnecessary to consider the thinning amount of the bent portion in the design. Next, penetrant test(PT) on the outer surface of the bending pipes were also confirmed to be acceptable. Subsequently, metallographic samples were taken from the outer surface of the extrados side, neutral side and intrados side of the pipe bending portion. Metallographic observation confirmed that the microstructures were normal at all the three selected positions. After induction bending, the pipe was subjected to solution treatment. Thereafter, tensile tests and creep rupture tests were carried out on samples that were cut from the extrados side, neutral side and intrados side of the pipe bending portion. Tensile strength satisfied the minimum tensile strength indicated in the regulatory study for advanced thermal power plants report of Japan. Each creep rupture strength was the almost same regardless of the solution treatment conditions. From the above, it was possible to establish a induction bending technique for HR 6W large piping.

Metallographic tests, micro-hardness tests, mechanics performance tests and Energy Dispersion Spectrum (EDS) were conducted for a 2.25Cr-1Mo main steam pipe weldment served for more than 32 years. Microstructural evolution of the 2.25Cr-1Mo base metal and weld metal was investigated. Degradation in micro-hardness and tensile properties were also studied. In addition, the tensile properties of subzones in the ex-service weldment were characterized by using miniature specimens. The results show that obvious microstructural changes including carbide coarsening, increasing inter lamella spacing and grain boundary precipitates occurred after long-term service. Degradation in micro-hardness is not obvious. However, the effects of long term service on tensile deformation behavior, ultimate tensile strength and yield stress are remarkable. Based on the yield stress of micro-specimens, the order of different subzones is: WM>HAZ>BM, which is consistent with the order of different subzones based on micro-hardness. However, the ultimate tensile strength and fracture strain of HAZ are lower than BM.

Welded joints of Ni-base alloys are often the critical part of components operated under high temperature service conditions. Especially welds in thick-walled structures are susceptible to various crack phenomena. Creep rupture and deformation behavior of different similar welds of Alloy 617B, both circumferential and longitudinal, were determined in many research German projects with the aim to qualify the nickel alloys and its welded joints for the use in highly efficient Advanced Ultra Supercritical (AUSC) power plants. Damage mechanisms and failure behavior have also been investigated within these projects. In order to reduce the welding residual stresses in thick-walled components a post weld heat treatment (PWHT) for Alloy 617B is recommended after welding. This PHWT reduces not only residual stresses but causes changes in the damage mechanisms and failure behavior of welded joints of Alloy 617B. Improving effects of PWHT have been investigated in this study and results of microstructural investigations were correlated with the material behavior.

Components such as tubes, pipes and headers used in power generation plants are operated in a creep regime and have a finite life. During partial replacement, creep exhausted materials are often welded to virgin materials with superior properties. The aim of this study was to identify a suitable weld filler material to join creep aged X20CrMoV12-1 to a virgin P91 (X10CrMoVNbV9-1) steel. Two dissimilar joints were welded using the gas tungsten arc welding (GTAW) process for the root passes, and manual metal arc (MMA) welding for filling and capping. The X20 and the P91 fillers were selected for joining the pipes. The samples were further heat treated at 755°C to stress relief the samples. Microstructural evolution and mechanical properties of the weld metals were evaluated. The average hardness of X20 weld metal (264 HV10) was higher than the hardness measurement of P91 weld metal (206 HV10). The difference in hardness was attributed to the high carbon content in X20 material. The characterisation results revealed that the use of either X20 or P91 weld filler for a butt weld of creep aged X20 and virgin P91 pipes material does not have a distinct effect on the creep life and creep crack propagation mechanism. Both weld fillers (X20 and P91) are deemed to be suitable because limited interdiffusion (<10 μm) of chromium and carbon at the dissimilar weld interface was observed across the fusion line. The presence of a carbon ‘denuded’ zone was limited to <10 μm in width, based on the results from local measurements of the precipitate phase fractions using image analysis and from elemental analysis using EDS. However the nanoindentation hardness measurements across the fusion line could not detect any ‘soft’ zone at the dissimilar weld interface. The effect of the minute denuded zone was also not evident when the samples were subjected to nanoindentation hardness testing, tensile mechanical testing, Small Punch Creep Test (SPCT) and cross weld uniaxial creep testing.

Dissimilar metal welds between T91 ferritic steels and TP347H austenitic alloys are commonly used in fossil power plants in China. Premature failure of such dissimilar welds can occur, resulting in unplanned plant outages that can cause huge economic losses. In this article, microstructural evolution of T91/TP347H dissimilar welds after different service conditions were studied, mechanical properties before and after service were also analyzed, a full investigation into the failure cause was carried out. The results show, the dissimilar metal welds in the as-welded condition consists of a sharp chemical concentration gradient across the fusion line, failure is attributed to the steep microstructural and mechanical properties gradients, formation of interfacial carbides that promote creep cavity formation.

A new method of creep life assessment was developed to consider heat-to-heat variations of welded joints of materials used in power plants. This paper explains a scheme of the assessment method and also describes an actual implementation of the method for Grade 91 steel. In the method, creep properties of the welded joints are related to those of each base metal because the heat-to-heat variations of welded joints strongly depend on the creep properties of the corresponding base metals. To estimate the creep properties of each base metal of the target pipe, microstructure analyses and small punch creep tests were conducted using small samples cut from the base metals in service, and evaluations were done on the basis of material data base obtained using standard test samples of long-term service exposed pipes. It is expected that the precision of the life assessment of pipes will be significantly improved using the developed method because it can consider the heat-to-heat variations of their materials, which are not considered in existing life assessment methods.