The objective of this study was to determine the typical range of weld metal cooling rates and phase transformations during multipass gas-tungsten arc (GTA) welding of Grade 23 (SA-213 T23) tubing, and to correlate these to the microstructure and hardness in the weld metal and heat affected zone (HAZ). The effect of microstructure and hardness on the potential susceptibility to cracking was evaluated. Multipass GTA girth welds in Grade 23 tubes with outside diameter of 2 in. and wall thicknesses of 0.185 in. and 0.331 in. were produced using Grade 23 filler wire and welding heat input between 18.5 and 38 kJ/in. The weld metal cooling histories were acquired by plunging type C thermocouples in the weld pool. The weld metal phase transformations were determined with the technique for single sensor differential thermal analysis (SS DTA). The microstructure in the as-welded and re-heated weld passes was characterized using light optical microscopy and hardness mapping. Microstructures with hardness between 416 and 350 HV 0.1 were found in the thick wall welds, which indicated potential susceptibility to hydrogen induced cracking (HIC) caused by hydrogen absorption during welding and to stress corrosion cracking (SSC) during acid cleaning and service.

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.

Haynes 282 alloy is a relatively new Ni-based superalloy that is being considered for advanced ultrasupercritical (A-USC) steam turbine casings for steam temperatures up to 760°C. Weld properties are important for the turbine casing application, so block ingots of Haynes 282 alloy were cast for properties studies. Good, sound welds were produced using Haynes 282 weld-wire and a hot gas-tungsten-arc welding method, and tensile and creep-rupture properties were measured on cross-weld specimens. In the fully heat-treated condition (solution annealed + aged), the tensile properties of the welded specimens compare well with as-cast material. In the fully heat-treated condition the creep-rupture life and ductility at 750°C/250MPa and 800°C/200MPa of the cross-weld specimens are similar to the as-cast base metal, and repeat creep tests show even longer rupture life for the welds. However, without heat-treatment or with only the precipitate age-hardening heat-treatment, the welds have only about half the rupture life and much lower creep ductility than the as-cast base metal. These good properties of weldments are positive results for advancing the use of cast Haynes 282 alloy for the A-USC steam turbine casing application.

Coal burning power companies are currently considering FeAlCr weld overlay claddings for corrosion protection of waterwall boiler tubes located in their furnaces. Previous studies have shown that these FeAlCr coatings exhibit excellent high-temperature corrosion resistance in several types of low NOx environments. In the present study, the susceptibility of FeAlCr weld overlay claddings to hydrogen cracking was evaluated using a gas-tungsten arc welding (GTAW) process. Microsegregation of alloying elements was determined for the FeAlCr welds and compared to a currently used Ni-based superalloy. Long-term gaseous corrosion testing of select weld overlays was conducted along with the Ni-based superalloy in a gaseous oxidizing/sulfidizing corrosion environment at 500°C. The sample weight gains were used along with analysis of the corrosion scale morphologies to determine the corrosion resistance of the coatings. It was found that although there were slight differences in the corrosion behavior of the selected FeAlCr weld coatings, all FeAlCr based alloys exhibited superior corrosion resistance to the Ni-based superalloy during exposures up to 2000 hours.

A new nickel-base superalloy GH750 has been developed as boiler tube of advanced ultrasupercritical (A-USC) power plants at temperatures about and above 750°C in China. This paper researched the weld solidification of GH750 filler metal, microstructure development and property of GH750 welded joint by gas tungsten arc weld. Liquid fraction and liquid composition variation under non-equilibrium state were calculated by thermo-dynamic calculation. The weld microstructure and the composition in the dendrite core and interdendritic region were analyzed by SEM(EDX) in detail. The investigated results show that there is an obvious segregation of precipitation-strengthening elements during the weld solidification. Titanium and Niobium are the major segregation elements and segregates in the interdendritic region. It was found that the changing tendency of the elements’ segregation distribution during the solidification of GH750 deposit metal is agree with the thermodynamic calculation results. Till to 3,000hrs’ long exposure at 750°C and 800°C, in comparison with the region of dendrite core of solidification microstructure, not only the coarsening and the accumulation of γʹ particles are remarkable in the interdendritic region, but also the small quantity of the blocky and needle like η phases from. The preliminary experimental results indicate that the weakening effect of creep-rupture property of the welded joint is not serious compared with GH750 itself.

This paper summarizes recent efforts to improve creep performance in Grade 91 (Mod. 9Cr-1Mo, ASTM A387) steel weldments via non-standard heat treatments prior to welding. Such heat treatments offer a potential solution for minimizing Type IV failures in creep strength enhanced ferritic (CSEF) steels. A lower temperature tempering (LTT, 650°C) of the 9Cr steels prior to gas tungsten arc welding (GTAW) resulted in improved creep-rupture life at 650°C compared to the samples tempered at a standard condition (HTT, 760°C) before welding. From detailed characterization of precipitation kinetics in the heat affected zone, it was hypothesized that M 23 C 6 carbides in the fine-grain heat-affected zone (FGHAZ) in the LTT sample were fully dissolved, resulting in re-precipitation of strengthening carbides during post weld heat treatment (PWHT). This was not the case in the HTT sample since M 23 C 6 in the FGHAZ was only partially dissolved prior to welding, which caused coarsening of existing M 23 C 6 after PWHT and premature creep failure in the FGHAZ. However, it was also found that the LTT raised the ductile-brittle transition temperature above room temperature (RT). Two different thermo-mechanical treatments (TMTs); two-step tempering and aus-forging/aus-aging, of the modified 9Cr-1Mo steels were attempted, in order to control the balance between creep properties and RT ductility, through control of precipitation kinetics of the M 23 C 6 carbides and/or MX carbo-nitrides. The hardness map of the TMT samples after GTAW and PWHT were evaluated.

This study investigated the creep rupture strength and microstructure evolution in welded joints of high-boron, low-nitrogen 9Cr steels developed by NIMS. The welds were fabricated using the GTAW process and Inconel-type filler metal on steel plates with varying boron content (47-180 ppm). Creep rupture tests were conducted at 923K for up to 10,000 hours. Despite their higher boron content, these steels exhibited good weldability. Welded joints of the boron steel displayed superior creep properties compared to conventional high-chromium ferritic steel welds like P92 and P122. Notably, no Type IV failures were observed during creep testing. Welding introduced a large-grained microstructure in the heat-affected zone (HAZ) heated to the austenite transformation temperature (Ac3 HAZ). This contrasts with the grain refinement observed in the same region of conventional heat-resistant steel welds. Interestingly, the grain size in this large microstructure was nearly identical to that of the base metal. Analysis of the simulated Ac3 HAZ revealed crystal orientation distributions almost identical to those of the original specimen. This suggests a regeneration of the original austenite structure during the alpha-to-gamma phase transformation. Simulated Ac3 HAZ structures of the boron steel achieved creep life nearly equivalent to the base metal. The suppression of Type IV failure and improved creep resistance in welded joints of the boron steels are likely attributed to the large-grained HAZ microstructures and stabilization of M 23 C 6 precipitates. The optimal boron content for achieving the best creep resistance in welded joints appears to lie between 90 and 130 ppm, combined with minimized nitrogen content.

The use of high-nickel superalloys has greatly increased among many industries. This is especially the case for advanced coal-fired boilers, where the latest high temperature designs will require materials capable of withstanding much higher operating temperatures and pressures than current designs. Inconel alloy 740H (UNS N07740) is a new nickel- based alloy that serves as a candidate for steam header pipe and super-heater tubing in coal-fired boilers. Alloy 740H has been shown to be capable of withstanding the extreme operating conditions of an advanced ultra-super-critical (AUSC) boiler, which is the latest boiler design, currently under development. As with all high nickel alloys, welding of alloy 740H can be very challenging, even to an experienced welder. Weldability challenges are compounded when considering that the alloy may be used in steam headers, where critical, thick-section and stub-to-header weld joints are present. This paper is intended to describe the proper procedures developed over years of study that will allow for ASME code quality welds in alloy 740H with matching composition filler metals.

This paper presents results and analyses from long-term creep-rupture testing of alloy CCA617 (also known as alloy 617B) in wrought and welded forms at temperatures and stresses relevant to power generation under advanced steam conditions. The refined controlled chemical composition of CCA617 resulted in increased creep-rupture strength compared to the conventional alloy 617 chemistry at applied stress levels of ~150 MPa and above. Long-term creep rupture testing of weldments (in one case, over 100,000 h) showed that their creep-rupture lives were dependent on the welding process. Gas-tungsten-arc and shielded metal-arc weldments of CCA617 performed nearly equivalent to standard alloy 617 base metals in creep, but there was some debit in creep-rupture resistance when compared to CCA617 base metal. Submerged arc welding produced weldments that were notably weaker than both versions of alloy 617 base metal under creep conditions, possibly due to lack of optimization of filler wire composition and flux.

A new martensitic steel was developed for power generation applications. Tenaris High Oxidation Resistance (Thor) is an evolution of Grade 91, designed to have improved steam oxidation resistance and better long-term microstructural stability, with equal or better creep strength. Based on consolidated metallurgical knowledge of microstructural evolution mechanisms, and extensive development performed in the last decade, Thor was engineered to overcome temperature limitations of Grade 91, yet it can be processed in the same fashion, permitting the use of existing best practices for Grade 91 boiler fabrication. Welding trials were performed on Thor tubes and pipe using welding procedures that are routinely employed in the construction of Grade 91 steel components. A summary of relevant results is presented, demonstrating the applicability of long-established and tested welding procedures to components manufactured with Thor steel.

The presented work summarizes the results of more than 10 years of research at TU Graz and TU Chemnitz and partners on a martensitic boron and nitrogen stabilized 9Cr3W3Co (MARBN) steel grade. The design philosophy of MARBN steels is presented and critical issues regarding boron and nitrogen balance are discussed. Microstructural characterization of two different laboratory heats, is presented and efforts in European projects towards an upscaling of melts are presented. Base material creep testing data at 650 °C up to 50.000 hours is presented and assessed to commercial alloys such as ASTM grades P91 and P92. An increase of creep rupture stress of more than +20% was recorded. Oxidation tests in steam at 650°C revealed an anomalous response of the material. Several specimens exhibited excellent oxidation resistance commonly only seen for grades of higher chromium content. The anomalous oxidation behaviour is identified and discussed, although the causes remain yet unclear. Results of manufacturing, characterization and testing of different MARBN welds, including gas-tungsten-arc-, gas-metal-arc-, friction stir and electron beam welds reveal a microstructure memory effect in the heat affected zone, so that no uniform fine-grained zone is present. The behaviour of crosswelds during long-term creep testing at 650 °C up to more than 32.000 hours is assessed and the susceptibility to Type IV cracking is discussed. The manuscript summarizes research of more than 10 years, presents current research activities on MARBN and describes open questions for an alloy identified as a promising martensitic steel grade for elevated temperature components.

Welding of collector pipes, flat heads, dished ends and connector pipes performed with high temperature and creep-resistant steels most often has been performed using GTAW process combined with MMA processes. Progress in GMAW process and availability of high quality filler materials (solid wires) enables welding of the above connections also using this method. In order to prove its efficiency, this article presents the results of related tests. The range of tests was similar to that applied during the qualification of welding procedure. The investigation also involved microscopic and fractographic examinations and creep tests. The results reveal that welding with GMAW is by no means inferior to a currently applied SMAW method yet the time of the process is shorter by 50%. The article presents the world’s first known positive results in welding of P92 grade steel using GMAW welding method.

Nimonic 263 alloy was selected for gas turbine combustor transition piece due to its excellent high temperature mechanical performance. In present work, Nimonic 263 alloy plate with thickness of 5mm was welded using 263 filler metal by GTAW, then post weld heat treatment of 800℃/8h/air cool was carried out. The hardness and impact toughness of welded joints were measured, and the microstructure evolution after aging at 750℃ for 3000h was investigated by scanning electron microscopy(SEM). The results show that, during the aging process, the hardness of weld metal increases firstly and then decreases. The impact toughness decreases significantly at first and then increase. Furthermore, some fluctuations can be detected in hardness and impact toughness after long-term thermal exposure. The significant decrease in the impact toughness of the aged welded joints mainly results from the precipitation of η phase around grain boundary and intergranular MC phase. The hardness of weld metal increases due to the precipitation of more carbides and γ′ phase after 1000h aging, then decreases owing to the growth of γ′ phase after 3000h aging.

India's current installed power generating capacity is about 225,000 MW, of which about 59% is coal based. It is projected that India would require an installed capacity of over 800,000 MW by 2032. Coal is likely to remain the predominant source of energy in India till the middle of the century. India is also committed to reducing the CO 2 emission intensity of its economy and has drawn up a National Action Plan for Climate Change, which, inter alia, lays emphasis on the deployment of clean coal technologies. With this backdrop, a National Mission for the Development of Advanced Ultra Supercritical Technology has been initiated. The Mission objectives include development of advanced high temperature materials, manufacturing technologies and design of equipment. A corrosion test loop in an existing plant is also proposed. Based on the technology developed, an 800 MW Demonstration A-USC plant will be established. Steam parameters of 310 kg/cm 2 , 710 °C / 720 °C have been selected. Work on selection of materials, manufacture of tubes, welding trials and design of components has been initiated. The paper gives details of India's A-USC program and the progress achieved.

Fossil fuels continue to be the primary source of energy in the U.S and worldwide. In order to improve the efficiency of fossil power plants, advanced structural materials need to be developed and deployed to meet the need of high temperature creep resistance and corrosion resistance. Examples include creep strength enhanced ferritic (CSEF) steels, austenitic stainless steels, nickel-based superalloys, and oxide dispersion strengthened alloys. Welding is extensively used in construction of fossil power plants. The performance of the weld region can be critical to the safe and economical operation of fossil power plants. Degradations in performance such as reduced creep strength and premature failure in the weld region (e.g. Type IV failure in ferritic steels) are examples of longstanding welding and weldability problems for boiler and other components. In the past, extensive studies have been carried out to characterize the different microstructures in different regions of a weld, and to a certain extent, to establish the correlations between the microstructure and the creep strength. However, the metallurgical or microstructural induced local stress/strain variations have been seldom quantified. In addition, it has been long recognized that, due to the sharp microstructure and property gradients in the weld and HAZ, the standard creep testing procedure for the base metal can produce erroneous results when used for weld testing.

Austenitic stainless steels have been used for boiler tubes in power plants. Since austenitic stainless steels are superior to ferritic steels in high temperature strength and steam oxidation resistance, austenitic stainless steel tubes are used in high temperature parts in boilers. Dissimilar welded joints of austenitic steel and ferritic steel are found in the transition regions between high and low temperature parts. In dissimilar welded parts, there is a large difference in the coefficient of thermal expansion between austenitic and ferritic steel, and thus, thermal stress and strain will occur when the temperature changes. Therefore, the dissimilar welded parts require high durability against the repetition of the thermal stresses. SUPER304H (18Cr-9Ni-3Cu-Nb-N) is an austenitic stainless steel that recently has been used for boiler tubes in power plants. In this study, thermal fatigue properties of a dissimilar welded part of SUPER304H were investigated by conducting thermal fatigue tests and finite element analyses. The test sample was a dissimilar welded tube of SUPER304H and T91 (9Cr-1Mo-V-Nb), which is a typical ferritic heat resistant boiler steel.

Dynamic development of steels used in power engineering industry for the production of boilers characterised by supercritical parameters poses new welding challenges. The introduction of new combinations of alloying agents aimed at obtaining the best possible mechanical properties, including creep resistance, affects the weldability of new steels. Each of the latter have to undergo many tests, particularly as regards bending and welding, in order to enable the development of technologies ensuring failure-free production and assembly of boiler systems. Martensitic steels containing 9% Cr, used in the manufacturing of steam superheaters, are characterised by good creep resistance and, at the same time, low oxidation resistance at a temperature in excess of 600°C. In turn, steels with a 12% Cr content are characterised by significantly higher oxidation resistance, but accompanied by lower strength at higher temperatures, which translates to their limited application in the production of boilers operating at the highest parameters. The niche between the aforesaid steels is perfectly filled by austenitic steels, the creep resistance and oxidation resistance of which are unquestionable. This article presents experience gained while welding dissimilar joints of advanced steels TEMPALOY AA-1 and T92, with the use of EPRI P87, Inconel 82 and Inconel 617 filler metals. The tests involving the said steel grades belong to the very few carried out in the world.

A Japanese national project has been undertaken since Aug. 2008 with the objective of developing an advanced ultra-supercritical power plant (A-USC) with a steam temperature of 700°C. Fe-Ni and Ni-based alloys, namely HR6W, HR35, Alloy617, Alloy740, Alloy263 and Alloy141, were taken as candidate materials for piping and superheater/reheater tubes in an A-USC boiler. Weldments of these alloys were manufactured by GTAW, after which long term creep rupture tests were conducted at 700°C, 750°C and 800°C. Weldments of HR6W, HR35 and Alloy617 showed similar creep strength as compared with these base metals. Weldments of Alloy740 tended to fail in the HAZ, and it is considered that voids and cracks preferentially formed in the small precipitation zone along the grain boundary in the HAZ. The creep strength of Alloy263 in weldments exhibited the highest level among all the alloys, although HAZ failure occurred in the low stress test condition. A weld strength reduction factor will be needed to avoid HAZ failure in Alloy740 and Alloy263. Also, to prevent premature failure in weld metal, optimization of the chemical composition of weld filler materials will be required.

The harsh operating conditions of Advanced Ultra-Supercritical (A-USC) power plants, i.e., steam operation conditions up to 760°C (1400°F)/35 MPa (5000 psi), require the use of Ni-based alloys with high temperature performance. Currently, the U.S. Department of Energy Fossil Energy program together with Electric Power Research Institute (EPRI) and Energy Industries of Ohio (EIO) is pursuing a Component Test (Comets) project to address material- and manufacturing-related issues for A-USC applications. Oak Ridge National Laboratory (ORNL) is supporting this project in the areas of mechanical and microstructure characterization, weld evaluation, environmental effect studies, etc. In this work, we present results from these activities on two promising Ni-based alloys and their weldments for A-USC applications, i.e., Haynes 282 and Inconel 740H. Detailed results include microhardness, tensile, air and environmental creep, low cycle fatigue, creep-fatigue, environmental high cycle fatigue, and supporting microstructural characterization.

In recent years continuous and extensive research and development activities have been being done worldwide on 700°C A-USC (Advanced Ultra Super Critical) power plants to achieve higher efficiency and reduce the CO 2 emission. Increasing steam temperature and pressure of such A-USC boilers under consideration require the adoption of Ni based alloys. In the Japanese national project launched in 2008, Ni based alloy HR6W (45Ni-23Cr-7W-Ti, ASME Code Case 2684) is one of the candidate materials for boiler tube and pipe as well as Alloy617, Alloy263 and Alloy740H. The most important issues in A-USC boiler fabrication are the establishment of proper welding process for thick wall components of these alloys and verification of the long term reliability of their weldments. In our previous study, the weldability of HR6W was investigated and the welding process for Ni based thick wall pipe was established with the narrow gap HST (Hot wire Switching TIG) welding procedure originally developed by Babcock-Hitachi K.K. In this paper, creep rupture strengths of HR6W weldment were verified by the long term test up to 60,000 hours for tube and 40,000 hours for pipe. In Japanese national project, narrow gap HST welding process was also applied to the welding test for the other Ni based candidate pipe materials. Furthermore, as the practical A-USC boiler manufacturing trials, header mockup test was conducted and qualified for HR6W.