Carbon migration in narrow-gap welding joints of dissimilar steels has been studied using bead-on-plate specimens to determine the factors that influence the formation of a soft ferrite structure in the carbon-depleted zone. Carbon migration was found to occur during tempering, with a ferrite structure formed at the intersection of multiple layers due to severe carbon migration. This was attributed to a steep gradient in Cr content caused by the low fusion penetration at the intersection. Experimental results and the relationship between fusion penetration and weld bead alignment confirmed that low fusion penetration is the main cause of ferrite-structured carbon depleted zones.

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.

Unpredictable failures near the phase boundary in Grade 91 dissimilar metal welds (DMW) with nickel based filler metals represent a significant problem for the power generation industry. In order to determine the root cause for these failures, it is necessary to understand the formation of the microstructure in the weld regions around the site of failure. Thermal histories were therefore measured inside the Grade 91 steel heat affected zone (HAZ) of an autogenous weld and of a DMW in the form of bead on plate with Alloy 625 to study the effect of the weld thermal cycle on microstructural formation. It was found that the HAZ in the DMW experienced longer dwell time at high temperatures because of the latent heat of fusion released during Alloy 625 solidification (1350 - 1125 °C). This allowed longer time for carbide dissolution and phase transformations in the DMW than in the autogenous weld. Additionally, the 625 filler metal created a large chemical potential gradient for carbon, which when combined with longer dwell times, yielded carbon depletion in the heat affected zone. Retention of δ ferrite in the coarse grained HAZ (CGHAZ) of DMWs was found to be an indicator for these mechanisms.

In this study, creep rupture behaviors and rupture mechanisms of dissimilar welded joint between Inconel 617B and COST E martensitic steel were investigated. Creep tests were conducted at 600 ℃ in the stress range 140-240 MPa. Scanning electron microscopy (SEM) and micro-hardness were used to examine the creep rupture behaviors and microstructure characteristics of the joint. The results indicated that the rupture positions of crept joints shifted as stress changed. At higher stress level, the rupture position was located in the base metal (BM) of COST E martensitic steel with much plastic deformation and necking. At relatively lower stress level, the rupture positions were located in the fine-grained heat affected zone (FGHAZ) of COST E or at the interface between COST E and WM both identified to be brittle fracture. Rupture in the FGHAZ was caused by type Ⅳ crack due to matrix softening and lack of sufficient precipitates pinning at the grain boundaries (GBs). Rupture at the interface was related to oxide notch forming at the interface.

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 trial weld joint of COST F and COST FB2 steels was produced using the GTAW HOT-WIRE method in conditions used in industry for production of welding steam turbine rotors. Conventional long-term creep tests (CCT) to the rupture of this weldment and the base materials were carried out at temperatures ranging from 550 °C to 650 °C in the stress range from 70 to 220 MPa (the longest time to rupture was above 52,000 hours). Creep rupture strength was evaluated using Larson-Miller parameter model. Assessment of microstructure was correlated with the creep strength. Precipitation of Laves phase and structure recovery during creep exposures were the main reasons for the failure which occurred in the heat affected zone of steel COST F. The recently developed simulative accelerated creep testing (ACT) on thermal-mechanical simulator allows the microstructural transformation of creep-resisting materials in a relatively short time to a state resembling that of multiyear application under creep conditions. ACT of samples machined from various positions in the weldment was performed at 600 °C under 100 MPa. Changes in the hardness and the microstructures of the samples, which underwent both types of creep tests, were compared. Small sample creep test (SPCT), another alternative method how to obtain information about the creep properties of materials when only a limited amount of test material is at disposal, were performed. It was shown that the same stress-temperature dependence and relationships are valid in the SPCT as in the CCT. Using a simple load-based conversion factor between the SPCT test and the CCT test with the same time to rupture, the results of both test types can be unified.

Dissimilar joints between modern 10% chromium steels and low-alloy steels are unavoidable in new installations or upgrades of steam turbine components. Welds between 10CrMo9-10 (P22) and X10CrMoVNb9-1 (P91) steel pipes are often required. This paper studies this heterogeneous weld from a steam turbine manufacturer's practical perspective. Two types of filler materials were used: P22- and P91-based weld metals. The integrity and mechanical properties of the prepared heterogeneous welds were evaluated according to the welding standard EN 288-3. Both approaches yielded satisfactory results. Additionally, creep rupture strength was evaluated. The creep rupture strength of both joints fell within the -20% scatter band of the P22 base material's creep rupture strength. The weld design with P91 filler material appeared to slightly outperform the P22-based approach for longer exposure times.

This paper presents RAFAKO S.A. experience within the field of research and implementation of new group of creep-resistant steel grades with addition of tungsten, including the selection of filler metal, welding procedures and selected results of mechanical properties testing carried out during the implementation of welding process for elements of steam superheaters of the boilers with supercritical parameters. It summarizes the experience and results of research works carried out in RAFAKO, Silesian Technical University in Katowice and Welding Institute in Gliwice within the framework of COST 522 program concerning the influence of simulation of element temperature operating conditions on Chaгpy V notch toughness, HV10 hardness, microstructure of simulated HAZ's, assessment of steel weldability, strength properties. The welded joints structure stability and HAZ after stress relief annealing and after ageing - simulation of element operating conditions are presented in the form of graphs and prints of microstructures.

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.

As flux cored wires for gas metal arc welding offer several technical and economic advantages they are becoming more and more popular. Matching flux cored wires for welding P92 have already been available for several years. A matching flux cored wire for welding the Co-alloyed cast steel CB2, which is used for turbine and valve casings operating at steam temperatures of up to 620°C, was developed recently. To connect casings with P92 pipes, dissimilar welding of CB2 to P92 is necessary. This can be done with filler metal that matches either CB2 or P92. Pre-tests have confirmed that flux cored arc welding (FCAW) can generally be used for dissimilar joint welding of CB2 to P92. To evaluate creep rupture strength dissimilar welds were performed with filler metal matching CB2 and P92, respectively. TIG welding was used for the root and the second pass and FCAW for the intermediate and final passes. Cross-weld tensile tests, side bend tests and impact tests of weld metals and heat-affected zones were carried out at ambient temperatures after two post-weld heat treatments (PWHT), each at 730°C for 12 hours. Creep rupture tests of cross-weld samples were performed at 625°C. This study compares the results of the mechanical tests at ambient temperature and the creep rupture tests, and discusses why P92 filler metals are preferred for such welds.

Carbon Capture and Storage (CCS) has become promising technology to reduce CO 2 emissions. However, as a consequence of CCS installation, the electrical efficiency of coal fired power plant will drop down. This phenomenon requires increase in base efficiency of contemporary power plants. Efficiency of recent generation of power plants is limited mainly by maximum live steam temperature of 620°C. This limitation is driven by maximal allowed working temperatures of modern 9–12% Cr martensitic steels. Live steam temperatures of 750°C are needed to compensate the efficiency loss caused by CCS and achieve a net efficiency of 45%. Increase in the steam temperature up to 750°C requires application of new advanced materials. Precipitation hardened nickel-based superalloys with high creep-rupture strength at elevated temperatures are promising candidates for new generation of steam turbines operating at temperatures up to 750°C. Capability to manufacture full-scale forged rotors and cast turbine casings from nickel-based alloys with sufficient creep-rupture strength at 750°C/105 hours is investigated. Welding of nickel-based alloys in homogeneous or heterogeneous combination with 10% Cr martensitic steel applicable for IP turbine rotors is shown in this paper. Structure and mechanical properties of prepared homogeneous and heterogeneous weld joints are presented.

In order to improve thermal efficiency of fossil-fired power plants through increasing steam temperature and pressure high strength martensitic 9-12%Cr steels have extensively been used, and some power plants have experienced creep failure in high temperature welds after several years operations. The creep failure and degradation in welds of longitudinally seam-welded Cr- Mo steel pipes and Cr-Mo steel tubes of dissimilar metal welded joint after long-term service are also well known. The creep degradation in welds initiates as creep cavity formation under the multi-axial stress conditions. For the safety use of high temperature welds in power plant components, the complete understanding of the creep degradation and establishment of creep life assessment for the welds is essential. In this paper creep degradation and initiation mechanism in welds of Cr-Mo steels and high strength martensitic 9-12%Cr steels are reviewed and compared. And also since the non-destructive creep life assessment techniques for the Type IV creep degradation and failure in high strength martensitic 9-12%Cr steel welds are not yet practically established and applied, a candidate way based on the hardness creep life model developed by the authors would be demonstrated as well as the investigation results on the creep cavity formation behavior in the welds. Additionally from the aspect of safety issues on welds design an experimental approach to consider the weld joint influence factors (WJIF) would also be presented based on the creep rupture data of the large size cross-weld specimens and component welds.

In the late 1980s, the domestic utility industry experienced failures in dissimilar metal welds (DMWs) between low-alloy ferritic tubing and austenitic tubing in superheaters and reheaters. Extensive research by EPRI found that nickel-based filler metals provided significant service life improvements over 309 stainless steel filler metals. Improved joint geometries and additional weld metal reinforcement were determined to extend service life further. A new nickel-based filler metal was also developed, exhibiting thermal expansion properties similar to the low-alloy base metal and a low chromium content that would result in a smaller carbon-depleted zone than currently available fillers. However, this new filler metal was never commercialized due to a tendency for microfissuring, resulting in less than desired service life. This paper discusses further investigation into the filler metal microfissuring issue and examines long-term testing to determine the filler's suitability for high-temperature applications.

Metallurgical factors affecting the fusion boundary failure and damage mechanism of DMWs (Dissimilar Metal Welds) between the CSEF (Creep Strength Enhanced Ferritic) steels and austenitic steels were experimentally and theoretically investigated and discussed. Long-term exservice DMWs up to 123,000 hours were investigated; the precipitates near the fusion boundary were identified and quantitatively evaluated. Comparing with the other generic Ni-based weld material, MHPS original filler metal HIG370 (Ni bal.-16Cr-8Fe-2Nb-1Mo) showed superior suppression effect on fusion boundary damage of DMWs, which was verified by both of the microstructure observation and thermodynamic calculation. Based on the microstructure observation of crept specimen and ex-service samples of DMWs, temperature, time and stress dependence of fusion boundary damage of DMWs were clarified. Furthermore, fusion boundary damage morphology and mechanism due to precipitation and local constituent depletion was discussed and proposed from metallurgical viewpoints.

In order to develop an Fe/Ni dissimilar-weld rotor structure for an Advanced Ultra Super Critical turbine, fundamental studies on the metallurgical properties of Fe/Ni welds are needed. In the work reported in this paper, we studied the microstructure evolution and creep rupture properties of Fe/Ni weld joints with different compositions. Investigation of thermally aged Fe/Ni diffusion couples revealed that Fe-based ferritic steel and Alloy 617 weld joints with a large difference in Cr content showed strong C diffusion at the weld interface. This decreased the creep rupture life of the weld joint, caused by coarsening of a martensitic structure near the interface. Analysis using Fe/Ni diffusion couples and thermodynamic calculations suggested that the driving force of C diffusion is the chemical potential gradient at the interface, and the difference in Cr content between Fe and Ni accelerates the C diffusion.

Qualifying welding procedures for repair of components in high temperature service requires careful consideration of factors including identification of the materials involved, existing mechanical properties and service operating parameters such as temperature, pressure and environment. Selection of weld metals to match, under match or overmatch base material as well as direct and indirect consequences on the heat-affected zone also require evaluation. Application of post weld heat treatment and ramifications where dissimilar base materials are involved are discussed plus the necessity of conducting tests at the operating temperatures and conditions where information is not available from the literature. Each of these factors is discussed and examples provided.

Enhancement of the steam conditions is one of the most effective measures to achieve the goal of higher thermal efficiency. 700°C class A-USC (Advanced Ultra Super Critical Steam Conditions) power plant is one of the remarkable technologies to achieve the goal and reduce CO 2 emissions from fossil fuel power plants. Toshiba has been working on the A-USC development project with subsidy from METI (Ministry of Economy, Trade and Industry) and NEDO (New Energy and Industrial Technology Development Organization). In this project, A-USC power plants with steam parameters of 35MPa 700/720/720°C were considered. To date, various materials have been developed and tested to verify their characteristics for use in A-USC power plants. And some of these materials are being investigated as to their suitability for use in long term. Together with members of the project, we carried out the boiler component test using a commercially-operating boiler. We manufactured a small-scale turbine casing made of nickel-based alloy, and supplied it for the test. In addition, we manufactured a turbine rotor for turbine rotation tests, and carried out the test at 700°C and rotating speed of 3,600rpm conditions. In this paper, we show the results of the A-USC steam turbine development obtained by the project.

To meet worldwide emission targets many Government policies either avoid the use of coal burning plant for future energy production, or restrict emissions per kilogram of coal consumed beyond the capability of most conventional plant. As a result this has accelerated current worldwide developments of steel and nickel alloys for coal-fired plant to operate at temperatures in excess of 625°C. Within the UK a modified 9%Cr steel has been developed which is based on the MarBN steel first proposed by Professor Fujio Abe of NIMS Japan, and has been designated IBN-1. The steel is modified by additions of, typically, 3% cobalt and tungsten with controlled additions of boron and nitrogen. While development of 9%Cr steels has continued since the last EPRI high temperature material conference in 2016 (Portugal), parallel developments in nickel alloy castings for even higher temperature and pressure applications have also continued. This paper summarises the latest developments in both of these material types.

Power generation technology selection is driven by factors such as cost, fuel supply security, and environmental impact. Coal remains a popular choice due to its global availability, but efficient, reliable, and cost-effective methods are essential. In Europe, efforts focus on advancing coal-fired steam power plants to ultrasupercritical conditions, with boilers and turbines now operating at up to 600°C. This has improved efficiency and maintained reliability comparable to subcritical plants. Orders are in detailed planning for plants exceeding 600°C, thanks to improved high-temperature steels for components like turbine rotors, casings, steam pipes, and boiler tubes, which undergo rigorous development and testing. Further efficiency gains are expected by increasing steam temperatures to over 700°C using nickel-based alloys. Test facilities are being built for pilot components, leading to a full demonstration plant. This systematic approach to materials development and proven design principles ensures operational reliability.