Search Results for chromium-tungsten-molybdenum-vanadium-niobium alloys

The rising global energy demand has led to a surge in the construction of high-efficiency power plants with advanced steam parameters. National and international projects indicate that fossil fuels will continue to be the primary source of power generation in the coming years, despite significant efforts and progress in utilizing alternative energy sources. Economic pressures and climate protection concerns necessitate more cost-efficient and environmentally sustainable energy production. Achieving this requires reducing specific fuel and heat consumption per kilowatt-hour, making it essential to improve the efficiency of new power plants beyond those commissioned in Germany between 1992 and 2002. While new construction and process innovations contribute to efficiency gains, the primary factors driving improvement are increased steam pressure and temperature. Current design parameters include steam temperatures of 605 °C (live steam) and 625 °C (hot reheat steam), along with pressures of 300 bar (live steam) and 80 bar (hot reheat steam), which have become critical for obtaining building and operating licenses in Germany. However, the European Creep Collaborative Committee’s (ECCC) 2005 reassessment of the creep strength of steel T/P92 (X10CrWMoVNb9-2) has placed limitations on further increasing steam temperatures beyond 625 °C.

The use of flux-cored arc welding (FCAW) is rapidly gaining acceptance in a variety of industries. Much of the gains are due to advances in manufacturing technology that result in superior wires that satisfy both technical and operability concerns. Additionally, productivity gains and the ability to use unsophisticated welding equipment have made these wires very popular. This paper concentrates on FCAW wires that have been formulated to address chromium-molybdenum, nickel base, and stainless steels for high-temperature and environmental applications. Mechanical properties, including creep rupture strength and ductility data, as well as corrosion in environmental components, are discussed.

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 identified that nickel-based filler metals significantly improved service life compared to 309 SS filler metals. Additionally, optimized joint geometries and increased weld metal reinforcement were found to further enhance durability. A new nickel-based filler metal was developed with thermal expansion properties similar to the low-alloy base metal, along with a low chromium content designed to minimize the carbon-denuded zone. However, this filler metal was never commercialized due to its tendency to microfissure, which resulted in a shorter-than-expected service life. This paper explores further investigations into the microfissuring of this filler metal and examines long-term testing to assess its suitability for high-temperature applications.

This paper presents comprehensive test results of thick-walled VM12 steel pipes containing 12% chromium, vanadium, and tungsten, with cobalt addition. The primary objective was to verify welding technologies for boiler superheater thick-walled components and characterize the strength, technological properties, and microstructure of welded joints produced at RAFAKO S.A. The extensive research program encompassed a broad range of tests on both parent material and welded joints, including mechanical property assessments at room temperature, creep resistance evaluations, low-cycle fatigue testing at room temperature and 600°C (1120°F), and detailed macro- and microstructural examinations. Furthermore, the investigation included a comprehensive microstructural stability assessment using light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), conducted after fatigue resistance testing at room and elevated temperatures, following additional annealing at 700°C (1,920°F), and after 1,000 hours of exposure for both parent material and welded joints. These investigations were conducted as part of the COST 536 Action, representing a collaborative effort to understand and characterize high-temperature creep-resistant steels like VM12 for advanced power generation applications.

The T/P91 and T/P92 steel grades were developed as a result of a demand of high creep strength for advanced power plants. Nevertheless, their operating temperature range is limited by their oxidation performance which is lower compared with usual 12%Cr steels or austenitic steels. Moreover, the new designed power plants require higher pressure and temperature in order to improve efficiency and reduce harmful emissions. For these reasons, Vallourec and Mannesmann have recently developed a new 12%Cr steel which combines good creep resistance and high steam-side oxidation resistance. This new steel, with a chromium content of 12% and with other additional elements such as cobalt, tungsten and boron, is named VM12. Manufacturing of this grade has been successfully demonstrated by production of several laboratory and industrial heats and rolling of tubes and pipes in several sizes using different rolling processes. This paper summarizes the results of the investigations on base material, including creep tests and high temperature oxidation behavior, but also presents mechanical properties after welding, cold bending and hot induction bending.

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.

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.

High-pressure valves and fittings used in coal-fired 600/625 °C power plants are hardfaced for protection against wear and corrosion and to provide optimum sealing of the guides and seats. Stellite 6 and Stellite 21 are often used for hardfacing, which is carried out by build-up welding, usually in several layers. The valve materials are generally heat-resistant steels such as 10CrMo9-10 (1.7380), X20CrMoV1 (1.4922), or Grade 91 / Grade 92 (1.4903 / 1.4901). In recent years, cracks or delaminations have frequently occurred within the hardfaced layer. The influence of cycling operation is not well understood. Other essential factors are the chemical composition of the base material and of the filler metal; especially in terms of the resulting iron dilution during the deposition of the welding overlays. The research project was initiated to investigate the crack and delamination behavior and to understand the involved damage mechanisms. Thermostatic and cyclic exposure tests have shown that cracking is favored by the formation of brittle phases due to iron dilution from the substrate material during the manufacturing process. Recommendations for the welding process of hardfaced sealing surfaces of fittings were derived from the investigation results.

ASTM Grade 23 is a 2.25Cr-0.3Mo-1.5W-V-Nb-B steel widely used for the fabrication of boiler components of the most recent ultra super critical power plants; it combines high creep resistance, enhanced oxidation and corrosion resistance and good weldability. Microstructural, mechanical, and creep properties of seamless tubes and pipes after normalizing and tempering heat treatment are compared with those obtained after cold bending and hot induction bending. The creep resistance is obtained through the precipitation of fine carbides after tempering. A broad program of TEM investigations on crept samples has been carried out in order to assess the evolution of the microstructure and its phases after long term high-temperature exposure, in terms of chemical composition, size and distribution of precipitates.

Improvement of thermal efficiency of new power plants by increasing temperature and pressure of boilers has led us to the development of high creep strength steels in the last 10 years. HCMA is the new steel with base composition of 1.25Cr-0.4Mo-Nb-V-Nd, which has been developed by examining the effects of alloying elements on microstructures, creep strength, weldability, and ductility. The microstructure of the HCMA is controlled to tempered bainite with low carbon content and the Vickers hardness value in HAZ is less than 350Hv to allow the application without preheating and post weld heat treatment. The HCMA tube materials were prepared in commercial tube mills. It has been demonstrated that the allowable stress of the HCMA steel tube is 1.3 times higher than those of conventional 1%Cr boiler tubing steels in the temperatures range of 430 to 530°C. It is noted that creep ductility has been drastically improved by the suitable amount of Nd (Neodymium)-bearing. The steam oxidation resistance and hot corrosion resistance of the HCMA have been proved to be the same level of the conventional 1%Cr and 2%Cr steels. It is concluded that the HCMA has a practical capability to be used for steam generator tubing from the aspect of good fabricability and very high strength. This paper deals with the concept of material design and results on industrial products.

The use of the bainitic class of creep strength enhanced ferritic steels T/P23 and T24 has increased over the last decade in a wide range of applications including replacement headers, superheater and reheater tubing and in waterwall tubing. Many issues have been reported in one or both of these materials including hydrogen induced cracking, reheat cracking and stress corrosion cracking. To appropriately address these issues, work has been initiated that includes a literature review, development of a database of phase transformation temperatures, investigation of tempering behavior, and an analysis of the effect of phase transformation on residual stresses. Such information will be provided in the context of understanding why these two materials appear highly susceptible to these cracking mechanisms.

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.

There is a constant need for improved knowledge of the influence of non-standard processing on the expected performance of creep strength enhanced ferritic (CSEF) materials as the total installed tonnage of these materials is rapidly increasing across the power generation industry. Cr-Mo-V steel grades micro-alloyed with niobium and titanium designed for pressurized equipment operating in the supercritical steam range proved to be very sensitive to relative minor variations in the principal heat treatment parameters time and temperature, when compared to the traditional Cr-Mo-V grades. A key component for successful welds is optimised post weld heat treatment (PWHT). Under certain conditions premature failures of welds can occur when incorrect weld and heat treatment performance result in a reduction of specified mechanical properties and high temperature creep performance, it is therefore of significant importance to have a good understanding of actual material properties for effective operation and plant life studies. This study investigated the effect and impact variations of post weld heat treatment time and temperature on mechanical properties of tungsten inert gas (TIG) and manual metal arc (MMA) welds on Grade 91 pipes from a set of reference samples. This is in preparation of establishing a benchmark set of tests to determine the integrity and expected long-term performance of butt-welds from limited site sample volumes, providing a non-intrusive methodology to identify welds suspected to have received non-standard PWHT cycles on Grade 91 pipework systems.

This study investigates a novel approach to addressing the persistent Type IV cracking issue in Grade 91 steel weldments, which has remained problematic despite decades of service history and various mitigation attempts through chemical composition and procedural modifications. Rather than further attempting to prevent heat-affected zone (HAZ) softening, we propose eliminating the vulnerable base metal entirely by replacing critical sections with additively manufactured (AM) weld metal deposits using ASME SFA “B91” consumables. The approach employs weld metal designed for stress-relieved conditions rather than traditional normalizing and tempering treatments. Our findings demonstrate that the reheat cycles during AM buildup do not produce the substantial softening characteristic of Type IV zones, thereby reducing the risk of premature creep failure. The study presents comprehensive properties of the AM-built weld metal after post-weld heat treatment (PWHT), examines factors influencing deposit quality and performance, and explores the practical benefits for procurement and field construction, supported by in-service data and application cases.

This paper presents three recent example cases of cracking in Grade 91 steel welds in longer-term service in high temperature steam piping systems: two girth butt welds and one trunnion attachment weld. All the cases were in larger diameter hot reheat piping, with the service exposure of the welds ranging from approximately 85,000 to 150,000 hours. Cracking in all cases occurred by creep damage (cavitation and microcracking) in the partially transformed heat-affected zone (PTZ, aka Type IV zone) in the base metal adjacent to the welds. The location and morphology of the cracking are presented for each case along with operating conditions and potential contributors to the cracking, such as system loading, base metal chemical composition, and base metal microstructure.