A 10%Cr martensitic steel with 3%Co and 0.008%B exhibits extremely long creep rupture time of approximately 40000 h under an applied stress of 120 MPa at a temperature of 650°C. The steel’s microstructure after creep tests interrupted at different creep stages was examined by transmission and scanning electron microscopy. It was shown that superior creep resistance of this steel was attributed to slow increase in creep rate at the first stage of tertiary creep whereas the rapid acceleration of creep rate took place only at the short second stage of tertiary creep. Transition from minimum creep rate stage to tertiary creep was found to be accompanied by coarsening of Laves phase particles, whereas M 23 C 6 – type carbides demonstrated high coarsening resistance under creep condition. Strain-induced formation of Z-phase does not affect the creep strength under applied stress of 120 MPa due to nanoscale size of Z-phase particles.

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.

Increasing the temperature capabilities of ferritic/martensitic 9-12% Cr steels can help in increasing the operating temperature of land-based turbines and minimize the use of expensive high-temperature alloys in the hot section. A creep resistant martensitic steel, JMP, was developed with the potential to operate at or above 650°C. The design of the alloys originated from computational modeling for phase stability and precipitate strengthening using fifteen constituent elements. Cobalt was used for increased solid solution strengthening, Si for oxidation resistance and different W and Mo concentrations for matrix strength and stability. The JMP steels showed increases in creep life compared to MARBN/SAVE12AD at 650°C for testing at various stresses between 138 MPa and 207 MPa. On a Larson-Miller plot, the performance of the JMP steels surpasses that of state-of-the-art MARBN steel. Approximately 21 years of cumulative creep data are reported for the JMP steels which encompasses various compositions. The relationships between composition-microstructure-creep properties are discussed including characterization of microstructures after >20,000 hours in creep.

In advanced ultra-supercritical (A-USC) power plants, which operate at steam temperatures of 700 °C or higher, there is a need to replace 9 to 12Cr martensitic steels with high-strength nickel-base superalloys or austenitic steels for components exposed to the highest temperatures. However, due to the high cost of nickel-base superalloys, it is desirable to use 9 to 12% Cr martensitic steels for components exposed to slightly lower temperatures, ideally expanding their use up to 650 °C. Key challenges in developing ferritic steels for 650 °C USC boilers include enhancing oxidation resistance and long-term creep rupture strength, particularly in welded joints where resistance to Type IV cracking is critical for constructing thick-section boiler components. The current research aims to investigate the creep deformation behavior and microstructure evolution during creep for base metals and heat-affected-zone (HAZ) simulated specimens of tempered martensitic 9Cr steels, including 9Cr-boron steel and conventional steels like grade 91 and 92. The study discusses the creep strengthening mechanisms and factors influencing creep life. It proposes an alloy design strategy that combines boron strengthening and MX nitride strengthening, avoiding the formation of boron nitrides during normalizing heat treatment, to improve the creep strength of both base metal and welded joints.

This study explores methods to enhance the creep strength of 12%Cr martensitic/ferritic steels. The approach focuses on utilizing various precipitates to hinder microstructure coarsening and dislocation movement. A combination of Laves phase (slow precipitation) and MX carbonitrides (dislocation pinning) is used for sustained strengthening. Different MX-forming elements (V, Ta, Ti) are investigated to identify the optimal combination for high quantities of finely distributed strengthening particles. Additionally, cobalt and copper are employed to promote a fully martensitic microstructure and potentially slow down diffusion or provide nucleation sites for Laves phase precipitation. Long-term creep tests confirm the effectiveness of Laves phase precipitation, particularly with tungsten present. Tantalum's influence on both MX precipitation and the Laves phase is also observed. Combining multiple MX-forming elements (V/Ta, V/Ti, Ta/Ti) further improves creep strength, supported by predictions of high MX carbonitride formation from Thermo-Calc calculations. Partially replacing cobalt with copper (1%) also demonstrates positive effects on creep properties.

A creep resistant martensitic steel, CPJ7, was developed with an operating temperature approaching 650°C. The design originated from computational modeling for phase stability and precipitate strengthening using fifteen constituent elements. Approximately twenty heats of CPJ7, each weighing ~7 kg, were vacuum induction melted. A computationally optimized heat treatment schedule was developed to homogenize the ingots prior to hot forging and rolling. Overall, wrought and cast versions of CPJ7 present superior creep properties when compared to wrought and cast versions of COST alloys for turbines and wrought and cast versions of P91/92 for boiler applications. For instance, the Larson Miller Parameter curve for CPJ7 at 650°C almost coincides with that of COST E at 620°C. The prolonged creep life was attributed to slowing down the process of the destabilization of the MX and M 23 C 6 precipitates at 650°C. The cast version of CPJ7 also revealed superior mechanical performance, well above commercially available cast 9% Cr martensitic steel or derivatives. The casting process employed slow cooling to simulate the conditions of a thick wall full-size steam turbine casing but utilized a separate homogenization step prior to final normalization and tempering. To advance the development of CPJ7 for commercial applications, a process was used to scale up the production of the alloy using vacuum induction melting (VIM) and electroslag remelting (ESR), and underlined the importance of melt processing control of minor and trace elements in these advanced alloys.

9-10%Cr-3%Co martensitic steels are the prospective materials for elements of boilers, tubes and pipes for fossil power plants which are able to work at ultra-supercritical parameters of steam (T=620-650°C, P=25-30 MPa). The effect of creep on the microstructure of the 10 wt.%Cr-3Co- 3W-0.2Re martensitic steel was investigated in the condition of 650°C and an applied stress of 140 MPa, time to rupture was more than 8500 h. Previously, this steel was subjected to the normalizing at 1050°C and tempering at 770°C. This heat treatment provided the hierarchical tempered martensite lath structure with the mean size of prior austenite grains of 59 μm and with high dislocation density (2×10 14 m -2 ) within martensitic laths. Boundary M 23 C 6 and M 6 C carbides and randomly distributed within matrix Nb-rich MX carbonitrides were detected after final heat treatment. The addition of Re in the steel studied positively affected creep at 650°C/140 MPa and stabilized the tempered martensite lath structure formed during 770°C-tempering. The formation of the subgrains in the gage section was accompanied by the coarsening of M 23 C 6 carbides and precipitations of Laves phase with fine sizes during creep. No depletion of Re and Co from the solid solution during creep was revealed whereas W content decreased from 3 to 1 wt.% for first 500 h of creep. Reasons of improved creep as well as mechanisms of grain boundary pinning by precipitates are discussed.

The size and distribution of the Laves phase particles in a 9.85Cr-3Co-3W-0.13Mo-0.17Re- 0.03Ni-0.23V-0.07Nb-0.1C-0.002N-0.008B steel subjected to creep rupture test at 650°C under an applied stresses of 160-200 MPa with a step of 20 MPa were studied. After heat treatment consisting of normalizing of 1050°C and tempering of 770°C, M 23 C 6 and Fe 3 W 3 C carbides with the mean sizes of 67±7 and 40±5 nm, respectively, were revealed along the boundaries of prior austenite grains and martensitic laths whereas round NbX carbonitrides were found within martensitic laths. During creep metastable Fe 3 W 3 C carbides dissolved and the stable Laves phase particles precipitated; volume fraction of Laves phase increases with time. The Laves phase particles nucleated on the interfacial boundaries Fe 3 W 3 C/ferrite during first 100 h of creep and provided effective stabilization of tempered martensitic lath structure until their mean size less than 150 nm.

This work deals with the potential of microstructurally based modeling of the creep deformation of martensitic steels. The motivation for the work stems from the ever increasing demand for higher efficiency and better reliability of modern thermal power plants. Service temperatures of 600°C and stress levels up to 100 MPa are currently the typical requirements on critical components. High creep and oxidation resistance are the main challenges for a lifetime 10+ years in steam atmosphere. New materials may fulfill these requirements; however, the save prediction of the creep resistance is a difficult challenge. The model presented in this work takes into consideration the initial microstructure of the material, its evolution during thermal and mechanical exposure and the link between microstructural evolution and creep deformation rate. The model includes the interaction between the relevant microstructural constituents such as precipitates, grain- lath- and subgrain boundaries and dislocations. In addition, the material damage is included into the model. The applicability of the model is then demonstrated on standard creep resistant alloys. Contrary to phenomenological models, this approach can be tested against microstructural data of creep loaded samples and thus provides higher reliability. Nevertheless, potential improvements are discussed and future developments are outlined.

Within the pursuit of improved economic electricity production with reduced environmental pollution, the European research activities COST 501/522 aimed to develop advanced 9-12%Cr steels for highly stressed turbine components by increasing thermal efficiency through higher steam temperatures up to 600/625°C. One such modified Cr steel, a tungsten-alloyed 10%Cr steel, has been in industrial production for several years in steam and gas turbine applications. This paper firstly discusses experiences in manufacturing, non-destructive testing, and mechanical properties achieved in forgings of this COST grade E steel. Secondly, it reports on the manufacturing of a trial melt of a later 9%Cr steel containing cobalt and boron from COST development, describing its long-term creep behavior, microstructural features responsible for superior creep resistance, and test results including short-term properties, detectable flaw size, and initial creep results for a full-size trial rotor forging.

In this paper, the dissimilar metal welds (DMWs) between 617B nickel-based alloy and 10%Cr martensitic heat-resistant steel filled by 617 filler metal was studied, focused on the high temperature creep rupture properties. The high temperature creep rupture properties of welded joints with different welding processes were tested, and the microstructure of welded joints before and after the creep rupture test was observed by OM and SEM. The results showed that, there were three failure modes: base metal failure, type W failure and interface failure, among which interface failure caused the most serious life reduction. The welded joints using ER NiCr-3 filler metal reduced the strain concentration at the interface, so the fracture location shifted from the interface to HAZ of 10%Cr martensitic heat-resistant steel under high temperature and low stress conditions, and creep rupture life was improved. Similarly, weld cap shifted the creep crack propagation path by changing the groove form, so as to altered the stress state of joint and prolong the creep rupture life.

The long-term performance of superheater super 304h tube during the normal service of an ultra-supercritical 1000mw thermal power unit was tracked and analyzed, and the metallographic structure and performance of the original tube sample and tubes after 23,400h, 56,000h, 64,000 h, 70,000 h and 80,000 h service were tested. The results show that the tensile strength, yield strength and post-break elongation meet the requirements of ASME SA213 S30432 after long-term service, but the impact toughness decreases significantly. The metallographic organization is composed of the original complete austenite structure and gradually changes to the austenite + twin + second phase precipitates. With the extension of time, the number of second phases of coarseness in the crystal and the crystal boundary increases, and the degree of chain distribution increases. The precipitation phase on the grain boundary is dominated by M 23 C 6 , and there are several mx phases dominated by NbC and densely distributed copper phases in the crystal. The service environment produces a high magnetic equivalent and magnetic induction of the material, the reason is that there are strips of martensite on both sides of the grain boundary, and the number of martensite increases with the length of service.

9-12%Cr martensitic steels can be applied to the next highest temperature components such as boiler tracts, steam pipelines and turbines of advanced ultra-supercritical power plants with steam temperatures of 650°C. New 10%Cr martensitic steels with high B and low N contents can be a worthy candidate for use in production because it has superior creep resistance. At the same time, resistance to cyclic and dynamic loads is very important. In this work, we studied the low cycle fatigue (LCF) properties at room and elevated (500-650°C) temperatures and Charpy impact toughness at temperatures ranging from -196…100°C of advanced 10% Cr martensitic steel with high B and low N contents. The effect of new alloying scheme and corresponding peculiarities of M 23 C 6 carbides on the low cycle fatigue resistance and impact toughness of the 10%Cr martensitic steel is analyzed. It is revealed that fine and densely distributed carbides has no effect on the fatigue resistance except for the slight improvement of fatigue life at small strain amplitudes and shift the ductile-brittle transition temperature (DBTT) to higher but satisfactory value of +10°C as compared to other high-chromium martensitic steels.

To save fossil fuel resources and to reduce CO 2 emissions, considerable effort has been directed toward researching and developing heat-resistant materials that can help in improving the energy efficiency of thermal power plants by increasing their operational temperature and pressure conditions. Instead of conventional 9-12Cr ferritic heat-resistant steels with a tempered martensitic microstructure, we developed “Precipitation Strengthened 15Cr Ferritic Steel” based on a new material design concept: a solid-solution treated ferrite matrix strengthened by precipitates. Creep tests for 15Cr-1Mo-6W-3Co-V-Nb steels with ferrite matrix strengthened by a mainly Laves phase (Fe 2 W) showed that the creep strengths of 15Cr ferritic steel at temperatures ranging from 923 K to 1023 K were twice as high as those of conventional 9Cr ferric heat-resistant steel. 15Cr steels have higher steam oxidation resistance than that of conventional steel in the same temperature range as the creep tests. Thus, the new material design concept of heat-resistant steel pro- vides improved creep strength and steam oxidation resistance. We are attempting to determine the optimum compositions, especially that of carbon, in order to improve the high-temperature creep strength.

To enhance long-term creep strength at 650°C, stabilization of the lath martensitic microstructure near prior austenite grain boundaries has been investigated for a 9Cr-3W-3Co-0.2V-0.05Nb steel. This was achieved by adding boron to stabilize M 23 C 6 carbides and dispersing fine MX nitrides. Creep tests were conducted at 650°C for up to approximately 3 × 10 4 hours. Adding a large amount of boron exceeding 0.01%, combined with minimized nitrogen, effectively stabilized the martensitic microstructure and improved long-term creep strength. The amount of available boron, free from boron nitrides and tungsten borides, is crucial for enhancing long-term creep strength. Reducing the carbon concentration below 0.02% led to a dispersion of nano-sized MX nitride particles along boundaries and in the matrix, resulting in excellent creep strength at 650°C. A critical issue for the 9Cr steel strengthened by MX nitrides is the formation of Z-phase, which degrades long-term creep strength. Excess nitrogen additions of 0.07 and 0.1% promoted Z-phase formation during creep. The formation of a protective Cr-rich oxide scale was achieved through a combination of Si addition and pre-oxidation treatment in argon, significantly improving the oxidation resistance in steam at 650°C.

Advanced martensitic 9% chromium steels have been identified as the most favored group of materials for high temperature applications in thermal power plants. To extend the temperature range of martensitic steels up to 650°C large effort was put on the development of new alloy concepts. The so-called MARBN concept (Martensitic steel with defined Boron/Nitrogen relation) provides increased creep rupture strength due to higher solid solution strengthening and improved microstructural stability. The major improvement is the reduction of type IV cracking in welded joints, which shifts the focus to the creep rupture strength of the weld metal. This paper illustrates the process experience of the steel foundry for production of heavy cast components in latest state of the art 9-12%Cr-MoCoVNbNB-alloyed cast steel grades and the newest state of development and prototype components in MARBN cast steel grades. Metallurgy, solidification, heat treatment and welding are main items to be considered for development of new, complex steel grades for foundry processing with the help of empiric processing in test programs and thermo-physical simulation. As welding is an essential processing step in the production of heavy steel cast components a good out-of-position weldability is required. Moreover a stress-relieve heat-treatment takes place subsequently after welding for several hours. This contribution also deals with the development of matching welding consumables for the production of heavy cast components and discusses the challenges of defining appropriate welding and heat treatment parameters to meet the requirements of sufficient strength and toughness at ambient temperature. Additionally, first results of creep rupture tests are presented.

In order to evaluate long term creep strength of modified 9Cr ferritic steels, the system free energy of creep ruptured specimens at both 650 and 700 °C is evaluated as the sum of chemical free energy, strain energy and surface energy, which are obtained by a series of experiments, i.e., chemical analysis using extracted residues, X-ray diffraction, and scanning transmission electron microscopy. Change ratio of the system free energy and creep stress showed the relationship with one master curve irrespective of creep conditions, indicating that the steel ruptures when the applied stress exceeds a limited stress depending on the microstructural state expressed by the change ratio of system free energy. Furthermore, it was found that dominant factor of the change ratio was the chemical free energy change. On the basis of these results, long term creep strength of the steel was evaluated at 700 °C, for example, 19MPa at 700 °C after 10 5 h. It is concluded that long term creep strength of modified 9Cr ferritic steels can be predicted by the system free energy concept using the ruptured specimens with various creep conditions.

The microstructures of an advanced Ta-added 9Cr-3Co-2W-Mo steel with increased boron content that has been homogenized at different temperatures were investigated. The chains of coarse W-rich particles were observed in the steel after homogenization at 1150°C for 24 h. These particles remained in the microstructure after normalization and tempering. Such additional dispersion hardening in the initial state of the studied steel decreased the creep rate in transient region. However, the duration of steady state creep and overall creep time was increased in the samples homogenized at 1200°C. Despite of the presence of coarse W-rich particles, the impact toughness of the low-temperature- homogenized steel in the tempered condition was significantly higher than that of the steel homogenized at 1200°C

Microstructure in the gage sections of ruptured GX12CrMoWVNbN10-1-1 cast steel specimens was examined after creep tests under applied stresses ranging from 120 to 160 MPa at T=893 K. The microstructure after tempering consisted of laths with an average thickness of 332 nm. The tempered martensite lath structure was characterized by M 23 C 6 -type carbide particles with an average size of about 105 nm, and MX carbonitrides with an average size of about 45 nm. Precipitation of Laves phase occurred during creep test. The structural changes in the gauge section of the samples were characterized by the evolution of relatively large subgrains with remarkably lowered density of interior dislocations within former martensite laths. MX carbonitrides and M 23 C 6 -type carbide particles increase in size slightly under long-term creep. Microstructural degradation mechanisms during creep in GX12CrMoWVNbN10-1-1 cast steel are discussed.

The recently developed 12%Cr steel VM12-SHC is characterized by very good creep properties at temperatures up to 620°C. This new material development exhibits an excellent oxidation resistance in steam atmospheres at the typical application temperature but also at temperatures up to 650°C. In comparison to the existing 9% Cr grades T/P91 and T/P92, VM12-SHC steel opens due to its excellent oxidation behavior, new possibilities for its application as a heat exchanger boiler component. It was found that outside its application temperature range VM12-SHC also shows, as all 9-12%Cr steels, the appearance of the so called Z-phase. This effect was investigated to understand its influence on creep properties of this class of ferritic/martensitic steels aiming at controlling the microstructure stabilities for future grade developments. Creep testing has been carried out in the temperature range between 525°C and 700°C. Selected crept specimens have been investigated using light optical microscopy, SEM with EDX and TEM. In this study, the oxidation behavior of a number of typical martensitic 9-12%Cr steels was compared with the newly developed 12% Cr steel VM12-SHC. The compositions and morphologies of oxide scales formed after 5000 h exposure steels in simulated steam environments as function of temperature were characterized by light optical metallography and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX).