Search Results for martensitic microstructure

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.

This study presents a characterization of the microstructural evolutions taking place in a 9%Cr martensitic cast steel subjected to fatigue and creep-fatigue loading. Basis for this study of investigation is an extensive testing program performed on a sample heat of this type of steel by conducting a series of service-like high temperature creep-fatigue tests. The major goal here was to systematically vary specific effects in order to isolate and describe relevant damage contributing mechanisms. Furthermore, some of the tests have been interrupted at several percentages of damage to investigate not only the final microstructure but also their evolution. After performing those tests, the samples were examined using transmission electron microscopy (TEM) to characterize and quantify the microstructural evolutions. The size distribution of subgrains and the dislocation density were determined by using thin metal foils in TEM. A recovery process consisting of the coarsening of the subgrains and a decrease of the dislocation density was observed in different form. This coarsening is heterogeneous and depends on the applied temperature, strain amplitude and hold time. These microstructural observations are consistent with the very fast deterioration of creep properties due to cyclic loading.

The present study presents a detailed investigation on the evolution of the microstructure during welding on virgin and long-term service exposed (creep aged 1 = 535°C; 16.1 MPa; 156 kh and creep aged 2 = 555°C; 17.0 MPa; 130 kh) 12% Cr (X20CrMoV11-1) martensitic steel. This study was carried out in order to understand the impact of welding on prior creep exposed Tempered martensite ferritic (TMF) steel and to explain the preferential failure of weldments in the fine grained heat affected zone (FGHAZ) of the creep aged material side instead of the new material side. Gleeble simulation (Tp = 980°C; heating rate = 200 °C/s; holding time = 4 seconds) of the FGHAZ was performed on the materials to create homogeneous microstructures for the investigation. Quantitative microstructural investigations were conducted on the parent plate and simulated FGHAZ materials using advanced electron microscopy to quantify: a) voids, b) dislocation density, c) sub-grains, and d) precipitates (M 23 C 6 , MX, Laves, Z-phase) in the materials. Semi-automated image analysis was performed using the image analysis software MIPARTM. The pre-existing creep voids in the creep aged parent material and the large M 23 C 6 carbides (Ø > 300 nm) in the FGHAZ after welding are proposed as the main microstructural contributions that could accelerate Type IV failure on the creep aged side of TMF steel weldments.

TAF steel is a Japanese high-boron 10.5% Cr martensitic stainless steel known for its exceptional high-temperature creep strength. Its high boron content (300-400 ppm) limited practical applications due to reduced hot workability in large turbine components. Recent research suggests that increasing boron content while adjusting nitrogen levels could enhance creep properties by promoting fine vanadium carbonitride formation while preventing boron nitride formation. This study presents microstructural investigations, particularly using transmission electron microscopy, focusing on precipitation characteristics and long-term precipitate evolution within the COST 536 framework.

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 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.

A 9Cr-3W-3Co-VNbBN steel, designated MARBN ( MAR tensitic 9Cr steel strengthened by B oron and N itrides), has been alloy-designed and subjected to long-term creep and oxidation tests for application to thick section boiler components in USC power plant at 650 o C. The stabilization of lath martensitic microstructure in the vicinity of prior austenite grain boundaries (PAGBs) is essential for the improvement of long-term creep strength. This can be achieved by the combined addition of 140ppm boron and 80ppm nitrogen without any formation of boron nitrides during normalizing at high temperature. The addition of small amount of boron reduces the rate of Ostwald ripening of M 23 C 6 carbides in the vicinity of PAGBs during creep, resulting in stabilization of martensitic microstructure. The stabilization of martensitic microstructure retards the onset of acceleration creep, resulting in a decrease in minimum creep rate and an increase in creep life. The addition of small amount of nitrogen causes the precipitation of fine MX, which further decreases the creep rates in the transient region. The addition of boron also suppresses the Type IV creep-fracture in welded joints by suppressing grain refinement in heat affected zone. The formation of protective Cr 2 O 3 scale is achieved on the surface of 9Cr steel by several methods, such as pre-oxidation treatment in Ar gas, Cr shot-peening and coating of thin layer of Ni-Cr alloy, which significantly improves the oxidation resistance of 9Cr steel in steam at 650 o C. Production of a large diameter and thick section pipe and also fabrication of welds of the pipe have successfully been performed from a 3 ton ingot of MARBN.

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.

Modified 9Cr-1Mo steel was manufactured via laser powder bed fusion (LPBF) using gas atomized powders under various building conditions. Dense samples were obtained at an energy density of 111-125 J/mm 3 . As-built samples were subjected to a normalization and tempering heat treatments. The microstructure of the as-built sample exhibits a duplex structure, comprising coarse columnar δ-ferrite grains and fine martensite grains. In addition, a small amount of retained austenite phase was observed at the interface between δ-ferrite and martensite. The formation of δ-ferrite is attributed to the extremely rapid solidification that occurs during the LPBF process, while martensite is obtained through the phase transformation because of the thermal cycles experienced during the process. The area fraction of δ-ferrite and martensite can be controlled by adjusting the LPBF parameters. Typical as-built microstructure morphology characterized by the columnar δ- ferrite was eliminated after the heat treatments, resulting in a tempered martensitic microstructure that is identical with that obtained through the conventional process. However, an increase in prior austenite grain size was observed when the area fraction of δ-ferrite in the as-built condition was high, due to faster phase transformation kinetics of martensite than that of δ-ferrite during the normalization. This suggests that the prior austenite grain size can be controlled by optimizing the area fraction of δ-ferrite and martensite in the as-built microstructure.

Inflection is observed at 50% of 0.2% offset yield stress, that is HALF YIELD, on the relation between stress and creep rupture life of creep strength enhanced ferritic steels with tempered martensitic microstructure. Similar shape is generally recognized on the ferritic steels with martensitic or bainitic microstructure, in contrast to ferritic steels with ferrite and pearlite microstructure, as well as austenitic steels and superalloys except for several alloys. Ferritic steel with martensitic or bainitic microstructure indicates softening during creep exposure, however, hardening due to precipitation takes place in the ferritic steels with ferrite and pearlite microstructure and austenitic steels. This difference in microstructural evolution is associated with indication of inflection at half yield. Stress range of half yield in the stress vs. creep life diagram of creep strength enhanced ferritic steels is wider than that of conventional ferritic creep resistant steels with martensitic or bainitic microstructure. As a result of wide stress range of boundary condition, risk of overestimation of long-term creep rupture strength by extrapolating the data in the high-stress regime to the low-stress regime is considered to be high for creep strength enhanced ferritic steels.

This paper reports the results of a collaborative investigation of an ex-service grade 91 bend carried out by the UK generating companies Centrica, SSE, Engie and RWE. As part of the handover exercise for Centrica’s Langage power station in 2009 a number of routine checks were carried out on the main steam and hot reheat grade 91 steam pipework. In some cases low hardness readings were found with subsequent metallurgical replication showing the presence of an aberrant non martensitic microstructure. This led to a more extensive inspection programme on the steam lines and the discovery of other areas of suspect material. A review of the operating capability of the plant, including detailed pipework stress analysis and a pipework peaking assessment, along with the assumption that lower strength grade 91 material was present, led to the steam lines being down rated and returning to service under these revised conditions. At the first C inspection in December 2012, after the HRSG and associated pipework had operated for 18720 hours, a bend with a soft weld, along with a section of the straight pipe on either side, was removed from service. An investigation was undertaken to establish how long this component would have survived, had it been left in service, and to consider the implications for the future operation of the plant.

In this study, a possibility of application of advanced 9%Cr steel containing 130 ppm boron for boiler components utilized at around 650 °C to higher temperature steam turbine rotor materials has been investigated by means of reduction in silicon promoting macro-segregation in the case of large size ingots, using laboratory heats. Tempered martensitic microstructure without proeutectoid ferrite in all steels studied is obtained even at the center position of a turbine rotor having a barrel diameter of 1.2 m despite lower amounts of nitrogen and silicon. The strength at room temperature is almost the same level of practical high Cr steels such as X13CrMoCoVNbNB 9-2-1 for ultrasuper critical steam turbine rotors. The toughness is sufficient for high temperature rotors in comparison with CrMoV steels utilized as sub-critical high pressure steam turbine components. The creep rupture strength of the steels is higher than that of the conventional 9-12Cr steels used at about 630 °C. The creep rupture strength of 9%Cr steel containing 130 ppm B, 95 ppm N, 0.07 % Si and 0.05 % Mn is the highest in the steels examined, and it is therefore a candidate steel for high temperature turbine rotors utilized at more than 630 °C. Co-precipitation of M 23 C 6 carbides and Laves phase is observed around the prior austenite grain boundaries after the heat treatments and the restraint of the carbide growth is also observed during creep exposure. An improvement in creep strength of the steels is presumed to have the relevance to the stabilization of the martensitic lath microstructure in the vicinity of those boundaries by such precipitates.

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.

For high-strength steels developed at the National Institute for Materials Science (NIMS) in Japan, a dispersion of nano-sized MX nitride particles along boundaries and in the matrix is achieved by reducing carbon concentration below 0.02%. This structure results in excellent creep strength at 923K, approximately two orders of magnitude longer rupture time than P92. Additionally, adding a large amount of boron exceeding 0.01% combined with minimized nitrogen effectively improves creep rupture strength by stabilizing the martensitic microstructure during creep. Efforts have been made to enhance the steam oxidation resistance of these 9Cr steels strengthened by boron and fine MX nitrides. A combination of 0.7% Si, 40-60 ppm S, and pre-oxidation treatment was applied. Steam oxidation tests were conducted at 923K for up to 4000h. Pre-oxidation treatment in argon gas at 973K for 50h significantly improved oxidation resistance in steam at 923K by forming a protective Cr-rich oxide layer. The pre-oxidized steels exhibited much lower mass gain in steam at 923K than Mod.9Cr-1Mo steel at 873K, and lower than T91 at 873K after 1000h. After 4000h, their mass gain was about zero, much lower than P91 at 873K and 923K. SEM/EDS analysis and low mass gain suggest a protective Cr-rich oxide scale formed on the pre-oxidized steel surface, exhibiting excellent oxidation resistance in steam at 923K.

With the desire for higher operating temperatures and pressures to improve the thermal efficiency of new power generating plant there have been significant changes in the materials used. For operation up to 620°C, a new range of ferritic steels with 9-13%Cr has been developed. With proper control of composition and heat treatment these materials, including Grades 91 and 92,exhibit predominantly martensitic microstructures and a good balance between strength and ductility. However, fabrication processes such as welding and bending, normally combined with extreme operating conditions have resulted in in-service damage. Examples of factors leading to accelerated creep, creep fatigue and oxidation damage are described.

Long-term creep strength property of creep strength enhanced ferritic steels was investigated. Stress dependence of minimum creep rate was divided into two regimes with a boundary condition of macroscopic elastic limit which corresponds to 50% of 0.2% offset yield stress (Half Yield). High rupture ductility was observed in the high stress regime above Half Yield, and it was considered to be caused by relatively easy creep deformation throughout grain interior with the assistance of external stress. Grades T23, T/P92 and T/P122 steels represented marked drop in rupture ductility at half yield with decrease in stress. It was considered to be caused by inhomogeneous recovery at the vicinity of prior austenite grain boundary, because creep deformation was concentrated in a tiny recovered area. High creep rupture ductility of Grade P23 steel should be associated with its lower creep strength. It was supposed that recovery of tempered martensitic microstructure of T91 steel was faster than those of the other steels and as a result of that it indicated significant drop in long-term creep rupture strength and relatively high creep rupture ductility. The long-term creep rupture strength at 600°C of Grade 91 steel decreased with increase in nickel content and nickel was considered to be one of the detrimental factors reducing microstructural stability and long-term creep strength. The causes affecting recovery of microstructure should be elucidated in order to obtain a good combination of creep strength and rupture ductility for long-term.

Recent advances in materials technology for boilers materials in the advanced USC (A-USC) power plants have been reviewed based on the experiences from the strengthening and degradation of long term creep properties and the relevant microstructural evolution in the advanced high Cr ferritic steels. P122 and P92 type steels are considered to exhibit the long term creep strength degradation over 600°C, which is mainly due to the instability of the martensitic microstructure strengthened too much by MX carbonitrides. This can be modified by reducing the precipitation of VN nitride and by optimizing the Cr content of the steels. An Fe-Ni based alloy, HR6W strengthened by the Fe2W type Laves phase is found to be a marginal strength level material with good ductility at high temperatures over 700°C and to be used for a large diameter heavy wall thick piping such as main steam pipe and hot reheat pipe in A-USC plants, while Ni-Co based alloys such as Alloys 617 and 263 strengthened by a large amount of the y’ phase are found to be the high strength candidate materials for superheater and reheater tubes, although they are prone to relaxation cracking after welding and to grain boundary embrittlement during long term creep exposure. A new Ni based alloy, HR35 strengthened by a-Cr phase and other intermetallic phases has been proposed for piping application, which is specially designed for a good resistance to relaxation cracking as well as high strength and a good resistance to steam oxidation and fire-side corrosion at high temperatures over 700°C.

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.

In thermal power plants, weldments of all currently used martensitic 9% chromium steels are prone to Type IV cracking in the fine-grained region of the heat-affected zone (HAZ). Japanese researchers have introduced a new martensitic steel for ultra-supercritical (USC) steam conditions that demonstrates resistance to Type IV cracking. This study compares a modified version of this boron-nitrogen balanced advanced 9Cr-3W-3Co steel with CB2, the most promising 9% Cr steel developed through the European research initiative COST, in terms of weldability. The HAZ was analyzed using the "Heat-Affected Zone Simulation" technique with a Gleeble 1500 thermo-mechanical simulator. Basic optical microscopy was complemented by advanced electron microscopy techniques, including energy-filtered TEM (EFTEM), electron energy loss spectroscopy (EELS), electron backscatter diffraction (EBSD), and energy-dispersive X-ray analysis (EDX). Phase transformations in the HAZ were directly observed using in situ X-ray diffraction with synchrotron radiation at the Advanced Photon Source (APS) of Argonne National Laboratory, IL, USA. Although both steels exhibited similar transformation behavior, their resulting microstructures after the weld thermal cycle differed significantly. At peak temperatures above 1200°C, delta ferrite formed and remained stable down to room temperature due to rapid cooling in both steels. While CB2 exhibited conventional coarse-grained (CG), fine-grained (FG), and intercritical HAZ regions, the boron-nitrogen balanced 9Cr steel did not develop a fine-grained HAZ. Since Type IV cracking primarily occurs in the FGHAZ, this alloy shows strong potential for eliminating Type IV cracking as a major life-limiting factor in heat-resistant steel weldments.

This paper reports the results of a collaborative small scale creep testing exercise carried out by the UK generating companies Centrica, SSE, Engie and RWE as part of an investigation of an ex-service grade 91 bend.