Search Results for martensite phase

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.

Effects of Ni content and heat treatment condition on impact toughness and creep strength of precipitation strengthened 15Cr ferritic steels were investigated in order to discuss a possibility of improvement in both mechanical properties. Both creep strength and impact toughness of the developing steels were improved drastically by solid solution treatment with water quenching. However, an addition of Ni reduced the long-term creep strength of the steels, though Ni was effective in improvement in impact toughness. It was found that water quenching suppressed formation of coarse block type particles and precipitate free zones around them, and precipitation of plate type fine particles and thermal stability of them within ferrite phase were promoted by solid solution treatment with water quenching. However, martensite phase with sparsely distributed coarse block type particles were formed in the Ni added steels, and such microstructure reduced the precipitation strengthening effect slightly. On the other hand, increase in impact values of the steel indicated no relation to volume fraction of martensite phase. It was supposed that the impact toughness of ferrite phase itself was improved by solid solution treatment and addition of Ni.

The microstructures and mechanical properties of T122 steel used for superheater tube of the boiler in a 1000 MW ultra supercritical power plant after service for 83,000h at 590℃ were investigated, and compared with data of that served for 56,000h in previous studies. The results show that compared with T122 tube sample service for 56,000h, the tensile properties at room temperature and the size of precipitated phase exhibit few differences, but the lath martensites features are apparent, and the Brinell hardness value are obviously higher. SEM and TEM experiments show that the substructure is still dominated by lath martensite. A few lath martensites recover, subgrains appear and equiaxe, and the dislocation density in grains is relatively low. A large number of second-phase particles precipitated at boundaries of original austenite grains and lath martensite phases, which are mainly M 23 C 6 and Laves phases.

High-temperature shape memory alloys (HTSMAs) are expected to be utilized for actuators in high temperature environments such as thermal power plants and jet engines. NIMS has designed TiPd shape memory alloys because high martensitic phase transformation temperature of TiPd around 570 ° C is expected to be high-temperature shape memory alloys. However, the strength of the austenite phase of TiPd is low and the perfect recovery was not obtained. Then, strengthening of TiPd by addition of alloying elements has been attempted, but the complete recovery was not obtained. Therefore, high entropy alloys (HEA, multi-component equiatomic or near equiatomic alloys) were attempted for HTSMA. The severe lattice distortion and the sluggish diffusion in HEA are expected to contribute strong solid-solution hardening of HTSMA. In this study, multicomponent alloys composed of Ti-Pd-Pt-Ni-Zr were prepared and the phase transformation, shape memory properties, and mechanical properties were investigated.

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.

Prediction of long-term creep strength is an important issue for industrial plants operated at elevated temperatures, although the creep strength of high Cr ferritic steels depends on their microstructural evolution during creep. The state of microstructure in metallic materials can be expressed as numerical values based on a concept of system free energy. In this study, in order to evaluate long term creep strength of 9Cr ferritic steel containing B, change in the system free energy during creep of the steel 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. The system free energy decreases with creep time. Change in the energy is expressed quantitatively as a numerical formula using the rate constants which depend on applied stress. On the basis of these facts, long term creep strength of the steel can be evaluated at both 948K(675°C) and 973K(700 °C).

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.

The delivery state of austenitic heat resistant steel boiler tubes is paramagnetic, such as TP304H, TP347H and S30432, the material state, however, appears obviously magnetic after long-time high-temperature service. Vibrating Sample Magnetometer (VSM) has been employed to test the magnetism difference after high-temperature service, and XRD, SEM, TEM, SAED and EDS has been adopted to observe and analyze their microstructure, phase structure and composition. The research results show that compared with the delivery state, the lath α´-Martensite and sometimes the lamellar ε-Martensite will occur in areas adjacent to grain boundaries due to martensite transformation in the microstructure of austenitic heat resistant steel boiler tube after high temperature service. There are high density dislocations tangled together in the substructure of α´-Martensite, and lamellar stacking faults arrayed orderly by a large number of dislocations in the substructure of ε-Martensite. The magnetism of α´-Martensite, its internal stress and carbides is the reason why the austenitic heat resistant steel boiler tubes appear obviously magnetic after high temperature service, and the α´-Martensite plays a major role.

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.

Hardfacing alloys are commonly used for wear- and galling-resistant surfaces for mechanical parts under high loads, such as valve seats. Cobalt-based Stellite, as well as, stainless-steel-based Norem02 and Tristelle 5183 alloys show similar microstructural features that correlate with good galling resistance. These microstructures contain hard carbides surrounded by a metastable austenite (fcc) phase that transform displacively to martensite (hcp or bcc or bct) under deformation. As a result, the transformed wear surface forms a hard layer that resists transition to a galling wear mechanism. However, at elevated temperature (350°C), the stainless steel hardfacing alloys do not show acceptable galling behavior, unlike Stellite. This effect is consistent with the loss of fcc to bcc/bct phase transformation and the increase in depth of the heavily deformed surface layer. Retention of high hardness and low depth of plastic strain in the surface tribolayer is critical for retaining galling resistance at high temperature.

A study of P92 steel's creep-rupture behavior at 625°C revealed distinct relationships between phase chemistry and stress rupture properties across two regions: high-stress/short-term (180-150 MPa for 30-454 h) and low-stress/long-term (140-110 MPa for 2881-10,122 h). Using EPMA-EDS with Multiphase Separation Method (MPSM), researchers analyzed how M 23 C 6 and Laves phase coarsening and chemistry (focusing on Cr, W, and Mo distribution) varied between these regions. This multi-region analysis established a framework for more efficient creep testing and improved extrapolation of short-term results to predict long-term rupture strengths, while providing reference phase chemistry data for future studies.

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.

Creep deformation behavior of the T122 type steels with different matrix phases such as α’ (martensite) and α’+δ (martensite and delta-ferrite) at different stress levels has been studied comparing with those of the model steels with the initial microstructures consisting of the various combination of matrices such as ferrite (α), martensite (α’) and austenite (γ), and precipitates such as MX and M 23 C 6 . The heterogeneous creep deformation is found to be pronounced at lower stress level in the steel with a dual phase matrix of α’+δ, resulting in a complex sigmoidal nature in the creep rupture life. The creep deformation process of the steel with the dual phase matrix is similar to that of the model steel with the α phase matrix which exhibits a typical heterogeneous creep deformation and the early transition to the acceleration creep at a very small creep strain. Such a heterogeneous creep deformation is much pronounced along the interfaces between the soft δ ferrite and the hard martensite (α’) phases, and has a viscous nature in creep deformation which was first identified in P91 steel. It is concluded that the homogeneous microstructure is a key for achieving the long-term creep strength in the advanced ferritic steels at elevated temperatures over 600°C.

The development of 9-12% chromium steels during the last twenty years is reviewed. The significant increases in creep strength that have been achieved by minor alloying additions of V, Nb, W, Mo, N and B are discussed and the mechanisms by which the individual elements contribute to the long-term creep strength are evaluated. The basic strengthening is provided by the martensitic transformation that allows the formation of a sub-grain structure from the martensite laths. The sub-grain boundaries are stabilized by precipitates, mainly M 23 C 6 ; within the sub-grains, fine nitride and carbonitride precipitates interact with dislocations, thereby enhancing the strength. The relative contributions of the martensitic transformation and the various precipitates to the overall creep strength of the steels are assessed. Of particular importance for the long-term creep strength is the stability of the microstructure, especially the time dependent coarsening of the various precipitates and the possible formation of additional phases, such as Laves phase (Fe 2 (W,Mo) and the Z phase (CrNbN). It is shown that microstructural changes that occur during exposure at anticipated service temperatures have a large impact on the strength and these changes must be taken into account in the derivation of long-term design stresses. Finally, the potential for achieving further increases in the creep strength of 9-12% chromium steels is discussed, especially in view of the need for higher chromium contents to ensure adequate steam oxidation resistance.

The complex nitride Z-phase, Cr(V,Nb)N, has recently been identified as a major cause for premature breakdown in creep strength of a number of new 9-12%Cr martensitic steels, especially the high Cr variants. A thermodynamic model of the Z-phase has been created based on the Thermo-Calc software. The model predicts the Z-phase to be stable in all of the new 9- 12%Cr martensitic steels, and this has generally been confirmed by experimental observations. Z-phase precipitation seems then to be a kinetic problem, and driving force calculations, using Thermo-Calc with the developed model, have been used to predict steel compositions, which could delay Z-phase precipitation. The model also predicted the existence of a new niobium free Z-phase variant, which has since been discovered in a niobium free 12CrMoV steel.

The creep resistance of 9-12% Cr steels is significantly influenced by the presence and stability of different precipitate populations. Numerous secondary phases grow, coarsen and, sometimes, dissolve again during heat treatment and service. Based on the software package MatCalc, the evolution of these precipitates during the thermal treatment of the COST 522 steel CB8 is simulated from the cooling process after cast solidification to heat treatment and service up to the aspired service life time of 100.000h. On basis of the results obtained from these simulations in combination with a newly implemented model for evaluation of the maximum threshold stress by particle strengthening, the strengthening effect of each individual precipitate phase, as well as the combined effect of all phases is evaluated - a quantification of the influence of Z-Phase formation on the long-term creep behaviour is thus made possible. This opens a wide field of application for alloy development and leads to a better understanding of the evolution of microstructural components as well as the mechanical properties of these complex alloys.

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.

Creep strength enhanced ferritic steels like T/P 91 and T/P 92 are widely used for the fabrication of pressure vessel components in the petro-chemical and thermal power industry. Today, a new generation of 9-12% Cr CSEF steels like MARBN, Save12AD, G115 and Super VM12 are entering into the market. All CSEF steels require an accurate post-weld heat treatment after welding. This paper discusses the impact of chemical composition on Ac1 as well as the transformation behavior during post-weld heat treatment in a temperature range below and above Ac1. The Ac1 temperature of weld metals with variations in chemical composition has been determined and thermodynamic calculations has been carried out. Simulations of heat treatment cycles with variations in temperature have been carried out in a quenching dilatometer. The dilatation curves have been analyzed in order to detect any phase transformation during heating or holding at post weld heat treatment. Creep rupture tests have been carried out on P91 and Super VM12 type weld metals in order to investigate the effect of sub- and intercritical post weld heat treatment on creep rupture strength.

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.

Martensitic 9Cr steels have been developed which are strengthened by boron in order to stabilize the microstructure and improve their long-term creep strength. Boron plays a key role in these steels by stabilising the martensitic laths by decreasing the coarsening rate of M 23 C 6 carbides, which act as pinning points in the microstructure. In this work two modified FB2 steel forgings are compared. Both forgings have similar compositions but one underwent an additional remelting process during manufacture. Creep tests showed that this additional processing step resulted in a significant increase in time to failure. In order to investigate the effect of the processing route on microstructural evolution during aging and creep, a range of advanced electron microscopy techniques have been used including ion beam induced secondary electron imaging and High Angle Annular Dark Field (HAADF) imaging in the Scanning Transmission Electron Microscope. These techniques have enabled the particle population characteristics of all the second phase particles (M 23 C 6 , Laves phase, BN and MX) to be quantified for materials from both forging processes. These quantitative data have enabled a better understanding of how the processing route affects the microstructural evolution of FB2 steels.