Search Results for X-ray diffractometer

The A-USC technology is still under development due to limited number of materials complying with the requirements of high creep strength and high performance in highly aggressive corrosion environments. Development of power plant in much higher temperatures than A-USC is currently impossible due to the materials limitation. Currently, nickel-based superalloys besides advanced austenitic steels are the viable candidates for some of the A-USC components in the boiler, turbine, and piping systems due to higher strength and improved corrosion resistance than standard ferritic or austenitic stainless steels. The paper, presents the study performed at 800 °C for 3000 hours on 3 advanced austenitic steels; 309S, 310S and HR3C with higher than 20 Cr wt% content and 4 Ni-based alloys including: two solid-solution strengthened alloys (Haynes 230), 617 alloy and two (γ’) gamma - prime strengthened materials (263 alloy and Haynes 282). The high temperature oxidation tests were performed in water to steam close loop system, the samples were investigated analytically prior and after exposures using Scanning Electron Microscopy (SEM) coupled with Energy Dispersive Spectrometry (EDS), and X-Ray Diffractometer (XRD). Mass change data have been examined every 250 hours.

A new ferritic steel branded as Thor 115 has been developed to enhance high-temperature resistance. The steel design combines an improved oxidation resistance with long-term microstructural stability. The new alloy was extensively tested to assess the high-temperature time- dependent mechanical behavior (creep). The main strengthening mechanism is precipitation hardening by finely dispersed carbide (M 23 C 6 ) and nitride phases (MX). Information on the evolution of secondary phases and time-temperature-precipitation behavior of the alloy, essential to ensure long-term stability, was obtained by scanning transmission electron microscopy with energy dispersive spectroscopy, and by X-ray powder diffraction on specimens aged up to 50,000 hours. The material behavior was also tested in service conditions, to validate the laboratory results: Thor 115 tubing was installed in a HRSG power plant, directly exposed to turbine flue gasses. Tubing samples were progressively extracted, analyzed and compared with laboratory specimens in similar condition. This research shows the performance of Thor 115 regarding steam oxidation and microstructure evolution up to 25,000 exposure hours in the field. So far, no oxide microstructure difference is found between the laboratory and on field tubing: in both cases, the oxide structure is magnetite/hematite and Cr-spinel layers and the oxide thickness values lay within the same scatter band. The evolution of precipitates in the new alloy confirms the retention of the strengthening by secondary phases, even after long-term exposure at high temperature. The deleterious conversion of nitrides into Z phase is shown to be in line with, or even slower than that of the comparable ASME grade 91 steel.

A paste, which contains Pt or Pt-xIr (x = 0-30 at%) alloy nano-powder was sprayed on some Ni-based single crystal superalloys. Then the annealing diffusion treatment at 1100 °C for 1 h in flowing Ar atmosphere was conducted to develop Pt and Pt-Ir diffusion coatings. Cyclic oxidation tests were carried out at 1150 °C in still air in order to investigate the thermal stability and oxidation behavior of the coatings and they were compared with electroplated diffusion coatings. It was found that Ir can retard the formation of voids in both the coatings and substrates. In addition, by replacing the electroplating method to the paste coating method, the crack problem due to the brittle feature of electroplated Pt-Ir coatings could be solved. Therefore, the Pt-Ir diffusion coating prepared by the paste- coating method is promising as the bond-coat material due to suppression of voids, cracks and stable Al 2 O 3 on the surface. The Pt-Ir paste diffusion coatings introduced above have several further advantages: they are easy to recoat, cause less damage to substrates, and offer comparable oxidation resistance. Thus, the method can be applicable to the remanufacturing of blades, which may extend the life of components. The future aspect of the paste coating will also be discussed.

Prior to utilizing new advanced materials in coal power plants, a large number of experimental testing is required. Test procedures are needed in specialized high temperature laboratories with state of the art facilities and precise, accurate analytical equipment capable of performing tests at a variety of temperatures and environments. In this study, the results of a unique technique involving salt spray testing at high temperatures are presented. The Haynes 282 gamma – prime (γ’) strengthened alloy fabricated by means of three different manufacturing processes: HAYNES 282 WROUGHT alloy, Haynes 282-SINT alloy, and finally Haynes 282-CAST alloy have been tested. The materials have been exposed to a salt spray corrosion atmosphere using 1% NaCl - 1% Na 2 SO 4 . Post exposure investigations have included SEM, EDS and XRD examinations. The test using salt spray of 1% NaCl - 1% Na 2 SO 4 water solution at 550 °C for 500 hours indicted no influence on the corrosion products formation, where Cr 2 O 3 has been developed in all three alloys, whereas NiO has been found only in Haynes 282-CAST material. On the other hand, it has been found that the fabrication process of HAYNES 282 alloy strongly influences the corrosion products formation under the high temperature exposures.

Cr-based alloys have potential as heat-resistant materials due to the higher melting point and lower density of Cr. Although oxidation and nitridation at high temperatures are one of the drawbacks of Cr and Cr-based alloys, addition of Si has been reported to enhance the oxidation and nitridation resistance. This study focuses on the microstructure and mechanical properties in the Cr-Si binary alloys with the Cr ss + Cr 3 Si two-phase structure. The Cr-16at.%Si alloy showed an eutectic microstructure and hypoeutectic alloys with the lower Si composition exhibited a combination of the primary Cr ss and the Cr ss /Cr 3 Si eutectic microstructure. Compression tests at elevated temperatures were conducted for the hypoeutectic and the eutectic alloys in vacuum environment. Among the investigated alloys, the Cr-13at.%Si hypoeutectic alloy including the Cr 3 Si phase of about 40% was found to show the highest 0.2% proof stress of 526 MPa at 1000 °C. Its specific strength is 78.1 Nm/g which is roughly twice as high as that of Ni-based Mar-M247 alloy. It was also confirmed that the 0.2% proof stress at 1000 °C depends on not only the volume fraction of the Cr 3 Si phase, but also the morphology of the Cr ss + Cr 3 Si two-phase microstructure.

NiAl precipitates with the B2 structure are known to be effective in increasing the strength of ferritic heat-resistant steels. The strengthening mechanism by the NiAl precipitates was examined using Fe-21Al-2Ni and Fe-23Al-6Ni (at%) single crystals. As a result, the difference in primary slip system between the bcc matrix and the NiAl precipitates is responsible for strong hardening. The B2-NiAl phase was precipitated in the bcc matrix satisfying the cube-on-cube orientation relationship with small misfit strain. The primary slip direction of the bcc matrix and the NiAl precipitates are <111> and <001>, respectively. However, in the ferritic alloys, the NiAl precipitates were cut by paired 1/2<111> dislocations in the bcc matrix, resulting in the hardening. The size and volume fraction of the NiAl precipitates strongly influenced the strength. The stress increase by the NiAl precipitates was also discussed quantitatively based on the precipitation hardening theory. Based on the experimental results obtained by the single crystal study, we developed Fe-Al-Ni-Cr-Mo ferritic heat-resistant alloy containing the NiAl precipitates. The alloy exhibited excellent creep properties at 923 K.

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.

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.

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.

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.

In this work, the effects of phosphorus addition on the creep properties and microstructural changes of wrought γ’-strengthened Ni-based superalloys (Haynes 282) were investigated, focusing on the effects of carbides precipitation. In an alloy with a phosphorus content of 8 ppm, precipitation of M 23 C 6 carbides was observed in both grain boundaries and the grain interior prior to the creep tests. Grain boundary coverage by carbide increased with phosphorus content up to approximately 30 ppm. On the other hand, the amount of M 23 C 6 in the grain interior decreased with phosphorus content. The results of the creep tests revealed the relationship between the time to rupture and the grain boundary coverage by carbides. The microstructure of the crept specimens showed the existence of misorientation at the vicinity of grain boundaries without carbides, as demonstrated via electron backscattered diffraction (EBSD) analysis. These results suggest that the observed improvement in the time to rupture is due to a grain-boundary precipitation strengthening mechanism caused by grain boundary carbides and that phosphorus content affects the precipitation behavior of M 23 C 6 carbides in the grain interior and grain boundaries. These behaviors were different between alloys with the single addition of phosphorus and alloys with the multiple addition of phosphorus and niobium.

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.

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.

Modern polycrystalline Ni-base superalloys for advanced gas turbine engines have been a key component that has contributed to technological advances in propulsion and power generation. As advanced turbine engine designs are beginning to necessitate the use of materials with temperature and strength capabilities beyond those exhibited by existing materials, new alloying concepts are required to replace conventional Ni-base superalloys with conventional γ-γ’ microstructures. The phase stability of various high Nb content Ni-base superalloys exhibiting γ-γ’-δ -η microstructures have been the subject of a number of recent investigations due to their promising physical and mechanical properties at elevated temperatures. Although high overall alloying levels of Nb, Ta and Ti are desirable for promoting high temperature strength in γ-γ’ Ni-base superalloys, excessive levels of these elements induce the formation of δ and η phases. The morphology, formation, and composition of precipitate phases in a number of experimental alloys spanning a broad range of compositions were explored to devise compositional relationships that can be used to predict the microstructural phase stability and facilitate the design of Ni-base superalloys.

Precipitation of Z-phase, Cr(V,Nb)N, is known to negatively affect creep properties of 9-12%Cr steels for power plant applications as it dissolves finely distributed MX particles, (V,Nb)N, especially in high Cr steels. As the Z-phase precipitates slowly as large particles, this causes a net drop in precipitation strengthening. Two model alloys containing 9 and 12%Cr, but otherwise having similar composition, were produced in order to quantify the difference in Z-phase precipitation speed at different Cr levels. The nitride precipitation behavior was followed at different temperatures using TEM and XRD, allowing for a quantification of the Z-phase precipitation. The Z-phase was found to precipitate 20-50 times faster in the 12%Cr steel compared to 9%Cr steel in the temperature range 600- 650°C. The transformation of MX into Z-phase was followed in a Ta containing alloy without V or Nb. In this alloy the Z-phase precipitates very quickly, and thus appears as finely distributed particles which have the same strengthening effect as MX particles. Investigations using atomic resolution microscopy showed how Cr diffuses from the matrix into the TaN MX particles and gradually transforms them both chemically and crystallographically into Z-phase CrTaN particles.