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1.
In the present work, Alloy 617 was subjected to creep in the absence and presence of hot corrosion (i.e. with and without a deposit of sodium salts) under different load magnitudes at 850 °C. Additionally, a sample of the alloy was exposed to hot corrosion without applying creep. The results revealed that the creep behavior of Alloy 617 was substantially affected by hot corrosion which accelerated creep and led to a considerable reduction in the fracture ductility and creep life of the alloy. Microscopic examinations demonstrated that the strain-to-rupture decreased due mainly to the damage accumulation during tertiary creep. Various mechanisms of hot-corrosion/creep interaction were discussed that highlighted the destructive role of M23C6 precipitates in the interaction. It was explained that hot corrosion contributed to the reduction of grain-boundary cohesive-strength followed by extensive cavitation at transvers grain boundaries. 相似文献
2.
The present work was carried out to investigate the effect of long-term service exposure on microstructure and mechanical properties of a gas turbine hot gas path component, made of Alloy 617. The results showed significant service-induced microstructural changes, such as excessive grain boundary Cr-rich M23C6 carbides formation and some oxidation features in the exposed material in compare with the solution-annealed material. Also it was found that the yield strength and hardness of the alloy have increased while the ductility of the alloy has decreased. In the similar test conditions, the stress-rupture life of the exposed alloy decreased considerably compared to the solution-annealed sample, which could be attributed to the microstructural degradation, especially formation of continuous M23C6 carbides on grain boundaries. 相似文献
3.
The purpose of this study was to characterize the precipitation behaviour of AISI type 316 steel in hydrogen. The different precipitates (M23C6, M6C), the intermetallicχ-phase and the martensitic phase (α′,ε) were determined by using transmission electron microscopy (TEM) and X-ray diffraction techniques. All the specimens were sensitized at 650? C for 24 h. Some samples were carburized up to 2 wt% C. Additions of carbon content decrease the time required for sensitization. Short-term (24 h) exposure of this steel to sensitization temperature results in a complex precipitation reaction of various carbides and intermetallic phases. Hydrogen was introduced by severe cathodic charging at room temperature. This study indicates that by conventional X-ray techniques it is possible to detect those precipitates and their behaviour in a hydrogen environment. The zero shift as observed by X-ray diffraction from the carbides (M23C6, M6C) and the intermetallicχ-phase, indicates that those phases absorb far less hydrogen than the austenitic matrix. TEM studies reveal that hydrogen inducesα′ martensite at chromium-depleted grain-boundary zones, near the formation of the carbides. 相似文献
4.
Inconel X-750 is a Ni-based precipitation-hardened superalloy having large tensile and fracture strengths. In the study, X-750 wires were cold drawn to different extents. Small angle neutron scattering was employed to quantitatively measure the size and volume fraction of the γ′ phase as a function of the cold drawing ratio (DR) and aging temperature. The presence and size of γ′ precipitates were confirmed by transmission electron microscopy. The drawing ratio had an important effect on the volume fraction of the γ′ precipitates. However, the size of the precipitates was independent on the drawing ratio. The specimen with the minimum drawing ratio (DR0) produced the largest volume fraction of γ′ as compared with large drawing ratio (DR) specimens such as DR17 and DR42. The small volume fraction of the γ′ phase for a sizeable drawing ratio was associated with the large amount of nucleation sites for secondary carbides, M23C6, and the fast diffusion path, i.e., dislocation, needed to form M23C6. A Cr depletion zone around the secondary carbides raised the solubility of γ′. Therefore, the significant drawing ratio contributing to the large volume fraction of the secondary carbides decreased the volume fraction of the γ′ precipitates in Inconel X-750. 相似文献
5.
The carbide behaviour of a directionally solidified cobalt-base superalloy has been investigated after low cycle fatigue at 900°C. During fatigue, primary carbides, M7C3 and MC, decomposed sluggishly and a great amount of secondary carbide, chromium-rich M23C6 precipitated. The inhomogeneous distribution of M23C6 brought about a different dislocation substructure. In the vicinity of the primary carbides, densely-distributed fine M23C6 pinned up dislocations effectively, resulting in a uniform distribution of dislocations, while in the interior of grains, since precipitates were coarse and scarce, dislocations were arranged in a planar array and piled up in the front of the precipitates. M23C6 also acted as an obstacle deflecting fatigue crack. Primary carbides on the surface of specimens were oxidizied preferentially, causing a precipitate depletion around them. The oxidized primary carbides were crack initiation sites. The primary carbides hindered fatigue crack propagation, causing the formation of shear steps. 相似文献
6.
《材料科学技术学报》2019,35(7):1240-1249
The microstructure, precipitation and mechanical properties of Ferrium S53 steel, a secondary hardening ultrahigh-strength stainless steel with 10% Cr developed by QuesTek Innovations LLC, upon tempering were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and tensile and impact tests. Based on these results, the influence of the tempering temperature on the microstructure and properties was discussed. The results show that decomposition occurred when the retained austenite was tempered above 440 °C and that the hardening peak at 482 °C was caused by the joint strengthening of the precipitates and martensite transformation. Due to the high Cr content, the trigonal M7C3 carbide precipitated when the steel was tempered at 400 °C, and M7C3 and M2C (5–10 nm in size) coexisted when it was tempered at 482 °C. When the steel was tempered at 630 °C, M2C and M23C6 carbides precipitated, and the sizes were greater than 50 nm and 500 nm, respectively, but no M7C3 carbide formed. When the tempering temperature was above 540 °C, austenitization and large-size precipitates were the main factors affecting the strength and toughness. 相似文献
7.
High chromium (18%) white irons solidify with a substantially austenitic matrix supersaturated with chromium and carbon. The austenite is destabilized by a hightemperature heat treatment which precipitates chromium-rich secondary carbides. In the as-cast condition the eutectic M7Ca3 carbides are surrounded by a thin layer of martensite and in some instances an adjacent thicker layer of lath martensite. The initial secondary carbide precipitation occurs on sub-grain boundaries during cooling of the as-cast alloy. After a short time (0.25 h) at the destabilization temperature of 1273 K, cuboidal M23C6 precipitates within the austenite matrix with the cube-cube orientation relationship. After the normal period of 4 h at 1273 K, there is a mixture of M23C6 and M7C3 secondary carbides and the austenite is sufficiently depleted in chromium and carbon to transform substantially to martensite on cooling to room temperature. 相似文献
8.
B Sasmal 《Bulletin of Materials Science》1984,6(3):617-623
Thin foil electron microscopy studies were made on the precipitation of lamellar M23C6 during aging at 973 K and 1073 K in water-quenched specimens of two austenitic stainless steels. After the precipitation
on incoherent twin boundaries M23C6 formed on coherent twin boundaries and in the regions adjacent to incoherent twin boundaries. These precipitates showed lamellar
morphology and were aligned in a specific manner with respect to the twin boundaries. Such lamellar precipitates were absent
in the specimens which were isothermally treated at 1073 K after being transferred from the solution treatment temperature.
The lamellar morphology of M23C6 is suggested to be developed by the influence of residual specific stress field around twin boundaries resulted from quenching. 相似文献
9.
A microscopic study was carried out to examine the microstructural and compositional features of an Alloy 600/182 weldment. The grain boundaries of the Alloy 600 base metal consisted of low angle boundaries (4.4%), coincidence site lattice boundaries including twins (46.6%), and random high angle grain boundaries (49.0%). The precipitates in the matrix of Alloy 600 were identified as Cr7C3, irrespective of the intergranular and intragranular ones. The microstructure of the Alloy 182 weld metal consisted of cellular dendrites in the grains epitaxially solidified from the heat affected zone, and the Mn concentration increased periodically across the dendritic interfaces. Contrary to the Alloy 600 base metal, most of the grain boundaries of the Alloy 182 weld metal were low angle and random high angle grain boundaries, with a negligible fraction of coincidence site lattice boundaries. Tiny Cr-rich M23C6 and Nb carbides were distributed on the grain boundaries in the weld metal, and a Cr-depletion zone was formed due to the Cr carbide precipitation. Precipitates of (Nb,Ti)C, Al2O3 type and TiO2 type oxides were found in the Alloy 182 weld metal. 相似文献
10.
Microstructure and mechanical properties of Incoloy 800 superalloy before (as-received) and after 15 years service exposure were evaluated. The metallurgical variations such as formation and growth of the secondary precipitates, phase transformation of titanium carbide to Cr23C6 + Ni16Ti6Si7 and decomposition of primary carbides were characterized during the long-term service of Alloy 800. It was shown that some of chromium and iron elements were moved from solid solution to the carbides during the service exposure. It was found that due to the formation of precipitates during service, the strength and hardness of Alloy 800 were improved, while the ductility and toughness were reduced. 相似文献
11.
In this research, the phenomenon of strain-induced precipitation of M23 (M = Cr, Fe, Mo) C6 precipitates in 347 austenite stainless steel was systematically studied. During annealing at 650 °C, the experimental results exhibited significantly earlier formation of M23C6 precipitates in the plastically strained specimen, than in the no-strain specimen. The various microstructural characterizations showed that the accelerated formation of M23C6 precipitates occurred around deformation bands that were generated by deformation twinning. Extensive investigation suggested that the strain-induced precipitation of M23C6 is attributed to the formation of strain-induced α′ martensite during plastic deformation near deformation bands. 相似文献
12.
《Materials Characterization》2007,58(1):72-77
The carbides present after creep testing a 9Cr–1Mo steel at 566 °C over a range of stress levels giving rupture times of up to 7300 h have been characterized and identified using a transmission electron microscopy, energy-dispersive X-ray spectroscopy and electron diffraction. The initial carbide precipitates present were M7C3, (NbV)C and VC and it was determined that M6C carbide precipitates were present in all specimens after elevated temperature exposure for greater than approximately 1700 h. No precipitation of M23C6 was detected. The evolutionary sequence from the initially present carbides during high temperature exposure involved the formation of the stable M6C carbide directly, without the intermediate formation of M23C6, as is reported to occur in other Cr–Mo steels. 相似文献
13.
The main existing intergranular phase in the alloy observed is thick lamellar and small plate-like M6C. Owing to the presence of M6C precipitates, intergranular M23C6 nucleates at these locations and coherently precipitates to form a thin lamellar and large-sized filmy single crystal. The misfit of M23C6 with M6C obtained from the moiré pattern is 3%. Intragranular M6C and M23C6 twin along their own planes (111), and the resulting twins keep a coherent relationship with austenite. The misfit of VC precipitated along dislocations with the matrix is smaller than that of dispersed VC with the matrix. Intragranular Laves phase together with intragranular M6C and VC strengthens the alloy grains. 相似文献
14.
Techniques for quantitative estimation of various secodary phases present in austenitic stainless steels were developed using electrochemical and chemical separation procedures. The secondary phases like delta-ferrite, sigma-phase and M23C6 carbide were obtained in pure forms by the electrochemical technique and their anodic polarization behaviours were studied in a sulphuric acid-ammonium thiocyanate medium. The dissolution behaviour of the secondary phases was studied in bromine-methanol solution and the procedures for the separation of delta ferrite-carbide and sigma-carbide mixtures and the quantitative estimation of delta-ferrite, sigma- and M23C6 carbide-phases were standardized. 相似文献
15.
《Materials Science & Technology》2013,29(10):1221-1225
AbstractModern (9–12)%Cr steels designated for power plants with higher steam parameters show a pronounced time dependent change in microstructure during purely thermal or creep stress exposure at temperatures around 600°C that determines their properties in service. In addition to other microstructural parameters, the state of the precipitates plays an important role for microstructural stability which is a prerequisite for long term creep strength. In order to support theoretical studies on precipitation growth and coarsening with more reliable experimental data, in this study a method is introduced for the quantification of the state of precipitates in (9–12)%Cr steels which is based on the application of different TEM methods. Therefore up to about 33 000 h aged specimens of the martensitic cast alloy G-X12CrMoWVNbN-10-1-1 were investigated by means of electron microscopy. The application of energy filtering transmission electron microscopy (EFTEM) allowed a reliable quantitative distinction between M23C6, VN, and Laves phases to establish the size distribution of these precipitates in different specimens conditions. 相似文献
16.
H. Hildebrand 《Materialwissenschaft und Werkstofftechnik》2000,31(10):899-903
Semicoherent precipitates in a Fe‐Cr‐C alloy Precipitation processes in ferromagnetic materials can be recorded very well by measuring the sensitive coercive field strength. It should be tested, whether also semicoherent precipitates have a sufficient clear interaction with Bloch‐walls. For this purpose the mild‐magnetic alloy X1FeCr25 served. To carry out the evidence sensitively, a method based on differences between HCt (heat‐treated state at T = 600…︁700°C) – HC0 (quenched state from high temperature) = Δ HC was used. A quantitative record of the amount of precipitates (as particle size) is possible by a decomposition parameter Δ HC/Δ t. Plate‐like β′‐precipitates with planes {100}∥{100} in the α‐Fe solid solution have been proved by transmission electron microscopic investigations; this is the preparation state for the transition into the stable fcc phase M23C6. As a result, the quantitative electron microscopic proof of the β′‐phase can be supported by magnetic measurements, qualitatively and quantitatively. The estimated values of the activation energy for the process in the 1st maximum of precipitation in X1FeCr25 are higher than for the stable phases as the orthorhombic M3C or the cubic complex M6C in other steels and give a hint to the difficult processes related to nucleation as to the transition into M23C6. 相似文献
17.
P. Parameswaran M. Vijayalakshmi P. Shankar V. S. Raghunathan 《Journal of Materials Science》1993,28(20):5426-5434
Transmission electron microscopic studies aimed at elucidating the effect of carbon level on the tempering behaviour of 2 1/4 Cr 1 Mo steels have been carried out. Specimens with two different carbon levels (0.06% and 0.11 %) were cooled in flowing argon gas (AC) from an austenitization temperature of 1323 K and tempered at 823, 923 and 1023 K for times ranging from 2 to 50 h. The tempering behaviour at these temperatures for the two carbon levels is found to differ in the nature of secondary hardening at lower temperatures, variation in the time to peak hardness and the saturation level of hardness at long tempering times. Based on a detailed study, using analytical electron microscopy, on the morphology, crystallography and microchemistry of secondary phases, the factors governing the observed variations in tempering behaviour are related to the difference in the dissolution rate of bainite, nucleation of acicular M2C carbides and transformation rate of primary carbides into secondary alloy carbides. The carbides which promote softening were identified as M7C3, M23C6 and M6C, whereas hardening is mainly imparted by M2C. 相似文献
18.
R. A. Varin 《Journal of Materials Science》1979,14(4):811-816
From a comparison of the isothermal precipitation curve for M23C6 grain-boundary precipitates in an austenitic stainless steel and of the spreading times of extrinsic grain-boundary dislocations (EGBDs), it has been shown that M23C6 precipitates cannot directly nucleate on EGBD lines lying on random high-angle grain boundaries. This process can occur only on coherent and incoherent twin and other special boundaries. 相似文献
19.
《Materials Science & Technology》2013,29(11):1239-1246
AbstractThere is clear evidence that creep damage in power plant steels is associated with grain boundary precipitates. These particles provide favourable nucleation sites for creep damage such as grain boundary cavities and microcracks. Monte Carlo based grain boundary precipitation kinetics is combined with continuum creep damage mechanics (CDM) to model both the microstructural evolution and creep behaviour in power plant metals. It is found that grain boundary precipitates, such as M23C6 in most Cr containing ferritic steels, are harmful to the creep properties of the material, in line with experimental observations. It is also found that to improve the creep behaviour of the material, means should be found either to increase the proportion of MX type particles, such as VN, or to decrease or remove the larger grain boundary precipitates, such as M23C6. Hafnium has been ion implanted into thin foils of a 9 wt-%Cr ferritic steel to study the effect of hafnium on the grain boundary precipitation kinetics. It is found that the implantation of hafnium to the steel completely prohibits the formation of the common grain boundary M23C6 particles. Instead, two new types of precipitates are formed. One is hafnium carbide, which is an MX type precipitate, and is very small in size and has a much higher volume fraction as compared with the volume fraction of VN in conventional power plant ferritic steels. The other is Cr- and V-rich nitride of formula M2N. CDM modelling shows that implantation of hafnium can markedly improve the creep property of the material. In addition, the replacement of M23C6 with hafnium carbide increases the concentration of Cr in the matrix and is expected to improve the intergranular corrosion resistance of the material. 相似文献
20.
Zhang Yongtao Han Haibo Miao Lede Zhang Hanqian Li Jinfu 《Materials Characterization》2009,60(9):953-956
It is usually difficult to quantitatively determine the mass fraction of each type of precipitates in steels using transmission electron microscopy and traditional X-ray powder diffraction analysis methods. In this paper the Rietveld full-pattern fitting algorithm was employed to calculate the relative mass fractions of the precipitates in 2.25Cr–1Mo–0.25V steel. The results suggest that the fractions of MC, M7C3 and M23C6 carbides were evaluated precisely and relatively quickly. In addition, it was found that the fine MC phase dissolved into the matrix with prolonged tempering. 相似文献