The influence of high temperature ageing on the Structure of Alloy 800

The report presents investigations of the microstructure of commercial Alloy 800 after isothermal ageing at 900°C, carried out by optical microscopy and by transmission electron microscopy using replica and thin foil techniques. The high temperature ageing was characterized predominantly by precipitation of carbides on grain and twin boundaries of the austenitic matrix, as well as on dislocations within the grains. The carbides were identified, using electron microscope diffraction and X-ray phase analysis of electrolytic extractions, as (Cr, Fe)₂₃C₆, Ti (C, N) and TiC.     

Thermal expansion of chromium-rich iron-based or iron /nickel-based alloys reinforced by tantalum carbides

Six alloys reinforced by TaC carbides based on iron (ferritic) or both iron and nickel (austenitic) were studied in thermal expansion between 100 and 1200°C for two microstructural orientations. The heating, isothermal, and cooling parts of the dilatometry curves were characterized. The thermal expansion of the ferritic alloys is less important than the thermal expansion of the austenitic alloys. A compressive deformation of the matrix subjected to stresses applied by the carbides network was observed. The importance of this phenomenon seems to depend more on the matrix nature than on the microstructural orientation.     

Precipitation of vanadium carbides in 0.8% C-13% Mn-1% V austenitic steel

The precipitation mechanism of vanadium carbides in 0.8% C-13% Mn-1% V austenitic steel was investigated by transmission electron microscopy and energy dispersive X-ray spectroscopy techniques. The precipitate size and nucleation sites were observed to differ with different heat-treatment cycles applied. Carbide particles were mainly seen to precipitate on grain boundaries, in the matrix and in association with stacking faults, depending on the ageing conditions. Characteristic stacking fault contrast was observed after ageing at low temperatures and stacking fault precipitation occurred in the samples that were water quenched and aged at high temperatures; whereas samples which were not water quenched showed only local matrix precipitation. Energy dispersive X-ray spectroscopy showed that the carbide particles are rich in vanadium. The observations suggest that the nucleation of these particles on stacking faults depends on the point defect concentration and dislocation density in the matrix, prior to ageing.     

Stress and crack propagation in the surface layer of carburized stable austenitic alloys during cooling

While operating in furnaces for thermal-chemical treatment, the furnace equipment cast from stable austenitic steel is exposed to many unfavourable factors that contribute to the formation of cracks and deformations, ultimately resulting in withdrawal of this equipment from further use. This study discusses the issue of microstresses formation in the carburized, surface layer of cast steel caused, during temperature changes, by different coefficients of thermal expansion of the structural constituents – carbides and austenitic matrix, of this steel and impact of the stresses on the development of cracks running from the surface to the core of the material. To study this problem a model of carbides network at the austenite grain boundary, for the carburized Fe-Ni-Cr alloy, with an oxides layer on its surface has been developed. It was used in the simulation analysis of the influence of carbides network depth and oxides layer thickness on stresses generated on the casts surface and in the subsurface zone during rapid cooling.     

Effect of carbides on erosion resistance of 23-8-N steel

Microstructure is one of the most important parameters influencing erosion behaviour of materials. The role of carbides in the matrix is very complicated in controlling the erosion rate of the materials. Conflicting results have been reported in the literature about the effect of carbides on erosion resistance. Carbides are of great importance especially as obstacles against the penetration of erosive particles into the material surface. However, they are susceptible to cracking and causing matrix decohesion which may increase the overall erosion rate. In 23-8-N nitronic steel, carbides present in the form of bands are observed to accelerate the erosion rate. Coarse carbides cause depletion of carbon in the austenite matrix which adversely affects the strain hardening tendency thus causing deterioration in erosion resistance of the bulk material. The dissolution of carbides in the austenitic matrix after solution annealing is observed to improve the erosion resistance of 23-8-N nitronic steel.     

GH2027铁基高温合金的第二相研究

运用电子显微分析和波谱分析等方法对ＧＨ２０２７合金的第二相进行了研究，结果表明，合金晶界相主要是片状Ｍ６Ｃ和薄膜状Ｍ２３Ｃ６，经波纹图样测得其错配度约为３％，晶内Ｍ６Ｃ和Ｍ２３     

Thermal reaction of SiC films with tungsten and tungsten–rhenium alloys

Solid-state reactions between SiC films and W–xRe (x = 0, 5 and 25 at%) substrates on thermal annealing between 1673 K and 1873 K for various durations have been investigated. SiC coatings were deposited on metallic wires by hot filament chemical vapour deposition (HFCVD) from a gas mixture of tetramethylsilane (TMS) and hydrogen at 1373 K under normal pressure. The interface zones were characterized using scanning electron and optical microscopies, X-ray diffraction and electron microprobe microanalysis. All analyses reveal that SiC reacts with substrates. Various metal silicides and carbides were formed in layered reaction and the presence of these phases was confirmed by electron probe microanalysis. The effects of rhenium on the reactivity were established by the determination of growth kinetics deducted from the thicknesses of reaction zones as a function of annealing time. It has been found that an increase in the diffusion kinetics and activation energy with the quantity of rhenium in the tungsten wire.     

Improvement of Stress-rupture Life for Modified-HR6W Austenitic Stainless Steel

Stress-rupture life of HR6W austenitic stainless steel modified with B and Mg additions was measured, and the microstructures were analyzed by optical microscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy equipped with energy dispersive spectroscopy. The results indicated that the enhancement of the stress-rupture life was mainly due to the precipitation with B in the elemental form at the grain boundaries, and the improvement of the form of carbides at grain boundaries and the removal of O and S elements by addition of Mg. The micro-alloying elements have a beneficial effect on stress-rupture life of the modified-HR6W austenitic stainless steel at high temperature.     

High rate deposition of VCTiC alloy carbides by activated reactive evaporation

VC and TiC form a complete series of solid solutions. These alloy carbides have a high strength-to-weight ratio; some of the compositions exhibit extremely high high temperature strengths and thus are of technological importance. We investigated the synthesis of VCTiC alloy carbides using the activated reactive evaporation process. A VTi alloy rod was evaporated in the presence of C₂H₂. The evaporation rate, the reactive gas pressure and the deposition temperature were the experimental variables. The deposits were characterized by X-ray diffraction, microhardness, scanning electron microscopy and microprobe analysis. It was shown that VCTiC alloy carbides with the same V:Ti ratio as in the evaporant rod can be synthesized by activated reactive evaporation.     

Microstructure evolution of hot-work tool steels during tempering and definition of a kinetic law based on hardness measurements

Microstructural evolutions of the 55NiCrMoV7 steel during tempering were investigated by transmission electron microscopy, scanning electron microscopy and X-ray diffraction in order to describe the main mechanisms of softening. The softening resistance is strongly associated with evolution of obstacles to the movement of dislocations (prior austenitic grain boundary, lath boundary, secondary carbides, etc.). Only the average size of carbides was found to be influenced by tempering conditions. Moreover, a strong correlation observed between the hardness measured after tempering and the average size of carbides showing that this easy test could in this case partially characterize the state of the microstructure after tempering. Performing hardness measurements at the as-quenched, tempered and annealed states, a kinetic law of tempering based on the work of Johnson, Mehl and Avrami has been proposed. This law was validated in the case of complex tempering and for other steels and can well describe the evolution of hardness during tempering.     

Carbothermic reduction as a potential means for the direct production of Fe-WC and Fe-TaC,NbC metal-matrix composites

The carbothermic reduction of wolframite and columbite ores has been studied as a means of direct production of Fe-WC and Fe-TaC/NbC composite materials. In both cases reduction with graphite at 1600°C produces a good dispersion of carbides in an austenitic iron matrix. Gangue oxide materials appear to separate, largely as a slag phase. Some residual porosity has however been found in the product materials.     

16Cr奥氏体不锈钢晶间腐蚀的敏感性

为了研究1Cr17Mn6Ni5N奥氏体不锈钢(16Cr奥氏体不锈钢)的晶间腐蚀行为,通过光学显微镜(OM)、X射线衍射仪(XRD)和晶间腐蚀试验研究了其在不同敏化温度和冷却方式下,晶间碳化物的析出和耐晶间腐蚀性能的变化。结果表明:16Cr奥氏体不锈钢在敏化温度区间内加热时,晶界碳化物随加热温度的上升而增加,加热温度为850℃左右时晶界析出碳化物最多,主要为Cr_(23)C_6和Cr_7C_3;在敏化温度区间内相同加热温度时,水冷可显著减少其晶界碳化物的析出;16Cr奥氏体不锈钢对晶间腐蚀不敏感。     

基于应力松弛实验对服役25Cr35Ni型耐热钢的高温性能评估

以服役的25Cr35Ni型钢为对象,研究利用应力松弛实验开展高温性能评估的方法以及对持久性能的预测效果。结果表明:由于高温服役后晶界处碳化物出现网链状和奥氏体基体内二次碳化物明显粗化,25Cr35Ni型耐热钢持久性能降低。通过得到的不同温度和应力的松弛蠕变速率曲线及外推关系,结合松弛蠕变速率-断裂时间关系方程,可以实现由松弛实验及少量蠕变持久实验开展持久寿命评估。与基于高温持久实验的预测结果比较,两者吻合较好。     

Effect of Long-term Thermal Exposure on Microstructure and Stress Rupture Properties of GH3535 Superalloy

The evolution of microstructure and the stress rupture properties of long term thermally exposed GH3535 alloy have been investigated. It was found that M6 C carbides presented in the solid solution heat treated samples. During long term thermal exposure at 700 C, fine M12 C carbides precipitated preferentially at grain boundaries. These carbides coexisted with the pre-exiting M6 C. The stress rupture life of700 C/1000 h exposed sample under creep testing at 650 C/324 MPa is 93 h. It is much longer than that of the solid solution samples. No noticeable changes could be detected in both the microstructure and stress rupture lives when the samples were exposed for time longer than 1000 h M12 C carbides were found to be beneficial to the creep properties. The cracks initiated at the interface of M6 C carbides and matrix, which led to a lower creep rupture life.     

HRTEM Study on Precipitates in High Co-Ni Steel

The secondary hardening reaction is accompanied with precipitation of fine carbides in high CoNi ultrahigh strength steel. The crystal structure of the precipitating carbides is unambiguously determined by microbeam diffraction in transmission electron microscopy. It is identified that the needle-shaped carbides are M2C with a hexagonal structure. The concentration of substitutional alloying elements in the carbides quantified by energy dispersive X-ray spectroscopy (EDS) also supports the result above. The spatial structure of M2C is identical with L'3 type. Metal atoms are in a close packed hexagonal structure, the carbon atoms partly distribute with random in the octahedral interstices and the filling probability is less than 1/2. Particular attention was paid to the relationship of needle-shaped carbides/ferrite matrix at secondary hardening peak tempered at 482癈 for 5 h. Observation by high resolution transmission electron microscopy (HRTEM) confirms that carbides with black-white contrast are ful     

Sinterability of Y2O3-Al2O3 particulate stainless steel matrix composites

P/M 316L austenitic stainless steel has been reinforced with yttria and alumina particles. In order to improve the sintering behaviour of these composite materials, chromium diboride and boron nitride were added. The sinterability of the different materials has been characterised through dilatometry and sintering curves (sintered density vs. sintering temperature). A metallographic study by SEM coupled with microprobe has also been performed. Composites materials present a good densification. Chromium diboride and boron nitride react with the matrix in different manners, but they both greatly improve the sinterability of reinforced materials. The optimum sintering temperature for these composites materials is 1250 °C.     

Structures of some 17% Cr(Mn,N) stainless steels

A study of the structures of 17 wt.% Cr(Mn, N) stainless steels has been made with the help of X-ray diffraction, optical and electron microscopic techniques. It reveals that these steels become entirely austenitic when 10% manganese or more is present in a steel that contains about 0.4% nitrogen and 0.1% carbon. No precipitation other than the grain boundary carbides is observed on prolonged exposure to high temperatures. Below 10% manganese, an unstable austenite is formed which undergoes a series of transformations at different temperatures. The mode of these transformations has been studied in detail from 1400 to 700‡C. A possible mechanism of transformation is discussed.     

Distribution of strong carbide forming elements in hard facing weld metal

Abstract

Carbides and the matrix microstructure in high carbon hard facing weld metals reinforced with strong carbide forming elements were investigated by means of electron probe microanalysis (EPMA), energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). The thermodynamics and the effect on the matrix of the formation of carbides were discussed. It was revealed that Nb, Ti, and V strongly influenced the distribution and existing state of carbon, induced obvious depletion of carbon in the matrix, and precipitation of carbides. However, when only V was alloyed as the carbide forming element, carbides were scarce and distributed along the grain boundary, and the hard facing metalwas easily hardened. The hard facing alloy reinforced with Nb, V, and Ti can form finely dispersed carbides and low carbon martensite matrix.     

Metallurgical investigation into the failure of a chopper blade used for cutting of hot-rolled steel coils

In the present work, failure investigation of a chopper blade received from an integrated steel plant has been presented. Chopper blades are used in chopping machines for cutting trimmed edges of hot-rolled coils into pieces to convert them into scrap. These blades are manufactured from hot forged or rolled billets or flats of high carbon high chromium cold work tool steel. The investigation consists of visual examination, chemical analysis, microstructural analysis through optical and scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and hardness measurement. The chemical analysis confirmed the steel as equivalent to D2 grade in AISI notation. Carbide volume fraction of the broken blade was in the normal range of 10–15% as commonly observed in D2 tool steel. Microstructural examination under light and scanning electron microscopy showed non-uniform distribution of large eutectic primary carbides of irregular morphology forming strings or bands in tempered martensite matrix preferentially aligned in a specific direction. The uneven carbide arrangement in the matrix made the structure highly anisotropic and susceptible to localized stress concentration. The carbides were identified mainly as M₂₃C₆ type. Cracks were observed to initiate at the edges of the blade and propagate to the interior through clustered zones of carbides. SEM study suggests that the crack initiation was associated with decohesion of carbide particles in the cluster which culminated into final fracture by the mechanism of void coalescence and subsequent crack growth.     

Influence of alloying elements silicon and tungsten upon the formation of chromium depleted zone in as-cast refractory steels

Annealing of as-cast, high carbon, refractory alloys induces the formation of chromium-depleted zones in the austenite near the carbide-austenite interface. This depletion is a consequence of the drag of solute caused by the coarsening of the carbides. Micro-analytical investigation has been carried out to illustrate the depletion profiles and to obtain a semi-quantitative analysis of the tiny secondary precipitates. The equipment used included an electron microprobe, a scanning transmission microscope equipped with energy dispersive X-ray spectrometer (STEM/EDX)VG501, a transmission electron microscope (JEM200CX), and a scanning electron microscope (JSM-35). Extrapolation of the depletion curve gives the solute content in austenite at the interface. The values were correlated with the available thermodynamic data.