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[en] This paper studies the microstructural and tribological differences of layers formed during the ferritic nitrocarburizing of AISI 4340 alloy steel and AISI 347 stabilized stainless steel. The samples were exposed to different times of immersion in a nitrocarburizing bath (60, 75, 90, 105 and 120 min) at 580 °C. Subsequently, they were subjected to an oxidation process at 480 °C in order to form a Fe3O4 layer. Surface microstructural studies were carried out by SEM-EDS and x-ray diffraction (XRD). Wear and friction coefficient of nitrocarburized samples and non-treated samples were studied by pin-on-disk test. The results show two well-differentiated zones in AISI 4340 steel: an outer oxides layer, a white layer or compound layer and a diffusion zone. However, the compound layer was not found in AISI 347 steel. In both steels, the specific wear coefficient (k) of nitrocarburated samples is about thirty times lower than the reference samples.
[es]Este trabajo estudia las diferencias microestructurales y tribológicas de las capas formadas durante la nitrocarburación ferrítica del acero aleado AISI 4340 y la nitrocarburación austenítica del acero inoxidable estabilizado AISI 347. Las muestras se sometieron a distintos tiempos de inmersión en un baño de nitrocarburación (60, 75, 90, 105 y 120 min) a 580 ºC. Posteriormente se sometieron a un proceso de oxidación a 480 ºC para formar una capa de Fe3O4. Los estudios de la microestructura de la capa nitrocarburada se realizaron por microscopía electrónica de barrido (SEM), espectroscopía de dispersión de energías (EDS) y difracción de rayos X (XRD). Se estudió el desgaste y el coeficiente de fricción de las muestras nitrocarburadas y las muestras no tratadas mediante el ensayo pink-on disk. Los resultados muestran tres zonas bien diferenciadas en el acero AISI 4340: una capa de óxidos externa, una capa blanca o de combinación y zona de difusión. Sin embargo, no se detectó la presencia de la capa de combinación en el acero AISI 347. En ambos aceros, el coeficiente específico de desgaste (k) de las muestras nitrocarburadas fue aproximadamente treinta veces menor que el de las muestras de referencia.
[en] The influence of carburization on the reoxidation of commercial iron ore pellets was studied. The pellets were H2-reduced up to 88% and then carburized in H2/CO/CO2 gas mixture, for 0, 12, 18, 25 and 32 min. at 700, 800, 900 and 1000 degree centigrade. After the reduction-carburization process, the pellets were reoxidized at 250, 300, 350 and 400 degree centigrade. It was found out that the higher reduction temperature the lower was the reoxidation degree. This effect was stronger at the reoxidation temperatures of 250 and 300 degree centigrade. The presence of iron carbide (Fe3 C) was more notorious when the reduction was carried out at 700 and 800 degree centigrade, and the carburization time was increased. Finally, it was observed that the higher the reoxidation temperature, the higher was the reoxidation degree. (Author) 15 refs
[en] Creep behaviors of solution treated, solution and precipitation heat treated, and solution, precipitation and carburization treated Nimonic 86 alloys were compared. Solution treatments were carried out at 1150 deg C for 2 hours and precipitation treatments were performed at 900 deg C for 200 and 1000 hours. Carburization treatments were performed during precipitation treatment using carburizing atmosphere. Creep tests were carried out up to 1200 hours at 850 and 950 deg C. Precipitation heat treatment decreased the creep resistance compared to the solution treatment, and the carburization treatment significantly increased the creep resistance, especially at 950 deg C. The results were explained by the relative amounts, size, and distribution of carbide precipitates. (Author)
[en] The high temperature corrosion behavior of two 9Cr and 12Cr ferritic-martensitic steel grades was studied under CO2 pressures varying from 1 to 250 bar for exposure times up to 8000 h. No 'breakaway' oxidation was observed. 9Cr steel grades suffered from fast parabolic uniform oxidation and fast carburization. Increasing CO2 pressure had very little effect on the oxidation rate but increased the carburization rate. The corrosion behavior of both 12Cr steel grades differed and might be influenced by gas composition, minor elements or surface finish. A corrosion mechanism coupling oxidation and carburization is proposed. (authors)
[en] Some 30 years ago, Yamasaki and Hirota have studied the carburization of the inner technological surfaces (ITS) of tubular coils used in ethylene cracking furnace heated by thermal agent supplied from a nuclear power plant. These coils were made up of centrifugally molded tubular elements of 25/20 type chromium/nickel steel with about 0.40% C. For a wall thickness of 10 mm, the depth of the carburizated zone reached 5 to 7 mm from the ITS and the carbon content (at the ITS level) raised from 0.40% up to 7 to 8%. Subsequently, Neitsch and Rubrecht, by studying the behaviour of the austenitic tubular coils operating in pyrolysis furnaces, established that due to utilization of thick-walled tubing the investment costs for furnaces amounted up to 10 to 15% of the global cost of the technological installation. These authors evidenced also other two physical phenomena accompanying this disastrous phenomenon of carburization, namely the increased steel hardening and the increased magnetic permeability. The carburization of austenitic steel has been studied by metallographic methods. The following results were obtained: 1. Due to maintenance of high temperature of tubing operating regime a precipitation of carbon results both in bulk of austenitic grains and in the form of intergranular continuous chains; 2. In the ITS zone a strong orientation of the crystals was observed which could be explained by intense carbon diffusion; 3. One can assume that in the microstructure of austenitic steels ferritic zones occurred which transform the paramagnetic steel into ferromagnetic steel following the local alteration of Cr/Ni ratio due to the different diffusion of the two elements, a phenomenon enhanced by the carbon diffusion. One should mention that nickel is gammagenous (γ) while chromium is alphagenous (α). This research was completed by realization of a device for determination of the carburation degree in austenitic steels called CARBOTEST and recorded in Romania as Patent No. 97.941
[en] The objective of the present works is to investigate the effect of reaustenitizing heat treatment on abrasive wear resistance of carburized SAE 8822H steel. For this investigation the specimens were gas carburized and reaustenitized to various heat treating parameters so as to obtain different amount of microstructural constituents in the case. Optical scanning electron electron microscopy and X-ray diffraction techniques were used to investigate the quantities of martensite retained austenite and carbides content in case structure. Experimental results indicates that double reaustenitize treatment results file microstructural constituents with highest abrasive wear resistance of carburized SAE 8822H steel. (author)
[en] High temperature corrosion in processing units of the chemical, petrochemical, and thermoelectric carbochemical is of high interest, due to the costs generated by sudden failures caused by deterioration in the mechanical properties of metals, being the carburization of the metallic matrix the most important corrosion mechanism resulting in loss of ductility and creep resistance. In this research a 9Cr-1Mo modified steel P91 was exposed to simulated atmospheres oxidant-fuel gas mixtures simultaneous in equilibrium of CO, CO2, CH4, H2O and H2, similar to those produced in the refining industry, at temperatures between 550 and 750 degree centigrade. The microstructural evolution of the metal matrix was analyzed, also the growth, evolution and behavior of oxide layers and carburized, establishing the mechanism of deterioration of the material and the extent of damage for times exceeding 700 h of exposure. (Author) 21 refs.
[en] In this study, a series of fatigue tests were conducted on C45 steel specimens with local surface cracks. The effects of carburizing treatment with different case depths were discussed under cyclic tensile and torsional loading. Moreover, the surface crack extension paths and crack propagation rates were discussed. In addition, the effects of additional static torsion under cyclic tensile loading and additional static tension under cyclic torsional loading were analyzed, and comparisons were made between the results of the carburized materials and those of the uncarburized materials. Some conclusions show that carburization can effectively retard crack extension and improve fatigue life. However, the improvement provided by the case depth on the fatigue life is limited to a certain range, and extensive case depths might decrease the fatigue strengths of materials. For the studied material, carburization had a more direct influence on the crack growth under cyclic tensile loading than cyclic torsional loading.