Journal of Metallurgical and Materials Engineering
Year:2011 | Volume:22 | Issue:2|Papers Count:7

Carburization is a common way to improve the performance of low carbon steel components used in industries. Accurate control of surface carbon content of carburized steels is the most important factor for the obtainment of desirable properties. A variety of methods exists for determination of surface carbon content, however, all of them are expensive and time consuming, and accordingly, all the samples cannot be tested. In this research, the capability of Eddy Current nondestructive method for determination of surface carbon content of steel components was evaluated. The Eddy Current tests were performed in order to determine the relationship between the surface carbon content and the normalized impedance, the input and output voltages. The results showed an appropriate relationship (with R2=0.82) between the surface carbon content and the normalized impedance. Moreover, the effect of test temperature on the mentioned relationship was also investigated. The relationship obtained in this way was suitable for the determination of the surface carbon content of the steel in a more accurate fashion.

Grain size distribution during grain growth in austenitic stainless steels was investigated using three different methods, namely serial sectioning analysis, 3D Monte Carlo simulation, and Abbruzzese-Lucke statistical approach. The results showed that the volumetric grain distribution can be well estimated by the log-normal and gamma distribution models for the low and high number of grains, respectively. The abnormal grain growth was taken place in 304H steel during annealing, probably due to the presence of chromium carbide on the grain boundaries within the initial microstructure. The segregation resulted from dissolution of these carbides decreased the grain boundary mobility. A time-dependent equation was obtained for the mobility of grain boundary in this steel after the estimation of chromium content in the grain boundary areas. Moreover, a method was introduced for quantitative evaluation of the error during the measurement of grain size using the metallographic cross sections.

In this investigation, the effect of gaiting system geometry during sand casting process has been studied. Sand erosion is one of the most important defects, which is caused due to the incorrect design of the gaiting system. In this study, a thin plate bench-mark was designed, and the pressure field was simulated for the cases of different slopes and cross-sections of the in-gate using Finite Volume Method (FVM). The dimensionless pressure parameter Cinp for the in-gate was calculated with the help of the simulation results. In the second stage, aluminium was molded in a sand mold with a Pyrex glass face. Then, the sand erosion phenomenon was examined using the photography technique, and through machining the upper surface of the aluminum thin plate. The results indicated that the maximum sand erosion during the casting process occurred for the in-gates having the slopes of 56 or 71 degree with a cross section experiencing a reduction of 25% from the initial cross-section.

In this research, the influence of austempering time on the microstructure, tensile and impact properties of the ultrahigh strength D6AC steel was studied. For this purpose, samples were first austenitized at 910° C for 40 minutes, and then austempering treatment at 330° C for 3.5, 5, 7.5, 10, 12.5, 15, 20 and 1440 minutes with the subsequent oil-quenching stage was conducted in order to obtain different steel microstructures. Steel microstructures were examined using the optical and scanning electron microscopes. Mechanical properties of steel samples were determined by performing the tensile and Charpy impact tests. The results showed that the yield and tensile strengths of the samples with lower bainite-martensite microstructure first increased and then decreased with increasing the austempering time when compared with the martensitic steel. However, its impact energy and ductility always increases with increasing the austempering time. Microstructural examinations showed that the onset of a peak in the strength-austempering time curve is due to the partitioning of the prior austenite grains by lower bainite and the plastic constraint which is induced on the lower bainite by the surrounding hard martensite. The fracture surface of impact samples with lower bainite-martensite microstructure showed a mixed mode including the quasi-cleavage and the ductile fracture represented by dimples. However, the fracture surface of the impact sample with martensitic microstructure exhibited the brittle intergranular fracture phenomenon.

Miserite is a kind of potassium-calcium silicate mineral, which can be synthesized by glass-ceramic route. This compound has a chain silicate structure, and is considered to be one type of the tough glass ceramic materials. In the present study, the effects of some important parameters on the formation of Miserite phase were studied, and the influence of CeO2, P2O5, Al2O3 and B2O3 additives was investigated. The results showed that CeO2 accelerated the formation of Miserite, whereas P2O5 suppressed the phase formation. However, it led to the formation of Fluor apatite, which is a bioactive phase. On the other hand, the presence of Al2O3 and B2O3 additives along with CeO2 resulted in an increase in the Miserite formation. Finally, some test samples with different amounts of Fluor were produced, and the formation of Miserite in these samples was investigated. The results of microstructural examinations showed that the produced Miserite has interlocking rod shape crystals with the diameter and length of 5 and 60 microns, respectively. This microstructure caused the strength of the compound to increase.

Carburization is a common way to improve the performance of low carbon steel components used in industries. Accurate control of surface carbon content of carburized steels is the most important factor for the obtainment of desirable properties. A variety of methods exists for determination of surface carbon content, however, all of them are expensive and time consuming, and accordingly, all the samples cannot be tested. In this research, the capability of Eddy Current nondestructive method for determination of surface carbon content of steel components was evaluated. The Eddy Current tests were performed in order to determine the relationship between the surface carbon content and the normalized impedance, the input and output voltages. The results showed an appropriate relationship (with R2=0.82) between the surface carbon content and the normalized impedance. Moreover, the effect of test temperature on the mentioned relationship was also investigated. The relationship obtained in this way was suitable for the determination of the surface carbon content of the steel in a more accurate fashion.

In present research, the graphitization behavior of the hypereutectoid steel CK100 with the initial martensitic microstructure has been studied. The optical and scanning electron microscopic observations showed that the graphite particles were formed within the ferritic matrix due to the dissociation of carbides during annealing of the martensitic steel. The graphitization behavior of the steel was examined at various temperatures using the dilatometric tests, and the results obtained from these tests were consistent with those predicted by the Johnson-Mehl-Avrami equation. Moreover, the TTT diagram corresponding to the graphitization phenomenon was obtained from the results of dilatometric tests and then, the optimum temperature of the graphitization was determined.