Papers by Keyword: Cementite

Abstract: Quenching and partitioning (Q&P) treatment was applied to a commercial low carbon martensitic stainless steel, AISI Type 410 (Fe-12Cr-0.1C). The condition of partial quenching and partitioning was optimized with consideration of the untransformed austenite fraction and stability of austenite (carbon concentration in solid solution). As a result, the amount of retained austenite could be increased up to approximately 15 vol%. Tensile testing revealed that the specimens after Q&P heat treatment exhibited lower yield stress and larger work hardening rate compared with quench-and-tempered (Q&T) specimens under the same tensile strength level, resulting in a significantly better strength-ductility balance. It was confirmed that the TRIP effect had contributed to the mechanical property.

Abstract: The evolution of microstructure with degree of deformation in a deformed wedge shaped hypereutectoid steel implement, containing 1.84 C, 0.06 Si, 0.01 S, 0.14 P, 0.11 Cu, 0.017 Zr and 0.05 Ce, has been characterized using optical and scanning electron microscopy (SEM) and electron back scattered diffraction (EBSD). Microstructures consisted of bulky carbides dispersed in a matrix of pearlitic, with grain boundary proeutectoid cementite, side plate Widmanstatten cementite and intragranular cementite were noted in region of low strain. The cast structure was broken down with increasing degree of deformation. The mechanism for the formation of spheroidization of cementite in hypereutectoid steel has been proposed. Microstructural analysis of high strain region consists of fine equiaxed ferrite grains surrounded by high angle boundaries.

Abstract: Lamellar cementite will be spheroidized in drawn pearlitic steel wire during galvanization process. To understand the evolution of the microstructure in this process, effects of isothermal time on microstructure of drawn pearlitic steel wires were investigated by using scanning electron microscope (SEM), transmission electron microscope (TEM) and DSC Technique. Experimental results showed that the lamellar cementite would transform to spheroidized cementite during the isothermal treatment. During the heating process, no endothermic or exothermic peak existed in pearlitic strand, while an obvious exothermic peak appeared in cold drawn pearlitic wire at about 380°C. It results from the spheroidization of lamellar cementite. The dislocation density was very low in pearlitic strand, but the dislocation density increased shapely after drawing. During the isothermal treatment at 450°C, the high dislocation density zone disappeared and some cementite became spheroidized. The cementite spheroidization phenomena first began at the boundary of pearlitic blocks or grains, and then in the high dislocation density zone in pearlitic blocks.

Abstract: Recent modifications in chemical composition have been applied commercially to high alloy tool steels, using different combinations of Cr, Si and Mo contents. Several reports have been published in the literature about the effects of such modifications on mechanical properties and tool performance, but only a few of these studies were concerned with the effects on secondary carbide formation. In previous papers, improvements in toughness and tempering resistance that were found in a 5% Cr tool steel (type H11 with lower Si contents) have been attributed to particular distributions of Cr-rich M₇C₃ particles. Although M₇C₃ carbides have been studied extensively in low alloy steels, some important differences have now been observed by the present authors for high alloy tool steels, especially regarding the effects of Si and Cr. The present work is concerned with the formation of Cr-rich M₇C₃ as well as Fe-rich M₃C particles in modified H11 tool steels, discussing the precipitation sequence and particle distributions developed during tempering within the martensite microstructure. By means of transmission electron microscopy, the effect of Si on M₃C cementite formation has been found to be responsible for a substantial change in the distribution of the M₇C₃ carbide phase, leading to a concentration of these particles at high energy interfaces in interlath and interpackage regions.

Abstract: In order to predict microstructures during vacuum carburizing, the model which simulates not only the carbon(C) diffusion but also growth/dissolution of cementite(θ) is required. For development of a new model we applied vacuum carburizing to low alloy steels and analyzed the distribution of C and θ by GD-OES and image analysis of microstructures. The C in retained austenite(γ) phase after carburizing was also measured by lattice constants obtained from XRD. We also simulated multi-component diffusion with γ matrix and θ layer to analyze a velocity of the moving interface. The new carburizing model was proposed based on the findings, which suggest that C in γ phase at the carburizing surface is supersaturated and corresponds to C concentration for metastable equilibrium condition to graphite. The growth and dissolution of the θ follow a square root of time with the coefficients controlled by diffusion of Si in γ and Cr in θ respectively. The required parameters such as diffusivity coefficients are obtained by the CALPHAD method. The calculated C distributions and volume fractions of θ represent the experimental results.

Abstract: The effects of annealing temperature and silicon content on mechanical properties on cold drawn pearlitic steel wires were investigated. Cold drawn steel wires, containing Si, 0.99 ~ 1.4%, were annealed at the temperature of 200 ~ 450°C with different annealing time. The variation of microstructural evolution with annealing temperature was not affected by silicon content. For steels containing high silicon content above 1.0%, the increase of silicon content did not cause the changes of peak temperature showing age hardening and age softening, except for the increase of tensile strength due to solid solution hardening.

Abstract: The effect of initial microstructure, cold reduction ratio, and annealing temperature on the spherodization rate of SK85 high carbon steel sheet was investigated. High carbon steel sheet fabricated by POSCO was soaked at 800oC for 2 hr in a box furnace and then treated at 570oC for 5 min in a salt bath furnace followed by water quenching to obtain a fine pearlite structure. Cold rolling was conducted on the sheets of fine pearlite by reduction ratios of 20, 30, and 40 % and heat treatment for spheroidization was carried out at 600 and 720oC for the various time intervals from 0.1 to 32 hrs. Area fraction of spheroidized cementite was measured with an image analyzer as a function of cold reduction ratios and duration times.

Abstract: A reduction from 1.0 to 0.3%Si has recently been shown to improve mechanical properties of H11-type hot work tool steels. The present paper shows that an important improvement in toughness can be explained by the effect of Si content on the precipitation sequence of secondary carbides during tempering after quenching. Carbide particle distributions were observed and identified by electron microscopy, allowing to relate the effect of Si on mechanical properties directly to its effect on cementite and subsequent alloy carbide formation during high temperature tempering.

Abstract: Reverse transformation has been frequently used to refine austenite grain size for refining ferrite, pearlite and martensite structures. However, kinetics and microstructure change during reverse transformation to austenite has not been examined systematically compared with the austenite decomposition reaction. Therefore, alloying effects of 1mass% Mn, Si and Cr on reverse transformation kinetics from pearlite and tempered martensite structures in Fe-0.6mass%C alloys were investigated in this study. Vickers hardness of all the specimens increases with increasing holding time at 1073K because reversely-formed austenite transforms to martensite by quenching. In the reverse transformation from pearlite structure, the kinetics of reverse transformation is hardly changed by the Mn addition while Si and Cr additions delay it. Kinetics of reverse transformation from tempered martensite structure becomes slower than from the pearlite structure in all the alloys. In particular, retarding effect by the Cr addition is most significant among those elements.

Abstract: Thermodynamic data for the substitution of silicon and manganese in cementite have been estimated using first-principles methods in order to aid the design of steels where it is necessary to control the precipitation of this phase. The need for the calculations arises from the fact that for silicon the data cannot be measured experimentally; manganese is included in the analysis to allow a comparison with its known behaviour. The calculations for Fe3C, (Fe11Si4c)C4, (Fe11Si8d)C4, (Fe11Mn4c)C4 and (Fe11Mn8d)C4 are based on the total energy all-electron full-potential linearized augmented plane-wave method within the generalized gradient approximation to density functional theory. The output includes the ground state lattice constants, atomic positions and bulk moduli. It is found that (Fe11Si4c)C4 and (Fe11Si8d)C4 have about 52 and 37 kJ greater formation energy when compared with a mole of unit cells of pure cementite, whereas the corresponding energy for (Fe11Mn4c)C4 and (Fe11Mn8d)C4 is less by about 5 kJ mol1. These results for manganese match closely with published trends and data; a similar comparison is not possible for silicon but we correctly predict that the solubility in cementite should be minimal.