Pearlite

About: Pearlite is a research topic. Over the lifetime, 6028 publications have been published within this topic receiving 65695 citations.

Transformation Kinetics of Bainite during Isothermal Holding and Continuous Cooling

Abstract: The isothermal and continuous cooling transformation kinetics of bainite were studied using an automatic quench dilatometer. It was found that the overall isothermal bainite transformation kinetics were well expressed by Johnson-Mehl type equation and that the transformation rate decreased with increase in the austenite grain size. The analysis of transformation kinetics and the optical microscope observations suggested that the nucleation site of bainite was both on the grain boundaries and inside the austenite grain. This effect of austenite grain size on the rate of isothermal transformation to bainite was found to be smaller than that to pearlite. Comparison of the observed cooling transformation kinetics with that predicted from the isothermal kinetic data showed that the transformed fractions were additive in bainite transformation. This result might be understood if it were assumed that the bainite transformation proceeds with repeated nucleation and growth of basic subunits, each attaining the limit size rapidly.

An attempt to establish the variables that most directly influence the austenite formation process in steels

Abstract: The aim of this work is to evaluate the influence of heating rate and initial microstructure on the anisothermal formation of austenite. In this sense, the start (Ac 1 ) and finish (Ac 3 ) temperatures of austenite formation have been determined on dilatometric curves obtained at various heating rates in steels with ferrite and/or pearlite initial microstructures. As it was expected, Ac 1 and Ac 3 temperatures rises linearly with heating rate, except for steels with a pure ferrite initial microstructure where the Ac 1 temperature is almost insensitive to heating rate over the range studied. Experimental results in steels with a pearlite and ferrite-pearlite initial microstructures also show that the elevation of the critical temperatures with heating rate is quite sensitive to the morphology of pearlite. It seems that the higher the heating rate is, the stronger the influence of morphology on the critical temperatures are. This experimental study and the knowledge of the mechanisms that control the austenite formation process have allowed to establish the variables that most directly influence this reaction in steels with pearlite and ferrite-pearlite initial microstructures. Those are the heating rate and the two parameters that characterise the morphology of pearlite, the mean true interlamellar spacing and the edge length of the pearlite colonies interface in pearlitic steels, together with the volume fraction of pearlite and the mean free distance of pearlite in ferrite plus pearlite initial microstructures. Likewise, two equations have been proposed for the determination of the start (Ac 1 ) and (Ac 3 ) finish temperatures of austenite formation as a function of those variables.

Effects of niobium addition on microstructure and tensile behavior of as-cast ductile iron

Abstract: The effects of niobium addition up to 0.11 wt% on the microstructure and tensile properties of as-cast ductile iron (ACDI) were investigated. Metallographic analyses by both optical microscopy (OM) and scanning electron microscopy (SEM) indicated that niobium (Nb) promoted the formation of pearlite, reduced pearlite lamellar spacing and decreased the extent of graphitization taking place in the Nb-alloyed ACDI. The nodularity and nodule counts of graphite changed insignificantly when the Nb content was less than 0.08 wt% in the ACDI. The analysis of precipitates by transmission electron microscopy (TEM) revealed that nano and micro sized (Nb, Ti)C carbides acted as nucleation site for graphites, and promoted the formation of large graphite nodules with low roundnesses as Nb content rose above 0.08 wt%. The results of tensile testing showed that the yield strength, ultimate tensile strength and elongation of the ACDI with 0.08 wt% Nb increased by 12.1%, 11.2% and 14.3% over those of the Nb-free ACDI, respectively. The optimum values of the yield strength, tensile strength and elongation of the Nb-alloyed ACDI were found to be 418 MPa, 746.0 MPa and 8.0%, respectively, at the Nb content of 0.08 wt%. The high strain hardening rates of the Nb-containing ACDIs implied that they were capable of spontaneously strengthening itself increasingly to a large extent, in response to a slight plastic deformation after yielding.

Phase change materials as quenching media for heat treatment of 42CrMo4 steels

Abstract: In the present work, paraffin phase change material is used as quenchant for the heat treatment of 42CrMo4 alloy and compared with water, air, and CuO doped paraffin. The samples were prepared based on ASTM E 8M-98 standard for tensile test and then heated up to 830 °C, kept for 4 h in an electric resistance furnace and then quenched in the mentioned media. Elastic modulus, yield strength, ultimate tensile strength, elongation, and modulus of toughness were determined according to the obtained stress-strain curves. Moreover, the hardness and microstructural evolution were investigated after the heat treatment at different media. The samples quenched in paraffin and CuO-doped paraffin are higher in ultimate tensile strength (1439 and 1306 MPa, respectively) than those quenched in water (1190 MPa) and air (1010 MPa). The highest hardness, with a value of HV 552, belonged to the sample quenched in CuO-doped paraffin. The microstructural studies revealed that the non-tempered steel had a ferrite/pearlite microstructure, while by quenching in water, paraffin and CuO-doped paraffin, ferrite/martensite microstructures were achieved. It is also observed that using the air as quenchant resulted in a three-phase bainite/martensite/ferrite microstructure.

Aging behavior and delamination in cold drawn and post-deformation annealed hyper-eutectoid steel wires

Abstract: The effects of drawing strains and post-deformation annealing conditions on the aging behavior and the occurrence of delamination in cold drawn hyper-eutectoid steel wires were studied. At low annealing temperatures the increased tensile strength for a short annealing time would be attributed to age hardening, which came from the decomposition of the unstable cementite in deformed pearlite. The decrease of tensile strength at the high annealing temperature was due to age softening, which would be attributed to the decreased carbon content in lamellar ferrite through the spheroidization or the re-precipitation of cementite, and recovery or recrystallization of ferrite. The extent for the occurrence of delamination during annealing expanded to the high temperature region with the increased amount of deformation in steel wires. At high annealing temperatures, the decreased carbon content dissolved in lamellar ferrite due to age softening would result in the decrease of tensile strength and suppress the occurrence of delamination. The total magnitude of carbon content dissolved in lamellar ferrite either by the partial dissolution of lamellar cementite during wire drawing or by the partial decomposition of lamellar cementite during post-deformation annealing would control the occurrence of delamination in cold drawn steel wires.