Papers by Keyword: Pearlite

Abstract: Currently, heavy-haul and passenger rails are joined by a welding process, which can be either flash-butt welding or thermite. The joining process has increased the overall rails strength, but the welding parameters optimization is tricky and must be performed and studied to improve the weld quality. Heavy-haul rails are high carbon steels, containing alloying elements and as such, the weld presents a series of difficulties. On one side, martensite should be avoided during the cooling step, while on the other, the HAZ should be minimized as it is known to be prone to localized wear and rolling contact fatigue. Finite element simulations were performed to map the weld cooling rates and corresponding heat-affected zone (HAZ) width. CCT curves of rail steels were determined using dilatometry for different austenitizing temperatures. Comparing the simulations with the CCT data, processing windows able to prevent martensite formation were determined, usually corresponding to a critical cooling rate of 2 °C/s. The correlation with the simulations showed that the shorter the HAZ length, the greater the chance of martensite formation due to the higher cooling rate. The methodology developed and presented in this paper can be used for simulations considering phase transformations or determining the microstructure formed from different thermal welding cycles, depending on the distance from the heat source during the welding process.

Abstract: In this study, an attempt has been made to produce ductile iron or spheroidal graphite iron and to study its important properties with a view to reduce the import of machinery parts made of ductile iron. Locally available compressor scrap (i.e. the compressor cylinder) which is made from grey cast iron was used to produce ductile iron using a crucible furnace that is fired by oil. Also, recycling of the grey cast iron to ductile iron was investigated and its effect on the microstructure, chemical composition, mechanical properties and chip shape. The mechanical and structural characteristics of the ductile irons that alloyed by the supplement of Ni, Mo, Mg, and Cr were studied In this study, four kilograms of the scrap were charged into an oil-fired crucible furnace. The scrap was heated to 1400°C with using a temperature controller to monitor the temperature with an inserted thermocouple. For desulphurization, the mixture of 3 wt.% burnt lime with 1 wt.% fluorspar of scrap weight was added to the molten at 1400°C by direct tapping into the molten. Then, 2.75 wt.% nickel element, 0.75 wt.% ferromolybdenum and 0.5wt.% ferromanganese of the scrap weight were added. Also, 1.25 wt.% spheroidizing alloy (FeSiMg9) and 1wt.% inoculant alloy of scrap weight were used to treat the iron melt at 1450°C. The analysis of scrap sample and product sample was done to determine their chemical composition, tensile strength, impact strength, hardness, and microstructure. The scrap and the as-cast product analysis determine its chemical composition, tensile, impact, hardness and microstructure. The microstructures revealed that the scrap contains flake graphite and the as-cast product contains spheroid graphite. An increase of the ultimate tensile stress (537.17 MPa), elongation (10%), hardness value (480.4 HB) and impact value (11.21 J) was observed for the alloyed ductile iron as compared with the mechanical properties of grey cast iron scrap, including (247.75 MPa), (6%), (400.3 HB) and (5.66 J), respectively. One of the important conclusions is the plunge container manufactured, and that was used according to the plunging technique followed in this investigation proved successful

Abstract: Comprehensive metallographic studies of steel forgings with different machinability by cutting with an edge tool were also completed. Structural features and properties of steel were revealed, having adversely influence on tool life and the process of chip formation during cutting. Metal Science criteria have been given for assessing the manufacturability of steel at machining operations. Microstructures of steel with satisfactory and unsatisfactory machinability are presented. The technological parameters of heat treatment of steel 18HGR have been established, causing a show of banding of ferrite-pearlite structure. The thermokinetic diagram shows an area of development of the segregation banding structure. An important role in assessing the manufacturability of steels is shown of the microhardness of individual structural components and the difference in values between them. The best results in machinability by cutting are observed when the microhardness of pearlite is not more than 350 HV, ferrite is not more than 210 HV and the difference in microhardness between these components is not more than 80 HV.

Abstract: The article deals with the problems of withstanding harsh temperatures by steel and iron. The authors of the work discuss iron denser high-temperature of γ modification and maximums and minimums of impact. In addition, the article analyses the transformations of iron and anomalies of properties: peak of heat capacity, acceleration of diffusion, etc. The authors take into account the consensus on the causes of polymorphism and the theoretical model of ferromagnetism. Besides, there is a consideration of "transformation" in interaction between Fe atoms that produce anomalies of steel properties. It is necessary to note the transformation detected by anomalies of any properties including mechanical. In the presented work the authors have made an attempt to prove transformations in iron at ~650 °C on the basis of extreme values of hardness and microhardness, metallographic structure, parameters of fine structure, precipitation resistance force depending on temperature. Therefore, the analysis of literature sources on physical and mechanical properties of iron and its derivatives has been made.

Abstract: Cold drawn wires were produced by drawing the pearlitic wire rod (5.5 mm diameter). Cold drawing involved multiple stages to a final drawing strain of ≈ 2.5. The cold drawing alters the pearlite morphology. During the wire drawing, the change in morphology is location dependent. This will create the gradient in stain and strain mode between the surface and the center. This led to have a strain partition among ferrite and cementite phases. The strain partitioning plays a major role in the final tensile and torsional performance of the cod drawn wire. The present work dealt with the experimental and their numerical simulations of stress gradients and the role of pearlite morphology on tensile and torsional properties of the pearlitic steel wire.

Abstract: We have determined different phase fractions from microscopy images using semi-automated image analysis fitting technique, and in addition we have classified each phase according to its hardness. The distribution of grayscale pixels of different phases is first characterised separately for each phase, which are sampled from the microscope image. After this the distributions of the separate phases are fitted to give the corresponding distribution of the whole image. The microhardness measurement provides reliability on the classification of the different phases to ferrite, bainite or martensite. In addition to describing the applied techniques in detail, we present the results obtained from the analysis for one steel subjected to isothermal holding experiments at different temperatures.

Abstract: The reverse transformation of austenite from pearlite was observed in-situ by using a sophisticated EBSD system. Quantitative information on the nucleation sites and orientation selection of the austenite was obtained. Initially, the nucleation sites were restricted to high-angle grain boundary (HAB) and all the austenite orientations were selected by Kurdjumov-Sachs (K-S) orientation relationship. Latterly, the constraints were relaxed as new nucleation sites were involved. The γ growth was preferentially into the pearlite grains without K-S relation. Nucleation and growth at HAB edge may contribute to γ coarse grains, whereas nucleation at pre-existing γ boundary is important to grain refinement of γ structure.

Abstract: The main characteristics of metal long products quality include mechanical properties, depending on the microstructure state. The key indicator for evaluation the micro-structure of high carbon steel can be considered the 1st grain grade lamellar pearlite B_fp. For the currently used pearlite dispersion method, according to GOST 8233-56, it is characterized by the subjectivity of the view fields choice for the microstructure evaluation, that reduces the quality of the obtained results. The study purpose was to improve the method of high-carbon steel wire rod microstructure estimation, to reduce the error magnitude in determining the 1st grain grade lamellar pearlite. The experiments were carried out on samples of high-carbon steel wire rod with a carbon content of 0,58 – 0,77 %. The pearlite dispersion was evaluated in 27 view fields, located on mutually perpendicular diagonals of the sample cross section. The study results showed the possibility of reducing the error in determining the estimated value of the high-carbon steel wire rod microstructure pearlite dispersion. Microstructure evaluation in the five view fields should be carried out, taking into account the weight coefficients, determined by the ratio of the zones length, occupied by pearlite with a certain percentage of the 1st grade grain pearlite to the wire rod radius. The proposed method of the microstructure evaluation increases assessment accuracy, without complication of its implementation process.

Abstract: The effect of the characteristics of austenite interface with ferrite on the pearlite transformation behaviour after intercritical annealing was investigated. Most austenite grains were situated mainly on ferrite grain boundaries and had the Kurdjumv-Sachs (K-S) or near K-S relationship to one of the neighbor ferrite grains before pearlite transformation. The pearlite transformation started mainly from the austenite grain boundary faced to ferrite. The pearlite transformation showed stasis. This indicates that some austenite is stabilized thermally against the pearlite transformation. The fraction of austenite having only the K-S or near K-S interface to neighbor ferrite grains was correspond to the fraction of austenite grains which does not include pearlite. The pearlite transformation was difficult to start from austenite interface having the K-S relationship to ferrite since the interface between austenite grains and ferrite grains was stabilized energetically in the case of their interface having the K-S relationship.

Abstract: A model of isothermal pearlite reaction kinetics in high-carbon ternary Fe–Cr–C alloys (cast irons) containing (Cr, Fe)₇C₃ carbide phase is presented. The model is based on traditional Avrami approach completed with a simple type temperature dependence of K coefficient. The model parameters for individual alloys were determined from F. Maratray and R. Usseglio-Nanot’s experimental data covering composition range from 2.1 to 4.3 %C and from 12 to 26 %Cr. The dependence of parameters (Avrami exponent n, activation energy U and time-scale constant C) on gamma phase composition by the end of austenitization (calculated with account for kinetics of carbide dissolution) is determined. The model thus permits to calculate the isothermal pearlite Ccurve (TTT diagram) for an alloy of arbitrary composition after given austenitization regime. The comparison of calculation results to experimental data shows their sufficient correlation.