Papers by Keyword: Eutectoid Steel

Abstract: Equal Channel Angular (ECAP) pressing has been showed as an attractive route to produce fine and ultrafine-grained metals and alloys with high strength and fracture toughness. ECAP is a simple process for applying severe plastic deformation (SPD) to metals that can be done with common laboratory equipments (mechanical tests machines) and an adequate die. In the present work, an eutectoid steel was processed by ECAP in a 120° die. Mechanical behavior of samples deformed by ECAP was compared to the same material processed by rolling. The hardness level obtained after a single pass of ECAP was comparable to an 84% reduction by rolling in a single pass. The hardness level obtained after 1 ECAP pass on a patented steel was higher than 4 ECAP passes on the same steel without patenting. The metallographic analysis showed intense alterations on the microstructure by the ECAP processing.

Abstract: Three different Fe-C alloys were prepared in vacuum using the arc melting method: hypereutectoid (1.4% C), eutectoid (0.76% C) and hypoeutectoid (0.4% C). Unlike commercial steels, which they always contain Mn and other impurities, our samples were prepared by using high quality powders (99.999 wt.%). The samples were heat-treated and then observed with optical and scanning electronic microscopy (Zeiss EVO MA10). Selected samples were tested by microidentation (microhardness test). After isothermal transformation at 350 °C fine bainite nanostructures were observed in the hypoeutectoid sample, the mean size of which was found to be 50 nm. With the eutectoid sample, following different heat treatment procedures different micro-and nanostructures were measured: pearlite lamellar spacing, spheroidized cementite particles, and martensite needles, whose mean size is 145 nm, 290 nm and 200 nm respectively. The nanostructure of hypereutectoid sample after isothermal transformation at 650 °C, reveals the eutectoid and proeutectoid cementite lamellas with a mean spacing of 390 nm. X-ray diffraction pattern of eutectoid sample indicated the existence of cementite (Fe₃C) content which is also confirmed by carbon mapping of pearlite colonies performed with Energy Dispersive X-ray Spectroscopy. The Vickers hardness of the samples compares well to the one of corresponding commercial steels.

Abstract: The aim of this research is to investigate the effect of Cr and Al (strong ferrite formers) on the strain-induced γ-to-pearlite transformation in eutectoid steels. The microstructure evolution during the hot deformation of three eutectoid steel grades was investigated using hot torsion testing. More specifically, the steels were deformed to strain levels varying from ε = 0,5 to ε = 1,5 at their specific Ar₁ temperature. Hot deformation of the undercooled austenite leads to strain-induced γ-to-pearlite transformation and to the almost instantaneous spheroidization of the formed carbides. The corresponding microstructures consist of submicronic cementite particles and ferritic grains that are 1-5 μm in size. It is shown that 1,5% Cr addition and 0,5% Al addition increase the equilibrium transformation temperature but slower significantly the kinetics of the strain-induced transformation and consequently reduce the kinetics of cementite spheroidization and of ferrite recrystallization.

Abstract: Differential geometry and toplogy-based three-dimensional (3D) analysis was conducted to understand pearlite spheroidization mechanism in an eutectoid steel. Morphological change during aging below A1 tempearture was examined in terms of Gaussian(K)/mean curvatures(H), genus and Euler characteristics based on 3D images.The holes presentnaturally grown cementite lamella caused shape instability andinduced shape evolution of the lamellar structure during spheroidization. 3D visualization demonstrated that the intrinsic holes played an important role in the initiation and development of pearlitespheroidization. The hole coalescence and expansion causedthe breakup of large cementite lamellae into several long narrow ribbons. H-K plot actually suggested that the number of thses holes decreased with increasing aging period. In addition, small cementite particles and narrow rod cementite decreased during aging. These microstractural evolutions were discussed from the view point of ferrite/cementite interfacial energy.

Abstract: In this study the effect of thermo-mechanical controlled rolling and continuous cooling of different grades of steel wire rod (e.g. high-carbon for cold drawing applications, medium-carbon micro-alloyed for cold forming) has been analysed through the application of a set of integrated mathematical models simulating hot rolling and continuous cooling, and a laboratory work involving hot rolling simulation on a pilot plant and heat treatments on a laboratory scale. The samples have been characterised by means of instrumented tensile tests, metallographic analyses including determination of pearlite interlamellar spacing, and controlled compression tests. The results show that: - The mechanical strength of high-carbon steel is essentially related to interlamellar pearlite spacing, and can be enhanced through the control of continuous cooling. Improvements in cold drawability can be obtained by means of prior austenitic grain size (PAGS) reduction, through the application of thermo-mechanically controlled hot deformation processes. - In the case of medium-carbon micro-alloyed steels for cold forming, the reduced PAGS achieved by means of thermo-mechanically controlled process reflects on a closer control of as-rolled mechanical properties, avoiding hardness hot spots asking for annealing treatments before cold forming. Moreover, the finer ferrite grain size could affect the forces needed during forming at the same deformation level.

Abstract: Eutectoid steels present a wide range of interesting mechanical properties (high strength, wear resistance, ductility and toughness) and could be a cheaper alternative to high strength low-alloyed steels (HSLA) in applications where weldability is not a critical requirement. The mechanical properties of pearlite are mainly dictated by the interlamellar spacing and the spheroidization of cementite. In this work, the spheroidization kinetics during annealing of a fully pearlitic steel produced in an electric arc furnace (EAF) is investigated. More specifically, the influence of a prior cold deformation and of the interlamellar spacing is studied using image analysis and hardness tests. It is shown that spheroidization is faster in fine pearlite than in coarse pearlite. Prior cold deformation strongly accelerates the spheroidization kinetics in fine and coarse pearlite. The tensile properties corresponding to different pearlite microstructure were measured and are compared to the hardness evolution during annealing.

Abstract: Diamond-like carbon (DLC) is an amorphous hard carbon, which has very high hardness, high resistivity, and dielectric optical properties. Economically and technologically attractive properties have drawn almost unparalleled interest towards the coatings. Eutectoid steel is a kind of material that has been widely used in shafts and various kinds of industrial components. Three kinds of fatigue specimens with different DLC conditions were used in this study. Fatigue test had been performed to investigate the effects of DLC on fatigue properties of eutectoid steel. The fractography was analyzed by a scanning electron microscope (SEM), and surface hardness was also evaluated. The fatigue limits of the DLC coated specimens did notincrease after DLC process, though the compressive residual stress which produced by DLC process can prevent fatigue fracture. According to the results of fatigue test, the optimal DLC method for improving the fatigue properties of eutectoid steel is determined and the relationship between fatigue limits and coating bias are obtained.

Abstract: Investigations of microstructural differences of tempered eutectoid steel strips are presented. The constituent phases – fine mixture of ferrite and cementite and a small amount of retained austenite – considerably affect the mechanical properties, especially the resistance to high cycle fatigue. The amount of retained austenite was determined by XRD and EBSD analysis. Thermoelectric power (TEP) measurements were performed for 270 different batches of C75S type eutectoid steel strips. TEP measurement shows characteristically the fine microstructural differences. The TEP of the investigated samples varied between 5650 and 7030 nV/°C. The amount of retained austenite can be significantly higher at the surface (20 %) than at the internal part (0-5 %). The sensitivity of XRD analysis was reduced because of the presence of coarse cementite particles. Using Cu anode, the measurements were more successful than in case of using Co anode. The EBSD analysis showed that also samples immeasurable with XRD contain retained austenite, but its detectability with XRD analysis declines with the coarsening of cementite.

Abstract: Cold-drawing is employed to fabricate wires and rods, which are mainly used as structural reinforcements in construction as well as in the tyre industry. As a consequence of processing, a residual stress profile is developed. In this paper, residual stress profiles are measured by neutron diffraction in cold-drawn pearlitic steel rods subjected to different deformations (true strain from 0.3 to 1.7). The results show that the residual stress profile produced by cold-drawing is similar in all the samples, irrespective of the degree of deformation.

Abstract: Cooperative growth of pearlite is simulated for eutectoid steel using the multi-phase field method. This allows to take into account diffusion of carbon not only in γ phase, but also in α phase. The lamellar spacing and growth velocity are estimated for different undercoolings and compared with experimental results from literature and theoretical results from analytical models. It is predicted, that diffusion in ferrite and growth of cementite from the ferrite increase the kinetics of pearlitic transformation by a factor of four as compared to growth from austenite only, which is assumed by the classical Zener-Hillert model. Further on the effect of stress due to inhomogeneous carbon distribution in austenite and due to transformation strain is discussed shortly.