Microstructure Optimization in δ-TRIP Steels with Al and Nb

δ-TRIP steel is a recent concept and has been developed over the last ten years aiming to combine good mechanical strength and ductility. This class of steels is multiphase and contains δ and α ferrites, as well as austenite, bainite and/or martensite. The TRIP (Transformation Induced Plasticity) effect is influenced by those phases proportion, which depends on alloying contents. This paper investigates a chemical composition that allows adequate proportion among the phases, optimizing the microstructures by means of computational methods. These microstructures are designed to contain between 10 to 50% austenite, 10 to 70% α-ferrite and 20 to 80% δ-ferrite at the eutectoid temperature. The ThermoCalc Software [1] was used to predict the fractions of the microconstituents, producing graphs describing areas of interest of microconstituents as function of alloying elements variations that leads to the desired microstructure. Results indicate that the designed volume of the phases can be found for certain proportions among the alloying elements, higher concentrations of Al and Nb combined with C allow or not the occurrence of carbides and other phases in smaller quantities.

Eutectoid Transformation in Zn-Al Alloys Solidified by Rapid Cooling

Rapid solidification represents a very attractive approach to develop new Al alloys in an economically convenient way. The lower segregation content, refined grains, higher ultimate tensile and yield strengths combined to a good ductile properties confer to these materials an interesting position also in the so critical automotive and/or aeronautical applications. The current paper presents results of an analysis concerning Zn-Al alloys with a new metastable microcrystalline structure, where Copper has been used as alloying element. With addition of elements as Ti and B modification of the microstructure has been reached. In order to study the influence of the cooling rate on the microstructure and structural transformations castings has been realized with melt spinning technique, in both steel and sand moulds. For morphological investigations optical and scanning electron microscopy has been employed. By dilatometric analysis and X-Ray diffraction technique the thermodynamic factors, the kinetic effects, phase transformations and the volume changes related to the transformations produced at the eutectoid temperature have been monitored. For the aforementioned field of applications the most favourable composition has been chosen: based on the up to date outcomes, by modifying the original alloy with some elements a quite homogeneous structure combined with good mechanical behaviour has been obtained.

Magnetic Field-Induced Granular Pearlite at Early Stages of Phase Transformation

ffects of high magnetic fields (HMF) up to 19.81T on pearlite phase transformation are studied by examination of the microstructures of a Fe-0.47C-2.3Si-3.2Mn (wt %) alloy partially isothermally processed above the eutectoid temperature. The results show that granular pearlite (GP) can be obtained at earlier transformation stages. The evolution of the granular pearlite is always accompanied by the formation of lamellar pearlite. TEM analysis reveals the existence of sub-grain boundaries within GP colonies and indicates that the nucleation of ferrite matrix in GP belongs to multiple nucleation mechanism. Most of carbides at the early stage of pearlite formation are found to precipitate at the α/γ interface--the growing front of ferrite phases, and some of coarse carbides can further develop into thin lamellar cementite.

Nitrocarburising of Low Alloyed Case Hardening Steels Applying Three Different Temperatures

Nitriding is one of the most important processes among surface technologies. The purpose of this process is to produce a wear resistant layer on the surface of components, to meet the complex requirements. Ferritic nitrocarburising was carried out at three different temperatures (520, 570 and 620°C) and with five different process times, to study the effects on process parameters . The experiments proved that in case of ferritic nitrocarburising, it is possible to produce larger diffusion zone during the same time. If the temperature is higher than the iron-nitrogen eutectoid temperature, the diffusion is deeper, but we should consider the softening of the core.

Effect of Divorced Eutectoid Transformation Temperature on Behaviour of Cementite Growth in GCr15 Steel

Cementite spheroidization of GCr15 bearing steel includes granulating and growth of cementite. The effects of divorced eutectoid transformation (DET) temperatures on growth of cementite were studied. It was found that the particles of cementite are fine and uniform when DET occurs at 690～720°С, and alloy elements mostly precipitate at different temperatures during cementite growth of GCr15 bearing steel; Cr and Si mostly precipitate in the eutectoid temperature upper limit, while Mn is in the eutectoid temperature lower limit.

Effect on Flow Stress of a Rapid Phase Transition in AISI 1045 Steel

New experimental data on AISI 1045 steel from the NIST pulse-heated Kolsky Bar Laboratory are presented. The material is shown to exhibit a nonequilibrium phase transformation at high strain rate. An interesting feature of these data is that the material has a stiffer response to compressive loading when it has been preheated to a testing temperature that is below the eutectoid temperature using pulse-heating than it does when it has been preheated using a slower heating method. On the other hand, when the material has been pulse-heated to a temperature that exceeds the eutectoid temperature prior to compressive loading on the Kolsky bar, it is shown to exhibit a significant loss of strength. A consequence of this behavior is that fixed-parameter constitutive models, such as the well-known Johnson-Cook model, cannot be used to describe this constitutive response behavior. An argument is made that the phase transition does not occur during high-speed machining operations, and suggestions are made as to how to modify the Johnson-Cook model of Jaspers and Dauzenberg for this material in order to obtain improved temperature predictions in finite-element simulations of high-speed machining processes.