Ferrite growth from austenite in C–Mn steels during continuous cooling

Abstract

Experiments are carried out to examine the ferrite formation in four lean construction steels of low carbon and low alloy contents during continuous cooling. The variation of the heat capacity during the transformation is measured by means of differential thermal analysis, and the heat capacity is converted to the volume fraction ferrite as a function of temperature. The ferrite growth is modelled in two ways: (i) according to an interface mobility model, and (ii) according to a diffusion control model. Different assumptions are made to model the geometry of the microstructure. It is shown that the interface mobility model, applying the proper driving force and geometry, can account for the experimental ferrite growth at the early stage of transformation, but carbon diffusion has to be taken into account quantitatively when the carbon concentration profile becomes significant. The simulation according to the diffusion control model is performed using the Dictra program based on the local equilibrium hypothesis. The resulting discrepancies with the experiments are partially due to the simple spherical geometry that is assumed.     

Effect of different cooling rates on the microstructure of Cu–Al–Fe alloy

Abstract

The phases obtained in copper aluminium bronze alloy (Cu–10Al–2Fe) cast into a permanent die were investigated. The parameters examined were the preheating temperatures of the die and the graphite coating thickness. The phases α and γ₂ were detected as well as the metastable phases β′ and γ′. The intermetallics of the system Fe–Al were obtained in various stoichiometric compositions. The different cooling rates of the casting resulted in two mechanisms of transformation to α grains out of the unstable β phase, one being nucleation and growth producing needle shaped α grains and the other exhibiting a massive transformation to spherical α grains.

On a étudié les phases obtenues dans l’alliage de cuivre aluminium bronze (Cu–10Al–2Fe) coulé dans un moule permanent. Les paramètres examinés incluaient les températures de préchauffage du moule et l’épaisseur du revêtement de graphite. On a détecté les phases α et γ₂ ainsi que les phases métastables β′ et γ′. On a obtenu les composés intermétalliques du système Fe–Al sous des compositions stoechiométriques variées. Les différentes vitesses de refroidissement du moulage ont résulté en deux mécanismes de transformation en grains α à partir de la phase β instable, l’un étant la nucléation et la croissance produisant des grains α en forme d’aiguille, l’autre exhibant une transformation massive en grains α sphériques.     

Stress affected bainitic transformation in a FeCSiMn alloy

Isothermal transformation experiments are reported in which the formation of bainitic ferrite occurs under the influences of stresses below the yield strength of the austenite. The response of the transformation was monitored by simultaneously measuring the longitudinal and radial transformation strains. This enabled the dilatational and deviatoric strain components to be deconvoluted from the total transformation strain. The data have been analysed by comparison with a theoretical model for the stress-assisted growth of bainite. The results confirm that the microstructure readily responds to stresses well below the yield strength of the parent phase. Furthermore, those crystallographic variants which are favoured by the stress grow first in the sequence of transformation. Experiments where the stress just exceeds the yield strength are also reported.     

Phase Transformations in Mn–Al and Mn–Bi Magnets by Repeated Heat Treatment

Transactions of the Indian Institute of Metals - A stepped heating cycle in a tube furnace at 1000 °C for 1 h followed by holding at 400 °C for 1 h...     

FCC → FCT martensitic transformation in two-phase Mn–Cu alloys

The fcc → fct martensitic transformation in Mn–Cu alloys, which contain two isomorphous fcc phases with different manganese contents, is considered. The influence of the ratio of contents of these phases, the phase dispersity, and the state of interphase boundaries on the development of the martensitic transformation in the volume of the alloys is described. As a result of these factors, the martensitic transformation can cover the entire volume of an alloy, including particles with the manganese content that is lower than the critical content required for the martensitic transformation to occur in single-phase solid solutions, and can take place only in manganese-rich particles. In the first case, a general tetragonal structure modulated with respect to lattice parameter c forms in the volume of a two-phase alloy.     

New C–Si–Mn–Cr–V–Mo powder wires for roller surfacing

Laboratory data indicate the positive effects of introducing fluorocarbon and nickel in 25Kh5FMS steel powder wire for the surfacing of rollers. In particular, the replacement of amorphous carbon by a material containing fluorocarbon lowers the content of nonmetallic inclusions in the applied layer, while introducing nickel in the powder wire reduces the austenite grain size and ensures the formation of small carbide particles. That increases the thermal stability of the applied layer. Increase in the carbon equivalent of the 25Kh5FMS powder wire boosts the hardness of the applied metal layer.     

Kinetic Transition During Ferrite Growth Induced by Interfacial Solute Segregation in an Fe–C–Mn–Si Alloy

The kinetic transition of partitionless proeutectoid ferrite transformation from austenite, experimentally reported earlier in an Fe–C–Mn–Si alloy, is simulated incorporating interfacial segregation of carbon and alloy elements. The time-dependent diffusion equations of solutes are solved within the α/γ interface to evaluate the transient effects of solute accumulation on the migration of interface. The carbon concentration at the interface in the matrix decreased faster and the interface migration ceased, or the so-called stasis occurred, when the carbon concentration gradient in the immediate front of the interface turned to null or reversed. This can happen earlier than the partitionless-to-partitioned growth transition predicted from conventional theory in the absence of interfacial segregation, depending upon austenite grain size, i.e., the extent of soft impingement of carbon diffusion fields in the matrix in which a large carbon supersaturation remained. The subsequent transformation may be resumed accompanying the bulk partitioning of Mn (and probably Si) and/or nucleation of new ferrite crystals.

     

Influence of the packing-defect energy on the abrasive wear resistance of Fe–12Mn–1.2C steel castings cooled at different rates

The influence of the cooling rate of castings on the abrasive wear resistance is considered. It is shown that the wear resistance depends on the layer hardened by deformational twinning at the wear surface. According to the results, the maximum thickness of the hardened layer on the casting is formed at both low and high cooling rates. This is associated with decrease in the packing-defect energy, which, in turn, depends on the concentration of manganese, chromium, and silicon. At a moderate cooling rate, the degree of alloying of the austenite is such that the packing-defect energy is increased. Deformation twinning (TWIP) is hindered, and the hardened layer is of minimal thickness, if it appears at all.     

Phase Formation in the Ti–Al–C System during SHS

Russian Journal of Non-Ferrous Metals - The phase formation of Ti–Al–C powder mixtures with compositions close to MAX phases during self-propagating high-temperature synthesis (SHS) is...     

Composition of C–Si–Mn–Cr–W–V powder wire and quality of surfacing

The influence of the composition of powder wire on the properties of the applied layer on steel samples is studied in the laboratory. If amorphous graphite in 35V9Kh3SF powder wire is replaced by material containing carbon and fluorine, the porosity of the applied layer is reduced, and fewer nonmetallic inclusions (including row oxide inclusions and undeformable silicates) are present. Statistical analysis of the experimental data illustrates the influence of the carbon equivalent of the 35V9Kh3SF powder wire on the hardness of the applied layer (including the mean surface hardness and the microhardness of martensite). With increase in the carbon equivalent calculated by the Paton Institute’s formula, the hardness of the applied layer linearly increases.     

Tensile and fatigue behaviour of sinter hardened Fe–1·5Mo–2Cu–0·6C steels

Abstract

In the present work the tensile and axial fatigue behaviour of sintered hardened Fe–1·5Mo–2Cu–0·5C at three density levels (6·8, 7·0 and 7·2 g cm^–3) have been studied. The materials were tested under the as sintered condition, and after tempering at 180 and at 240°C. The results show that steels under the as sintered condition posses a high hardness but a brittle tensile deformation and fracture behaviour. Tempering at 180 and 250°C induces the disappearance of brittleness and tensile fracture is thus ductile although very localised at the necks. Fatigue strength is determined by the resistance of the materials to the internal damage evolution due to the nucleation of small cracks at the pores edges, and their coalescence into a long crack. Tempering induces an increase in the fatigue resistance. The greatest fatigue strength at 2 × 10⁶ cycles is displayed by the steel with a density of 7·2 g cm^–3 and tempered at 180°C.