Pearlite

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

Effect of pack-chromizing temperature on microstructure and performance of AISI 5140 steel with Cr-coatings

Abstract: Two different types of Cr-coatings were prepared on the AISI 5140 steel at different temperatures via induction heating chromizing (IHC). Their phases and microstructures were characterized by X-ray diffraction (XRD), backscattering electron imaging (BSEI), energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD) techniques. The results show that the phase and microstructure of the Cr-coatings are highly dependent on the chromizing temperature. At low temperature (1000 °C), the Cr-coating is an alloy pearlite. As the temperature rises to 1300 °C, the Cr-coating becomes a Cr-Fe solid solution (Cr-Fe SS). Alloy pearlite coating has a higher density of geometrically necessary dislocations (GNDs) than the steel substrate, while the bulk Cr-Fe SS coating has a less density of GNDs than the steel substrate. The Cr-Fe SS coating formed at high temperature has a higher hardness and better corrosion resistance but poorer ductility compared with the alloy pearlite coating developed at low temperature. The formation mechanisms of these two distinct types of coatings were revealed, and the cause of the difference in dislocation density between the coating and the substrate was discussed.

Detailed study of the transformation mechanisms in ferrous TRIP aided steels

Abstract: Development of TRIP aided ferrous alloys is one answer to the demand for weight decrease in the automotive industry. The microstructure of hot rolled and cold rolled TRIP steels is quite complex and the optimisation of such steel products requires a detailed understanding of the mechanisms of phase transformation, during thermomechanical treatment as well as during mechanical testing or metal forming. We present in this paper the results obtained at Irsid concerning the study of austenite stabilisation through bainitic transformation during thermal treatment and its transformation into martensite during mechanical testing. First of all, the characterisation methods are presented. An effort has to be put on this point due to the refinement of the microstructure of TRIP steels, especially the size of austenite and martensite islands. Carbon replicas for the observation by means of transmission electron microscopy (TEM) are used to analyse the morphological features of the microstructure - nature of the constituents, size and shape - and the composition of cementite particles present in the steels. The mean value for this carbon content in retained austenite is deduced from X-ray diffraction measurements. Then the kinetics of bainitic transformation are discussed as well as cementite precipitation. The typical composition of the steel studied is 0.5 % C, 1.5 % Mn. The use of 0.5 % C steels facilitates the study of bainitic transformation by avoiding the ferrite formation usually occurring in TRIP steels. Cementite nucleation appears at the ferrite/austenite interface without any partitionning of substitutional elements. To satisfy thermodynamic equilibrium at the interface, the silicon content on the cementite side is very low and high on the austenite side. Then, carbon diffusion towards austenite is delayed and, as a consequence, cementite growth is also delayed. As the diffusion kinetics are low at 400 °C, cementite keeps this non partitioned composition, even after 3 hours holding. At 500 °C, diffusion kinetics are higher and cementite composition approaches that predicted by equilibrium. Finally, the stability of retained austenite during mechanical testing is studied. Before and after mechanical testing the morphological characteristics of the microstructure (austenite island size and elongation) are analysed by TEM replicas and image analysis. There is a high density of very small austenite islands but they represent only a small fraction of the total retained austenite. These results confirm and quantify the size effect on austenite stabilisation during deformation.

On the determining role of microstructure of niobium-microalloyed steels with differences in impact toughness

Abstract: The relationship between microstructure and impact toughness was investigated for niobium-microalloyed steels with similar yield strength. The nominal steel composition was similar and any variation in processing history was unintentional. The general microstructure of the investigated steel was similar and consisted of 85% polygonal ferrite and 15% pearlite. Despite these similarities, they exhibited variation in toughness and were classified as high- and low-toughness steels. Detailed microstructural investigation including stereological analysis and electron microscopy implied that toughness is strongly influenced by mean intercept length of polygonal ferrite and pearlite colony, and their distribution, interlamellar spacing, and degenerated pearlite.