CHARACTERIZATION OF LOW-ALLOY STEELS BY MEANS OF DIFFERENT TECHNIQUES

Abstract

Two low carbon steels with different concentrations of carbon, one with manganese as its principal alloy component, were studied using electrical resistivity, Differential Scanning Calorimetry (DSC), Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) as experimental techniques to study the kinetic of phase changes   occurring during a non-isothermal heating in normalized and austenized microstructures. The measurements of electrical resistivity were used to test the austenization process at 1100ºC, showing how sensitive this  method  is  to  the  concentration  of  solute  in  the  samples.  The  resistivity  being  slightly  larger  at  the  onset,  possibly  due  to  the precipitation of carbides, and becoming slightly erratic and diminished as the aging time increases. For DSC, measurements were taken at  different  heating  rates,  which  demonstrated  different  exothermic  and  endothermic  transformations,  indicative  of  the  initial microstructure  of  the  samples.  Austenization  introduces  a new endothermic  reaction, not present in  the normalized  samples,  very localized and basically associated with the dissolution of martensite. Optical and scanning electron microscopy allowed us to visualize the granular state of the samples and follow the sequence of evolution of phases present in these steels, the normalized state showing a matrix  rich  in  ferrite  and  pearlite,  and  all  the  treated  samples  showing  a  matrix  rich  in  martensite,  product  of  rapid  tempering.  The activation energies, for the endothermic reactions corresponding to the allotropic transformations α → α + γ and α + γ →γ., calculated by isoconversión methods are in the neighborhood of 22.2 y 26.7 Kcal/mol for the first and second endothermic processes, respectively.