The Micro-Structure of Pearlite formed by the Isothermal Transformation of Carbon Steel

The pearlite in carbon steel appears in different forms according to the conditions cooling from γ-phase. Using three carbon steels, I have obtained different micro-structures by the isothermal transformation in several temperatures quenched from several maximum heating temperatures (below 950°) and its results are shown in charts. The forms obtained are the globular pearlite, lamellar pearlite, sorbite, sorbitic globular, sorbitic lamellar, dotted lamellar, Atols (named by Osmond) and the mixture of these forms. In this study is discussed the formation of these different structures. In hypereutectoid steel, the upper limit of the maximum heating temperature, from which the globular pearlite is formed on cooling, reaches to the temperature immediately below the Acm point, and the lower limit of the transformation temperature is 710° in pearlite steel previously globulized and 700° in those of the other structures. The uniform globular pearlite is obtained by transformation at 710° on cooling from 825° The form of pearlite is remarkably influenced by the form of the cementite in steel before heating. In 0.76% C steel, globular pearlite is formed at the maximum heating temperatures below 825° in globular pearlite steel, but does it not occur in lamellar pearlite steel. In 0.40% C steel, fine globular cementite is formed from globular pearlite and martensite steels heated, but in those of the other structures the coarse globular cementite is formed.