Effect of Carbon Concentration in Austenite on Cementite Morphology in Pearlite

Abstract

To understand the formation mechanism of degenerate pearlite, the effect of carbon concentration on the cementite morphology in pearlite was investigated in hypoeutectoid C-Mn steels with fully pearlite and ferrite-pearlite duplex microstructures. The carbon concentration in untransformed austenite was enriched by the precipitation of proeutectoid ferrite during isothermal holding after austenitization and could be controlled based on local equilibrium theory. Consequently, it was found that the morphology of cementite in the pearlite formed by the decomposition of the untransformed austenite continuously changed from lamellar to fine rod or spherical shapes by decreasing the carbon concentration. The critical carbon concentration for the cementite morphology transition was evaluated at approximately 0.42 mass% at 773 K. The ferrite growth rate increases with decreasing carbon concentration in austenite, which leads to non-cooperative growth between ferrite and cementite in the eutectoid reaction, resulting in the formation of degenerate pearlite. The critical carbon concentration and its temperature dependence for lamellar and degenerate pearlite transition can be estimated by a simple competition model of growth kinetics between ferrite and pearlite formations. In addition, it was found that the softening of degenerate pearlite during annealing after the decomposition was much faster than that of lamellar pearlite because the constrictions of cementite lamella do not exist for Ostwald ripening.

Effect of quenching in aqueous polyvinylpyrrolidone solutions on the microstructure and pitting corrosion resistance of AISI 1045 carbon steel

“…Notably, previous studies have reported that degenerate pearlite generally forms at slightly lower temperatures than lamellar pearlite. [ 22–24 ] In the 30%PVP specimen (Figure 2c), primary ferrite and lamellar pearlite phases were observed. The surfaces of the primary ferrite phases were smooth; however, the lamellar structure of the ferrite and cementite phases made the surfaces of pearlite appear rough.…”

“…The formation of degenerate pearlite was attributed to the fact that the transformation occurred at a lower temperature compared to that of lamellar pearlite. [22][23][24] Few martensite phases were observed in the 20%PVP specimen (see Figure S1); therefore, it is believed that the pearlite and bainite transformations were largely complete. In the 30%PVP specimen, the cooling rate was further reduced; hence, a structure comprising primary ferrite and lamellar pearlite was obtained.…”

Transformation Mechanism and Precipitation Behavior of Nanoscale Cementite in Carbon Steels during Ultrafast Cooling

“…Here, the ferrite formation rate is obtained using Equation (6) or (8), and the growth rate of pearlite can be described by Equation (11) when lamellar pearlite grows in the steady state, [ 9,28 ] as follows where , , and S are the ferrite, cementite volume fractions, and lamellar spacing in pearlite, respectively. The initial carbon concentration, C 0, indirectly affects the pearlite growth rate by affecting the , , and S , and the effect is not significant.…”

“…Here, the ferrite formation rate is obtained using Equation ( 6) or (8), and the growth rate of pearlite can be described by Equation (11) when lamellar pearlite grows in the steady state, [9,28] as follows…”

“…Yasuda et al proposed a competition model of the transformation kinetics between ferrite and pearlite formation to understand the formation mechanism of degenerate pearlite and the effect of carbon concentration on the cementite morphology. [ 9 ] They observed that the cementite morphology in the pearlite decomposed from untransformed austenite continuously changed, from a lamellar to a spherical shape, as the austenite carbon concentration decreased from 0.54% C to 0.25% C. These studies were carried out using the isothermal transformation method, which does not involve the processing of continuous cooling after rolling nor the influence of degenerate pearlite on the mechanical properties. However, the manufacturing process of the steel plate is a combination of deformation and dynamic cooling, and the study of the transformation behavior of degenerate pearlite under continuous cooling conditions is more instructive for actual production.…”