Austenitization of hypoeutectoid steel starting from of spheroidized structure and of normalized structure.

Author(s)

Wilson de Sousa Paulino

• Document type: Master's Dissertation
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Escola Politécnica (EP/BC)
• Defense date: 2002-12-16

Examining board members

Ivan Gilberto Sandoval Falleiros; Amilton Sinatora; André Paulo Tschiptschin

• Advisor: Ivan Gilberto Sandoval Falleiros

Abstract

The influence of the previous state, spheroidized or normalized, on the austenite formation in a steel hypoeutectoid was studied by dila tometry in various temperatures. The process was interrupted for growing times. The obtained structures were analyzed by optic microscopy (including quantitative metallography) and by scanning electronic microscopy, which, with the dilatometric measurements, helped to identify the reaction mechanisms. In all of the studied conditions, the normalized structure has faster kinetics of austenitization than the spheroidized. The austenitization of the spheroidized material (ferrite plus dispersed carbides) begins with the formation of austenite grains close to the carbides and with the growth of the austenite grains consuming the ferrite and dissolving carbides, being this last one the slowest process; during growth the relative quantity of carbon in the austenite is heterogeneous, tending to homogenize with the time. The hardenability of the austenite is growing with the time. The austenitization of the normalized material (pro-eutectoid ferrite, plus fine pearlite) begins with austenite formation in the pearlitic colonies, without detected heterogeneity of carbon distribution, when consuming the pro-eutectoid ferrite areas the austenite show heterogeneity of carbon distribution; the final stage of the process is homogenization of the austenite. The hardenability of the austenite decreases during the process of ferrite transformation, before the homogenization. The global kinetics, measured by dilatometry and by quantitative metallographic, is presented in the form of isothermal curves (TTT) of austenitization for the two previous studied structures. (AU)