ESTUDO DA CINÉTICA DE “FRAGILIZAÇÃO DE 475°C” E SEUS EFEITOS NAS PROPRIEDADES MECÂNICAS DOS AÇOS INOXIDÁVEIS AISI 430 E SAF 2507

Abstract

The goal of this work is to evaluate the effects in the transformation kinetics and in the mechanical properties of the AISI 430 (16-18% Cr) ferritic and the SAF 2507 (24-26% Cr) superduplex stainless steels, when they undergo aging heat treatments at 475°C, thus embrittled by the formation of the alpha prime phase. This phenomenon is known as “475°C embrittlement” and it occurs in the ferrite of ferritic and duplex stainless steels, and in martensitic stainless steels aged between 300°C and 550°C. The monitoring and analysis of the kinetics of α’ formation in the AISI 430 and SAF 2507 steels was done based on mathematical fitting by means of nonlinear regression of seven sigmoidal models of general application, whose were compared to the JMAK model of diffusive phase transformations elaborated by Johnson, Mehl, Avrami and Kolmogorov. Ferritic steel samples were submitted to solubilization heat tratments in a tubular furnace at 1050°C for 30 minutes, and at 1150°C during 30 minutes for the duplex steel. Posterior aging treatments will hold at 475°C by various periods. The evolution of the “475°C embrittlement” and, indirectly, the formation of α’, were verified by the way of Vickers hardness, tensile and Charpy instrumented impact tests. Microstructural and fractographic analyses were done by optical and scanning electron microscopies (FEG-SEM); crystallographic analyses by electron backscattered diffraction (EBSD); and chemical analyses by energy dispersive spectroscopy (EDS). The superduplex steel, containing approximately 25,2% Cr, showed a faster embrittlement kinetics than the ferritic steel, with about 16,6% Cr. The sigmoidal model that better fitted to the hardness data of both steels was the Boltzmann one; which also showed greater versatility of information and a better fit and predictive ability than the JMAK model. The trends for rising aging times of the AISI 430 steel were the increase in the yield and ultimate tensile stresses, having oscillation in the breaking stress in function of the aging time, yet the literature claims that there is also a increase in the breaking stress. The change in the mechanical behavior from ductile to brittle (cleavage) of the AISI 430 steel, in function of the following of the “475°C embrittlement”, was more severe in the impact test specimens, compared to the tensile specimens.