TEM CHARACTERIZATION OF THE MICROSTUCTURES OF FATIGUED ROLLED DUPLEX STAINLESS STEEL

M.G. MOSCATO; M.F. GIORDANA; I. ALVAREZ-ARMAS

Rosario

Congreso; 10th Inter-American Congress of Electron Microscopy 2009, CIASEM 2009/1st Congress of the Argentine Society of Microscopy, SAMIC 2009; 2009

CIASEM

The studied duplex stainless steel was the standard type UNS S32750 DSS in the form of hot and cold-rolled plate. The main purpose of this work is to evaluate the formation of fatigue microcracks in the present steel due to surface microstructure formed during rolling. Strain-controlled cyclic tests were carried out at room temperature at total strain ranges of 0.8% and 1.0% in flat specimens cut parallel (L), perpendicular (T) and at 45º (D) to the rolling direction (RD). The dislocation structures in the bulk were analyzed by transmission electron microscopy for the different specimen orientations. Additionally, the microstructure developed near the free surface of the three type of specimens was studied by surface thin foils technique [Int J Fatigue 29 (2007) 758]. It is widely accepted that the slip in the ferritic matrix has a wavy character. According to this, it has been found that the dislocation structures developed in the bulk correspond to vein structures in the ferrite and planar arranges of dislocations in the austenite. Nevertheless, in the grains near the free surface, the observed microstructure is quite different. The results show martensite plates in the austenitic phase while the ferritic phase shows a characteristic microstructure that consists of planar bands oriented in the trace of the {110} and {112} planes. This last structure on the {112} plane trace resembles the structure of fine microtwins. In general, it was found that the underlying microstructure beneath the free surface of the DSS plate due to rolling process plays an advantage role regarding the fatigue life of the hot and cold-rolled S32750 plate.