ON THE CYCLIC BEHAVIOR OF AGED AND UNAGED SUPER DUPLEX STAINLESS STEEL AT DIFFERENT TEMPERATURES

M. C. MARINELLI; A. F. ARMAS; I. ALVAREZ-ARMAS; S. HEREÑÚ

Gijón

Conferencia; XVIII International Colloquium MECHANICAL FATIGUE OF METALS; 2016

Duplex stainless steels are a category of alloys that combine high strength with excellent corrosion resistance. The superior properties of the duplex stainless steels come primarily from approximately equivalent amounts of austenite and ferrite phases. However, these types of steels are intrinsically subject to embrittlement when exposed to temperatures above 300◦C because of solid-state reactions within the ferrite phase. The cyclic behavior of type UNS S32750 super duplex stainless steel in unaged (as-received) and aged (100 hours at 475ºC) conditions was investigated between room temperature and 500ºC. Previous results show completely different cyclic softening-hardening behaviors of this steel under both thermal conditions when it is cycled in this temperature range. The following figures, plotted both with same scales, show such differences.1101001000100004006008001000 20ºC 200ºC 300ºC 400ºC 475ºC 500ºCUnaged Samplesp= 0.3%Stress Amplitude (MPa)Number of Cycles, N1101001000100004006008001000 20ºC 200ºC 300ºC 400ºC 475ºC 500ºCAged Samplesp= 0.3%Stress Amplitude (MPa)Number of Cycles, NBesides TEM observations of specimens after interrupted and completed cyclic tests, the corresponding cyclic flow stress components, the friction, and the back stresses, have been analyzed in order to explain the main physical mechanisms inducing hardening or softening behaviors. The cyclic mechanical behavior, under both thermal conditions, is mainly governed by the back stress. Only in unaged samples at 20ºC the cyclic softening is attributed to the friction stress. The pronounced cyclic hardening observed in unaged samples between 200 and 475ºC is associated with the activation of dynamic strain aging (DSA) in the ferritic phase. On the other hand, during the entire fatigue life aged samples only show cyclic softening behavior attributed to the cyclic softening of the back stress. It is observed that, after a brief period of smooth hardening, this stress softens up to the failure of the sample independent of the temperature of the test. The secondary cyclic hardening observed at 475ºC for both thermal conditions was attributed to DSA but in the austenitic phase. The aim of this work is also to correlate the observed dislocation structure in tested aged specimens with the crystallographic structure of spinodal decomposition.