COMPORTAMIENTO DE LA RELAJACION DE LA TENSION EN UN ACERO INOXIDABLE DUPLEX EN ENSAYOS DE FATIGA A TEMPERATURAS INTERMEDIAS

S. HEREÑÚ; A. F. ARMAS; I. ALVAREZ-ARMAS

San Nicolas

Conferencia; 3 era CONFERENCIA SOBRE USOS DEL ACERO DEL IAS; 2006

Instituto Argentino de Siderurgia

The austenite-ferrite microstructure of duplex stainless steels (DSSs) promotes an excellent combination of toughness and mechanical resistance as well as good corrosion properties. The progressive increase in the nitrogen content in DSSs is beneficial in improving the yield stress and corrosion resistance. Thus, nowadays DSSs may be classified in three commercial generations: a first one given by DSSs whose nitrogen content is lower than 0.10 wt%; a second one consisting of DSSs with a percentage in nitrogen between 0.10 and 0.20 wt%; and a third one or latter, usually referred to as superduplex, corresponding to materials with more than 0.20 wt% in nitrogen. Some applications of DSSs usually imply operations at temperatures above 300oC, as it is the fabrication of components of primary water coolant system in nuclear power plants, where this material is susceptible to aging embrittlement. This phenomenon is sometimes ascribed to the spinodal decomposition of the á phase. Moreover, it is well-known that single-phase austenitic and ferritic steels display manifestations of dynamic strain aging (DSA) at intermediate temperatures. Nevertheless, only a few investigations were devoted to this theme in DSSs. The real conditions that structural components of steels experience in service are not well simulated by low cycle fatigue tests with uninterrupted cycling.  Low cycle fatigue tests with “hold times” approach better to a machine operation at elevated temperatures. It is well known that many mechanical components used in gas turbines, nuclear reactors, power and chemical industries operate in situations where low cycle fatigue in combination with steady state conditions are present. Under such mechanical situation a reduction of service life is observed. Therefore, the fatigue life not only depends on temperature but also on the loading waveform imposed. When hold periods on strain are imposed, the total strain is kept constant at the peak value and the stress relaxation takes place in each hold period.  Therefore, in this work cyclic tests with “hold time” in different types of duplex stainless steels (DSSs) in the temperature range 20-500oC have been carried out. The DSSs selected for the study were DIN W. Nr 1.4460 (first generation) y SAF 2507 (third generation). Both steels exhibit an anomalous cyclic hardening rate and an inverse dependence of the peak tensile stress with strain rate at 300-400oC, typical manifestations of dynamic strain aging ( DSA). The analysis of the stress relaxation during the “hold time” provides a new manifestation of DSA not reported in published data. In this respect, the negligible relaxation of the stress at 300-400oC as a function of time could be rationalized through the DSA mechanism operating in these steels. Moreover, in the DSA temperature range the difference of cyclic hardening and stress relaxation rate suggest a preponderant role of the ferritic phase in this phenomena.