ON THE EMBRITTLEMENT PROBLEM IN DUPLEX STAINLESS STEELS FATIGUED AT HIGH TEMPERATURE

A.F. ARMAS,S. DEGALLAIX,G. DEGALLAIX,S. HEREÑÚ,C. MARINELLI, I. ALVAREZ-ARMAS

San Nicolás, Argentina

Congreso; 3er conferencia sobre el uso del acero del IAS; 2006

IAS

Due to their excellent performance under severe conditions Duplex Stainless Steels (DSSs) have become the main competitor of the standard austenitic grade AISI 316L. The advantages of DSS are high mechanical and fatigue strength combined with high corrosion resistance. Their high chromium concentration enhances excellent resistance to pitting and crevice corrosion and to chloride stress corrosion. The two-phase presence also reduces the risk of intergranular attack; for the same reason, they are not prone to solidification cracking during welding.  Iron-chromium body-centered cubic solutions have a tendency at certain temperatures for like atoms to cluster; at temperatures below a critical value, the solid solution tends to undergo spinodal decomposition into chromium-rich and iron-rich regions. High chromium ferritic stainless steels thus become susceptible to the so-called ´475°C embrittlement´, which is caused by this clustering process. This aging effect results in a severe drop of impact toughness and ductility. DSS, because of their high Cr concentration, are prone to the 475°C embrittlement described above within its ferrite phase, so their application is frequently confined to temperatures below about 300°C. However, the operating temperature of cast DSS components for the cooling system of pressurized water reactors is about 320°C. Therefore, further insights into the mechanical behavior of DSS at higher operating temperatures are needed in order to conclude about the reliability of these steels under such conditions. Several studies were found in the literature about the aging effects on the evolution of mechanical properties of DSS. On the other hand, aging effects on the fatigue properties of DSS have been studied much less extensively and the few available reports are mainly concentrated on the evolution of the fatigue properties of aged DSS at room temperature. In accordance with these studies, fatigue tests performed in our laboratory on an aged DSS at room temperature also show strong differences with the as-received material. In all the tests, a strong increase in stress level produced by the aging treatment is evident. An initial cyclic hardening period more pronounced for the aged steel was accompanied by load drops and audible twinning noises during each cycle. This initial hardening period was followed by a pronounced softening that ended with a catastrophic failure occurring much earlier than for the as-received material. The fracture surface revealed cleavage fracture in the ferrite and ductile fracture in the austenite. To our knowledge no work exists in the literature about the cyclic behavior of thermal aged DSS at high temperature. In this sense, preliminary tests performed in our laboratory on both as-received and aged DSS show that the synergetic effect between temperature and mechanical cycling could produce beneficial effects on the lifetime of embrittled samples. After a similar hardening period for both materials, the aged steel shows a pronounced and very short softening stage that continues up to reach a cyclic behavior similar to that presented by the as-received material. The most surprising and interesting fact is that embrittled DSS evidences a comparable lifetime that the as-received steel. Moreover, this work reveals that the lifetime of samples fatigued at 400°C matches the usual Manson-Coffin curve obtained for room temperature, showing that at least for this temperature, the aging does not seem to affect the fatigue performance of the steel. This paper proposes that at high temperature the to and fro movement of dislocations can dilute at least partially the chemical composition fluctuations, promoting an erasure of the spinodal decomposition formed in aged steels. The wavy slip character of the deformation in a bcc crystal helps to the dilution of segregated regions. Additionally, in the as-received material, the synergetic effect of temperature and mechanical cycling will impede the developing of those fluctuations. Key words: duplex stainless steel, fatigue, embrittlement.