FATIGUE OF A LOW ACTIVATION STEEL

M AVALOS; I. ALVAREZ ARMAS; A.F. ARMAS

San Nicolás. Buenos Aires. Argentina

Conferencia; 2nd Conferencia sobre usos del acero; 2004

Instituto Argentino de Siderurgia

Structural metallic materials subjected to cyclic strain situations may either soften or harden. These phenomena illustrate the inadequacy of using tensile properties in fatigue based design. The initial high strength of hardened steels can be seriously compromised even after a low number of loading cycles, reducing their original strength and load carrying capabilities.   Normalized and tempered 9-12% Cr ferritic/martensitic steels exhibit attractive tensile strength properties up to approximately 6000C. In addition they have the ability to resist the effects of high doses of irradiation by the complete or partial substitution of critical alloying elements by others with relative short radioactive decay times. Much of this performance is based on high dislocation densities in a fine, well dispersed precipitate distribution. From the literature [1] it is known that 9Cr-1Mo steels subjected to the standard normalising and tempering treatment are microstructurally stable during subsequent ageing at temperatures up to 5500C for periods up to 1100 hours. However, under cyclic loading conditions, dislocation annihilation and rearrangement could occur thereby leading to break up of the martensite laths and the gradual development of an equiaxed substructure. Alloys with such characteristics are prone to softening during cyclic loading. The present work is an examination of the fatigue response of EUROFER 97 in the temperature range between RT and 5500C with particular attention to the relationship between  cyclic mechanical behaviour and  the Start Martensitic transformation temperature (Ms ).   The examination of a martensitic structures by transmission electron microscopy shows two types of martensite:  one of them with structure of twins denominated acicular martensite and another one with a high dislocation density denominated lath martensite.  The primary factor that controls the volumetric fraction of one or another type of martensite is, in appearance, the temperature of start martensitic transformation or Ms [ 1 ].  The majority of  the elements of a ferrous alloy has influence on this temperature.  Several expression have been proposed in order to consider the Ms value from the concentration of alloy elements. Nevertheless the influence of the Ms temperature in the mechanical properties of martensitic stainless steel is an open question today.  The present work is part of a project  that proposed  to analyse the relation betwee cyclic mechanical behaviour of martensitic stainless steel and  the Ms temperature.   In this work the cyclic behaviour of EUROFER 97 was also compared with AISI 420 in order to make evident the differences in the observed cyclic softening. Low cycle fatigue tests were performed in the temperature range between room temperature and 550°C in a low activation quenched and tempered EUROFER steel. After the first few cycles a pronounced cyclic softening that continues up to failure is observed for this steel. Almost all cyclic curves never reach a saturation stress and they converge in a common softening stage independent of the strain range. The mechanism governing this softening stage is almost independent of the temperature below 450°C.   The cyclic softening observed in the reduced activation steels is more pronounced that the softening observed in the commercial AISI 420 steel. The microstructural examination showed that the softening during cycle was associated with a mechanism of lath boundaries annihilation. This mechanism is atermal and independent of strain amplitude. In the present study it is proposed that the differences observed in the cyclic softening of these steels  is related to the difference in Ms  ( Martensitic Start temperature)