Creep-Fatigue Interactions in a 9 Pct Cr-1 Pct Mo Martensitic Steel: Part II. Microstructural Evolutions
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TRODUCTION
THE microstructure of 9 to 12 pct Cr steels is frequently considered to be made of several scales (Figure 1). The largest scale consists of the former austenitic grains coming from the normalizing heat treatment. Their diameter usually varies between 10 and 60 lm depending on this heat treatment.[1–4] Inside each former austenitic grain, one or several packets of laths are formed during the martensitic transformation. Such packets are made of several blocks of laths having the same {111}c habit plane. These blocks consist of martensitic laths coming from the same variant. Inside these laths, subgrains are formed during the tempering treatment.[5,6] The very fine martensitic microstructure of these steels is relatively stable under aging at temperatures lower than 550 C. However, for aging at 593 C (during 50,000 hours), Gieseke et al.[8] reported an increased mass fraction of precipitates, these precipitates’ coarsening being even more pronounced at higher temperatures.[9,10] B. FOURNIER, PhD, Research Engineer, is with CEA/DEN/ DANS/DMN/SRMA, Bat. 455, 91191 Gif-sur-Yvette, Cedex, France, and ENSMP, Centre des Mate´riaux P.-M. Fourt, UMR CNRS 7633, BP 87, 91003 Evry, France. Contact e-mail: [email protected] M. SAUZAY, PhD, F. BARCELO, A. RENAULT, PhD, T. COZZIKA, and L. DUPUY, PhD, Research Engineers, are with CEA/DEN/DANS/DMN/SRMA. A. PINEAU, PhD, Professor, is with ENSMP, Centre des Mate´riaux P.-M. Fourt. E. RAUCH, PhD, Professor, is with INP Grenoble-SIMaP-CNRS-UJF, BP 46, 38402 Saint Martin d’Heres, Cedex, France. Manuscript submitted February 21, 2008. Article published online December 12, 2008 330—VOLUME 40A, FEBRUARY 2009
When mechanical loading is applied at high temperature, these microstructural evolutions are accelerated. In addition to the evolution of the precipitation state, a coarsening of the laths and subgrains and a decrease of the dislocations density are also observed. This microstructural coarsening has often been reported and quantified after creep but much more seldom after fatigue or creep fatigue. Table I reports the measures given in the open literature for several steels and experimental conditions. Such data were mainly gathered using detailed transmission electron microscopy (TEM) observations. The first part of this study[25] showed that, under fatigue and creep-fatigue (CF) loadings, a very fast deterioration of the creep properties of 9 to 12 pct Cr steels is observed. Even though fatigue damage might be partly responsible for this deterioration, previous studies on other materials[26,27] showed that microstructural changes occurring during fatigue tests can also affect the creep properties. In addition, the work softening effect observed under cyclic loadings on 9 to 12 pct Cr steels has already been qualitatively attributed to the coarsening of their microstructure.[20,28–31] This study proposes a quantitative multiscale characterization of the microstructural evolutions taking place on a P91 steel subjected to pure fatigue (PF), relaxationfatigue (RF), and CF tes
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