Effect of the metallurgical variables on the yield stress of PH 13-08 steel
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I. INTRODUCTION
STAINLESS steel PH 13-08 was originally developed for aeronautic applications that require a high level of strength, ductility, and toughness. Recently, the applicability of this alloy in pressurized water nuclear power plants has been questioned. For both types of applications, the alloy can be exposed to intermediate temperatures (300 °C to 400 °C), which may involve aging phenomena in the martensite. This aging leads to an increase of hardness and strength but also to some loss of ductility and moreover an increase of the ductile-brittle transition temperature (DBTT).[1,2,3] The amount of such an increase depends on the precise chemical composition as well as on the particular microstructure of the alloy.[4,5] Increases by about 200 °C of the DBTT have been observed in PH 13-08 stainless steels after long-term treatments in the temperature range 300 °C to 400 °C.[6] The technological response to such a problem is often to anticipate the amount of the DBTT shift by adapting the initial transition temperature. For that purpose, the initial DBTT is fixed to a low level in such a way that at the end of service the DBTT remains acceptable. This is done through the precise design of chemical composition of the alloy during production and necessitates tools able to predict the DBTT for given composition and metallurgical state. After classical thermomechanical treatment, PH 13-08 steel is completely martensitic. During the aging treatment, aluminum forms precipitates of the ordered -NiAl phase (B2 structure) that harden the martensite. However, when the heat treatment is too long (overaging during the treatment or due to the high-temperature service), two main reasons may explain the degradation of mechanical properties. Those are the coarsening of NiAl precipitates and the partial transformation of martensite to austenite (which is called the reverted austenite). In the present work, we have studied more closely the influence of both the aluminum content and the reverted austenite volume fraction on the mechanical properties. The yield strength of the PH 13-08 alloy is calculated depending on the metallurgical variables JEAN-MARC CLOUÉ, Senior Scientist, is with FRAMATOME ANP, Direction Conception et Ventes, 69456 Lyon, Cedex 06, France. BERNARD VIGUIER, Associate Professor, and ERIC ANDRIEU, Professor, are with the Centre Interuniversitaire de Recherche et d’Ingénierie des Matériaux (CIRIMAT), 31077 Toulouse, Cedex 4, France. Contact e-mail: eric.andrieu@ ensiacet.fr Manuscript submitted December 15, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
and a satisfactory relation is obtained. This calculation is finally extended in order to calculate the DBTT. II. EXPERIMENTAL PROCEDURE A. Material and Treatments For the present study, 13 casts of PH 13-08 Mo have been produced and tested. The chemical composition of these alloys are given in Table I. All the ingots have been given a double remelting under vacuum (vacuum induction melting then vacuum arc remelting). They present a very low density of in
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