ABSTRACT

The influence of ferrite and heat treatment condition on the toughness of ASTM(1) A 351 Grade CF3M (0.03C-19Cr-9Ni-2Mo) and CF8M (0.08C-19Cr-9Ni-2Mo) castings used in valve bodies in both Cold -200F<T<-50F (-128C<T<-45C) and Cryogenic Services -325F<T<-200F (-198C<T<128C) was studied. The intent was to understand if the low ferrite limits commonly associated with acceptable toughness in austenitic welds can be used on ASTM A 351 Grade CF3M and CF8M castings. The ASME B31.3 Code accepts A 351 CF3M and CF8M material down to -425F (-255C) in low temperature toughness applications, as long as the cited material contains C<0.1%w, and it’s the annealed condition. There is no mentioning of ferrite content and influence on toughness. Additionally, no details of the heat treatment condition are provided in the Code for these materials to be acceptable for low-temperature, toughness applications. The results of this study show that ferrite content has little influence on the toughness of those materials at low temperatures. However, heat treatment condition does. Depending on the casting grade and heat treatment, the formation of Chi or a carbide phase in the interdendritic space negatively influences the toughness at low temperatures. Annealing at a minimum of 1,925 F±25F (1050C +/-15C) and 2,000F±25F (1095C +/- 15C), for CF3M and CF8M grades, respectively, followed by a water quench is required to obtain acceptable toughness. Acceptable toughness is correlated to the disappearance of Chi and/or carbide phase in those castings.

BACKGROUND

ExxonMobil(2) completed a study of the influence of ferrite and heat treatment condition on the toughness of ASTM A 351 Grade CF3M (0.03C-19Cr-9Ni-2Mo) and CF8M (0.08C-19Cr-9Ni-2Mo) castings used in valve bodies in both Cold -200F<T<-50F (-128C<T<-45C) and Cryogenic Services -325F<T<-200F (-198C<T<128C). The intent of the study was to understand if the low ferrite limits commonly associated with acceptable toughness in austenitic welds1,2 can be used on ASTM A 351 Grade CF3M and CF8M castings3. The ASME(3) B31.3 Code4 accepts A 351 CF3M and CF8M material down to -425F (- 255C) in low temperature toughness applications, as long as the cited material contains C<0.1%w, and it’s the annealed condition. There is no mentioning of ferrite content to influence toughness. Additionally, no details of the heat treatment condition are provided for these materials to be acceptable for low-temperature, acceptable toughness applications. When the Code requires toughness measurements on austenitic stainless steels at a targeted temperature, it is expressed in terms of lateral expansion with a minimum of 0.015 in (0.4 mm) per Charpy V-notch tests carried in accordance with ASTM A 3705

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