Discontinuous creep deformation in a type 316 stainless steel casting
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TYPES304 and
316 austenitic stainless steel are elevated-temperature structural materials for both nuclear and nonnuclear energy systems. These steels are c o m m o n l y used in the wrought condition. However, because of their intricate shapes some components can be used only in the as-cast condition. At service temperatures of a b o u t 773 to 973 K, these steels undergo creep deformation, which is a time-dependent deformation of the material under constant stress or load. The generation of creep data for austenitic stainless steels has been a part of a continuing mechanical properties test program at Oak Ridge National L a b o r a t o r y (ORNL) for the past several years. Both types 304 and 316 stainless steel have been tested in the wrought condition at temperatures in the range 755 to 1033 K and for rupture times of a few hours to 65,000 h duration. Tests were conducted on twenty different heats of type 304 and 10 heats of type 316. One heat of each type stainless steel was also tested in at least bar, plate, and pipe forms of various sizes. The material was creep tested additionally in the thermally exposed condition. In all creep tests the specimens continuously strained as a function of time, yielding classical creep curves containing primary, secondary, and tertiary creep regions. However, when creep tests were conducted on specimens taken f r o m a square casting of type 316 stainless steel, the creep deformation was both continuous and discontinuous based on the test conditions and specimen location. The discontinuous creep curves showed "strain b u r s t s " (instantaneous increases in strain) and shorter times to rupture. This paper describes our observations of discontinuous creep d e f o r m a t i o n in specimens taken f r o m a square casting of type 316 stainless steel. V. K. SIKKA and S. A. DAVID are Research Staff Members, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830. Manuscript submitted August 7, 1980. METALLURGICAL TRANSACTIONS A
MATERIAL AND EXPERIMENTAL DETAILS We conducted tests on a 305 by 305 by 305 m m static cast, 670 kg ingot of type 316 stainless steel (heat 30173). A transverse section 102 m m thick was cut from the center of the ingot. The section was 305 m m square, and the test specimens were cut along the two diagonals with their axes vertical. Samples 1 through 25 ran along the two diagonals with their axes vertical. S a m p l e s 1 through 25 ran along one diagonal, and 26 through 50 ran along the other, as shown in Fig. 1. Chemical analyses of specimens 1, 5, and 11 are summarized in Table I. Data show that the bulk chemical composition of the cast ingot was uniform f r o m the outer edge to the center of the ingot. The extent of ferrite in stainless steel castings can be estimated by computing the c h r o m i u m equivalentto-nickel equivalent (Cr e /Nie) ratio. The elemental compositions in weight percent of the c h r o m i u m equivalent, Cre, and nickel equivalent, Nie, are given by t Cre=Cr+
1.5 S i + M o + C o - 4 . 9 9 ,
and Nie
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