Grain Boundary Analysis of HT9 Steel after Accelerated Creep Testing
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Grain Boundary Analysis of HT9 Steel after Accelerated Creep Testing
Zhe Leng1 and David P. Field1 1 School of Mechanical and Materials Engineering, Washington State University, Pullman, WA, 99164-2920, U.S.A.
ABSTRACT Ferritic/martensitic steels are attractive materials for use as components in nuclear reactors because of their high strength and good swelling resistance. Grain boundary specific phenomena (such as segregation, voiding, cracking, etc) are prevalent in these materials so grain boundary character is of primary importance. Certain types of boundaries are more susceptible to thermal creep damage whereas others tend to resist damage. If more damage resistant boundaries can be introduced into the structures, this will result in steel that is more resistant to the processes of degradation that prevail in high-temperature environments. In this study, the grain boundary structure in HT9 steel was characterized by electron backscatter diffraction to identify boundaries that are resistant or susceptible to damage in extreme environments. It is found that intergranular damage is mitigated by a high fraction of low angle boundaries, and certain kinds of grain boundaries, such as the Sigma 3 boundary, are more favored by intergranular cracks. KEY WORDS HT9 steel, Creep damage, EBSD, Pole Figure, Intergranular crack INTRODUCTION For structural applications in fusion reactors, high-chromium(9-12 wt%) ferritic/martensitic steels have several advantages such as good creep strength and excellent swelling resistance in comparison with austenitic stainless steels [1,2]. HT9 steel is a typical ferritic/martensitic stainless steel which has been in use in nuclear reactors for many years [3]. During its service life, one of the major damage types is from thermal creep, thus the grain boundary character plays an important role in controlling intergranular damage. Various publications have shown that, for FCC structures, grain boundaries with certain crystallographic relationships are more resistant to creep damage while others are not [cf. 4-6]. However, little information is available for HT9 steels, and thus it is necessary to inspect the damaged grain boundaries and determine GB character for boundaries which are susceptible to damage and those which are more resistant. The main objective of this study was to characterize grain boundaries that are damaged under the accelerated creep test. The test was conducted under constant loading and the creep damage was observed using the electron backscatter diffraction (EBSD) method.
EXPERIMENTAL PROCEDURE Material and sample preparation Tested material is HT9 steel, a nominally Fe–12Cr–1Mo–0.5W–0.5Ni–0.25V–0.2C steel alloy, the chemical composition is shown in Table 1. Samples were machined in the form of tensile bars with a gage length of 24 mm and a cross section of 6 mm x 4 mm as shown in Fig.1. The samples were austenitized at 1050°C for 1 hour to dissolve the precipitates and carbides in the austenite phase, and air cooled after austenitizing. The resulting
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