Electron microscopic study of cr 2 n formation in thermally aged 316ln austenitic stainless steels
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INTRODUCTION
NITROGEN alloyed austenitic stainless steels are important as structural materials in nuclear reactors. Cr-MnN stainless steels are often considered to be the most promising material for use in fusion reactors. In the fast breeder nuclear fission reactor, many of the structural components like main vessel, safety vessel, inner vessel, auxiliary grid plate, fuel transfer mechanism, intermediate heat exchanger, core support structures, etc. are fabricated of CrNi-N stainless steels. The high susceptibility of carbon steels to intergranular corrosion has necessitated the use of extra low carbon steels. However, the accompanying reduction in the mechanical strength of the alloy with a decrease in carbon content has led to the development of nitrogen alloyed stainless steels. Apart from retarding the tendency for sensitization and pitting corrosion,Vl nitrogen plays a key role in improving several other mechanical properties. The yield strength, Young's modulus, and shear modulus of the material increase with increasing nitrogen content,t2-s3 accompanied with little or no decrease in ductility. The direct proportionality of thermal and atherrnal components of flow stress with the nitrogen content have been attributed to the possibility of short range ordering in these alloys.t2] The decrease in minimum creep rate on nitrogen alloying has been attributed to either solid solution strengthening or presence of short range ordered zones, tr] A substantial increase in fatigue life has been found in nitrogen steels resulting from an increased tendency for coplanar slip.[7-m Coplanar slip can be promoted by (a) decrease in stacking fault energy (SFE), (b) Cr-N complexes pinning down the dislocation, or (c) presence of short range ordered domains distributed in the matrix. Estimates of stacking fault energy with varying nitrogen levels have been done using X-ray diffraction and transmission electron microscopy (TEM) studies. The decrease in SFE due to nitrogen was found to be only about 2 to 6 ergs/sq, cm, too insignificant to account for the ob-
servations.[12'13'141No definitive evidences for the presence of short range ordered clusters have been reported in the literature. In spite of the large number of articles published during the last decade on the mechanical properties of low nitrogen stainless steels, the mechanism by which nitrogen influences the properties has remained an enigma. Cr2N precipitation has been observed by TEM only in alloys containing greater than about 1600 ppm of nitrogen.V5.~6] A tendency for cellular precipitation of Cr2N has been observed in high nitrogen steels, tl71 No reports on the pre-precipitation behavior of these steels are available to the best of our knowledge. The only findings on the presence of complexes at room temperature in alloys containing greater than 1600 ppm of nitrogen have been reported based on extended X-ray absorption fine structure (EXAFS) analysis and field ion microscopy (FIM) investigations,vs,~glFor nuclear reactors, austenitic stainless steels conta
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