Study of microstructure of low-temperature plasma-nitrided AISI 304 stainless steel
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RODUCTION
STAINLESS steel is widely used in chemical, food, and other industries due to its corrosion-resistance properties. However, because of its low hardness or poor wear-resistance properties, its application is greatly limited. Scientists have paid much attention to the surface modification of stainless steel. Many attempts have been made to increase the surface hardness of this material using such techniques as plasma source ion implantation (PSII), plasma ion implantation (PII), and plasma nitriding[1–3]. An increase in surface hardness is often accompanied by a decrease in corrosion properties due to precipitating CrN, especially in the traditional plasma nitriding used widely in industry production. Zhang and Bell[4] reported for the first time that the S-phase formation on the surface by reducing treatment temperature of the plasma nitriding can provide both wear resistance and corrosion resistance. This S-phase has become known as g N in later research because the basic fcc structure (g austenite) is retained. However, despite many investigations[5–8] the exact nature of the microstructural changes responsible for this improvement is not very well understood. Using most techniques, the lattice parameter of the so-called g N is greater than that of the g 8-Fe4N (a 5 0.379, nm) and its nitrogen content more than 20 at. pct.[7,8] It is uncertain whether the supersaturated solid solution is based on the g 8Fe4N or g -austenite or on both. In other words, there would be two kinds of supersaturated solid solution respectively based on either g 8-Fe4N or g -austenite. However, there is a difference between g 8-Fe4N and g -austenite in crystal structure. The former has nitrogen atoms occupying octahedral interstitial sites of cube center in a fully ordered manner, and the latter has nitrogen atoms randomly distributed XIAOLEI XU, LIANG WANG, and ZHIWEI YU, Associate Professors, JIANBING QIANG, Master, and ZUKUN HEI, Professor and Director, are with the Institute of Metal and Technology, Dalian Maritime University, Dalian 116024, People’s Republic of China. Manuscript submitted December 11, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
throughout all interstitial octahedral sites. In this article, we will address this issue using transmission electron microscopy (TEM). In order to understand the present work more easily, it is necessary to pronouncedly define the phases dealt with in the article as follows. g -austenite—the solid solution of nitrogen in g -Fe (fcc),[9] in which nitrogen atoms occupy the octahedral holes of fcc lattice in a disordered manner; g 8-Fe4N—the ordered solid solution, in which the metal atoms arrangement is fcc, but the nitrogen atoms occupy one-quarter of the number of octahedral holes in a fully ordered manner;[9] S or g N —the supersaturated solid solution of nitrogen in g austenite or the nitrogen expanded austenite, in which nitrogen atoms occupy the octahedral holes in a disordered manner;[4] g 8N — the supersaturated solid solution of nitrogen in g 8-Fe4N, which is defined by
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