Effect of Treatment Time on the Microstructure of Austenitic Stainless Steel During Low-Temperature Liquid Nitrocarburiz
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AUSTENITIC stainless steel is one of the widely used high alloy steels in the world for a range of applications, in the chemical, nuclear, coal, and oil industries. However, because these steels have low hardness and poor wear resistance, their applications are seriously limited.[1–4] In order to improve the alloy’s tribological properties, thermal-chemical surface hardening techniques such as nitriding, carburizing, carbonitriding, and nitrocarburizing are extensively studied to mend the surface tribological properties of austenitic stainless steels.[5–16] Compared to conventional gas-nitrocarburizing, liquid nitrocarburizing (LNC) treatment is regarded as an effective, low-cost method with many advantages, such as low treatment temperature, short treatment time, high degree of shape and dimensional stabilities, and reproducibility.[17–22] Funatani[21] insisted that LNC technology solved the environmental pollution problems that pose increasingly serious constraints on many surface technologies, and that it can be applied to the hardening of stainless and high alloy steels with high reaction efficiency. But ordinary LNC technology is JUN WANG, Associate Professor, and HONGYUAN FAN, Professor, are with the School of Manufacturing Science and Engineering, Sichuan University, Chengdu 610065, P.R. China. Contact e-mail: [email protected] YUANHUA LIN, Professor, and DEZHI ZENG, Associate Professor, are with the State Key Lab of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, P.R. China. QIANG ZHANG, Senior Engineer, is with Southwest Oil and Gasfield Company Research Institute of Natural Gas, Chengdu 610023, P.R. China. Manuscript submitted November 6, 2012. Article published online June 30, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
usually carried out at 853 K to 923 K (580 C to 650 C). At these temperatures, the great mass precipitation of CrN results in a depletion of Cr from the matrix. This induces a strong decrease of corrosion resistance, which greatly degrades the beneficial effect of increased hardness by ordinary LNC.[22–26] Some reports insisted that to avoid the precipitation of CrN, nitriding or nitrocarburizing must be carried out at a lower temperature. The modified layer of the stainless steel samples nitrided at low temperatures contains expanded austenite, which has better corrosion resistance and high microhardness.[1–6] But, due to the high natural melting point of most compositions of LNC, the development of lowtemperature LNC was difficult. Recently, ‘‘Palsonite’’ technology, an infrequently reported commercialized low-temperature LNC technology in Japan, greatly improved the wear performance without impairing the corrosion resistance of stainless steel.[14] More recently, authors also conducted low-temperature LNC on AISI 321 stainless steel by a self-designed liquid medium, which was mainly composed of M2CO3 (M denotes the elements K, Na, and Li), CO(NH2)2, and some trace nontoxic components.[27] Some typical characterizations of low-temperature
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