Study on Low-temperature Carburizing of Austenitic Stainless Steel by the Gas Carburizing Method

• Main Authors: Shuo-Han Hsu, 徐碩韓
• Other Authors: 陳永傳
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/11375772547653487334

碩士 === 國立臺灣大學 === 機械工程學研究所 === 100 === Austenitic stainless steel is an alloy steel containing large amounts of nickel, chromium and other alloying elements. The surface will form a thin layer of Cr2O3 passive film, which is resistant to corrosion, so that this type of stainless steel can be applied in petroleum engineering, chemical engineering, medical equipment, and the food industry etc. However, this stainless steel will easily be scratched due to its low hardness; therefore in the application of mechanical parts, its wear resistance often fails to meet the requirements of the users. Carburization is one of the surface hardening techniques commonly used in industries, but the traditional high temperature carburization (800℃~1100℃) will lead to the precipitation of chromium carbides (Cr7C3) onto the grain boundaries, which will significantly reduce the corrosion resistance of the stainless steel. To prevent the precipitation of the chromium carbides, this research focuses on utilizing a carburization technique with long duration at low temperature, lower than or at 520°C, to increase the surface hardness of the stainless steel. At low temperatures, substitutive chromium atoms are not easy to move, but interstitial carbon atoms can move more easily. Therefore, carbon atoms can dissolve into the stainless steel, which can increase its surface hardness and wear resistance. In addition, the chromium atoms within the stainless steel cannot precipitate to form carbides; therefore, preventing from the decreasing of the corrosion resistance of the stainless steel. This research utilizes the mixture of nitrogen and methanol dissociation gas to carburize the surface of the stainless steel. Before the carburization process, the passive film on the surface of the stainless steel should be removed through the so-called activation treatment. This research uses a pre-oxidation process with hydrochloric acid vapor to replace the conventional activation treatment, so that the former can save more time than the latter. Within ten minutes, the passive film will be completely removed, and then the stainless steel can be carburized. After the stainless steel is pre-oxidized by the hydrochloric acid vapor under appropriate conditions, and then carburized at a low temperature, the effects of the carburization on the stainless steel are evaluated from the microstructure, hardness distribution, wear resistance and corrosion resistance of the carburized layer.The results are as follow: 1. The carburized layer created at a carburizing temperature of 470°C mainly consists of a super saturated solid solution of carbon atoms and only a minute amount of carbide; On the contrary, the carburized layer created at a carburizing temperature of 520°C is a mixture which consists of a super saturated solid solution and a noticeable amount of carbides. 2. Major carbides generated from AISI 304 in the carburized layer are Cr7C3. In addition to Cr7C3, another type of carbide generated from AISI 316 is Mo2C. 3. When comparing the carburizing rate and the carburized layer’s thickness of both stainless steels, AISI 316 is better than AISI 304, overall. 4. When the stainless steel is carburized at low temperatures, the deterioration of its corrosion resistance is not significant. The carburizing of AISI 304, on the contrary, will increase the corrosion resistance to the hydrochloric acid. When closely observing the effect of carburizing temperature on the corrosion resistance, the higher carburizing temperature of 520°C produces more carbides, which results in poor corrosion resistance. As opposed to a lower carburizing temperature of 470°C, less carbides are produced, resulting in better corrosion resistance. 5. When the low temperature carburized specimen is compared with the raw one, its wear resistance is improved significantly. The main reason is due to a large amount of compressive stress generated on the surface of the carburized specimen because of the super saturation of carbon atoms, therefore resulting in significant improvement in surface hardness and wear resistance.