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Title: | 以氣體滲碳法實施沃斯田鐵系不銹鋼低溫滲碳之研究 Study on Low-temperature Carburizing of Austenitic Stainless Steel by the Gas Carburizing Method |
Authors: | Shuo-Han Hsu 徐碩韓 |
Advisor: | 陳永傳 |
Keyword: | 沃斯田鐵系不銹鋼,低溫滲碳,預氧化處理,酸浸漬,耐蝕性,耐磨性, Austenitic Stainless Steel,Low-teperature Carburizing,Pre-oxidation Treatment,Acid Dip,Corrosion Resistance,Wear Resistance, |
Publication Year : | 2012 |
Degree: | 碩士 |
Abstract: | 沃斯田鐵系(austenitic)不銹鋼因含有多量的Ni、Cr等合金元素,表面會生成一層緻密且性質穩定的Cr2O3鈍化膜,故其耐蝕性佳,被廣泛使用於石油、化工、醫療器材、食品工業等領域。但不銹鋼的硬度低,表面容易刮傷,製作機械零件時,其耐磨性也往往不能滿足使用上的需求。
滲碳是工業上常用的一種表面硬化技術,但傳統的高溫滲碳(800℃~1100℃)會使不銹鋼析出鉻碳化物(Cr7C3)而大幅降低其耐蝕性。為了避免鉻碳化物的析出,本研究採用低溫長時間滲碳來提高不銹鋼的表面硬度,將滲碳溫度降至520℃以下。由於溫度低,置換式的Cr原子不容易移動,只有插入式的C原子可移動。因此C原子可以滲入不銹鋼中,但不容易析出Cr的碳化物,可以在不降低耐蝕性的前提下,增加不銹鋼的表面硬度及耐磨性。 本研究使用含有N2+木精分解氣來實施不銹鋼的滲碳處理。但滲碳之前必須將不銹鋼表面的鈍化膜去除,亦即實施所謂的「活化處理」。而本研究使用鹽酸蒸氣配合預氧化處理取代一般之活化處理,能節省較多的時間,僅在十幾分鐘內就能完全的去除鈍化膜,並使不銹鋼成為可滲碳的狀態。而不銹鋼試片在適當的條件下實施鹽酸蒸氣預氧化處理及低溫滲碳處理後,觀察其滲碳層的顯微組織、表層硬度分佈,並施以磨耗試驗及腐蝕試驗等,以了解低溫滲碳對不銹鋼表面各種性質的影響,主要結果如下: 1. 在滲碳溫度470℃下生成的滲碳層主要為含碳過飽和的固溶體與極少量之碳化物,在滲碳溫度520℃下生成的滲碳層為過飽和固溶體以及碳化物的混合相。 2. AISI 304在滲碳層內生成的碳化物主要為Cr7C3,而AISI 316所生成的碳化物除了Cr7C3之外,尚有Mo的碳化物Mo2C。 3. 就滲碳速率以及滲碳層生長厚度而言,AISI 316優於AISI 304。 4. 不銹鋼經低溫滲碳後,其耐腐蝕性並沒有顯著劣化。對AISI 304而言,甚至可大幅提升對鹽酸的耐腐蝕性。若細看滲碳溫度對耐腐蝕性的影響,較高滲碳溫度520℃下因碳化物生成的量較多,導致耐腐蝕能力較差,反觀在滲碳溫度470℃下碳化物生成的量極少,故其耐腐蝕能力較佳。 5. 經低溫滲碳的試片與原材相較之下,其耐磨耗能力有顯著提升,主要是因為滲碳試片表面受到溶碳過飽和的影響,在表面產生極大的壓應力,造成硬度及耐磨耗能力大幅的提升。 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. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/65287 |
Fulltext Rights: | 有償授權 |
Appears in Collections: | 機械工程學系 |
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