沃斯田鐵系不銹鋼之低溫氣體滲碳與電漿拋光之研究

Study on Low-temperature Gas Carburizing and Plasma Polishing of Austenitic Stainless steel

江嘉瑋(Chia-Wei Chiang)

指導教授 ： 陳永傳

國立臺灣大學/工學院/機械工程學研究所/碩士(2015年)

https://doi.org/10.6342/NTU.2015.10151

摘要

沃斯田鐵系不銹鋼因含有豐富的鉻元素，表面容易形成緻密的Cr2O3鈍化膜，能夠提高耐蝕性。但不銹鋼硬度較低，表面容易產生刮痕，耐磨性也無法滿足工業上的應用，因此常利用表面改質處理來增強其硬度與耐磨性。本研究利用低溫氣體滲碳作為改善不銹鋼表面性質的方法，但由於表面Cr2O3鈍化膜會阻擋C原子的滲入，因此需先施以活化處理移除表面鈍化膜。接著使用氮氣加上木精分解氣作為滲碳氣氛進行低溫滲碳處理，在表面形成溶碳過飽和的沃斯田鐵層，使其硬度、耐蝕性及耐磨性都獲得提升。
而低溫滲碳後的試片表面因積碳會有黑層(black film)的產生，本研究利用低汙染的電漿拋光處理方法，移除表面黑層，使不銹鋼表面恢復光亮。電漿拋光時使用低濃度的鹽類水溶液作為拋光液，將試片通以高壓的直流電源，使試片表面的拋光液游離形成電漿蒸氣層，隔絕試片與拋光液造成電位差而產生輝光放電，達到拋光的效果。主要的實驗結果如下：
1. AISI 304 不銹鋼在低溫滲碳後，表面會形成滲碳層，其硬度、耐蝕性及耐磨性相較於原材都獲得改善。而滲碳溫度較高，時間較長時，會得到較厚的滲碳層。
2. 電漿拋光需要足夠的電壓與拋光液溫度才能在試片表面產生電漿蒸氣層，本實驗裝置可拋光的條件為340V的直流電源與溫度為80~90℃的拋光液。
3. 隨著拋光時間的增加光澤度也會逐漸變高，表面粗糙度逐漸下降，但到一定程度之後不再有顯著變化。
4. 經470℃滲碳後的不銹鋼試片能夠在短時間拋光至光亮狀態，且還存有一定厚度的滲碳層，耐蝕性與耐磨性仍遠優於原材；而經520℃滲碳的不銹鋼因晶界上有碳化物析出，長時間拋光後表面仍無法達到光亮狀態。

關鍵字

沃斯田鐵系不銹鋼 ； 活化處理 ； 低溫氣體滲碳 ； 電漿拋光

並列摘要

Austenitic stainless steel will form a layer of Cr2O3 passive film on the surface protecting the inside material from corrosion; however this stainless steel will be easily scratched due to its low hardness. Low-temperature gas carburizing has been used in surface-modification of austenitic stainless steel to enhance its surface hardness and wear resistance, but the passive film will obstruct the diffusing of carbon atoms in the carburizing process. Therefore, the passive film should be removed by activation treatment before low-temperature gas carburizing. Nitrogen and methanol dissociation gas has been used as carburizing atmosphere in this research. The carburized layer will be formed on the surface of specimen with high hardness, great corrosion and wear resistance.
After low-temperature gas carburizing, the surface of the austenitic stainless steel was covered by a layer of black thin-film causing high roughness and low brightness. The plasma polishing is an environment friendly method to remove the black thin-film and brighten the surface. The specimen immerses in the low concentration, non-toxic and pre-heating electrolyte. Then a high voltage DC power is applied to ionize the electrolyte and form an ionized water-steam film on the entire surface. The film separates the metal surface from electrolyte and causes the voltage drop to form glow discharge with polishing effect. The appropriate polishing parameter and the properties of carburized layer after polishing has been investigated in this study. The results are as follow:
1. After the low-temperature carburizing, AISI 304 stainless steel forms a carburized layer on the surface with high hardness, good corrosion resistance and wear resistance. The thickness of carburized layer is relative to the processing temperature and time.
2. The ionized water-steam film will not form on the metal surface unless there are enough working voltage and electrolyte temperature. With this equipment of plasma polishing, the working voltage and the temperature of electrolyte set as 340V and 82~90℃.
3. The surface roughness and brightness are improved when polishing time is longer, but won’t be changed apparently while surface performance is getting optimized.
4. Austenitic stainless steel carburized at 470℃ becomes brightness after short time polishing and consists enough carburized layer which possesses better hardness, corrosion resistance and wear resistance than the raw material; However the specimen carburized at 520℃ will form grain boundary carbide. As a result of this phenomenon, this polishing method is not capable of improving the brightness of the specimen.

並列關鍵字

Austenitic stainless steel ； Activation treatment ； Low-temperature gas carburizing ； Plasma polishing

參考文獻

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