沃斯田鐵系不銹鋼之表面活化處理與低溫氣體滲氮之研究

Study on Surface Activation Treatment and Low-temperature Gas Nitriding of Austenitic Stainless Steel

楊宗澤(Tsung-Tse Yang)

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(2013年)

https://doi.org/10.6342/NTU.2013.11002

摘要

活化處理是沃斯田鐵系不銹鋼能進行後續表面改質(滲碳、滲氮等)的必要製程，也是最關鍵的技術。本研究藉由在不銹鋼表面產生貧Cr層來達成活化的目的，貧Cr層因為含Cr量較低而不易產生緻密的Cr2O3，可以使活化後之不銹鋼表面進行後續之氣體滲氮處理。
本實驗得出之AISI 316L理想活化方法，是利用鹽酸蒸氣作為活化氣氛，在560℃下活化10min。經過此活化處理後能夠快速去除不銹鋼表面之鈍化膜，並留下鈍化膜下方之貧Cr層，活化效率和泛用性遠高於一般常見之活化方法，如：離子活化、電漿活化、酸洗、噴砂等。
這種利用產生貧Cr層來達成沃斯田鐵系不銹鋼表面活化的方式，可以讓經過活化處理之不銹鋼在大氣中放置多天後還能夠實施低溫滲氮處理，此種活化和滲氮可以分開進行的優勢，有利於目前工業界實際生產流程的安排。
本研究將AISI 316L不銹鋼施以上述活化處理後，採用低溫(430℃)氣體滲氮法來提升其表面硬度，不僅製程、設備簡單，更可以大量生產，實用上來說是理想的沃斯田鐵系不銹鋼之表面硬化法。主要實驗結果如下：
1. AISI 316L不銹鋼滲氮層之硬度可達1200 HV0.01，耐磨性是原材的200倍。
2. AISI 316L滲氮層中S相所能固溶之N元素含量達到飽和時(6wt%)，容易因為內應力過大而產生許多裂紋，而且會使插入性元素(例如：C)被推擠到晶界或是滲氮層和母材交界處，造成滲氮層整體耐蝕性下降。
3. 預分解能夠使氨氣部份分解成N2＋H2，有效降低滲氮氣體之氮勢，進而大幅改善316L不銹鋼滲氮層之耐蝕性，而且滲氮完之不銹鋼表面並不會產生氮化物而維持原有的金屬光澤，具有輝面熱處理的效果。

關鍵字

沃斯田鐵系不銹鋼 ； 鈍化膜 ； 活化處理 ； 貧Cr層 ； 低溫氣體滲氮 ； 預分解

並列摘要

Activation treatment is a critical technology that allows austenitic stainless steel can be surface-modified by carburizing, nitriding, etc.
This research used a unique activation treatment to form a chromium depleted layer on the surface of austenitic stainless steel, the less concentration of chromium in this layer lead to the growing difficulties of Cr2O3 passive film, as a result, the nitriding process can be carried out on the stainless steel which has been activated.
In this research, the passive film could be completely removed and the so called chromium depleted layer can formed after the activation process at 560℃ for 10min, using hydrochloric acid vapor as activation atmosphere. The activation efficiency is much better than that of the conventional activation treatment such as ion activation, plasma activation, pickling, sandblasting, etc.
Attribute to the chromium depleted layer, the activated austenitic stainless steel, can keep in the ordinary atmosphere for a few days before processing nitriding. It is convenient for the arrangement of the production process.
This research focuses on the application of gas nitriding at low temperature (430℃) because of its economic efficiency. It is considered the most appropriate process to increase the surface hardness of the austenitic stainless steel.
The results are as follow:
1. The hardness of the AISI 316L nitrided layer can reach 1200 HV0.01, and its wear resistance is 200 times better than that of the raw one.
2. When the N contents in the 316L nitrided layer reach the super saturated limit (6wt%), it will caused numerous cracks because of the excessive internal stress. On the other hand, the C elements may be pushed to the grain boundary or the interface of nitrided layer and the matrix, owing to the insertion of N atoms. This phenomenon will reduce the corrosion resistance significantly.
3. Pre-decomposition of ammonia allows NH3 to decompose into N2＋H2. This process effectively reduces the nitrogen potential of the nitriding gas and avoids the formation of carbides and nitrides. Consequently, the corrosion resistance of 316L nitrided layer can be greatly improved. Moreover, the surface of the stainless steel after the nitriding process, still maintains the original metallic luster, with the effect of bright heat-treatment.

並列關鍵字

Austenitic stainless steel ； Passive film ； Activation treatment ； Chromium depleted layer ； Low-temperature gas nitriding ； Pre-decomposition

參考文獻

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1993, pp.288-232.

3. Whelan, M. J., P. B. Hirsch, et al. “Dislocations and Stacking Faults in Stainless Steel” Proceedings of the Royal Society of London Series a-Mathematical and Physical Sciences 240(1223), 1957, pp.524-&.

4. C.-O.A. Olsson, D. Landolt, “Passive films on stainless steels-chemistry, structure and growth” Electrochimica Acta 48(9), 2003, pp.1093-1104.