無鎳高氮不銹鋼中錳含量對孔蝕性質影響之研究

Abstract

本研究利用合金設計的方式，嘗試以低氮壓電弧熔煉爐製備不同錳含量之Fe-15Cr-12~22Mn-2.8Mo無鎳高氮不銹鋼，發現當錳含量增加時，合金的氮含量也隨之增加，可得到最高之氮含量約為1.4wt%，且完全不具磁性的單一沃斯田鐵相結構。鑄錠經裁切後分別以50%及75%軋延量於1150℃進行熱輥軋，得到晶粒大小不同的兩組試片，接著於3.5wt% NaCl溶液中，以0.3mV之掃描速率，分別進行動態極化掃描試驗。結果發現，氮含量較高的試片，孔蝕電位也較高。50%軋延量下，試片經電化學腐蝕測試後的結果與其錳含量有關，錳含量高於鉻含量的試片鈍化效果並不明顯，且孔洞於合金內部的擴散能力強，在表面形成佈滿細小孔洞的薄層；錳含量低於鉻含量的試片鈍化效果較為明顯，其孔洞於局部漸漸成長，顯見合金的錳含量對於鈍化層的形層的形成有不良影響，進而使得合金抗腐蝕能力下降。對此現象，本研究提出合理的模型，並與過去的研究相互得到應證。75%軋延量下，由於晶粒較小，晶界面積增加，腐蝕首先發生在晶界，且部分合金於晶界處產生層狀的Cr2N析出物，此析出物的生成溫度約在800~900℃間，此析出物對於機械以及抗腐蝕性有害，熱輥軋時必須注意溫度的控制以避免其生成。 此外，活性元素(SAE)如硫元素，其含量越多，熔煉後合金的氮含量越少，故製備無鎳高氮不銹鋼時，須注意原料的清淨度，以確保能達到較高的氮含量。

Different compositions of nickel free high nitrogen stainless steels (Fe-15Cr-12~22 Mn-2.8Mo-N) are prepared by low pressure arc melting process, nitrogen content is increased with Mn addition from 0.34wt% to 1.4wt%. Ingots are properly cut and hot-rolled under 1150℃ to 50% and 25% of their original thickness. The potentiadynamic polarization measurement were performed in a conventional three electrode cell with 3.5wt% NaCl medium. The result shows that pitting corrosion potential increased with nitrogen content. For 50% hot-rolled samples, pit morphology vary with their Mn content : in higher Mn content ones, pits growth beneath the surface toward some specific direction, left extensive lace-like cover region; in lower ones, pits tend to growth locally by etching grains, lace-like cover region is relatively small. Hence, the passivity and corrosion properties of stainless steels is sensitive to Mn content. For 75% hot-rolled samples, corrosion tends to start from grain boundaries, and in some cases, Cr2N precipitates from grain boundaries, it can be avoided be by carefully temperature control. Nitrogen content varies with different sulfur level, surface active element (SAE) like sulfur should be removed before or during melting.