AISI 440C不銹鋼深冷及隨後回火相變態之電子顯微鏡微結構研究

Abstract

摘 要 AISI 440C麻田散鐵系不銹鋼，其成份主要為高含量的碳及鉻元素，使其硬化能很高，均質化處理後採用空冷即可避過肥粒鐵的相線區，但也導致其Ms溫度低於室溫，故需以深冷處理方能相變得到麻田散鐵組織，但基底仍殘有大量沃斯田鐵，故再施以回火處理消除殘留沃斯田鐵，並改善麻田散鐵相變後之韌性與機械性質，期望能得到最佳的性質。 本次實驗著重在深冷後的板片狀麻田散鐵組織及回火處理後的回火板片狀麻田散鐵組織，逐一觀察各項處理後所形成之組織變化，並以穿透式電子顯微鏡分析各相微觀組織。觀察後發現板片狀麻田散鐵基地是由雙晶所組成，有中脊面(midrib)的痕跡及伴隨大量的疊差區；回火板片狀麻田散鐵基地除了雙晶和中脊面外，有多種的合金碳化物得析出，並發現部分中脊面兩側的雙晶有小角度的轉折。除此之外，回火麻田散鐵板片週圍所觀察到的中低碳區麻田散鐵板條狀組織，來推斷殘留沃斯田鐵分解後基底重新分配後的相變結果。

Abstract AISI 440C is an important alloy steel and is widely applied in medical appliances, cutting tools and turbine blades. The steel typically contains 1 wt% carbon and 17% Cr, so that the MS Temperature is expected to be much lower than the room temperature. After quenching into liquid nitrogen, It gives plate martensite, which has a burst formation with Zig-Zag morphology. There is still remaining a great quantity of retained austenite in the matrix. In order to deliminate the retained austenite, the tempering treatment is necessarily carried out. In the study, plate martensite and tempered martemsite has been examined after different cycles of tempering. The orientation relationship between the twins within the plate martensite has been investigated through the electron diffraction of TEM. In addition, it has found that after multi-tempering cycles the MS temperature gradually rises with increasing the repeated cycling numbers, and that austenite transforms into ferrite and carbides, step by step.