AISI440C透鏡狀麻田散鐵及其回火組織研究

Abstract

於本研究中，將針對AISI 440C不鏽鋼之透鏡狀麻田散鐵作相變態組織的研究。首先將AISI 440C不鏽鋼作1145˚C三天均質化處理，其後降至室溫下作觀察；並搭配DSC實驗結果，得知鋼材於此均質化溫度下的Ms溫度為-38.7˚C。為了觀察得到麻田散鐵組織，我們將後續作深冷處理，觀察深冷透鏡狀麻田散鐵及其後續回火的相變態過程。本研究將以OM、SEM、TEM對透鏡狀麻田散鐵板片之內部結構作觀察，根據研究結果顯示，透鏡狀麻田散鐵包含中脊區、雙晶拓展區和非雙晶區三個區域；中脊區具有高密度雙晶，會向兩旁延伸出雙晶密度較低的區域 (ie.雙晶拓展區)，且於非雙晶區內有看到差排的存在；這意味著透鏡狀麻田散鐵會同時藉由雙晶和滑移兩種形式來達到麻田散鐵相變所需維持的晶格不變應變。此外，於回火實驗中觀察到，隨著回火時間不同，將有不同種類的回火碳化物於透鏡狀板片基底內析出；650˚C回火30分鐘，析出的碳化物是以M3C為主，然而經60~90分鐘回火，基底的碳化物將轉變為M23C6。且由立體投影圖的分析顯示，M3C碳化物者和麻田散鐵基地維持Bagaryatsky的方位關係，M23C6則接近K-S的方位關係。 本研究中的另一個主題，是觀察多次循環回火對殘留沃斯田鐵的催化作用，並和單次回火作一比較。根據熱膨脹曲線實驗結果，得知循環回火冷卻的過程中，Ms溫度將逐漸提升，最終比單次回火之Ms溫度高；且觀察循環回火各階段對應之金相組織，發現隨著回火次數增加，殘留沃斯田鐵將逐漸轉變為新生麻田散鐵，由此可推斷，循環回火會比單次回火更能促進殘留沃斯田鐵的催化。

In this work , AISI 440C stainless steel has been investigated. It was homogenized at 1145˚C for 3 days, with subsequent quenching to room temperature. The Ms temperature of the homogenized specimen were measured to -38.7˚C via a DSC tester. In order to obtain the lenticular martensites, the specimens were then subzero cooled by liquid nitrogen (-196˚C).The substructure of subzero lenticular martensites and the following tempered structure have been investigated by OM, SEM and TEM. The results indicate that the substructures of lenticular martensites contained three regions: the midrib, extended twinned region and untwined region. The midrib is composed of high density of transformation{112} twins. The extended twinned region emanates laterally from the midrib region and give a lens-like feature. The untwinned region can be seen to have some dislocations. It should be note that the lenticular martensites transformation combine both twinning and slip modes；the midrib and extended twin keep the lattice-invariant deformation mode of twinning, whereas the untwinned region keep another deformation mode, slip. Additionally, the result of tempering experiments at 650˚C indicate that the different type of carbides precipitate from martensite matrix and had specific orientation relationships with the tempered matrix. M3C type carbide was dominant after tempering at 650˚C for 30 minutes but M23C6 type carbides was frequent after tempering for 60 and 90 minutes. Analysis the stereographic projection from diffraction patterns, it revealed that the Bagaryatsky OR was found between M3C carbides and ferrite, and Kurdjumov-Sachs OR was found between M23C6 carbide and ferrite. Another object in this research was to realize the effect of the decomposition of retained austenite, a single tempering and multi-tempering cycles had been investigated by dilatrometric experiments. The resulting dilatrometric curves showed the Ms temperature rises with increasing the repeated cycling numbers, and according to the corresponding optical metallographs of each tempering cycle, it shows that the retained austenite can be transfered into new born martensites due to the higher Ms temperature, during the multi-tempering cycles. As a result, the multi-tempering method would be much efficient to decomposite the retained austenite.