AISI 440C合金鋼透鏡狀麻田散鐵次微米結構分析

Abstract

本研究是以AISI 440C中生成的透鏡狀麻田散鐵為研究對象，首先試著控制均質化溫度，使碳化物融入基地中，探討基地中合金元素的含量對Ms溫度與麻田散鐵形貌的影響。透鏡狀麻田散鐵分別在沃斯田鐵晶界與未完全固溶的M7C3邊界優先成核，透鏡狀麻田散鐵的生長過程是先生成板片狀麻田散鐵再轉變成透鏡狀麻田散鐵，因此造成透鏡狀麻田散鐵含有三個區域；含有兩種缺陷-雙晶與差排，利用穿透式電子顯微鏡（TEM）與背向散射電子繞射分析（EBSD）微觀觀察其三個區域與misorientation的變化，並觀察透鏡狀麻田散鐵的2-D形貌，發現雙晶具有彎曲現象，與藉著聚焦式離子束顯微鏡(FIB)觀察透鏡狀麻田散鐵的3-D形貌為橢圓體，巨觀觀察透鏡狀麻田散鐵間相互連結的樣貌有spear, kink 及 zigzag array三種，均與沃斯田鐵基地呈現Kurdjumov-Sachs(K-S)方位關係。 以400ºC回火熱處理的透鏡狀麻田散鐵微結構變化，在同一透鏡狀麻田散鐵晶粒內有兩種雪明碳鐵生成，一種是needle型，與肥粒鐵呈現Isaichev OR，只具有一個方向性，不會隨時間轉變成M7C3；另一種為rice型，與肥粒鐵呈現Bagaryatskii OR，先在中脊區(雙晶)與無雙晶區(差排)生成，隨著時間拉長cementite越來越多、越來越大，為降低表面能而聚集形成圓形，再聚集經由in-situ機制轉變為平行四邊形的M7C3，前述兩種雪明碳鐵的成核機制分別為剪切機制與擴散機制。

This work mainly focuses on the investigation of the microstructure and transformation of lenticular martensite in AISI 440C stainless steel. The research can be divided into two parts. The goal of the first is to analyze the transformation and morphology of lenticular martensite; the goal of the second is to observe the evolution of carbide formation in lenticular martensite during tempering treatment. In the first part, by controlling the homogenization temperature, an appropriate lenticular martensite structure was obtained, facilitating the following observation. The influences of the alloying element content on Ms temperature and the transformed martensite morphology were analyzed with electrical microscopy. Due to the high alloying element content of AISI 440C stainless steel, even after homogenization treatment, the M7C3 carbides are still distributed in the austenite matrix and therefore become the nucleation sites of lenticular martensite. During the martensite transformation process, the initial nucleation product is plate martensite, which then grows into a lenticular martensite structure. Lenticular martensite contains three regions: a midrib (which can be seen as the former plate martensite structure), a twinned region, and an untwinned region. These three regions differ not only in their morphologies but also in their misorientation distribution. Transmission election microscopy (TEM) and electron backscatter diffraction (EBSD) techniques were applied to explore the misorientation in these regions and the crystal orientation relationship between austenite and martensite. In our observations, a single lenticular martensite grain shows an elliptical morphology. Furthermore, a specimen polished with the focus ion beam (FIB) method provides 3-D images information and confirms that lenticular martensite is shaped like a thick shell. The crystallography of lenticular martensite formed in coarse austenite grains was investigated using electron backscattered diffraction (EBSD). Although the spread in diffracted intensity within pole figures was significant, due to the orientation gradient within lenticular martensite, the trend of pole figures indicated that the lenticular martensite approximately adopted a Kurdjumov-Sachs (K-S) orientation relationship with respect to the austenite matrix. The orientation relationships of variant pairings (zigzag, spear, and kink types) have been analyzed. The aim of the second part of this research is to discuss the effects of tempering treatment on lenticular martensite. After tempering at 400ºC, there exist two different kinds of cementite: the 'needle' type, which does not transform into M7C3 even after a long tempering treatment, and the 'rice' type, which first nucleates at twins in the midrib region and dislocations in the untwinned region, continues to grow during tempering treatment, and finally becomes a parallelogram-shaped M7C3 carbide. The discrepancies between the two types of cementites in shape and crystallographic relationship can be attributed to the nucleation and growing mechanisms. With electrical microscopy analysis, the displacive mechanism and diffusion mechanism are applied to elucidate the formation of cementites in lenticular martensite.