17-7半沃斯田鐵系析出硬化型不鏽鋼之熱處理對顯微結構及機械性質之影響

Abstract

17-7不鏽鋼在室溫下主要含有沃斯田鐵與麻田散鐵兩相，而由於較少的鎳元素含量，使得沃斯田鐵在室溫下並非穩定相，是否將沃斯田鐵經由各種處理相變為麻田散鐵仍需取決於材料應用的環境。對於17-7不鏽鋼熱處理的工業規範，前人的研究已表明並非最佳的流程，本研究會對其進行改良，最終目標是藉由調控麻田散鐵的相比例以得到不同性質的材料。 論文前半部分會以參考文獻導入需要具備的背景知識，後半部分則討論實驗流程與結果。內文會分為五大重點：多道回火熱處理、深冷處理、冷抽加工、時效機械性質、析出物觀察，前三部分是調控相比例的手段，而後兩部分則會對其進行性質的總結。光學顯微鏡、掃描式電子顯微鏡、穿透式電子顯微鏡是本實驗的主要實驗工具，掃描式電子顯微鏡用以分析相比例及觀察破斷面，而穿透式電子顯微鏡則可觀察對沃斯田鐵、麻田散鐵的形貌及各種析出物。同時，局部方位差角分析也可用於定性解釋許多現象，有助於簡化問題。 由最終的結果可知，多道回火熱處理能得到將近100%的麻田散鐵，使材料的機械性質更接近麻時效不鏽鋼，但缺點是會有較嚴重的敏化；深冷處理仍會留下一定比例的沃斯田鐵，雖然強度較低，但能提供優秀的延性；冷抽加工導入的大量缺陷使材料呈現相對複雜的變形組織，時效前的強度就已獲得大幅提升。整體而言，材料的機械性質並非單純由麻田散鐵的比例決定，還需考慮應變場分布、組織細緻程度、析出物偏好的生成位置。17-7不鏽鋼的各種熱處理皆有其優缺點，因此最重要的是因地制宜選用不同特性的材料。

17-7 stainless steel mainly contains two phases at room temperature, which are austenite and martensite. Due to the low content of nickel element, austenite is not a stable phase at room temperature, and whether to transform austenite into martensite through various heat treatments still depends on the process conditions of the material. For the industrial specification of 17-7 stainless steel heat treatment, previous research has shown that it is not the best process. We improve industrial heat treatments in this research, and our ultimate goal is to obtain materials with different properties by tuning the content of martensite. In the first half of this thesis, references will be used to introduce the required background knowledge, and in the second half, the experimental process and results will be discussed. The text will be divided into five major points: repeated tempering heat treatment, subzero treatment, cold drawing process, mechanical properties after aging, and observation of precipitates. The first three parts are the means to control the phase ratio, and the latter two parts will summarize their properties. Optical microscope, scanning electron microscope and transmission electron microscope are the main experimental tools in this experiment. The scanning electron microscope is used to analyze the phase ratio and observe the fracture surface, while the transmission electron microscope can be used to observe the morphology of austenite and martensite, also, and various precipitates. At the same time, kernel average misorientation can also be used to qualitatively explain many phenomena, which helps to simplify the problem. From the final results, it can be seen that repeated tempering heat treatment can obtain nearly 100% of martensite, making the mechanical properties of the material closer to maraging stainless steel, but the disadvantage is that there will be serious sensitization; subzero treatment will still leave a certain proportion of austenite, although the strength is relatively low, it can provide excellent ductility; a large number of defects introduced by cold drawing processing make the material present a relatively complex deformation structure, and the strength before aging has been greatly improved. Various heat treatments of 17-7 stainless steel have their advantages and disadvantages. Therefore, the most important thing is to choose materials with different characteristics according to local conditions.