17-7 析出硬化型半沃斯田鐵系不鏽鋼相變態特性及顯微組織研究

Abstract

本實驗採用華新麗華公司提供的 17-7PH 不鏽鋼作為研究材料，進行相辨認以及析出強化之研究。17-7PH 不鏽鋼是一種析出硬化型不鏽鋼，具有優異耐疲勞、良好耐腐蝕性與良好成形性等優點。17-7PH 除了添加不鏽鋼常見的鉻和鎳元素之外，也添加鋁與氮元素，目的是使 AlN 達成晶粒細化和 NiAl 能實踐析出強化效果。 實驗的第一部分會先將華新麗華所提供的 HR 原材進行 EBSD、KAM、IQ 分析，以區別此材料複雜的相組織；第二部分進行 RH950 標準熱處理，淬火後的 17-7PH 鋼可以透過進一步深冷處理增加麻田散鐵的相變驅動力，根據標準規範，深冷處理藉由將材料冷至 –75℃ 持溫 8 小時實現。由前人研究報導可知 17-7PH 鋼種深冷處理過程涉及恆溫麻田散鐵相變，故於本論文一大工作目的是展現 17-7 PH 鋼中恆溫麻田散鐵形成的實驗證據；第三部分則是分別進行 510°C 持溫不同時間(0 至 4.5 小時)的時效處理，期望 NiAl 成核於麻田散鐵基地，實踐析出強化，過程中以 OM、TEM、 STEM 儀器進行觀察，發現 NiAl 僅需要經由短時間時效處理就會廣泛地均質成核於麻田散鐵基地內，且在後續時效階段會逐漸在缺陷處粗化。本研究的最後則進行拉伸試驗與破斷面觀察，得知此材料的破裂機制屬於延性與脆性破裂共存之混合模式。

This study used the 17Cr-7Ni PH stainless steel provided by Walsin Lihwa Company for phase identification analysis and precipitate hardening research. Alloy 17-7PH is precipitation-hardening stainless steel with excellent fatigue properties, corrosion resistance, and formability. Its composition contains Al and N elements for grain refinement and precipitate hardening by AlN and NiAl, respectively. In the first part, the HR samples were analyzed by the EBSD, KAM, and IQ techniques to distinguish the mixed phase of this material. In the second part, the RH950 standard heat treatment was conducted. Complete martensite transformation into martensite in as-quenched 17-7 PH steel can be achieved by increasing the driving force for martensite formation with further undercooling at sub-zero Celsius temperature. According to standard heat treatment specifications, it is performed by cooling the material to –75℃, followed by isothermal holding at this temperature for 8 hours. This work shows the direct experimental evidence of isothermal martensite in 17-7 PH steel. In the third part, 510°C aging treatment for various times (0~4.5h) was conducted to achieve the effect of precipitate hardening, as well as observed and analyzed statistically by OM, TEM, and STEM, so we can conclude NiAl precipitates nucleated widely in the martensitic matrix by homogeneous nucleation, and these precipitates coarsened at defects after longer aging time. Finally, the tensile test was conducted to evaluate the mechanical properties of this material, and fractographs were taken by SEM. The SEM result shows the fracture surface may be the mixed mode fracture.