The characteristics of microstructure phase transformation and mechanical properties in GPM A30 powder metallurgy high speed steel

本研究是針對榮剛重工生產的粉末高速鋼GPM－A30做特性、組織等方面的探討，企圖以穿透式電子顯微鏡組織為研究方向，配合掃描式電子顯微鏡組織觀察，以及熱膨脹曲線分析，探討高速鋼淬火及各次回火組織之相變態。 隨著淬火溫度的升高，碳化物固溶於基地的量急遽升高，使得基地中的碳含量提高，降低沃斯田鐵組織的Ms溫度，對沃斯田鐵有安定化的作用，故沃斯田鐵再淬火後的殘留比例也急遽升高。淬火溫度提高，強化基地的效果降低，再由於殘留沃斯田鐵增加，使硬度下降。 殘留沃斯田鐵在回火的過程中，分解產生碳化物，而使得Ms溫度提高，有利於麻田散鐵生成。由熱膨脹曲線可知，經三次回火後，殘留沃斯田鐵幾乎已完全分解而不存在。同時，由實驗結果曲線證明，一次回火的效率不大，只有多次回火才能使殘留沃斯田鐵完全（或幾乎完全）相變。 經由顯微組織觀察可以發現，殘留的沃斯田鐵在回火的冷卻過程中會轉變為麻田散鐵，這種新生的板片麻田散鐵（在高碳中，通常為雙晶麻田散鐵）是未經回火，對基地的韌性是有害的，所以必須再進行第二次的回火，使先前硬脆的新生麻田散鐵析出合金碳化物而轉變為強韌的回火麻田散鐵組織，並且兼具適當的硬度與韌性；實驗後證實回火期間沃斯田鐵之催化作用能有效地使基地重新調質，並利用多次回火冷卻的過程來消除殘留沃斯田鐵且逐步相轉變而得到最終回火麻田散鐵的穩定組織。The effect of tempering on the decomposition of retained austenite in a powder metallurgy (PM) high-speed steel, GPM A30, has been monitored with a high-speed dilatometer. The corresponding microstructures of specimens with different tempering cycles have been investigated by a combination of scanning electron microscopy and analytical transformation electron microscopy. The as-quenched structure of the steel studied is composed of retained austenite, untempered martensite and carbides. The results indicate that the complete transformation of retained austenite can be more nearly accomplished by two or triple tempering cycles than by a single long-time cycle. The possible transformation mechanism for the decomposition of retained austenite during multiple tempering cycles is attributed to the invariant-plane-strain of the prior martensitic transformation extending accommodation defects to the adjacent retained austenite, which favors further transformations in the subsequent tempering operations. There has been intensive research work on retained austenite and lath martensite in low-carbon alloy steels. However, little TEM research work has been carried out on twinned plate martensite with retained austenite in high-carbon alloy steels . It is naturally very difficult to produce electron-transparent samples for TEM (because the high-carbon alloys are quite brittle), but TEM investigation continues to assume greater significance in research. GPM A30 is a high-carbon grade in commercial P/M high speed steels, and is widely used in such applications as metal cutting tools and metal forming dies. In this work, dilatometric experiments were performed to investigate the tempering response in quench-treated specimens of GPM A30 high speed steel. The resulting scanning electron micrographs and transmission electron micrographs were examined to elucidate the microstructural evolution.目錄 第一章 前 言............................. 1 第二章 文獻回顧............................. 4 2.0 簡介..................................... 4 2.1傳統冶煉鑄造高速鋼與粉末高速鋼之比較...... 4 2.1.1 GPM A30（相當於AISI ASP30）粉末高速鋼...5 2.2 麻田散鐵相變態 ............................6 2.2.1 麻田散鐵的特徵..........................7 2.2.2 麻田散鐵的晶體結構......................8 2.2.3 麻田散鐵的結晶學........................9 2.2.4 麻田散鐵的型態學........................12 2.2.4.1 板條狀麻田散鐵（Lath Martensite）.....13 2.2.4.2 板片狀麻田散鐵 (Plate Martensite).....14 2.2.5 殘留沃斯田鐵 ............................15 2.3合金元素對高速鋼之影響.....................16 2.3.1碳(C)....................................16 2.3.2鎢(W)....................................17 2.3.3鉬(Mo)...................................18 2.3.4鉻(Cr)....... ............................18 2.3.5釩(V)....................................19 2.3.6鈷(Co)...................................19 2.4高速鋼碳化物的探討.........................20 2.4.1高速鋼中碳化物的種類.....................20 2.4.2合金碳化物對高速鋼的影響.................22 2.4.3 合金鋼碳化物相變態模式..................22 2.5高速鋼之熱處理.............................23 2.5.1高速鋼的淬火硬化與深冷處理...............23 2.5.2含V、Mo、Cr等合金鋼之回火，通常可以分成下列五個階段： .....................................24 2.5.3高速鋼的回火熱處理.......................26 第三章 實驗方法..............................34 3.1實驗材料...................................40 3.2 實驗流程..................................41 3.2掃描式電子顯微鏡(SEM)的觀察和EDAX之半定量成份分析 ...........................................42 3.3熱處理試棒的製作及使用儀器.................42 3.4穿透式電子顯微鏡（TEM）的試片製作及觀察....43 第四章 粉末高速鋼GPM A30相變態研究............49 4.1最佳淬火溫度之選擇.........................49 4.1.1前 言...................................49 4.1.2晶粒大小與淬火溫度之關係.................49 4.2熱膨脹曲線之探討...........................53 4.2.1前 言...................................53 4.2.2高速工具鋼熱處理之膨脹曲線...............53 4.3顯微組織觀察...............................57 4.3.1原有狀態（As Received）的顯微組織........57 4.3.2淬火狀態的顯微組織.......................58 4.3.2.1固溶淬火溫度的影響.....................58 4.3.2.2 1160℃×3min淬火後之SEM顯微組織........58 4.3.2.3 1160℃×3min淬火後之TEM顯微組織........59 4.3.3回火狀態的顯微組織.......................61 4.3.3.1 SEM顯微組織...........................61 4.3.3.2 TEM顯微組織...........................64 第五章 結 論...............................106 第六章 建議未來工作.........................108 參考文獻.....................................10