粉末高速鋼之顯微組織相變態特性及其相關機械性質之研究

Abstract

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.