粉末射出成形工具鋼之超固相線液相燒結研究

Abstract

粉末射出成形具有製造形狀複雜且難以加工的工具鋼合金之優勢，由於工具鋼材料之燒結技術仍未成熟，目前對440C麻田散鐵不銹鋼與SKD11、M2工具鋼的燒結僅有10~20°C的燒結溫度區間(燒結窗)，過低或過高的燒結溫度將使工件密度過低或變形，主要原因是這三者的超固相線液相燒結區(L+gamma+carbide)的溫度區間過窄，工件常因溫度的些微變化產生大量的液相而導致變形，所以工業界必須使用一溫度均勻的特製燒結爐才可提升良率，故本研究嘗試添加碳化物與合金元素來改善其燒結窗，並輔以熱力學計算軟體Thermo-Calc來瞭解與預測其燒結行為，希望能改善燒結窗過窄之問題並從中尋找熱力學參數與燒結組織之關聯性。實驗結果顯示Ti或Nb可生成TiC及NbC等難固溶於基地之碳化物，或直接加入TiC、NbC，均可擴寬(L+gamma+carbide)之區域，如此可提升原始材料的燒結窗至30~60°C之寬度，並可細化晶粒組織與改善其機械強度，最後由相圖及熱力學數據與實際燒結的實驗結果作比較，我們發現Thermo-Calc可成功地預測添加碳化物與合金元素對超固相線燒結材料如SKD11、M2及440C燒結窗範圍之影響，並成功設計出適合燒結製程之新穎工具鋼材料。

The typical sintering window of supersolidus liquid phase sintered 440C stainless steels and SKD11 and M2 tool steels is less than 10~20 K. To enlarge the sintering window of these materials and obtain parts with good dimensional control, carbides and elements were added and the results were compared to those predicted from the phase diagrams calculated using Thermo-Calc software. Both calculated and experimental results indicate that the addition of titanium and niobium and their carbides broaden the liquid+gamma+carbide region. As a result, the amount of liquid present during sintering becomes less sensitive to the temperature variations. The TiC and NbC provided or formed due to the reaction between carbon and Ti and Nb during sintering are quite stable and help reduce the grain growth. With enlarged liquid+gamma+carbide region and fine grains, the sintering window for retaining the part shape is broadened to 30~60 K and hence the problems of dimensional control of 440C, SKD11, and M2 are alleviated. These results demonstrate that it is possible to predict the effect of the addition of various alloying elements and carbides on the geometry preservation of the high alloyed steels and find new modified alloy compositions that have a wide sintering window during supersolidus liquid phase sintering