淬火冷卻速度及回火持溫時間對熱作工具鋼機械性質之影響

Abstract

本研究是以SKD61之規範鋼及改變規範鋼中某些合金元素含量的五種改良鋼材為實驗材料，在固定的淬火溫度和保溫時間下，用不同的冷卻速度淬火，再以590℃鹽浴爐分成0.5hr、3hr、6hr、10hr四個不同的回火持溫時間，各施以兩次回火，回火完成後水冷至室溫。最後量測各個試片之硬度值和衝擊值，以比較不同熱處理條件下各材料的機械性質，並以光學顯微鏡、SEM與EPMA分析各試片之顯微組織與化學組成，探討淬火冷卻速度和回火持溫時間對各材料顯微組織的影響。 研究結果顯示，淬火冷速較快的試片其硬度值和衝擊值幾乎都比淬火冷速較慢的試片高，其中若含有較多碳、鉻、錳元素的試片，其硬度值也較好。隨著回火持溫時間的增長，各材料的硬度值都會跟著下降，含有較多釩、鉻、鉬的材料下降幅度較小，抵抗回火軟化的能力較佳。 此外，影響衝擊值主要有三個因素：(1)硬度值的下降使韌性變好，衝擊值提升；(2)回火時間增長或淬火冷速變慢，使較多碳化物在晶界上生成，造成韌性下降，衝擊值降低；(3)合金成份在回火時對韌性的影響。上述三個因素在不同的回火持溫時間下的效果強弱各有不同。

In this study, the effect of different quenching cooling rates and tempering holding times on the mechanical properties of hot work tool steels was investigated. SKD61 and its modified steels were used as experimental materials. Quenching temperature and holding time are fixed at 1030℃ and 1hr, respectively. There are two kinds of cooling rate during quenching. After quenching, specimens were dipped in a 590℃ salt bath for tempering, with four different holding times of 0.5, 3, 6 and10 hours. All specimens were tempered twice. Then the hardness and impact toughness of these specimens were measured. In addition, the microstructure and chemical compositions were examined using OM, SEM and EPMA, and the relationships between different heat treatment parameters and mechanical properties were revealed. As results, the specimens which were cooled faster during quenching have higher hardness and impact toughness mostly, and the specimens which contained more carbon, chromium and manganese possess higher hardness value. We found out that there are three main factors affecting the impact toughness value: (1) Impact toughness increases due to the decrease of hardness. (2) Impact toughness decreases because of the precipitation of carbides on the grain boundaries, when the tempering holding time gets longer or the quenching rate becomes slower. (3) the influence of alloy element on the toughness during tempering. The effect of the three factors will vary with the tempering holding time.