不同溫度區間的淬火冷速對熱作工具鋼衝擊韌性的影響

Abstract

本研究使用JIS SKD61的規範鋼種以及改良鋼種作為實驗材料，並在固定的淬火溫度(1020 oC)以及持溫時間下(1hr)，用不同的冷卻速率進行淬火，淬火時將其冷卻過程分為三個階段，分別為高溫區(1020oC~700oC)、中溫區(700oC~500oC)、低溫區(500oC~300oC)，為了模擬現場淬火作業厚度大於300mm之模具中心實際的溫度變化歷程，依照各鋼種的CCT曲線圖推測出適當的淬火冷速作為初始條件，並以每次只改變其中一個溫度區間的淬火冷速的方式將冷速降為初始條件的2/3或1/2，淬火後的試片接著回火至相同的目標硬度HRC47±_1^(0.5)，之後比較衝擊值，並以光學顯微鏡、SEM分析各試片的顯微組織與化學組成，探討各溫度區間冷速調降對衝擊韌性的影響。 研究結果顯示，各鋼種以低溫區冷速調降的條件淬火時最容易造成衝擊值下降，其次為高溫區冷速調降的條件，而以中溫區冷速調降的條件淬火後則對衝擊值影響不大。此外，原材組織中若有數量較多且粗大的網狀碳化物、碳化物顆粒、以及數量較多的巨觀非金屬夾雜物(20µm以上)，將會導致含有這些組織的鋼種在以各條件淬火回火後，衝擊值會有明顯的下降。

This study used the steel of JIS SKD61 and the improved steel as experimental materials. The quenching temperature (1020℃) and holding time (1hr) were fixed for quenching at different cooling rates. The cooling process was divided into three stage during quenching, which are the high temperature range (1020℃~700℃), moderate temperature range (700℃~500℃) and low temperature range (500℃~300℃). In order to simulate the actual temperature change in the mold center thicker than 300mm in the field quenching operation, the appropriate quenching cooling rate was reasoned from the CCT curve diagram of various steel grades as the initial condition. The quenching cooling rate of only one temperature interval was changed each time to reduce the cooling rate to 2/3 or 1/2 of initial condition. The quenched sample was tempered to the same target hardness HRC47±_1^(0.5) for impact test to compare the impact values. The microstructure and chemical composition of various samples were analyzed by optical microscope and SEM. The effect of reducing the temperature interval cooling rates on the impact toughness was discussed. The findings showed that the impact value is most likely to decrease when various types of steel are quenched in the condition of low temperature range cooling rate scale down, and then in the condition of high temperature range cooling rate scale down. The quenching in the condition of moderate temperature range cooling rate scale down has slight effect on the impact value. In addition, if the raw material structure contains considerable and continuous net carbide, as well as considerable macroscopic nonmetallicinclusions (above 20μm), the impact value of the steel containing these structures decreases markedly after quench tempering under various conditions.