可控制冷速之熱處理設備及其在熱作工具鋼淬火之應用

Abstract

本研究嘗試以PID控制器控制工件在管型爐內的移動，達到工件等速降溫的目的，並以此種等速冷卻的方式實施熱作模具鋼SKD61的規範鋼以及改良鋼的淬火，之後回火至相同硬度下比較各工件之韌性，並透過觀察顯微組織以及比較各材料的元素配置，以了解在此等速冷卻的條件下，合金成份對於韌性影響。 實驗的結果顯示，PID控制可以將工件淬火時的降溫曲線控制到相當平順的等速冷卻，而隨著淬火期間冷卻速率的降低，各鋼材韌性值皆隨之下降。在不同鋼種間，合金含量較高的改良鋼，在此種等速淬火的條件下，雖然由於硬化能較大而得到較高的淬火硬度，但是經過高溫回火至相同硬度後，其韌性表現大多不如合金元素含量較低的規範鋼。根據此種等速冷卻方式的淬火以及合金元素配置對於CCT圖的影響，推究其原因如下： 1.本等速冷卻的淬火方式相對於一般連續冷卻的淬火方式，在相同的平均冷速下，淬火初期（750℃以上）的冷速較慢，故生成粒界碳化物的機會大幅增加。 2.改良鋼由於Mo含量較多，使得淬火期間粒界碳化物形成的趨勢較高，在CCT圖中的表現即為粒界碳化物的生成曲線向左偏移（往短時間移動），使得相同冷速下淬火後得到的粒界碳化物較規範鋼為多，從而影響回火後的韌性表現。 3.高Mn低V的合金配置方式，由於Mn的增加而降低沃斯田鐵化的溫度，V的減少而使在高溫可抑制沃斯田鐵晶粒成長的VC較少，使得改良鋼在相同溫度淬火時，其沃斯田鐵的晶粒較為粗大而降低韌性。

To quench at a constant rate, in this research, a quenching equipment was designed by controlling the position of specimen in the furnace tube with a PID temperature controller. Standard and modified hot work tool steels were quenched with a constant cooling rate, then tempered to the same hardness to compare their toughness. After observing the microstructure and comparing the composition of each steel, the effect of alloying elements on the toughness in the constant rate quenching was varified. The result indicates that the cooling curves were well-controlled to be a smooth straight line by the designed equipment, and the toughness decreased with the reducing of cooling rate for each steel. Between different steels, the modified ones with higher alloy elements are with higher hardness after quenching because of their higher hardenability; however, their toughness performance are inferior to the standard ones. According to the effect of the constant-rate- quenching and composition on the CCT diagram of each steel, the reasons why the modified steels are inferior to the standard ones on toughness are presumed below: 1.The initial(above 750℃) cooling rate of constant-rate-quenching is far slower than the conventional quenching for the same average cooling rate, that is, the chance to form prior grain boundary carbide is higher than the conventional. 2.Because of the higher molybdenum, the modified steels are with higher tendency to form prior grain boundary carbide than the standard ones. This prior grain boundary carbide could severely cause the deterioration of toughness. 3.Compared with standard steels, the modified ones contain higher manganese and lower vanadium, causing lower austenizing temperature and fewer vanadium carbide to suppress coarsing of grains. Modified steels, quenching from the same temperature as standard ones, maybe overheated to form coarse grains, therefore, worse toughness performance is expectable.