镁对GH3625合金一次碳化物析出的影响

Effect of magnesium on precipitation of primary carbide in GH3625 alloy

GH3625合金中碳的质量分数约为0.05%,由于含有较高的铌、钼和铬元素,合金中会形成MC型、M_(6)C型和M_(23)C_(6)型碳化物,在冶炼凝固过程中由于选分结晶的原因,易产生碳化物偏聚问题。因为碳化物回溶温度偏高,在其可锻温度区间内很难消除,所以会导致合金棒材中存在碳化物条带状聚集的问题,对其服役性能影响较大。利用Thermal-calc热力学软件计算分析了GH3625合金平衡析出相及一次碳化物的析出规律,利用金相显微镜、扫描电镜等研究了镁对GH3625合金一次碳化物形貌、尺寸及分布的影响。结果表明,GH3625合金的基体为单一的奥氏体相,MC型碳化物作为一种高温析出相,直接从液相析出,其富含铌元素,其次还有少量的钛、钼等元素。而随着凝固温度的降低,铌质量分数逐渐升高;未加镁时,GH3625合金在二次枝晶间析出了大量长条状的一次碳化物,其平均直径和面积较大;加入质量分数为0.014%的镁后,镁通过改变碳化物相与基体相的比界面能关系,不仅有效地改善了合金枝晶间和晶界碳化物的分布及形貌,还减小了一次碳化物的尺寸;当镁质量分数增加到0.037%时,一次碳化物分布更加均匀弥散,此时镁细化、球化碳化物的效果最好;同时,试验合金在水冷和空冷的条件下,由于冷却速率比炉冷时更大,一次碳化物析出尺寸也相对更细小。

The mass percent of carbon in GH3625 alloy is about 0.05%.Because of the high content of Nb,Mo and Cr,carbides of MC,M_(6)C and M_(23)C_(6)type will be formed in the alloy,and carbide segregation will easily occur due to the selective crystallization during solidification.Because of high redissolution temperature,carbide is difficult to eliminate in the wrought temperature range,which will lead to the problem of carbide banded aggregation in alloy bars and have a great influence on its service performance.Thermodynamic software of Thermal-calc was used to calculate and analyze the equilibrium precipitated phase and precipitation law of primary carbide in GH3625 alloy.The effect of Mg on the morphology,size and distribution of primary carbide in GH3625 alloy was studied by metallographic microscope and scanning electron microscope.The results show that the matrix of GH3625 alloy is a single austenite phase,and MC carbide,as a high temperature precipitated phase,precipitates directly from the liquid phase,which is rich in Nb,followed by a small amount of Ti,Mo and other elements.With the decrease of solidification temperature,the mass percent of Nb gradually increases.When Mg is not added,a large number of strip-shaped primary carbides are precipitated between secondary dendrites in GH3625 alloy,and their average diameter and area are large.After adding 0.014%Mg,Mg not only improves the distribution and morphology of carbides at interdendritic and grain boundary,but also reduces the size of primary carbides by changing the specific interfacial energy relationship between carbide phase and matrix phase.When the mass percent of Mg increases to 0.037%,the distribution of primary carbides is more uniform and dispersed,and the effect of refining and spheroidizing carbides by Mg is the best.At the same time,under the conditions of water cooling and air cooling,the cooling rate of experimental alloy is higher than that of furnace cooling,and the size of primary carbide precipitation is relatively smaller.