材料热处理学报

研究了表面喷丸纳米化预处理对气体渗氮行为的影响。利用低碳钢试样单面表面超声喷丸纳米化处理 ,另一面保持原始晶粒 ,在 46 0℃、5 0 0℃、5 6 0℃、6 40℃四种温度不同时间气体渗氮 ,通过金相观察和X射线衍射法测定渗氮层的厚度和种类。对比发现在 5 6 0℃以下渗氮时 ,经过表面喷丸纳米化预处理 :可以提高扩散系数D和气 -固传递系数 β,降低氮势门槛值 ;使常规渗氮温度降低 5 0℃左右或者渗氮时间缩短 5 0 % ;使渗氮层厚度随渗氮时间增长在初期就符合抛物线规律x =At0 5。 6 40℃短时间渗氮时 ,表面纳米化预处理仍然可以起到一定的加速作用 ,但是随着渗氮时间的延长 ,表面纳米化预处理优势消失 ,甚至会阻碍渗层厚度的增长

The effects of surface ultrasonic shot peening (SUSSP) nanocrystallization on gas nitriding were studied. The mild steel plates were SUSSP nanocrystallized on one side but the original coarse\|grained condition on the opposite side. The relationship between the compound layer growth and nitriding time for two types of surface condition was obtained by gas nitriding at 460℃,500℃,560℃,640℃ respectively with different nitriding times. The thickness of compound layer was determined by metallographic observation and the phases appeared on the top surface were identified by x\|ray diffraction. When gas nitriding below 560℃, SUSSP nanocrystallization treatment could not only enhance both diffusion coefficient D and mass transfer coefficient β dramatically during the gaseous nitriding process but also reduce the threshold nitriding potential. With SUSSP pre\|treatment, the nitriding rate can be accelerated so dramatically that nitriding time can be reduced to the half, or the conventional nitriding temperature could be reduced approximately 50℃. The growth of compound layer with nitriding time assumed parabolic relationship from the start of nitriding process as long as the sample surface was pre\|treated by SUSSP. In the case of gaseous nitriding at 640℃, SUSSP pretreatment is still of some advantages to accelerating nitriding rate when nitriding time is shorter than 3 hours. With the further progress of nitriding, the advantage of SUSSP pretreatment wiel disappears and even on the contrary might slow nitride growth.