Effects of Surface Nanocrystallization Pre-treatment on Gas Nitriding Behavior of Mild Steel

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.