Study On The Gas Nitriding/Nitrocarburizing Process And The Modification Of Nitrocarburized Layers Of Low-carbon Steel

Nitrocarburizing is a process of infiltration of nitrogen and carbon on the surface of the workpiece. Nitriding and nitrocarburizing of iron and steel in order to improve the surface hardness, wear resistance, fatigue resistance, corrosion resistance and seizure resistance. The surface brittle compound layers(CLs) composed of ?-Fe2-3N phase and ?'-Fe4N phase were formed on the surface of low carbon steel by nitriding or nitrocarburizing. Moreover, porosity and other loose structural defects are common in the surface compound layers, which lead to the poor toughness and wear resistance of the nitrided layers. For this series of problems, this work put forward a complex heat treatment technology, a low-carbon steel was austenitized in inert gas atmosphere by heating up the furnace directly after gas nitricarburizing, followed by tempering in turn, i.e austenitizing after nitrocarburizing and follow-up tempering(AAN-T). The microstructure and properties of nitrided/nitrocarburized layers were characterized using optical microscope (OM), X-ray diffractometer (XRD), scanning electron microscope (SEM), electron probe (EP), transmission electron microscope (TEM) and multiple performance testers. The main conclusions are as follows:(1)It was found that the effect of nitriding potential on the compound layers (CLs) is dependent on the nitriding temperature in pure ammonia nitriding. When the temperature is not higher than 550?, high nitriding potential significantly increases the thickness of the CLs; when the temperature is not lower than 580?, although low nitriding potential delays the formation of the CLs, the CLs quickly grow after forming. The thickness of the CLs with low nitriding potential reaches and exceeds that of the CLs with high nitriding potential. The N concentration profile of the CLs with high nitriding potential is decreasing along the depth, and the N concentration is as high as 10wt% at the uppermost sublayer. The N concentration profile of the CLs with low nitriding potential is relatively uniform, and its value is about 5-6wt%. The CLs with high nitriding potential have excellent corrosion resistance, but its toughness and density are bad. The CLs with low nitriding potential have less defects and excellent toughness.(2)The surface compound layers (about 28?m) obtained by conventional nitrocarburizing alone at 580? to the low-carbon steels, exists structural defects like the pores and cracks while there is a "best relationship" between the compactness of the compound layers and nitriding potential. The compactness of the compound layers are excellent when the value of nitriding potential is between 0.42atm-1/2-0.68atm-1/2.(3)A complex process system was put forward in this work, i.e a low-carbon steel was austenitized under inert gas atmosphere by heating up the furnace directly after gas nitricarburizing, followed by tempering can get rid of the brittle compound layers composed of ?-Fe2-3N phase and ?'-Fe4N phase, and obtain surface well performance bainite layers, whose composite structure is mainly composed of (?-Fe+?'-Fe4N+?"-Fe16N2).(4)The phenomenon of denitriding of the surface nitrided layers is easy to occur at the stage of austenitizing in AAN-T process, which is to reduce the density of surface layers due to the formation of pores and cracks. When the higher the value of N concentration of the compound layers are (especially close to the surface), the acuter the degree of denitriding at the stage of austenitizing is, and defects of surface loose are more obvious. The compact surface bainite layers with the thickness above 40?m and the surface hardness above 1023HV0.05 are acquired by the AAN-T process when nitriding potential value and time are 0.68atm-1/2 and 1h at the stage of nitrocarburizing (i.e short time and low nitriding potential). The compact surface bainite layer owned more excellent fracture toughness and wear resistance than that of brittle compound layer which obtained by nitrocarburizing alone.(5)Compared with tempering alone after nitrocarburizing, the complex process of austenitizing and tempering after nitrocarburizing can not only improve the hardness and surface toughness of nitrided layers, but also more conducive to the regulation of the tensile properties and anti fatigue properties of the nitrocarburized specimens.(6)A low-carbon steel was austenitized and follow-up tempered at different temperatures after nitrocarburizing, the austenite decomposition products have the most excellent toughness after tempering at 120?-160?. The primary reinforcement phase of the surface bainite layers may be ?"-Fe16N2 phase other than nano ?'-Fe4N phase when the temperature of tempering is not higher than 180?.