Heat Treatment Simulation for Low Pressure Hyper Carburizing Process

Carburizing heat treatment is well used to increase the strength of gears and automobile drivetrain parts. When very high contact fatigue strength is required, hyper carburizing or carbonitriding is well used to improve the surface hardness and tempering softening resistance. Over saturate carburizing as called hyper carburizing can be used in a conventional ordinary carburizing furnace and investment of new gas supply equipment is unnecessary, while there are problems such as controlling precipitation of cementite and securing hardenability of the surface, etc. Also, problems such as secondary precipitation to control the material micro structure, its quality prediction method becomes important.In recent years the low pressure carburizing method has been widely used and the possibility of making hyper carburizing in low cost has been expanding. However, when hyper carburizing is performed by low pressure carburizng, reticulated cementite occurs at the edge of the part, there is a problem such as local increase of carbon concentration distribution and toughness. In this study, we tried to formulate precipitation when super carburizing was performed on steel parts using a low pressure carburizing furnace, formulated the surface hardness distribution, and tried to calculate the hardness and cementite precipitation size distribution.CALPHAD method is used to obtain the physical property values, graphite of an intermediate product was precipitated from the low pressure carburizing atmosphere on the surface layer, and it was assumed that carburization occurred therefrom. Estimating supersaturated carbon precipitates as cementite, the growth amount of the precipitate is calculated. The deposition rate was estimated by comparing the actual measurement with the calculated value. Estimating hardness is determined by the sum of the hardness of the precipitate and the hardness of the parent phase part, the hardness distribution was verified. As a result, the hardness distribution at the time of high concentration carburization in the test piece shape was predicted. The method was also applied to the actual part shape, and the quality difference between the edge part and the flat part of the gear and the heat treatment deformation quantity were estimated.