The influence of carburising temperature on the case depth and properties of vacuum carburised precision gears

Abstract

Low pressure gas carburising (LPGC) or vacuum carburising is a heat treatment process used in the aerospace and automotive industries to surface harden low alloy steel parts with carbon in the range of 0.1 to 0.25%. This project examined the LPGC process used to case harden precision gears and shafts used in various helicopter engines. Due to the stringent quality requirements specified by the OEMs (Original Engine Manufacturers) and by aerospace quality standards, approval can only be ensured through careful control of the carburising temperature, time and atmosphere during heat treatment. Final acceptance requires that the case depth, surface hardness, core hardness and component microstructure (related to the effectiveness of the quench and temper process) be controlled to within strict tolerances. This investigation utilised an industrial vacuum carburising furnace and an acetylene atmosphere to examine the effect of carburising temperature on the properties of the carburised surface layer in parts machined from 16NCD13 carburising steel. The project aimed to determine whether the total carburising time could be decreased by increasing the carburising temperature without adversely affecting the specified case depth, hardness values or microstructure. The predictions of published carbon diffusion models (taking into account the influence of temperature, changing carbon concentration and alloying element content on the diffusion coefficient of carbon in austenite) were compared with the carbon concentration profiles measured after carburising at four different combinations of time and temperature. The results showed that increasing the carburising temperature from 900°C to 940°C, while reducing the carburising time from 104 to 64 minutes, did not have any detrimental effect on the case depth, case hardness, core hardness, component microstructure or part dimensions, while resulting in a significant reduction in the total carburising time. The mechanical properties of the test pieces were within specification and the grain size was not adversely affected by the higher heat treatment temperature. Increasing the carburising temperature to 960°C (and simultaneously reducing the carburising time to 44 minutes), however, caused a reduction in mechanical properties to below specification. Published carbon diffusivity models that consider the influence of temperature, increasing carbon concentration and the alloy content of the component were found to predict the actual carbon concentration profiles to a reasonable degree of accuracy.