Hydrogen uptake during Carburizing and Effusion of Hydrogen at Room Temperature and during Tempering

Abstract: The carburizing atmosphere during the case hardening process contains a large proportion of hydrogen. Due to the rapid diffusion of hydrogen a high amount of hydrogen can be absorbed by the carburizing component. The amount of absorbed hydrogen is dependent on some factors such as for example the carburizing time and component dimensions. Hydrogen diffused in material can then cause hydrogen embrittlement and in some cases cause cracking under a static load. This hydrogen must therefore be removed. High amounts of hydrogen diffuse out spontaneously at room temperature. Tempering accelerates the process. The aim of this study was to experimentally measure the amount of absorbed hydrogen after case hardening and hydrogen content after storage at room temperature and also after tempering. The effect of the enriching gas in carburizing furnace on hydrogen absorption was investigated in this study. Three steel grades with different content of alloying elements were used in this investigation. Steel samples were case hardened by gas carburizing and tempering. The hydrogen content analyses included the measurement of hydrogen content before case hardening, after case hardening and after tempering using Leco-RHEN602. Based on the results in this study it was concluded that all steel grades used in this investigation absorb hydrogen during case hardening by gas carburizing. A major part of the absorbed hydrogen is then released by effusion after being stored at room temperature and during tempering. Around 50% of the absorbed hydrogen content during gas carburizing is due to the presence of the enriching gas in the carburizing atmosphere. Around 50 % of hydrogen diffuses out of the steel specimens after one day. It is likely that all of free diffusible) hydrogen has diffused out of the specimens of two steel grades after one week at room temperature or after tempering.