The Gaseous Carburisation of Austenitic Steels

Abstract

Abstract The paper is concerned with the corrosion behaviour of materials in high temperature gaseous carburisation environments and initially reviews the current ‘state‐of‐the‐art’ in this field. Results of investigations carried out on three 25 Cr‐20 Ni steels exposed to carburising H 2 ‐CH 4 mixtures with a controlled carbon activity of 0.8 at 825°C and 1000°C are then presented. Kinetic data for exposure periods of up to 1100 hours are given and it is shown that gross carburisation of these alloys has occurred after only short exposures at 1000°C. The importance of alloy composition and surface condition, particularly during exposure at 825°C, is demonstrated. The results of surface and cross‐sectional examinations using a range of optical, electron and X‐ray techniques are reported. The use of a nuclear microprobe technique has enabled accurate carbon concentration profiles to be established and diffusion coefficients for the three alloys have been determined. In addition, the use of a magnetic technique for the non‐destructive monitoring of the extent of sub‐surface carburisation has been investigated.