Low temperature aluminising of Fe-Cr-Ni super alloy by pack cementation

Abstract

A low temperature pack-aluminising diffusion coating was conducted on Fe-Cr-Ni super alloy at 500, 550, 600, 650 and 700 degrees C for 2, 3 and 4 h. The aluminising powder packs were prepared using Al powder as a source for depositing aluminium, A1203 powder as an inert filler and ammonium chloride NH4Cl as an activator. The microstructures of the coatings formed on the alloy surface were characterized by means of SEM/EDS and XRD analysis. SEM analysis revealed that coating layers were homogenous, compact and nonporous and there was a good bonding at the interface of the coating and matrix. Layer thickness variation was measured from the surface to the matrix and changed from 1 mu to 200 pm which was increased with increasing process time and temperature. It was seen that the lowest temperature 500 degrees C was not adequate for deposition of aluminium layer on the alloy. Besides this it is determined that at 700 degrees C for 4 h process time and temperature has provided to thickest coating layer. The growth kinetics of the layer is analysed by measuring the depth of aluminide layer as a function of the time and process temperature (550-650 degrees C). The activation energy calculated by using Arrhenius was 207 kJ/mol. The hardness of the coating layer increased to 850 HVN with the increasing process time and the temperature while the hardness of the matrix was 350 HVN.