Formation of Al and Cr Dual Coatings by Pack Cementation on SNCM439 Steel

Abstract

Two stage pack cementation processes are developed to form dual Fe–Al–Cr layers on surfaces of SNCM439 steels. The first 550°C treatment assists to modulate adequate aluminum activity for the formation of iron-rich intermetallics. In the second 750°C treatment stage, simultaneous chromizing and aluminizing treatments are achieved by first forming a FeAl ferritic layer and then a surface layer with higher Cr content at later time. The current study examines the effects of second stage 750°C holding time, activator concentration, and Cr:Al ratio on coating structures. Fe–Al coatings consisting of Fe₃Al and FeAl intermetallic phases are observed to form initially. This Fe–Al layer accounts for 25 to 32 μm thickness of the coatings and show good adherence with the substrates. The coating thickness increases parabolically with 750°C holding time. With prolonged treatment at 750°C, surface concentration of aluminum in powder packs drops with treatment time and increasing concentration of activator. A peak concentration exists at a depth below substrate surface. Aluminum is back diffused from the steel surface into the powder packs. The growth of Fe–Al intermetallics slows down. Surface layer then forms a thickness of 6 μm coating with 2–5 wt.% of chromium. Samples treated for longer than 6 h with over 12 wt.% of NH₄Cl activator concentration or with Cr:Al ratio higher than 90:10 induce earlier chromium infusion and lead to porous coating structure due to Kirkendall effect. Eventually chromium carbide forms to cease further growth of the dual coating structures.