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 μ m 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 μ m 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.

Modeling of Surface Modification of Stainless Steel by Halide Activated Pack Cementation Method

“…Halide activated pack cementation process is essentially an in situ chemical vapor deposition (CVD) coating process. A typical pack consists of four components (Chen et al 2012):…”

“…The diffusion elements modify the surface chemical composition resulting in phase transformations along the depth of the material (Casteletti et al 2009). Therefore, the coating composition is dependent on processing temperature, time, substrate composition, and atmosphere (Chen et al 2012).…”

Composition, Structure, and Properties of Ti, Al, Cr, N, C Multilayer Coatings on AISI W1-7 Alloyed Tool Steel

“…Multilayer multicomponent coatings with aluminum are particularly interesting [16][17][18]. It can be assumed that in titanium aluminizing and chrome aluminizing, the coatings will combine the heat resistance of aluminum compounds with a hardness of carbides, titanium nitride, and chromium (TiN, TiC, Cr 7 C 3 , and Cr 23 C 6 ) on the surface of carbon steel.…”

A study on preparation and corrosion behavior of nano rare earth oxide‐modified chromized coatings

“…However, these methods often require expensive equipment and huge operating costs. Fortunately, pack cementation is a cost‐efficient method to prepare metallic coatings, such as Al, Ti, and Cr, on the surface of metallic substrates with satisfactory interfacial strength . It is an in‐situ and self‐generated CVD process to deposit a diffusion coating in essence.…”