Effect Of Zirconia Addition On Microstructure And Surface Properties Of Chromoaluminized Steel

摘要

Pack cementation chromo-aluminizing coating was applied to the 41Cr4 steel using two powder mixtures; the first contained 35% Ferro-chrome, 8% Aluminium, 4% ammonium chloride and 53% kaolin while the second contained additional 8% zirconia. A two-step coating technique was applied by heating first at 925℃ for 3 hrs to allow for Al diffusion, followed by heating at 1150℃ for another 3 hrs to allow for Cr diffusion. The microstructural characterization of the two types of coating was carried out using light microscopy, XRD, SEM, and EDX techniques. The layer thickness and microhardness profiles were also measured. The oxidation resistance was tested at 800, 900, and 1000℃ using weight gain method for a total duration of 20 hrs. The addition of zirconia increased the chromo-aluminized layer thickness from 120 μm to 200 μm with Cr and Al concentrations on the surface increasing from 25% to 47% and from 2.5% to 4% respectively. The average concentration of Zr was 0.5%. The XRD analysis of the coating layer indicated the presence of (Cr,Fe)_7C_3, (Cr,Fe)_3C_2 and Fe_3Al phases. The surface modification due to Zr addition significantly improved scale resistance and led to increasing the surface hardness from 330 HV to 420 HV.