Microstructure And Properties Of Surface Layer On Q235Via Simultaneous Chromising And Nitriding By Pack Cementation Process

Nowadays, chromium nitrides are alternative coating materials which possess high hardness,good wear and corrosion resistance, high temperature oxide resistance simultaneously. Thisstudy aims to prepare chromium nitrides coatings in Q235via simultaneous nitriding andchromiuming by pack cementation, to increase its surface hardness and corrosion resistance. Allthe specimens were treated at1100°C. The pack powder mixtures used for the process consistedof Cr₂N as a source of both N and Cr, NH₄Cl as activator and Al₂O₃as inert filler andanti-sintered supplies; in some cases, Cr powder was added as an additional source of Cr.The effect of pack composition and duration time on the structure of coatings in Q235wasrespectively analyzed by the XRD and SEM/EDS methods. It was demonstrated that there werethree different structural coatings formed due to the change of Cr₂N and Cr content in packmixture: a top Cr₂N and a Cr-enriched diffusion layer underneath; a top （Cr, Fe）2N1-Xlayer witha middle Cr-deposited layer, and a inner Cr-enriched columnar layer; a top Cr₂N layer and aCr-enriched columnar layer underneath. All the surface coatings with the structure of a top Cr₂Nand a Cr-enriched diffusion layer underneath, contains strip precipitations in the diffusion layer.The82Al₂O₃-8Cr₂N-8Cr-2NH₄Cl（wt.%） pack was conformed as the best pack composition toform a top Cr₂N and a Cr-enriched diffusion layer underneath. With the best pack, the effect ofduration time on the surface coatings after pack chromising and nitriding was studied. The topCr₂N layer thickness was parabolic growth with the increasing of duration time in the zone of0.5⁴h. Nevertheless, increasing duration time beyond4h did not further increase the Cr₂N layerthickness, but caused the coarsening of precipitations in Cr-enriched diffusion zone. Thus, thebest pack cementation process to form a top Cr₂N and a Cr-enriched diffusion layer underneathwas confirmed to be with the pack composition of82Al₂O₃-8Cr₂N-8Cr-2NH₄Cl（wt.%） treated at1100°C for4h. With the best pack process, hardness values of more than1800HV at the surfaceand25³1GPa in the top Cr₂N layer was obtained. Three typical specimens were selected fromthe three series of specimens which contains different surface coatings. The corrosion resistanceof the three typical specimens with Q235was studied by the dynamic electrochemical test andimmersion test. It was demonstrated that, the corrosion resistance of the three typical specimenswas much better than Q235. The typical specimen with the coating structure of a top （Cr,Fe）2N1-Xlayer Cr-deposited layer, and a inner Cr-enriched columnar layer, exhibited the bestcorrosion resistance. And the typical specimen with the coating structure of a top Cr₂N and aCr-enriched diffusion layer, exhibited bad corrosion resistance due to the precipitations in theCr-enriched layer.Finally, the effect of different substrates on the structure of coatings by pack chromisingand nitriding was also studied. With the （75Al₂O₃-15Cr₂N-10Cr）-2NH₄Cl（wt.%） pack, a topCr₂N layer of about40μm thickness, and a Cr-enriched layer beneath without precipitations formed on AISI204austenitic stainless steel by packing at1100°C for6h. By pack nitriding andchromising, the coatings formed on Fe-2%Cr-1%Mo alloy were similar with Q235. The Cr₂Nlayer on Q235was the densest. The surface hardness of Cr₂N layer on both AISI204and Q235was much higher than Fe-2%Cr-1%Mo alloy.