Microstructural Investigations on Hot-Dip Aluminized and Subsequent Diffusion Treated AISI 321 Stainless Steel

Abstract

In this study, the formation of microstructural features during hot-dip aluminizing and subsequent diffusion treatment of AISI 321 stainless steel is investigated. The mechanism of microstructural evolution is compared with the low-carbon steel/Al and AISI 430 steel/Al system. The microstructural details are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffractometry (XRD). During solid-liquid interaction, an aluminide(intermetallic) layer is formed at the interface between steel and aluminum. The aluminide layer consisted of two phases; namely; Al13Fe4 and Fe2Al5 in case of the low-carbon steel/Al and the AISI 430 steel/Al system. The growth of the aluminide layer is parabolic following reaction-diffusion type of growth mechanism. In the case of AISI 321 steel/Al system, the constituent phases of the aluminide layer are found to depend on the dipping time. During short interaction time of 10 s, metastable microstructures were formed. These are FeAlm, multiple twinned Al13Fe4 formed in the aluminide layer and Al3(NiFe) formed in the topcoat as one of the eutectic phases with Al. With the increase in dipping time to 10 minutes, the aluminide layer consisted of nanocrystalline Fe2Al5, Al7Cr, and Al. crystalline approximant phases closely related to quasicrystals were observed. Ordered phases with ordering along [100] direction is observed. Two variants, five-layered and eight-layered ordered phases are present. OE type of Al-FeCr orthorhombic approximant phase was observed. The topcoat consisted of intermetallic phases such as Al7Cr and Al13Fe4 dispersed in an Al matrix. The mechanism of microstructural evolution in case of AISI 321 steel/Al system is found to be of dissolution-nucleation type. Diffusion treatment of the aluminized AISI 321 stainless steel was carried out to investigate the phase transformation in the coating during high-temperature exposure. The coating transformed into a layered structure consisting of four layers. The outermost layer consisted of Fe2Al5 phase. The next layer consisted of a mixture of three phases consisting of Fe2Al5, disordered FeAl and a new phase with a simple cubic structure. The lattice parameter of the simple cubic structure was measured to be 7.2 Å, and that of disordered FeAl is 4.8 Å. The phase with the simple cubic structure shared a cube on cube orientation relationship with the disordered FeAl. Towards the base metal side, FeAl and ferritic layers were observed.The ferritic layer formed by diffusion of aluminum atoms and precipitation of the NiAl phase. Finally, the hot-corrosion resistance of aluminized AISI 321 steel under the salt mixture of 60%V2O5 + 40% Na2SO4 at 700 ℃ and cyclic oxidation test under an open atmosphere at 900 ℃ was evaluated. The formation of adherent Al2O3 scales on the coated sample provided increased resistance to hot corrosion, while discontinuous oxide scales with cracks and spallation caused poor cyclic oxidation resistance.