Hot-Dip Aluminizing on AISI F55–UNS S32760 Super Duplex Stainless Steel Properties: Effect of Thermal Treatments

Abstract

Abstract:The behavior of super duplex stainless steels AISI F55-UNS S32760 in hot-dip aluminizing process has been studied, investigating the influence of cold working and of different initial microstructures obtained through a preliminary thermal treatment. The microstructural features examined are the secondary austenite precipitation, the static recovery of ferrite and the thermal dissolution of austenite within ferritic matrix. The hot-dip aluminizing temperature has been optimized through sessile drop tests. The treatment has been performed at 1100 • C for 300 s, 900 s and 2700 s. A strong chemical interaction occurs, generating intermetallic compounds at the interface. Molten aluminum interacts exclusively with the ferritic phase due to its much higher diffusivity in this phase coupled with its marked ferrite-stabilizer behavior. Thus, the influence of cold working is not remarkable, since the strains are mainly allocated by austenitic phase. The diffusivity of aluminum increases due to lattice defects thermally generated and, mainly, to influence given by grain boundaries, multiplied by secondary austenite precipitation, which act as short-circuit diffusion paths. Ni and Cr contents in the ferritic matrix have an influence but not highly relevant. Then, the best starting condition of the super duplex stainless steel substrates, to obtain a thick interfacial layer, are the thermal annealing at 1080 • C for 360 s/mm after a solution thermal treatment at 1300 • C for 60 s/mm.

A review on metallurgical features of hot-dip aluminized steel

“…Results of fundamental thermodynamic studies on the effect of various alloying or impurity elements demonstrate that the addition of elements can be classified into two groups-elements like Si, Cu, Ti, Mg, etc, which decrease interatomic interaction, resulting in a lowering in thickness of the intermetallic layer, and another group of elements like Mn, Ni, etc which result in a tendency of increase in the intermetallic layer [138]. The addition of copper as an alloying element in aluminium has been well-known for many years to enhance the strength properties.…”

“…Heat-resistant stainless steels, widely used in petroleum, chemical, nuclear and other applications, obtain a beneficial effect from aluminium: the high-temperature oxidation resistance is recognized as providing a protective alumina surface layer during high-temperature exposure. These aluminide coatings result in superior resistance to abrasion, oxidation, carburization, and sulfidation [138]. The interaction and wettability of molten aluminium with super duplex stainless steel UNS S32760 (Cr 24-26 wt%, Ni 6-8 wt%, Mo 3-4 wt%, W 0.5-1 wt%, Cu 0.5-1 wt%) in hot-dip aluminizing process had been studied.…”

“…It was envisaged that an aluminium-rich barrier coating would effectively resist the ingress of damaging species, including atomic hydrogen, which can cause embrittlement. A thorough study was undertaken to obtain a thick interfacial layer on Super Duplex Stainless Steel specimens with different processing backgrounds by hot dipping in pure aluminium [203]. Specimens were given different heat treatments prior to aluminizing.…”