Desulphurization of Nickel Base Super Alloys

A superalloy, or high-performance alloy, is an alloy able to withstand extreme temperatures that would destroy conventional metals like steel and aluminum. Nickel based superalloys are selected for use in certain applications due to their characteristics. These alloys have an exceptional combination of high temperature strength, toughness, and resistance to degradation in corrosive or oxidizing environments. Among the important characteristics are creep resistance at high temperatures, good surface stability, and corrosion and oxidation resistance. Because of these characteristics they are widely used in aircraft and power-generation turbines, rocket engines, nuclear power and chemical processing plants and other challenging environments.

Ni base superalloys are multicomponent complex alloys which, in addition to Ni, contain varying amounts of Cr, Mo, W, Nb, Al, Ti, Ta, Re, Hf, Zr, B, and C to obtain the desired strength, oxidation resistance, and corrosion resistance.

The presence of impurities, such as sulfur, phosphorous, oxygen, and nitrogen, in nickel-based alloy can combine with the host metal or other alloying elements to form precipitates that have detrimental effects on mechanical properties of the final product.

In order to meet the increasing demand on the quality and performance of nickel-based alloy, therefore, many efforts have been made to reduce the sulfur content below the critical values during the production of superalloy since 1970s. The mature desulfurization practices during vacuum induction melting (VIM) refining Ni-based alloy are the addition of NiCa and NiMg in a vacuum induction furnace and/or using a CaO crucible. In consideration of the limits of desulfurization in VIM, however, lots of attention has been given to electroslag remelting (ESR) in recent years. To the best of the authors’ knowledge, up to now, effect of CaF₂–CaO–Al₂O₃–MgO–TiO₂ slag with different TiO₂ content on the desulfurization of Inconel 718 alloy has not been reported. In the current study, therefore, the desulfurization behavior of the nickel-based alloy was investigated at 1773 K.

The effect of sulphur in superalloys:

• Shows a tendency to grain boundary segregation
• Forms a low melting (635°C) Ni₃S₂ phase at concentrations of 10 ppm and higher
• Decreases stress-rupture life dramatically
• Causes hot shortness

The origin of sulphur:

• Sulphur is a common tramp element in primary raw materials, in particular in chromium, iron and nickel ores

Current desulphurization practice:

• Addition of NiCa and NiMg in vacuum induction melting (VIM) just before casting
• Mg/S ratio >1 supports HT ductility

Disadvantages:

• Significant evaporation of calcium and magnesium in VIM
• low yield of desulphurization agents
• contamination of the furnace chamber
• safety issues
• Ca corrodes refractory lining and slag sticks to crucible wall
• decreases life cycle of the crucible
• Sulphur can be re-dissolved in subsequent melts
• Excess Mg can form MgNi₂ Laves phase, degrades creep rupture properties Melt Melt

Experimental studies by Richadson and Fincham and Denier have demonstrated that using lime (CaO) in the desulphurization process with top slag as reagent for the molten steel is achieved to fix the sulfur in the slag; (Equation 1).

In the FeNi case and following the theory, the results were the same, taking into account that molten FeNi behavior is similar than molten steel. Then, we were able to eliminate large amounts of sulfur from crude FeN4 if the metal remains deoxidized, (Equation 2).

Mo.S + (CaO)_Slag ↔ Mf + (CaS)_Slag ...(1)
(Ma)_Desoxidante + Mo.S + (CaO)_Slag → Mf + (CaS)_Slag + M_Oxidos ...(2)

Ma: Alloy or deoxidized metal
Mo: Crude metal (FeNi)
Mf: Refined metal
M_Oxidos: Oxides formed
Slag: Slag