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Abstract
High-nitrogen content austenitic stainless steel parts have been processed by injection moulding of a novel feedstock composed of gas atomized P558 stainless steel powder and biopolymer-based binder. The binder formulation consists of polyhydroxyalkanoate backbone polymer produced by bacterial fermentation, filler wax and surfactant. The solids loading was 60 vol.%. Tensile test specimens were injection moulded, debinded and sintered at 1270°C under nitrogen. The microstructure was characterized by metallographic observation, X-ray diffraction and X-ray computer tomography. Yield stress of 600 MPa, ultimate tensile strength of 900 MPa were achieved, meeting the reference values of the wrought alloy.
INTRODUCTION
Stainless steels for surgical and dental prosthetics must meet several requirements, among which are good biocompatibility, absence of ferromagnetism, high corrosion resistance and good combination of strength, ductility, fatigue endurance and wear resistance. In addition, the use of nickel-free stainless steels is important to avoid the harmful effect of nickel-ion release in the human body [1]. Several development works and production from the 90's have shown that nitrogen combined with molybdenum and manganese can effectively replace nickel as austenite formation element and increase the corrosion resistance [2-3]. In high-nitrogen austenitic stainless steels processed by powder metallurgy, nitrogen can be added via the sintering atmosphere [4]. A feedstock with polyacetal binder is commercially available, the application of which under different sintering and solid state nitriding conditions has been extensively studied [5].
Cost issues and market considerations have limited the use of PM nickel-free steels until now. In the case of MIM, the development of new environmental friendly systems can offer new opportunities for widespread use of nitrogen strengthened stainless steels. Compared with polymers derived from oil, natural polymers offer the advantage of better biocompatibility, biodegradable character and sustainability. MIM binders are usually multi-component, composed of a filler phase (which provides good flowability), a surface active phase (to ensure good wettability of the powder by the binder), and a backbone phase (which provides green part strength). Among the natural polymers which can be used as filler phase, they are natural waxes as carnauba wax. Castor oil, seed oil and stearic acid are biosourced surfactants. Finally, as a backbone phase, one can notably cite polyhyd roxya lkanoates, bio-derived polyethylene, polylactic acid and polysaccharides.
Polyhydroxyalkanoates (PHAs) are natural polyesters synthesised...