ABSTRACT
The availability of large components with high corrosion resistance and mechanical strength combined with high toughness is becoming an increasing need in oil and gas industry as well as for power and petrochemical applications. Duplex and super-duplex stainless steels provide a good combination of such properties but they are limited in thickness because of strong toughness reduction and possible presence of sigma phase on large sections. Because of the well-known beneficial effect of nitrogen on austenite stabilization, corrosion resistance, strength and toughness, high nitrogen austenitic stainless steels containing nitrogen above 0.5 % can provide an excellent alternative to duplex and super-duplex stainless steels, especially for a product thickness as large as 350 mm and greater. In this context, a high nitrogen steel designated as AF932N has been manufactured in large section sizes. Details of manufacturing process and characterization of AF932N alloy in terms of mechanical properties, microstructure and corrosion resistance, especially in regards to hydrogen embrittlement are presented and discussed in this paper.
INTRODUCTION
Nitrogen alloying in austenitic stainless steels improves austenite stability, mechanical properties and corrosion resistance. These beneficial effects of nitrogen were used to increase strength and corrosion resistance of austenitic stainless steels by developing nitrogen-alloyed austenitic stainless steels of AISI(1)-200 series and AISI-300 series. In these alloys, nitrogen was limited to about 0.5 % max, which approximately corresponds to the solubility of nitrogen in liquid pure iron.
Recent advances in melting technologies and alloying metallurgy have made it possible to produce alloys with nitrogen concentrations higher than 0.5 % and up to 1 %. These new materials are usually designated as high nitrogen austenitic stainless steels (HNASS) and possess an excellent combination of high strength and toughness, good ductility and corrosion resistance and good high temperature mechanical properties.1,2,3,4 These properties make HNASS very interesting for a wide variety of uses ranging from petrochemical industry to high temperature applications in power industry.
This paper describes the properties of a HNASS designated as AF932N with nitrogen in the range 0.8- 0.9 % and minimum 48 Pitting Resistance Equivalent Number (PREN).5 This alloy has been produced by air melting followed by AOD (Argon-Oxygen-Decarburization) refining in an industrial plant of 70 tons of capacity, thus providing large ingots up to 25 tons from which large components up to 800 mm (31.5 in.) thickness can be manufactured.
