P. J. Maziasz and J. T. Busby

ile strength

YS Yield strength

2.09.1 Introduction

Austenitic stainless steels are a class of materials that are extremely important to conventional and advanced reactor technologies, as well as one of the most widely used kinds of engineering alloys. They are austenitic Fe-Cr-Ni alloys with 15-20Cr, 8-15Ni, and the balance Fe, because they have a face — centered-cubic (fcc) close-packed crystal structure, which imparts most of their physical and mechanical properties. They are steels because they contain dis­solved C, typically 0.03-0.15%, and more advanced steels can also contain similar or greater amounts of dissolved N. They are stainless because they con­tain >13%Cr and Cr provides surface passivation for corrosion-resistance in various aqueous or corrosive chemical environments from room temperature to about 400 °C. At elevated temperatures of 500 °C and above, Cr provides oxidation resistance by the forma­tion of protective Cr2O3 oxide scales. Commercial stainless steels are complex alloys, with varying addi­tions and combinations of Mo, Mn, Si, and Ti as well as Nb to enhance the properties and behavior of the austenite parent phase over a wide range of temperatures. They can also contain a host of minor or impurity elements, including Co, Cu, V, P, B, and S, which do not have significant effects within certain normal ranges.

Typical commercial steel grades relevant to nuclear reactor applications include types 304, 316, 321, and 347. They can be fashioned into a wide range of thick or thin components by hot or cold rolling, bending, forging, or extrusion, and many are also available as casting grades as well (i. e., 304 as CF8, 316 as CF8M, and 347 as CF8C). These steels all have good combinations of strength and ductility at both high and low temperatures, with excellent fatigue resistance, and are most often used in the solution-annealed (SA) condition, with the alloying elements fully dissolved in the parent austenite phase and little or no precipitation. The steels with added Mo (316) or stabilized with Ti (321) or Nb (347) also have reasonably good elevated temperature strength and creep resistance. Additions of nitrogen (i. e., 316LN or 316N) provide higher strength and stability of the austenite parent phase to the embrit­tling effects of thermal — or strain-induced martensite formation and allow this grade of steel to be used at cryogenic temperatures. It is beyond the scope of this chapter to describe in detail the physical metallurgy of austenitic stainless steels, and adequate descriptions are found elsewhere.1,2 The remainder of this chapter focuses on the factors that broadly affect the properties of austenitic stainless steels in specific reactor environments, and highlights efforts to develop modified steels that perform significantly better in such reactor systems. These will likely be important in enabling materials for any new applica­tions of nuclear power.