2.09.2.1 General and Fabrication Behavior

Without the effects of irradiation, austenitic stainless steels are fairly stable solid-solution alloys that generally remain in the metallurgical condition in which they were processed at room temperature to about 550 °C. The typical austenitic stainless steel, such as type 304, 316, 316L, or 347 stainless steel, in the SA condition (1000-1050 °C), will have a wrought, recrystallized grain structure of uniform, equiaxed grains that are 50-100 pm in diameter, particularly in products such as extruded bar or flat-rolled plates (6-25 mm thick).1-3 Ideally, such products should be free of plastic strain effects and have dislocation-free grains, but for real applications, products may be straightened or bent slightly (1-5% cold strain), and thus have some dislocation substruc­ture within the grains. Stainless steel products with heavier wall thicknesses (>50 mm) would be forgings and castings, which would have coarser grain sizes, but probably not have additional deformation. Spe­cial stainless steel products would include thin foils, sheets, or wires (0.08-0.5 mm thick), which would have much finer grain-sizes (1-10 pm diameter) due to special processing (very short annealing times) and special considerations (5-10 grains across the foil/ sheet thickness).3 Typical fast-breeder reactor (FBR) cladding for fuel elements can be thin-walled tubes of austenitic stainless steel, with about 0.25 mm wall thickness, so they fall into this latter special pro­ducts category. Although austenitic stainless steels are highly weldable, welding changes their structure and properties in the fusion (welded and resolidified) and adjacent heat-affected zones relative to the wrought base metal, so they may behave quite dif­ferently than the base metal, which is what was described above. The detailed behavior of welds under irradiation is beyond the scope of this chapter, so the remainder of this chapter focuses on typical wrought metal behavior.

Another important aspect of austenitic stainless steel that defines it is the stability of the parent austenite phase. The addition of nickel and elements that behave like nickel including carbon and nitrogen to the alloy causes it to have the austenite parent phase and its beneficial properties, which is also the same fcc crystal structure found in nickel-based alloys. Otherwise, the steel alloy would have the natural crystal structure of iron and chromium, which is body-centered cubic (bcc) ferrite, as the parent phase, and alloying elements that make the alloy behavior like this include molybdenum, nio­bium, titanium, vanadium, and silicon. A stable aus­tenitic alloy will be 100% austenite, with no 8-ferrite formed at high temperature and no thermal or strain — induced martensite, whereas an unstable austenitic alloy may have all of these. A useful way of expressing these different phase formation tendencies at room temperature in terms of the alloy behaving more like Cr (bcc ferritic) or Ni (fcc austenitic) is a Schaeffler diagram, as shown in Figure 1. The fcc austenite phase is nonmagnetic and maintains good strength and ductility even at cryogenic temperatures, with no embrittling effects of martensite formation. The bcc phase by comparison is ferromagnetic, has a little less

Figure 1 Schaeffler diagram showing regions of stable austenite, martensite, and delta-ferrite in austenitic stainless steels at room temperature as a function of steel alloys compositional effects acting as the equivalent of Cr or Ni. Reproduced from Lula, R. A., Ed. Stainless Steel; ASM International: Materials Park, OH, 1986.

ductility (less active slip systems), and has a ductile- to-brittle transition temperature (DBTT), below which the steel has low ductility and impact resis­tance, with a brittle fracture mode. Maintaining suf­ficient carbon and adding nitrogen are two ways of imparting good, stable austenite phase behavior to the common grades of austenitic stainless steels, like 304LN or 316LN.