Generally, austenitic stainless steels that have no 8-ferrite stay austenitic from room temperature up to about 550 °C, at which temperature they can start to experience the effects of thermal aging. Aging causes the alloy to decompose from a solid solution into various carbide or intermetallic precipitate phases and a more stable austenite phase. The decomposition of a quaternary Fe-Cr-Ni-Mo alloy, typical of type 316 stainless steel at 650 °C, is shown in Figure 7, and

the time-temperature-precipitation (TTP) diagrams for aging of SA behavior of type 316 and 316L stainless steel at 500-900 °C are shown in Figures 8 and 9.12,13 For typical light water reactor (LWR) or fusion reactor applications, such high temperature aging behavior is not too important, but it does become important for understanding irradiation-induced or — produced precipitation behavior for FBR irradiation of compo­nents at temperatures 400-750 °C. As indicated in Figure 8, prolonged aging of 316 steel at 550 °C and above tend to produce precipitation of Cr-rich M23C6 in the matrix and along grain boundaries, while expo­sure at 600-750 °C eventually also produce precipita­tion of M6C, Laves (Fe2Mo), and s (FeCr) phases.12 Precipitation kinetics of these phases appears maxi­mum at 750-850 °C, and then at temperatures above 900-950 °C, none of these phases forms. The lower C content of 316L accelerates and shifts the formation of intermetallic phases relative to 316 steel, as indi­cated in Figure 9. Additions of Ti or Nb cause the formation of MC carbides at the expense of the Cr — rich M23C6 carbides, depending on whether the steel is fully stabilized or not, but can also accelerate the formation of intermetallic phases, such as s or Laves. If 8-ferrite is present in the alloy, it generally rapidly converts to s-phase during aging. CW effects tend to accelerate the formation and refine the dispersion of carbides, but they can also significantly enhance the formation of intermetallic phases at lower tempera­tures, particularly in 20% CW 316.12-15 However, careful alloy design and compositional modification

Time (h)

Figure 9 Time-temperature-precipitation diagram of solution-annealed 316L stainless steel during thermal aging. Dashed lines represent a lower solution anneal temperature (1090 °C vs. 1260 °C). Reproduced from Weiss, B.; Stickler, R. Metall. Trans. 1972, 4, 851-866.

of certain austenitic stainless steels, such as the HT — UPS steels, can result in alloys resistant to the forma­tion of а-phase during aging or creep for up to 60 000 h or more. The various precipitate phases that form in 300 series austenitic stainless steels during thermal aging or creep are listed below, with some information on their nature and characteristics.12,1 ,1

• M23C6 — fcc, Cr-rich carbide, that can also enrich Mo, W, and Mn, but is generally depleted in Fe, Si, and Ni relative to the 316 alloy matrix.

• M6C — diamond-cubic phase that can be either a carbide (M6C — filled, M12C — half-filled) or a silicide phase (M5Si — unfilled), depending on how carbon fills the atomic structure. It is generally enriched in Si, Mo, Cr, and Ni relative to the 316 alloy matrix.

• MC — fcc Ti — or Nb-rich carbide. The Ti-rich MC phase can also be very rich in Mo, or V and Nb, and may contain some Cr, but tend to contain little or no Fe, Si, and Ni. The Nb-rich MC is a fairly pure carbide phase that can enrich in Ti, but does not usually contain any of the other alloying elements in the 347 or 316 alloy matrix.

• Laves — hexagonal Fe2Mo-type intermetallic phase. Fe2Nb and Fe2W can also be found in steels contain­ing those alloying additions. Phase tends to be highly enriched in Si and can contain some Cr but is generally low in Ni relative to the 316 alloy matrix.

• а — body-centered-tetragonal intermetallic phase, consisting of mainly Cr and Fe. It can be enriched
somewhat in Mo, but is depleted in Ni relative to the 316 alloy matrix.

• w — bcc intermetallic phase, enriched in Mo and Cr, and containing mainly Fe, and depleted in Ni relative to the 316 alloy matrix.

• FeTiP or Cr3P — hexagonal or tetragonal phos­phide compounds that can be found in stainless steels containing higher levels ofP. FeTiP is found in the HT-UPS steels during aging.