The general mechanical behavior properties of austenitic stainless steels at room and at elevated temperatures are described. These provide the back­ground for their behavior in various reactor environ­ments. The mechanical properties of the various grades of the 300 series austenitic stainless steels are fairly similar, particularly at room temperature, so available data for type 316 or 316L steel are used as representative of the group. There is more variation in properties at elevated temperatures, particularly creep-resistance and creep-rupture strength, so important properties differences are noted, particu­larly for steels modified with Ti or Nb which have more high-temperature heat-resistance than type 316 steel. Some effects of processing on mechanical properties are noted, but generally properties are described for material in the SA condition.

Austenitic stainless steels such as types 304, 316, and 316L have yield strength (YS — 0.2% offset) of 260-300 MPa in the SA condition at room tem­perature, with up to 50-70% total elongation.1-7 Typ­ical YS values as a function of temperature for type 316 are shown in Figures 2 and 3. Other austenitic stainless steels developed for improved creep resistance at high temperatures, such as fine-grained 347HFG or the high-temperature, ultrafine precipitate — strengthened (HT-UPS) steels (Table 3), have very similar YS of about 250 MPa in the SA condition (typical thicker section pipes or plates), as shown in Figure 3. Many applications of type 304 and 316 stainless steels require a minimum YS of 200 MPa. However, small amounts of cold plastic strain, 1-5%, typical or straightening or flattening for various prod­uct forms, termed ‘mill-annealed,’ raise the YS to about 400 MPa, because austenitic stainless steels tend to have high strain-hardening rates. Large

amounts of cold work (CW) push the YS higher, with 20-30% CW 316 having YS of 600-700 MPa,8,9 but with very low ductility of only 2-3%. The very

fine grain sizes found in thin-sheet and foil products made from 347 steel also tend to push ambient YS to 275-300 MPa or above.7 The ultimate tensile strength (UTS) of SA 316 steel at room temperature is about 600 MPa, and can be higher (600-700 MPa) for steels such as 347HFG, HT-UPS, or some of the high — nitrogen grades. The UTSof20-30% CW 316orother comparable steels can be 700-800 MPa at room

4,8,9

temperature.

The impact-toughness and crack-growth resis­tance of SA 316 at room temperature and tem­peratures below 500 °C are excellent because of its high ductility and strain-hardening behavior. Charpy impact toughness values for SA 316 and 347 steel are about 150J at 22-400 °C, and tend to stay above 100J even at cryogenic (—196 °C) temperatures. Type 316 stainless steels also have good room — temperature fatigue resistance, exhibiting endurance limits for cyclic stresses below the YS.

At elevated temperatures, the YS of SA 316 declines with increasing temperature, reaching levels of about 150MPa at 600-650 °C (Figure 2), and going lower at 700-800 °C. More heat-resistant steels such as 347HFG or HT-UPS steels may be slightly stronger at 700 °C, and can have YS values of 300­350 MPa in the ‘mill-annealed’ (5% CW) (Figure 3). The UTS of SA 316 remains at about 500 MPa up to 500 °C, and then declines rapidly with increasing temperature until YS and UTS approach similar values (120-180MPa) at about 800 °C (Figure 2). More heat-resistant steel, such as 347HFG and the HT-UPS steels, can retain higher UTS values of 200­300 MPa at 800 °C. Unaged SA 316 generally have 30-60% total tensile elongation at temperatures up to 800 °C; similar steels with 20-30% CW can have 5-10% ductility until they recrystallize at tempera­tures of 800 °C or above.9

1000

10

18000 20000 22000 24 000 26000

Larson-Miller parameter

Figure 4 A plot of creep-rupture stress as a function of Larson-Miller parameter (LM P) for nine heats of SA 316 austenitic stainless steel tubing tested by the National Research Institute for Metals (now NIMS) in Japan. LMP10000 represents data for rupture after 10 000 h. LMP = (T[°Cj + 273) (20 + log tr), where T is creep testing temperature and tr is the creep-rupture life in hours. Reproduced from Data sheets on the elevated temperature properties of 18Cr-12Ni-Mo stainless steels for boiler and heat exchanger tubes (SUS 316 HTB), Creep Data Sheet No. 6A; National Research Institute for Metals: Tokyo, Japan, 1978.

At elevated temperatures, time-dependent defor­mation, or creep, becomes a concern for austenitic steels such as 304 and 316 above 500-550 °C. A Larson-Miller parameter (LMP) plot of creep — rupture strength for SA 316 is shown in Figure 4, and for 347HFG and HT-UPS steels in Figure 5. Long-term creep-rupture behavior is affected by precipitation behavior at elevated temperatures, as is described in the following section. Creep-rupture behavior (time to rupture or time to 1% strain) is far more limiting in design for high temperature integrity than tensile properties. The creep-rupture

Temperature for 100 000 h rupture life (°C)

580 620 660 700 740 780 820

strength of SA 316 in Figure 4 is comparable to creep-rupture strength of 347 steel in Figure 5, and both have less creep strength than 347HFG, a steel containing more Nb and C (Table 3). Types 304 and 316L steels would have less creep strength than 316 steel. By comparison, the triply stabilized (additions of Ti, V, and Nb) HT-UPS steel has outstanding creep — rupture resistance at 700-800 °C, comparable to that of the solid-solution Ni-based alloy 617. A more direct comparison of creep resistance at 700 °C and 170 MPa is shown in Figure 6. For this creep-rupture condi­tion, SA 316 ruptures after about 40 h, whereas the SA HT-UPS steel resists creep and rupture until 18 745 h.4,7 For elevated temperature creep behavior of heat-resistant stainless steels with additions of Ti and Nb, processing conditions are also important, including prior cold-strain and the SA temperature. The creep resistance of SA 304 and 316 steels is not affected significantly by different annealing tem­peratures, and both steels have less creep resistance in the 10-30% CW condition. By contrast, 347HFG and HT-UPS steels benefit dramatically from higher solution annealing temperatures (1050-1100 °C com­pared to 1150-1200 °C) and small amounts of CW, because these enhance the formation and stability of nano-dispersions of MC carbide precipitates, which are responsible for their high-temperature creep

4,7,10,11

resistance.

Figure 6 Direct comparison of creep-resistance of D9 and high-temperature, ultrafine precipitate-strengthened steels. Adapted from Swindeman, R. W.; Maziasz, P. J.; Bolling, E.; King, J. F. Evaluation of Advanced Austenitic Alloys Relative to Alloy Design Criteria for Steam Service: Part 1 — Lean Stainless Steels; Oak Ridge National Laboratory Report (ORNL-6629/P1); Oak Ridge National Laboratory: Oak Ridge, TN, May 1990; Data sheets on the elevated temperature properties of 18Cr-12Ni-Mo stainless steels for boiler and heat exchanger tubes (SUS 316 HTB), Creep Data Sheet No. 6A; National Research Institute for Metals: Tokyo, Japan, 1978; Teranishi, H.; etal. In Second International Conference on Improved Coal Fired Power Plants; Electric Power Research Institute: Palo Alto, CA, 1989; EPRI Publication GS-6422 (paper 33-1); Swindeman, R. W.; Maziasz, P. J. In Creep: Characterization, Damage and Life Assessment; Woodford, D. A., Townley, C. H. A., Ohnami, M., Eds.; ASM International: Materials Park, OH, 1992; pp 33-42.