Comprehensive nuclear materials 2 09 properties of austenitic steels for nuclear reactor applications

Comprehensive nuclear materials 2 09 properties of austenitic steels for nuclear reactor applications Comprehensive nuclear materials 2 09 properties of austenitic steels for nuclear reactor applications Comprehensive nuclear materials 2 09 properties of austenitic steels for nuclear reactor applications Comprehensive nuclear materials 2 09 properties of austenitic steels for nuclear reactor applications Comprehensive nuclear materials 2 09 properties of austenitic steels for nuclear reactor applications

2.09 Properties of Austenitic Steels for Nuclear Reactor Applications P J Maziasz and J T Busby Oak Ridge National Laboratory, Oak Ridge, TN, USA Published by Elsevier Ltd 2.09.1 2.09.2 2.09.2.1 2.09.2.2 2.09.2.3 2.09.2.4 2.09.2.5 2.09.3 2.09.4 2.09.4.1 2.09.4.2 References Introduction Properties of Unirradiated Alloys General and Fabrication Behavior Physical Properties Mechanical Properties Precipitation Behavior During Elevated Temperature Aging Corrosion and Oxidation Behavior Summary of How Properties Can Change During Irradiation Some Examples of Advanced Alloys for FBR and ITER/Fusion Applications FBR Application ITER/Fusion Application Abbreviations ASTM American Society for Testing and Materials bcc Body-centered cubic BWR Boiling water reactor CW Cold worked D-T Deuterium–tritium (fusion) DBTT Ductile-to-brittle transition temperature FBR Fast-breeder reactor fcc Face-centered cubic GenIV Generation IV reactors HFIR High Flux Isotope Reactor IASCC Irradiation-assisted stress-corrosion cracking ITER International Magnetic Fusion demonstration device, being constructed in Cadarache, France LWR Light water reactor MFR Magnetic fusion reactor NIMS National Institute for Materials Science (Japan) ORR Oak Ridge Research Reactor PCA Prime candidate alloy PWR Pressurized water reactor R&D Research and development RIS Radiation-induced solute segregation SA Solution annealed SCC Stress-corrosion cracking SEM Scanning electron microscopy TEM UTS YS 267 268 268 269 270 273 274 275 279 279 280 282 Transmission electron microscopy Ultimate tensile strength 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 facecentered-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 267 268 Properties of Austenitic Steels for Nuclear Reactor Applications temperatures They can also contain a host of minor or impurity elements, including Co, Cu, V, P, B, and S, which 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 2.09.2 Properties of Unirradiated Alloys 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 mm 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 mm 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 d-ferrite formed at high temperature and no thermal or straininduced 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 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 Properties of Austenitic Steels for Nuclear Reactor Applications 269 32 30 Ferrite, 10% Nickel equivalent, %Ni + 30 (%C) + 0.5 (%Mn) 28 26 Austenite (A) Ferrite, 5% 24 20% 22 Ferrite, 0% 20 40% 18 16 A+M X 14 80% o A+F 12 10 100% ferrite Martensite (M) A+M+F M+F F+M 0 Ferrite (F) 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 Chromium equivalent, %Cr + %Mo + 1.5 (%Si) + 0.5 (%Nb) Figure 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 ductileto-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 Table steel 2.09.2.2 Table Physical Properties Physical properties of 300 series stainless steels tend to be fairly similar, and the typical physical properties of 316L stainless steel are given in Tables and 2.1–3 The 316L stainless steel has a density at room temperature of 8000 kg mÀ3 and a melting temperature of slightly above 1400  C (Table 1) The elastic (Young’s) modulus at room temperature is 190–200 GPa, which is typical of a range of engineering alloys, including ferritic steels and solid-solution Ni-based superalloys At 100  C, the coefficient of thermal expansion of 316L is about 16 Â 10À6 cm cmÀ1  CÀ1 (Table 2), and values of that property may vary by up to 3–4% for types 316 and 347 steels The 300 series stainless steels Basic physical properties for 316L stainless Property Value Density Melting temperature Elastic modulus Shear modulus 8000 kg mÀ3 1390–1440  C 193 GPa 82 GPa Thermal properties for 316L stainless steel Property Temperature range Value Coefficient of thermal expansion 0–100  C 0–315  C 0–538  C 0–1000  C At 100  C At 500  C 0–100  C 15.9 Â 10À6  CÀ1 16.2 Â 10À6  CÀ1 17.5 Â 10À6  CÀ1 19.5 Â 10À6  CÀ1 16.3 W mKÀ1 21.5 W mKÀ1 500 J kgÀ1  C Thermal conductivity Specific heat capacity have much more thermal expansion than martensitic/ferritic steels or Ni-based superalloys, with the thermal expansion of 316L at 100  C being about Properties of Austenitic Steels for Nuclear Reactor Applications 50% higher than that of type 410 ferritic steel.3 The thermal conductivity of 316L stainless steel at 100  C is 16.3 W mKÀ1, which is to the higher end of the range for such alloys, with type 316 or 347 steel having 15–30% lower thermal conductivity Thermal conductivity of 300 series stainless steels is lower than that of ferritic steels or Ni-based superalloys If the 300 series stainless steel is fully (100%) austenitic, such as 316 or 347, then it has no ferromagnetic behavior, but if it contains ferromagnetic phases (like delta-ferrite or martensite), then such steels have some degree of ferromagnetic behavior Adding nitrogen to 316L produces fully stable austenitic phase structures 700 YLD UTS 600 500 Strength (MPa) 270 400 300 200 100 200 400 600 800 Temperature (ЊC) Mechanical Properties 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 and Other austenitic stainless steels developed for improved creep resistance at high temperatures, such as fine-grained 347HFG or the high-temperature, ultrafine precipitatestrengthened (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 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 Figure Plots of yield strength (YS) and ultimate tensile strength (UTS) as a function of tensile test temperature for nine heats of SA 316 austenitic stainless steel tubing tested by the National Research Institute for Metals (now NIMS) in Japan 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 450 25 ЊC 700 ЊC 400 Yield strength (MPa) 2.09.2.3 350 300 250 200 150 100 50 316 347HFG HT-UPS HT-UPS + 5% CW Austenitic stainless steel Figure Comparison of yield strength (YS) at room temperature and at 700  C for 316, 347HFG, and high-temperature, ultrafine precipitate-strengthened (HT-UPS) austenitic stainless steels, all in the solution-annealed condition, and for HT-UPS steel with 5% CW prior to testing 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 – Lean Stainless Steels; Oak Ridge National Laboratory Report (ORNL-6629/P1); Oak Ridge National Laboratory: Oak Ridge, TN, 1990; Teranishi, H.; et al 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) 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 Properties of Austenitic Steels for Nuclear Reactor Applications 271 Table Composition of various commercial or advanced/developmental austenitic stainless steel alloy grades and types (wt%) Cr Ni Mo Mn Si 304 18–20 8–12 – 1–2 0.75 316 16–18 10–14 2–3 0.75 321 347 D9 PCA HT-UPS CF3MN CF3MN (US) 17–19 17–19 14 14 14 17–21 17.6 9–13 9–13 15 16 16 9–13 12.6 – – 2.3 2.3 2.5 2–3 2.5 1–2 1–2 2