Austenitic stainless steels are a unique family of alloys renowned for their exceptional corrosion resistance and impressive mechanical properties. These alloys exhibit unparalleled strength, toughness, and ductility among commercial alloys across a wide temperature range, from cryogenic to high temperatures. Additionally, they are aesthetically appealing and environmentally safe. Table 1 lists the most common grades of austenitic stainless steels, including two grades with precipitation hardening capabilities. While a minimum of 10.5% chromium is needed to make iron alloys stainless, austenitic stainless steels contain at least 15% chromium, combined with nickel, manganese, carbon, and nitrogen to stabilize the face-centered cubic (f.c.c.) structure.
Since the 1920s, austenitic stainless steels have been widely used, with advancements in production making them cost-competitive with copper alloys, aluminum, and clad alloys. Key considerations in their development include long service life, low maintenance costs, biocompatibility, and recyclability.
Table 1: Main Grades of Austenitic Stainless Steels
Steel Grade | Carbon | Nitrogen | Chromium | Nickel | Silicon | Manganese | Molybdenum | Others |
| 201 | 15/0 | 25/0 | 16-18 | 5/5-5/3 | 0/1 | 5/7- 5/5 | – | |
| 301 | 15/0 | – | 16-18 | 6-8 | 0/1 | 0/2 | – | |
| 304 | 08/0 | 10/0 | 18-20 | 5/10- 8 | 0/1 | 0/2 | – | |
| 316 | 08/0 | 10/0 | 16-18 | 10-14 | 0/1 | 0/2 | 2-3 | |
| 17-4 PH | 07/0 | – | 15-17 | 3-5 | 0/1 | 0/1 | – | 3-5 Cu: |
| 17/7 | 09/0 | – | 16-18 | 75/7- 5/6 | 0/1 | 0/1 | – | 5/1: Al |
Processing of Austenitic Stainless Steels
The production of austenitic stainless steels involves advanced processing methods such as Argon-Oxygen Decarburization (AOD) and continuous casting, alongside hot and cold rolling. Electric melting using scrap and raw materials initiates the process. After melting, the melt is transferred to a chamber where AOD reduces carbon content from approximately 1% to 0.05%. A mixture of argon and oxygen gases is injected at 1700°C, with the argon-to-oxygen ratio adjusted to oxidize carbon instead of chromium. Silicon is added to recover chromium losses forming as slag.This processing enhances melting efficiency, reduces raw material costs, and ensures alloy cleanliness by minimizing carbon and sulfur. Vacuum AOD is sometimes used for stainless steel ingots. Continuous casting eliminates separate ingot production and reduces chromium oxidation. In hot rolling, single-pass rolling is preferred for its flexibility within a narrow temperature range. Cold rolling may be applied for specific applications, followed by annealing at 1000–1100°C to dissolve precipitates, with water cooling to prevent precipitation.
Structure of Austenitic Stainless Steels
Austenitic stainless steels are solid solution alloys, with chromium stabilizing ferrite and nickel stabilizing austenite. These elements are critical to achieving desired properties like corrosion resistance, strength, and cost-effectiveness. The alloys retain some ferrite phase after solidification, calculated using Equation 1:
| (1) | % ferrite = 3(Cr+1.5Si+Mo)- 2.8(Ni+0.5Mn) -84(C+N) – 19.8% |
Nickel and chromium equivalent criteria (Equations 2 and 3) assess solidification conditions, ferrite content, welding requirements, and hot workability:
| (2) | Cr eq=% Cr+1.37% Mo+1.5% Si +2% Nb+3% Ti |
| (3) | Ni eq=% Ni+0.31% Mn+22% C +14% N+% Cu |
A chromium equivalent of 1.55 minimizes impurities like sulfur, selenium, phosphorus, tin, and lead. During continuous casting, delta ferrite may form but dissolves during hot working, reducing risks like cracking and poor weldability.
Austenite Stability in Stainless Steels
Austenite stability is a defining feature of these alloys. Unstable austenite may transform into martensite during cooling or deformation, though the martensitic start temperature (Ms) is below room temperature in commercial alloys. Strain-induced martensite forms during processes like cold rolling, deep drawing, wire drawing, milling, or grinding, influenced by temperature, strain rate, and composition (Equation 4):
| (4) | Md30(C) = 497-462(% C+% N)-9.2% Si -8.1% Mn-13.7% Cr -20% Ni-18.5% Mo |
All alloying elements stabilize austenite, even those promoting ferrite during solidification. Austenitic grades offer good yield strength and ductility after annealing, with varying work hardening rates due to strain-induced martensite, a key factor in alloy selection for forming methods.
Carbide and Nitride Phases
Austenitic stainless steels contain up to 0.15% carbon, considered an impurity rather than an alloying element. Carbon increases yield strength and stabilizes austenite but can form M23C6 carbides with chromium and iron, precipitating at grain boundaries, twins, and within grains. This “sensitization” depletes chromium, reducing corrosion resistance. Grades with 0.08% carbon sensitize quickly, while those with less than 0.03% carbon take significantly longer. Nickel, molybdenum, and cold work accelerate precipitation, while nitrogen and manganese delay it. Low carbon content and carbide-forming elements like titanium and niobium mitigate sensitization.Carbide precipitation occurs between 600°C and 1000°C, with sigma phase precipitation possible in highly alloyed grades over longer periods, particularly in chromium-rich matrices (>18%).
Austenite Hardness in Stainless Steels
Interstitial elements like carbon, nitrogen, and boron strengthen austenite more effectively than grain size or substitutional elements. Nitrogen, with its high strengthening coefficient and solubility, is the most effective, also suppressing carbide precipitation and enhancing corrosion resistance. Strain hardening in austenitic stainless steels exceeds that of ferritic steels, with strain-induced martensite increasing the strengthening coefficient to nearly 1.0. Cold-worked austenitic stainless steels achieve tensile strengths close to 2000 MPa.
Precipitation-Hardening Stainless Steels
Precipitation-hardenable austenitic stainless steels, distinct from martensitic maraging steels, are strengthened by gamma-prime intermetallic phase precipitates formed from nickel, aluminum, and titanium. Aging at 725°C for 16 hours after homogenization at 1000°C is typical. Alloys like A286 and Discaloy are notable examples. Semi-austenitic grades, such as 17-7PH and 15-7PH, are strengthened by M23C6 carbide precipitation.
Formability of Austenitic Stainless Steels
Austenitic stainless steels are highly formable due to their soft nature, with stretch forming being the most common cold-forming method. In deep drawing, sheets are drawn into dies with or without restraint. Unlike carbon and ferritic stainless steels, which benefit from anisotropic b.c.c. structures, austenitic stainless steels are isotropic but excel in formability. Work hardening during deformation requires careful control to prevent tearing in thin sections. Grade 301 is ideal for applications like kitchen sinks and cylindrical sections, formed without intermediate annealing.Surface quality is critical, with fine-grained finishes free of defects like loader bands or scratches. Hydrogen embrittlement may occur if hydrogen diffuses during forming.
Corrosion Resistance of Austenitic Stainless Steels
The hallmark of austenitic stainless steels is their superior corrosion resistance, attributed to a thin, adherent chromium oxide layer. At least 11% chromium is required, with higher contents needed for aggressive environments. Corrosion typically manifests as pitting, forming small-diameter holes. Chromium, molybdenum, and nitrogen enhance pitting resistance, while low sulfur content is beneficial.
Oxidation and Creep Resistance
The chromium oxide layer provides excellent oxidation resistance due to its low ionic permeability. In grade 310, chromium and nickel improve oxidation resistance by 25% and 20%, respectively, with phosphorus and aluminum also contributing. Calcium and rare earth elements may reduce resistance by forming Cr23C6 precipitates. Austenitic stainless steels outperform ferritic steels in creep resistance, with interstitial elements like carbon and boron, and strong carbide formers like titanium, molybdenum, and zirconium, enhancing performance.
Applications of Austenitic Stainless Steels
Austenitic stainless steels are used in high-temperature, industrial, and food-related applications due to their unique properties. While more expensive than plain carbon steels, their performance justifies the cost in specialized applications.
Sources:
http://ui.adsabs.harvard.edu/abs/2001emst.book..406M/abstract
https://www.researchgate.net/publication/285111370_Austenitic_Stainless_Steels
References:
- Angel T 1954 J. Iron Steel Inst. London 177, 165
- Blank J 1988 Stainless Steels ’87. Institute of Metals, London
- Boyer H, Gall T 1985 Metals Handbook Desk Edition. American Society For Metals, Metals Park, OH
- Lacombe P, Baroux B, Beranger G 1990 Les Aciers Inoxydables. Les Editions de Physique, Paris
- Nicodemi W 1993 Innoation in Stainless Steel. Associazone Italiana di Metallurgia, Milan
- Nordberg H, Bjorklund J 1992 Applications of Stainless Steel. Kristianstads Boktryceri, Stockholm
- Peckner D, Bernstein I 1977 Handbook of Stainless Steel. McGraw-Hill, New York
- Pickering F, Irvine K, Gladman T 1969 J. Iron Steel Inst. London 1017
- Sedriks A 1979 Corrosion of Stainless Steel. Wiley-Interscience, New York
- Sedriks A 1984 Stainless Steels ’84. Institute of Metals, Go$ teborg, Sweden
Suutala N 1983 Metall. Trans. 191–7
[1] -Argon–oxygen decarburization (AOD)
[2] Hot reversing mills
[3] Tandem mills
[4] sensitization
