Did you ever wonder how the austenitic stainless steels differ from other categories of stainless steel? If so, here is a brief description of how the various types of stainless steels differ from each other.
1. Austenitic Grades Have Characteristics that are Distinct from Other Stainless Steels
Stainless steels are steel alloys with at least 11 percent (maximum 30%) chromium, which prevents corrosion of iron particles and enhances their ability to withstand higher temperatures. (Need the background? Read An Introduction to Stainless Steels.)
Austenitic grades belong to the family of stainless steels, and are the most popular among the five categories of these steels:
- Ferritic stainless steels have standard chromium content (11%) and a small amount of carbon (below 0.10% carbon). They have low corrosion resistance. Their magnetic property is due to the body-centered cubic (BCC) structure with a Fe atom at each corner and a Fe atom at the center. The limitations of ferritic stainless steels arise due to their lower hardness and average mechanical strength. However, ferritic steels are among the most highly useful and popular stainless steels. Their low nickel content explains why they are inexpensive.
- Martensitic stainless steels also contain mainly chromium, but unlike the ferritic variety, they contain a higher percentage of carbon (up to 1%). They are capable of being hardened and tempered, and are magnetic as well. Their main disadvantage is they can’t be welded easily and have lower corrosion protection compared to the ferritic and austenitic types. Martensitic stainless steels are preferred for applications where the corrosion risk is very low and high tensile strength and impact load capacity is a necessary requirement.
- Duplex stainless steels have a microstructure that is both austenitic (50%) and ferritic (50%). These steels are capable of taking higher loads than austenitic or ferritic due to their higher strength, and they have inherent resistance to stress corrosion cracking (SCC) and acidic corrosion. Duplex stainless steels show limited magnetic characteristics and weldability. Duplex grades are preferred materials for large storage tanks and containers storing and transporting chemicals.
- Precipitation hardened stainless steels are highly corrosion resistant (like austenitic grades). Their strength can be higher than that of martensitic stainless steels due to the addition of niobium, copper and aluminum up to 0.5%. Chromium addition is to the tune of 15% - 17.5%. Due to their mechanical strength these grades are the preferred materials for long shafts.
- Austenitic stainless steels contain chromium in the range of 16% to 30% and a moderate to high percentage of nickel. These grades cannot be heat treated to increase hardness. Austenitic stainless steels are non-magnetic. They are the most widely used grades.
2. Austenitic Grades Have a Hundred Years of History
Austenitic stainless steels and other stainless steels have a history of more than a hundred years. In the early 1800s, some researchers (Mallet, Faraday, Stodart and Bunsen) were able to observe and conclude that chromium iron alloys (similar to today’s chromium steel alloys) had resistance to oxidation and acid attack. In 1821, Pierre Berthier, a metallurgist of France, suggested the use of this chromium iron for making cutlery, for he too had discovered its resistance to acid attack.
In 1913, a steel alloy (the first stainless steel) containing 12.8% chromium was melted by Harry Brearley of England, who was trying to produce wear-resistant steel for gun barrels and accidentally ended up producing a grade that resisted both acid attack and rusting. By adding chromium to molten iron he produced what was known as “rustless steel." In 1914, rustless steel knives were marketed as stainless steel knives.
By 1912, Krupp engineers Enno Strauss and Eduard Maurer had already patented austenitic stainless steel as Nirosta. This invention had 7% nickel and 21% chromium. The grade of steel developed by Krupp engineers was acid corrosion resistant, ductile and easy to form. In 1919, a patent for martensitic stainless steel was obtained by E. Haynes. Meanwhile, the Germans invented austenitic grade 316. Soon these austenitic grades found applications in medical instruments, kitchens, vacuum cleaners and vehicles.
3. The Face-Centered Cubic Structure (FCC) is the Basic Feature of Austenitic Grades
In contrast, ferritic stainless steels have a body-centered cubic (BCC) structure. In a nickel-based alloy system, 8% nickel (with 18% chromium content) is the minimum requirement for conversion of a ferritic to an austenitic structure (face-centered cubic). These are called the 300 series (e.g., 304 grade). If the nickel content is less than 8%, then the addition of nitrogen can ensure the complete conversion to the austenitic structure. The low nickel austenitic grades are called the 200 series, which are comparatively inexpensive.
The presence of a large amount of nitrogen in austenitic grades can create disadvantages due to the generation of nitride precipitation. (Learn about other characteristics in A Look at High Nitrogen Stainless Steels.) Other elements such as manganese and copper are also added to austenitic grades to ensure the stability of the face-centered cubic (FCC) structure.
4. Austenitic Grades Can Be Work Hardened
Austenitic grades cannot be hardened through heat treatment. However, their hardness and mechanical strength can be enhanced through cold working. Other methods of enhancing hardness include adding nitrogen and through the precipitation hardening process. When austenitic stainless steels are work hardened, they rapidly gain strength.
By combining the work hardening process with annealing, fabricated parts and components can be finished according to the desired bendability and strength requirements.
5. Austenitic Grades Can Tolerate Cryogenic Temperatures
Austenitic grades can be used all the way from a cryogenic to a high temperature range due to their ductility, toughness and mechanical strength that is sustained over a wide temperature range. The toughness and ductility of metals at subzero temperatures is a measure of cryogenic temperature resistance. At subzero temperatures, austenitic stainless steels have a higher tensile load capacity than at normal ambient temperatures. However, the martensitic and ferritic grades can’t be used at subzero temperatures because their toughness is drastically reduced at these temperatures.
6. Austenitic Grades are Also Suitable for Elevated Temperatures
Austenitic stainless steels with a high chromium content maintain their mechanical strength at elevated temperatures. The grades that contain high silicon also have enhanced strength at high temperatures. A high silicon content also improves corrosion resistance to hot sulfuric acid at these high temperatures.
7. Stress Corrosion Cracking (SCC) can Damage Some of the Austenitic Grades
Although the austenitic grades have general corrosion resistance, they are susceptible to damage by stress corrosion cracking in corrosive environment. The presence of even a few parts per million of chloride particles can lead to damage due to SCC. Residual stresses caused during fabrication can also be one of the factors adding to the severity of SCC.
By increasing the nickel content, the SCC damage can be minimized. Similarly, the nickel to chromium ratio can be modified to optimize the formability of austenitic grades.
8. Austenitic Grades are Nonmagnetic
Because austenitic stainless steels are nonmagnetic, they can be used in the construction of buildings where large magnetic fields are likely to be produced, such as medical centers with MRI scanners.
However, a few grades of cold worked austenitic stainless steels (302 grade, for example) may turn into a weakly magnetic grade due to an increase in magnetic permeability. If cold worked, austenitic grades with a high nickel content (e.g., 316 grade) won’t become magnetic.
9. The Role of Molybdenum and Niobium
Niobium is added as a stabilizer in the content in order to minimize the likelihood of intergranular corrosion.
10. Low Carbon Austenitic Grades Have Better Weldability
As welding is generally a high temperature process, when welding thick sections of austenitic grades, any chromium may react with the carbon content in the stainless steel, resulting in the production of chromium carbide, which precipitates at the grain boundaries. (Related reading: The Role of Chromium in Intergranular Corrosion.) The consequent reduction in chromium content can cause severe localized corrosion. Chromium carbide formation can be minimized by using the austenitic grades with very low carbon content.
Austenitic grades are sometimes used as reinforcement for cement concrete construction in severely corrosive marine environments, either in the solid form or as cladding over the core of carbon steel. In such cladding applications, the risk of bimetallic corrosion must be carefully assessed.
11. Super Austenitic Stainless Steels Ensure Stronger Corrosion Protection
Super austenitic grades contain high molybdenum, high nickel, high chromium and nitrogen, to ensure superior corrosion resistance in highly corrosive environments (e.g., hot saltwater environments).
Super austenitic grades are iron-based austenitic face-centered cubic stainless steel alloys. While retaining the advantages such as formability, general corrosion resistance and strength of austenitic grades, these also ensure superior resistance to crevice corrosion and pitting (due to chlorides) as well as superior weldability.
Pitting resistance is measured in terms of a pitting resistance equivalent number (PREN). Higher pitting resistance is achieved by the addition of molybdenum and nitrogen. In solution-annealed conditions these grades have superior yield strength compared to austenitic grades.
12. Austenitic Grades are Widely Used by Industry
Typical applications of the different austenitic stainless steel grades include:
- Standard nickel-18/8 stainless steel (grade 304L and 304) are chosen to manufacture fittings and fasteners, structural parts of vehicles, mining equipment, cryogenic equipment, architectural components, storage tanks and piping systems containing corrosive fluids.
- Austenitic high chromium-molybdenum-nickel grade 316 is selected for chemical process machinery, lab equipment, textile processing equipment, drug manufacturing, heat exchanger parts, food preparation appliances, surgical instruments, surgical/medical implants, brewery equipment, saltwater boat fittings and structural parts for coastal environments.
- The low carbon, heat resisting grade 317L is chosen for petrochemical and chemical plant equipment and condensers of power houses.
- Austenitic high nickel chromium silicon grades (309S) are used for catalytic converter parts for vehicles, chemical processes and furnaces.
- Low nickel grades containing manganese (200 series) are used for cookware, household water tanks, automotive components, food production equipment, and home appliances such as washing machines.
- Super austenitic stainless steel grades are used in saltwater applications, condensers, marine heat exchangers, chemical plants, paper mills, wastewater treatment, desalination plants and fertilizer production.
Austenitic grades generally have 18% chromium and 8% nickel. However, grades containing higher levels of up to 26% chromium and 35% nickel can have superior corrosion resistance and durability. High durability is achieved at a high cost because nickel is expensive. The nickel content requirement can be minimized by adding manganese, nitrogen and copper to the alloy metal. The addition of molybdenum helps achieve better pitting corrosion protection in highly corrosive (chloride) environments. Some of the austenitic grades can be used in cryogenic or high temperature ranges.
Austenitic grades are extensively used in food production, storage of corrosive fluids, high pressure pipes and pressure vessels, cookware, architecture and the transportation industry. Super austenitic stainless steel grades are used to manufacture marine heat exchangers and desalination plant equipment.