Austenitic Stainless Steels
Austenitic Stainless Steels
Austenitic stainless steels are the most common and widely known types of stainless steels. They make up over 70 % of total stainless steel production. These steels contain around 16 % to 25 % chromium and sufficient nickel and/or manganese to retain an austenitic structure at all temperatures from cryogenic region to the melting point of the stainless steel. Austenitic stainless steels can also contain nitrogen in solution. Although nickel is the alloying element most commonly used to produce austenitic stainless steels, nitrogen can also be used to produce austenitic stainless steels. The austenitic stainless steels are more easily recognized because of their non magnetic properties. Austenitic steels are non magnetic since the face centered cubic structure of austenite is non magnetic. They are extremely formable and weldable, and they can be successfully used from cryogenic temperatures to the jet engines and red hot temperatures of furnaces.
The austenitic stainless steels can have compositions anywhere in the portion of the Schaeffer- Delong diagram labeled austenite shown in Fig. 1.
Fig 1 Schaeffer- Delong diagram
The family of austenitic stainless steels is shown in Fig 2.
Fig 2 Family of austenitic stainless steels
Austenitic stainless steels are mainly segregated into the following two series
- 200 series – Stainless steels with a low nickel and high nitrogen content are classified as 200 series. These are chromium-nickel-manganese austenitic stainless steels. Grade 201 is hardenable through cold working while the grade 202 is a general purpose stainless steel. Decreasing nickel content and increasing manganese results in weak corrosion resistance.
- 300 Series – The most common austenitic stainless steels are iron-chromium-nickel steels and are widely known as the 300 series. In this series the most widely used austenitic stainless steel is the grade 304, also known as 18/8 for its composition of 18 % chromium and 8 % nickel. The second most common austenitic stainless steel in this series is the grade 316, also called marine grade stainless steel, used primarily for its increased resistance to corrosion. A typical composition of 18 % chromium and 10 % nickel, commonly known as 18/10 stainless, is often used in cutlery and high quality cookware.
Besides the above two series there are super austenitic stainless steel grades which exhibit great resistance to chloride pitting and crevice corrosion because of high molybdenum content (> 6 %) and nitrogen additions. Higher nickel content ensures better resistance to stress-corrosion cracking than the stainless steels of the 300 series. The higher alloy content of super austenitic steels makes them more expensive.
The straight grades of stainless steel contain a maximum of 0.08 % carbon. In these grades, there is no requirement of minimum carbon in the specification.
The ‘L’ grades are used to provide extra corrosion resistance after welding. The letter ‘L’ after a stainless steel grade indicates low carbon (as in 304L). The carbon is kept to 0.03 % or under to avoid carbide precipitation. Carbon in steel, when heated to temperatures in what is called the critical range (430 deg C to 870 deg C) precipitates out, combines with the chromium and gathers on the grain boundaries. This deprives the steel of the chromium in solution and promotes corrosion adjacent to the grain boundaries. By controlling the amount of carbon, this is minimized. For weldability, the ‘L’ grades are used. However the ‘L’ grades are more expensive. In addition, carbon, at high temperatures imparts great physical strength.
The ‘H’ grades contain a minimum of 0.04 % carbon and a maximum of 0.10 % carbon and are designated by the letter ‘H’ after the steel grade. ‘H’ grades are primarily used at extreme temperatures as the higher carbon helps the material retain strength at extreme temperatures.
Austenitic stainless steels can also be classified into following three groups.
- Lean alloys – Stainless steels with less than 20 % chromium and 14 % nickel fall into this category. Examples of these alloys are grades 301, 304 and 201. These are the largest portion of all the stainless steels being produced. These stainless steels are generally used when high strength or high formability is the main objective since the lower, yet tailorable, austenite stability of these stainless steels gives a great range of work hardening rates and good ductility. Richer stainless steels (e.g. grade 305) with minimal work hardening are the high alloy steels. The general purpose stainless steel (grade 304) is within this group. Stainless steels of this category have sufficient corrosion resistance to be used in any indoor or outdoor environment. These stainless steels are easily weldable and formable and can be given many attractive and useful surface finishes.
- Chrome nickel alloy – These stainless steels are used when the objective is high temperature oxidation resistance. This can be enhanced by silicon and rare earths. If the application requires high temperature strength, carbon, nitrogen, niobium, and molybdenum can be added. Stainless steels of grades 302B, 309, 310, 347 and various proprietary alloys are in this group.
- Chromium, molybdenum, nickel, and nitrogen stainless steels – These stainless steels are used when corrosion resistance is the main objective. Elements such as silicon and copper are added for resistance to specific environments. This group of stainless steels includes 316L, 317L, and 904L, and many proprietary grades.
All austenitic stainless steels contain a small amount of ferrite. Conventional austenitic stainless steel grades may contain traces of delta ferrite, for improved weldability. Usually this amount of ferrite is not enough to attract a normal magnet. However, if the balance of elements in the steel favours the ferritic end of the spectrum, it is possible for the amount of ferrite to be sufficient to cause a significant magnetic response. Also, some types of stainless steels are deliberately balanced to have a significant amount of ferrite.
Properties and of stainless steels
Austenitic stainless steels are non magnetic and are not heat treatable. They cannot be hardened by heat treatment. However, they can be cold worked to improve hardness, strength and stress resistance. A solution anneal (heating within the range 1000 deg C to 1200 deg C followed by quenching or rapid cooling) restores the stainless steels original condition, including removal of alloy segregation and re-establishment of ductility after cold working. Stainless steels can be subjected to solution annealing. Due to the solution annealing the carbides, which may have precipitated (or moved) at the grain boundaries, are put back into solution (dispersed) into the matrix of the metal by the annealing process. ‘L’ grades are used where annealing after welding is impractical.
Austenitic stainless steels can be made soft enough (i.e. with yield strength of around 200 N/sq mm) to be easily formed by the same tools that work with carbon steel, but they can be made incredibly strong by cold work, up to yield strengths of over 2000 N/sq mm. Their austenitic (fcc, face centered cubic) structure is very tough and ductile down to absolute temperature. They also do not lose their strength at elevated temperatures as rapidly as ferritic (bcc, body centered cubic) iron base alloys.
Austenitic grades of stainless steels are the most common used grades, mainly because they provide very predictable level of corrosion resistance with excellent mechanical properties. The least corrosion resistant versions can withstand the normal corrosive attack of the everyday environment that people experience, while the most corrosion resistant grades can even withstand boiling seawater.
Austenitic stainless steels have good formability and weldability, as well as excellent toughness, particularly at low, or cryogenic, temperatures. Austenitic grades also have a low yield stress and relatively high tensile strength. They have excellent corrosion resistance and excellent high-temperature tensile and creep strength.
Austenitic stainless steels are not very strong materials. Typically their 0.2 % proof stress is about 250 N/sq mm and the tensile strength between 500 and 600 N/sq mm, showing that these steels have substantial capacity for work hardening, which makes working more difficult than in the case of mild steel. However, austenitic stainless steels possess very good ductility with elongations of about 50 % in tensile tests.
Austenitic stainless steels are also highly resistant to high temperature oxidation because of the protective surface film, but the usual grades have low strengths at elevated temperatures. Those steels stabilized with Ti and Nb, grades 321 and 347, can be heat treated to produce a fine dispersion of TiC or NbC which interacts with dislocations generated during creep. One of the most commonly used alloys is 25Cr20Ni with additions of titanium or niobium which possesses good creep strength at temperatures as high as 700 deg C.
Austenitic stainless steels are ductile over a wide temperature range, from cryogenic to creep temperatures. They do not display brittle fracture. Their tensile strength is high at low temperatures. They can be work hardened to high levels of strength by cold forming.
Austenitic stainless steels are less resistant to cyclic oxidation than are ferritic grades because their greater thermal expansion coefficient tends to cause the protective oxide coating to spall. They can experience stress corrosion cracking (SCC) if used in an environment to which they have insufficient corrosion resistance. The fatigue endurance limit is only about 30 % of the tensile strength (vs. 50 % – 60 % for ferritic stainless steels). This, combined with their high thermal expansion co efficient, makes them especially susceptible to thermal fatigue. However, the risks of these limitations can be avoidable by taking special precautions.
The salient feature of austenitic stainless steels is that as chromium and molybdenum contents are increased to increase specific properties, usually corrosion resistance, nickel or other austenitic stabilizers must be added if the austenitic structure is to be preserved.
The tensile properties in the annealed state not surprisingly relate well to composition. The 0.2 % yield strength applies to the austenitic stainless steels.
Austenitic stainless steels have many advantages from a metallurgical point of view. Their properties include good to excellent corrosion resistance. They can be work hardened. They can be easily machined and fabricated to tight tolerances. They have smooth surface finish that can be easily cleaned and sterilized. They are temperature resistant from cryogenic to high heat temperatures.
Usage of austenitic stainless steels
Nickel which stabilizes the austenitic structure of these steels restricts their widespread usage since nickel increases the costs of these stainless steels.
Other steels can offer similar performance at lower cost and are preferred in certain applications, for example ASTM A387 is used in pressure vessels but is a low-alloy carbon steel with a chromium content of 0.5 % to 9 %. Low-carbon versions, for example 316L or 304L, are used to avoid corrosion problems caused by welding. Grade 316LVM is preferred where biocompatibility is required (such as body implants and piercings).
Austenitic grades of stainless steels are the most commonly used grades, mainly because they provide very predictable level of corrosion resistance with excellent mechanical properties. Using them wisely can save the designer of a product significant cost. These steels are user friendly metal alloy with life cycle cost of fully manufactured products lower than many other materials.
Austenitic stainless steels are those steels which are commonly used for stainless application. Some of the applications for austenitic stainless steel include the following.
- Kitchen sinks
- Architectural applications such as roofing and cladding
- Interior decoration
- Roofing and gutters
- Doors and windows
- Kitchenware, cutlery and cookware
- Benches and food preparation areas
- Food processing equipment
- Heat exchangers
- Ovens and furnace parts
- Chemical tanks