Austenitic Stainless Steel

Austenitic stainless steel has austenite as its primary phase at both elevated and room temperature.

A stable austenite structured steel contains about 18% Cr, 8% ~ 10% Ni and about 0.1% C. Chrome-nickel austenitic stainless steel including the most familiar stainless steel 18Cr-8Ni (TP304) and the series of high Cr-Ni steel developed by increase Cr, Ni contents and add Mo, Cu, Si, Nb, Ti and other elements. Austenitic stainless steel is characterized by non-magnetic, high plasticity and ductility but lower strength. It can be only strengthened via cold working (by adding S, Ca, Se, Te and etc, austenitic stainless steel can have excellent machinability).

Grades:304/L/H/LN, 316/L/H/LN/Ti/LMod, 310S/H, 317/L, 321/H, 347H/HFG

Specifications:

  • ASTM A312 Standard Specification for Seamless, Welded, and Heavily Cold Worked Austenitic Stainless Steel Pipes
  • ASTM A790 Standard Specification for Seamless and Welded Ferritic/Austenitic Stainless Steel Pipe
  • ASME SA213 Seamless Ferrite and Austenitic Alloy Steel Pipe for Boiler, Superheater and Heat Exchanger
  • ASME SA249 Standard Specification for Welded Austenitic Steel Boiler, Superheater, Heat-Exchanger, and Condenser Tubes
  • ASME SA789 Standard Specification for Welded Austenitic Steel Boiler, Superheater, Heat-Exchanger, and Condenser Tubes

Austenitic steels are non-magnetic stainless steels that contain high levels of chromium and nickel and low levels of carbon. Known for their formability and resistance to corrosion, austenitic steels are the most widely used grade of stainless steel.

Features:

  • Excellent corrosion resistance
  • Good for cold working or hot working
  • High plasticity and ductility
  • Optimum general performance
  • A wide range of applications

There is a fifth specialist type Precipitation Hardening Steels.


Development diagram of common grades of austenitic stainless steel

Molecular Structure of Steel

The fundamental difference between them is their crystaline structure. Steel is an alloy of iron and carbon. At normal temeperatures the atomic structure is that of a cube with an atom of steel at each corner and a single atom in the centre of that cube.

This is known as "ferritic" and, is by the way, magnetic.

When heated to about 900o C the structure changes and each face has an atom at its centre. This is the austenitic structure and is non-magnetic.

When ordinary steel cools down gradually it reverts to a ferritic structure. If you cool it fast, it will adopt another structure with the carbon atoms being arranged in one direction. This is martensitic steel and in its "as quenched" condition is hard but brittle and generally requires further treatment before it can be used.


Strength Graph of 316L Mod at room and elevated temperature

Chemical Composition of Austenitic Steels

AISI grade C max. Si max. Mn max. Cr Ni Mo Ti Nb Al V
301 0.15 1.00 2.00 16-18 6-8
302 0.15 1.00 2.00 17-19 8-10
304 0.08 1.00 2.00 17.5-20 8-10.5
310 0.25 1.50 2.00 24-26 19-22
316 0.08 1.00 2.00 16-18 10-14 2.0-3.0
321 0.08 1.00 2.00 17-19 9-12 5 x %C min.
347 0.08 1.00 2.00 17-19 9-13 10 x %C min.
E 1250 0.1 0.5 6.0 15.0 10.0 0.25
20/25-Nb 0.05 1.0 1.0 20.0 25.0 0.7
A 286 0.05 1.0 1.0 15.0 26.0 1.2 ~1.9 ~0.18 ~0.25
254SMO 0.02 0.8 1.0 18.5-20.5 17.5-18.5 6-6.5 ~1.9 ~0.18 ~0.25
AL-6XN 0.03 1.0 2.0 20-22 23.5-25.5 6-7
Glossary

Annealing

Annealing is a process of heating the steel to a temperature slightly above its recrystallisation temperature and allowing it to cool at an appropriate rate - generally slowly - causing the crystals to reform without the defects caused by "working" the steel.

Annealing can restore the ductility of the steel and its corrosion resistance characteristics.

Carbide Precipitation

When steel contains higher levels of carbon there is a tendency for it to combine with the chromium as it cools - between 900oC and 500oC forming chromium carbide. This reduces the amount of chromium available to form the passive layer and creates intergranular boundaries that are accessible to corrosive chemicals.

This can be overcome by using Low carbon variants of the steel (designated by "L", eg 304L or 316L).

However, the lower carbon levels reduce the steel's performance at elevated temperature. If resistance to carbide precipitation and strength at high temperature are required then the addition of Titanium may be the solution. There are a number of grades available in this form - eg 316Ti.

Creep Strength

Steels perform very differently at elevated temperatures than they do at ambient temperatures. When they are bent at ambient temperatures to below their yield point, they will spring back. At elevated temperatures, they begin to stretch, but very slowly. Some steels have better resistance to this phenomenon than others.

Grain Size

Steel is made up of a lattice of crystals of iron interwoven with atoms of other materials. These crystals are called grains.

The grain size is important because it affects the machining, hardness, strength, and corrosion resistance amongst other things.

Grain size can be determined both by the addition of other alloying elements and by the careful regulation of the heating and cooling processes involved in the production of the steel and by further heat treatment (annealing and quenching) following initial production, welding or "working" on the steel.

Intergranular Corrosion

The atoms in metals are arranged into crystals (or grains) which are aligned closely with each other. In certain conditions, corrosion can attack the grain boundaries rather than the crystals themselves.

When stainless steel containing a higher percentage of carbon is heated it the chromium can react with the carbon to form chromium carbide thereby depleting the passive layer of chromium that protects the surface.

Passive Layer

The passive layer is what makes stainless steel "stainless". It is a microscopically thin layer of chromium oxide that is impenetrable to oxygen, very hard, resists corrosion itself and is virtually transparent. This prevents oxygen and other corrosive materials reaching the iron and reacting with it.

Chromium reacts readily with oxygen with the result that should it become scratched it will repair itself providing there is free oxygen available.

Pitting Corrosion

This is a very localised form of corrosion that arises particularly in high chloride conditions such as the marine environment. An initial breach in the passive layer of the steel is not "repaired" by the reformation of chromium oxide. The steel beneath this breach continues to corrode often leaving no obvious signs other than a light surface staining (sometimes called "tea staining) on the surface but continuing to deepen and widen below the surface.

While localised it can result in the penetration of the entire cross section of the steel.

High contents of chromium, molybdenum and nitrogen increase the resistance to pitting corrosion. The degree of resistance can be calculated as %Chromium + 3.5 x %Molybdenum + 16 x %Nitrogen to give a Pitting Resistance Equivalent Number (PREN).

316 has a PERN of between 22.6 – 27.9. Some Duplex steels have PRENs over 40. The span of the numbers given in certain grades is the result of the specification for the quantities of the relevant chemical having max and min figures.

Precipitation hardening

Also known as age hardening, is a process used to increase tensile strength. The alloy is first raised to a temperature that a produces a single phase with all the solute atoms dissolved and evenly distributed. It is then rapidly quenched before reheating to a lower temperature and holding it at that temperature for a predetermined time. At this temperature, the precipitates can clump together in a uniform and distributed manner. It is essential that the right temperature and duration at this stage of the process is correct. If the temperature is held too long it will result in oversize clumps and reduce the strength of the alloy. This is known as "over aging".

Sensitisation

Sensitisation is the process of Carbide Precipitation - See above.

Sigma phase embrittlement.

Is phase change that occurs in some stainless steels when they are heated above about 540C. This results in a dramatic loss of toughness and can lead to brittle fractures.

Stabilisation

Stabilisation is the process of removing or protecting the steel from sensitisation - the danger of carbide precipitation which can lead to stress corrosion cracking (SCC).

There are two approaches commonly used. Low carbon variants can be used; they are inherently more stable but they perform less well at higher temperatures.

Alternatively, steel can be chemically stabilised by alloying it with titanium, niobium (sometimes still called columbium). Both of these readily form carbides thereby preserving the chromium.

However, this may not be sufficient to stabilise the steel should it be held within the carbide forming temperature band 425oC to 850oC. Should this occur during the fabrication process the problem can generally be reversed by annealing at a higher temperature.

Stress Corrosion Cracking (SCC)

Stress corrosion cracking occurs when chemicals attack the intergranular boundaries within an alloy. When the metal is subject to tensile stresses it can result in the sudden failure in generally ductile materials. Because the corrosion only occurs at the grain boundaries it is likely to go unnoticed since the metal will generally maintain an apparently normal surface appearance.

Work Hardening

Work hardening is a term applied to any work done on the steel at a temperature below the metal's recrystallisation temperature.

This work includes any type of squeezing, bending, cutting/shearing or drawing.

These processes cause distortions in the crystalline structure of the metal reducing their ability to move within the metal and makes it more resistant to further deformation.

Hardening can be an advantage or a disadvantage.

  • It increases tensile strength and hardness
  • Reduces ductility and makes the steel more brittle.

The crystalline structure can be restored by annealing.

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.

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

Super Austenitic Stainless Steel

Super austenitic stainless steels contain high levels of chromium and higher levels of nickel together with additions of molybdenum and nitrogen. The result is a series of austenites, stronger than conventional 300 series stainless and with superior resistance to pitting, crevice corrosion, and stress corrosion cracking.

Super austenitic stainless steel is defined as Cr-Ni stainless steel with a Pitting Resistance Equivalent Number (PREN=[Cr]+3.3[Mo]+16[N])≥40%. It has better resistance to chloride pitting and crevice corrosion than Cr-Ni austenitic stainless steel with Mo >4% in highly corrosive media containing Cl-

Grades:

Features:

 Specifications:

Machinability

Work hardening produces hard surfaces and hard chips , which in turn lead to notch wear. It also creates adhesion and produces built-up edge (BUE). It has a relative machinability of 60%. The hardening condition can tear coating and substrate material from the edge, resulting in chipping and bad surface finish. Austenite produces tough, long, continuous chips, which are difficult to break. Adding S improves machinability, but results in lowered resistance to corrosion.


Rupture Strength Curve of S30432 Ultra-Supercritical Boiler Tube

Use sharp edges with a positive geometry.

Cut under the work hardened layer.

Keep cutting depth constant. Generates a lot of heat when machined.



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