Austenitic Stainless Steels
The austenitic stainless steels are among those most products and, given the numerous laboratory research and industrial, even those most studied. Austenitic stainless steels are normally defined as those iron-chromium-nickel alloys with chromium content of 12 to 30% and nickel of 7-35%, with fully austenitic structure. Chromium has an ferritic effect on steel. Adding nickel, the structure of the stainless steel becomes austenitic. This type of steels do not undergo any hardening treatment because would change the crystal structure of the steel. The definition is missing an element that has influenced the evolution of this class of steels: carbon.
The first austenitic steels containing a carbon content of 0.25%. This value reduced the resistance to intergranular corrosion. The first trick was to lower the carbon content to attenuate the formation of chromium carbides. With this arrangement have been developed stainless steels of the series 300. Further expedients were developed adding elements such as titanium and niobium in small percentages in order to form carbides of titanium and niobium and leave unchanged the content of chromium.
With the advent of an insufflation oxygen process and the availability of low-carbon alloys, it was possible to produce austenitic stainless steels with very favorable economic conditions.
With average values of 18% Cr and 9% Ni these stainless steels have a very low resistance to corrosion if they are installed in marine atmospheres and chemical oxidants.
Although it can be used in soft state but with ductility significantly lower than AISI 301, it finds its wider use in the annealed condition, in order to “divide” the market with the AISI 304, at least abroad.
After cold workingit is slightly magnetized.
Among the many end-use can be cited: moldings, equipment for the food industry (dairy, vinaria, beer, meat), sinks, cookware, tableware, cutlery, jewelery, transport tank acids and chemicals in general, facades and architectural panels, furniture, aeronautical, petrochemical, chemical, cryogenic plants.
Reel of a fishing rod
With mean values of 19% Cr and 9.5% Ni represents the type of stainless steel used in the most absolute sense.
It has a corrosion resistance slightly higher than that of the AISI 302 and acquires a slight magnetism with cold working.
It has a maximum content of C of 0.08%. much lower than the austenitic types previously described.
This fact limits the possibility of precipitation of carbides after welding by increasing the resistance to intergranular corrosion.
With the maximum value of nickel and minimum values of chromium it can identify another similar type which may be referred to as type 304PS, particularly suitable for very deep moldings.
The application for this kind of stainless steel are very wide.
In particular, it is noted the field of architecture and furnishings, the industry and the transport of beer, the food industry, pipes, transport of oxygen, hydrogen and nitrogen liquid reactors for chemical and nuclear.
Reactors for the chemical and nuclear
With 25% Cr and 21% Ni these stainless steels can reach temperatures of service higher than the previous and precisely than 1120°C in continuous service and of 1040 ° C in service intermittent.
Even in these cases, the S indicates a better weld ability.
We cite other end-uses: combustion chambers, filler metals for soldering, parts for gas turbines and jet engines, incinerators.
System of a gas turbine
Austenitic grades of this series differ from that of the series previously described for the addition of Mo (Molybdenum) which confers especially the possibility to resist corrosion in reducing environments, and to increase the resistance to corrosion in highly industrial and marine as well as in water brackish.
The molybdenum is able to counteract, in proportion to the percentage in which it is present in steel, the particular form of corrosion always possible in environments full of halides.
This is the pitting corrosion or pitting, extremely treacherous because the losses in weight are virtually nil but are sufficient some “pits”, often not visible, to lead to perforation of the artefact with very serious consequences also from the point of view of safety.The Molybdenum also helps to resist to stress corrosion and crevice.
Therefore the types austenitic Ni-Cr-Mo stainless steels are among the most resistant to corrosion both to the greater number of corrosive environments that in the various possible types of corrosion.
In series of the stainless steel with molybdenum, AISI 316 corresponds to AISI 304 and is produced in large quantities.
In Europe the original AISI is split in two type: type 316 with Mo 2.0 – 2.5% and a type 316S with 2.5 -3.0% Mo.
Compared to AISI 304, it has better performance against the creep. It can be mentioned some typical end-use: equipment for the chemical and paper industries, equipment development and printing of photographs; containers for wine industries, components of installations for the production of fertilizers, to the industry of the tomato.
Steriliser for the food industry
It can be assimilated to an AISI 304 with the presence of Ti. As already said the action of Ti is to engage the C during the creation of carbides of Ti, preventing it combining with Cr, thus avoiding the formation of Cr carbides during welding operations.
The end uses are those typical of the AISI 304 L for which use one or the other type is almost a fact of sympathy; usually in Germany it is preferred AISI 321.
It should be stressed that the surfaces of 321 are worse than those of the 304 L due to the presence of inclusions of titanium carbides specially introduced and therefore is not recommended the use of this steel in decorative applications.
The Ti, on the other hand, determines also a greater mechanical resistance for high temperature.
In fact, the AISI 321 is recommended for particular use at temperatures between 430° C and 900° C, which is the field in which occurs the precipitation of carbides Cr steels excluded for all other types L and those with Ti and Nb. Typical uses include: exhaust manifolds for aircraft, bodies of the boiler, chemical reactors, expansion joints, burners, fire walls , pressure vessels, pipes for armored resistances.
Exhaust manifolds for aircraft
In these two types it is added instead of Nb, Ti in order to decrease the possibility of the formation of Cr carbides detrimental to intergranular corrosion.
The second type differs from the first by a limitation of the levels of tantalum (always present together with Nb) and of cobalt, required for use in radioactive environments.
The end uses are similar to those of the AISI 321 and to a large extent to those of the AISI 304L. These types with Nb are distinguished, however, for a more marked resistance to hot creep for which excel between all stainless steels.
High Performance Austenitic Stainless Steels (HPASS)
Recently, new austenitic stainless steel with high corrosion resistance were placed on the market.
They showed convenience and performance in a variety of harsh environments and corrosive.
They have supported the engineers of materials with alloys suitable for new requirements in the energy and environmental sectors.
These new materials are called High Performance Austenitic Stainless Steels (HPASS).
They get their performance thanks to the alloying with levels of molybdenum and nitrogen higher than normal due to the use of new technologies such as iron and steel processes argon-oxygen decarburization (AOD), and vacuum-oxygen (VOD).
These solutions have made possible the achievement of a very low carbon content with production costs very low, a high alloying and better control of the composition (in particular of the nitrogen content).
One of the first alloy to take advantage of new technologies, the first member of HPASS, was the AISI 904L. The type 904L contains very low carbon content to provide a version of a weldable alloy for mergers with high resistance to acids.
In 1973, it was produced the first fully austenitic stainless steel resistant to salt water, AL-6X® containing 6% Mo and reduced carbon content to get very thin laminations and welded tubes.
Around the mid-70s, developments in the use and control of nitrogen led to advanced alloys with 6% Mo weldable with high thicknesses and resistant to the formation of harmful intermetallic phases that reduce the resistance to pitting.
Some representatives of these alloys are the 254 SMO® and AL-6XN®. The growing demand for high-performance alloys, but the cost, in emerging areas such as environmental and energy drove these stainless steels even higher in the 90s.
Two alloys with very high resistance to pitting in water were strongly chlorinated alloy 654 SMO® (S32654), containing 7.3% of Mo and 0.50% of N, and B66 alloy, containing 6% of Mo, 2% of W and 0.45% of N. These steels are close to the performance of some nickel alloys resistant to corrosion with costs significantly lower.
The HPASS retain the good overall mechanical properties of the standard types, with a resistance slightly higher and a slightly lower ductility in annealed condition.
The hardening increases their resistance.
Since work-harden, they have excellent formability and can absorb significant levels of energy in the event of breakage.
Their resistance is due to the effects of alloying elements in solid solution of substitution and interstitial hydrogen.
These alloys retain good properties at low and at high temperatures.
The HPASS have the highest corrosion resistance of stainless steels.
Due to the presence of molybdenum, the film in HPASS is made more resistant and therefore the steel becomes more resistant to dissolution than the case of the standard types.
Air filter made of HPASS
The substitution of nickel in austenitic stainless steels by the addition of manganese has been discussed for over 50 years in order to reduce the impact due to fluctuations in the price of nickel in the extra alloy.
This led to the development of the so-called series 200. These steels were designed and produced in times of war by nations that had problems of supply of nickel.
In the absence of nickel and with the maximum content of manganese, the percentage of nitrogen required to obtain the desired stable austenitic structure introduces difficulty in melting and treatment and affects in a negative way the properties of the final product.
Small amounts of nickel improve mechanical properties and corrosion resistance.
The nickel content ranges from 1% to 6%, while the manganese is between 6% and 10%.
AISI 201 can substitute satisfactorily in many applications the stainless steel AISI 301 and obviously it has a lower cost.
In the annealed condition it has a average breaking load of 70 kg / mm2 and a yield strength of 32 kg / mm2, elongation around 40%.
With the cold working becomes magnetic and can be hardened in the whole range of the hardened with minimum elongations from 20% for 1/4 hard to 4% for the completion hard.
In various states of hardness applies to carrying spars trains. The annealed is used for coatings, pottery that you can get with light moldings, automotive trim.
Coils made of AISI 201
Compared to the previous it presents higher percentages of Mn (9 instead of 7%) and Ni (5 instead of 4.5%).
The resulting austenite is more stable and with the cold working becomes slightly magnetic. It is comparable to AISI 302, and it is a cheaper alternative while being less ductile in the annealed condition.
It can be hardened only state 1/4 hard and has better corrosion resistance than the previous (AISI 201).
In the annealed condition it has the same uses of the AISI 201, but with decidedly better molding characteristics that make possible the manufacture of batteries kitchen; It can be used for containers for transport and storage of the milk.
Cookware made of AISI 202
Contains amongst Austenitic the highest proportion of Mn (15%) and the lowest percentage of Ni (1.5%).
Its austenitic structure is very stable, it hardens very little with the cold working, nor becomes magnetic.
It presents breaking loads (95 kg / mm2) and yield limits (50 kg / mm2) very high with excellent elongation (about 60%).
It is suitable for applications at very low temperatures and for special molding operations and by spinning.
At least in Europe it is still little used and like the previous two deserves to be better known, especially in Italy where the stainless steel AISI 304 is always used.
Coils made of AISI 205