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The family of stainless steels
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The family of stainless steels
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"Martensite"
MartinsÃ*ticos in stainless steel, the carbon concentration is such that allows the formation of martensite during cooling from the austenite at high temperatures.
The martensite phase is rich in carbon, brittle and hard. The stainless steel martinsÃ*ticos have characteristics common to magnetic and endurecÃ*veis by heat treatment, showing tempered when an acicular microstructure.
It is important to note that these steels are usually supplied by the steel industry in the state annealing, with reasonably good ductility.
Only after that is tempered become very hard and ductile little - but resistant to corrosion.
The most used of martensitic stainless steel in Brazil is the type 420. In state annealing (ferritic structure), does not show good performance against atmospheric corrosion. This is because during the operation of annealing, the approximate temperature of 760 ° C, the carbon and chrome combine to form chromium carbide, Cr23C6. Each molecule of chromium carbide contains, by weight, approximately 95% of chromium. Considering the high content of carbon and low chromium stainless steel of 420 (approximately 0.35% C and 12.50% Cr) - and all the carbon precipitates as chromium carbide during annealing, the precipitation of solid solution withdraw approximately half the available chrome. In this condition the material can not resist to corrosion and can not be properly considered as a stainless steel (as they do not have a minimum of 11% of chromium in solid solution).
Therefore, the 420 stainless steel is placed in service by the user only after a treatment of time. When tempered, the carbon is part of the martensitic phase and is not found in the alloy precipitate as chromium carbide. The high hardness and consequent resistance to wear guide the applications of this material, used in cutlery, brake discs, surgical and dental equipment and turbines.
If the quantity of high carbon stainless steel is inconvenient in 420 in state annealing, a logical solution is to reduce this level, which is done in Type 410 stainless. As this material has a maximum of 0.15% carbon, this quantity is not sufficient to remove both the chromium solid solution and therefore presents a good resistance to atmospheric corrosion in both the condition of annealing as temperate.
After treatment for tempering, the hardness achieved in this material are not high as those made by stainless steel 420. The main applications of stainless steel 410 are in equipment for refining oil, valves, pumps and components of cutlery. Increasing the amount of sulfur is obtained by the 420F stainless steel, a variety of 420, with good machinability.
Additions of carbon (to get even greater hardness) and chromium and molybdenum (improving the corrosion resistance) lead us to martensitic stainless steel type 440, used in cutting knives professional.
"Austenitic"
The austenitic stainless steels are not magnetic and can not be hardened by heat treatment. Are very ductile and have excellent weldability. The most popular is the Type 304, which contains basically 18% chromium and 8% nickel, with a carbon content limited to a maximum of 0.08%. Has great application in chemical, pharmaceutical, petrochemical, alcohol, aviation, shipbuilding, from architecture, food, transportation, and also used in flatware, serving set, sinks, coatings, elevators and many other applications.
In some ways, especially in containing chloride ions, the 304 stainless steel shows a tendency to call a corrosion corrosion by pites. It is an extremely localized form of corrosion, which in some points of the surface material in the aggressive environment can break the passive film and then progressing in depth. The growth of pites occurs in an autocatalytic process, and although the loss of weight can sometimes be negligible, generates a very insidious form of corrosion, as often pitting one is enough to make a device out of service.
The corrosion of cracks can be considered as a corrosion by pites artificial. The appearance is often similar to the corrosion by pites and the growth process is autocatalytic. But the existence of a crack is necessary for the occurrence of the phenomenon, which does not happen in pites corrosion. The same means able to cause corrosion by pites promote corrosion in stainless steel.
The molybdenum is introduced as an alloying element in stainless steel in order to reduce the susceptibility to these forms of corrosion. The presence of molybdenum allows the formation of a passive layer more resistant and, in cases where the stainless steel 304 can not resist the action of certain means, corroded by pites or cracks, the 316 and 317 stainless steels are an excellent solution. Steels are very use in chemical, alcohol, petrochemicals, pulp and paper, in prospeção oil and the textile and pharmaceutical industries.
When submitted for some time the temperature between 450 and 850 ° C, the austenitic stainless steels are subject to precipitation of carbides of chromium in their grain boundaries, which makes them SENSITIZER. The abundant precipitation of carbides - the sensitização - results in the decrease of content of chromium in the neighboring regions with lathes, regions that have thus its resistance to corrosion dramatically compromised, making the material susceptible to intergranular corrosion in certain media. Areas affected by heat welding operation are particularly sensitive to this form of corrosion because during the thermal cycle of welding of the material is maintained in the range of critical temperatures. The consideration of this phenomenon led to the development of extra low carbon austenitic stainless steels, 304L, 316L and 317L, in which the carbon content is controlled to a maximum of 0.03%, thereby reducing the possibility of extremely sensitização.
The use of stabilizers has the purpose to avoid the problem of sensitização. The titanium, added as an alloying element, inhibits the formation of chromium carbide due to the fact of having a greater affinity with carbon, for the affinity with chromium. Thus, it precipitates of titanium carbide and chromium remains in solid solution. With the same purpose may be used niobium. Both the titanium and the niobium are stabilizers of carbon and stainless steel obtained - the 321 and 347 - are known as stainless steels stabilized. The stainless steel 316 Ti is the stable version of Type 316. For applications in equipment operating between 400 and 900 ° C, the stabilized stainless steels are the most recommended, as better mechanical properties keep temperatures in the extra low carbon steels - especially the resistance to creep.
In 904 L stainless steel (20Cr-25Ni-4, 5Mo-1, 5Cu), the addition of alloying elements to improve not only the demand resistance to pitting corrosion resistance but also in reducing acid media. The high content of nickel also improves the performance against corrosion under tension.
Where you want a good mechanical strength there is great concern with intergranular corrosion, the 304H and 316H stainless steel with carbon contents in the range 0.04 / 0.10%, are recommended. The precipitation is fine network of chromium carbides, as harmful from the point of view of corrosion, it is beneficial when what matters are the mechanical properties.
Considerable increases in levels of chromium and nickel can increase the temperature of the formatting of scale (scaling) of austenitic stainless steels. The 304 stainless is recommended for work in air at temperatures below 925 º C with continuous service. Under the same conditions, the 310 stainless steel with chromium and nickel 24/26% 19/22%, resistant to temperatures up to 1150 º C. It is a material classified as stainless steel refractory.
Large increases in nickel, lead us to alloy Ni-Cr-Fe, where the element with greater presence in the material is no longer iron, but the nickel. These materials are not known as stainless steel and steel alloys, but the basis of nickel, and offer resistance to corrosion in various media and high temperatures. The high content of nickel also gives assurance of a good resistance to corrosion under stress.
The 304 stainless steel is a material with excellent ductility. In cases of extra-deep drawing, an increase in nickel content can further improve the ductility. To this end the Type 305 was developed.
Reductions in light of nickel content decreases the stability of austenite, allowing the appearance of martensite induced by cold deformation, thereby achieving excellent properties for structural applications. It is the Type 301, available in versions 1 / 4, 1 / 2, 3 / 4, completely hard and wide use in industries rail, subway trains and bus bodies.
The Type 303 result of increased sulfur content in the 304, with the purpose of improving the machinability. The ductility and corrosion resistance are compromised by this increase in the amount of sulfur.
The 200 series of steel resulting from a partial replacement of nickel by manganese. Are used in structural applications, showing resistance to corrosion than the 301.
BANDS OF CHEMICAL COMPOSITION OF stainless steel
PP1JL
Brasil
