Stainless steel

Stainless steel or stainless steel is the name given to a group of corrosion and acid resistant steels. Was invented by Harry Brearley in Anglo-Saxon, of his patent filed in 1913 ( granted until 1919 ). In the German Empire, however Eduard Maurer and Benno Strauss left out of the Krupp company in the autumn of 1912 by the officials Clemens Pasel a patent log on stainless steel ( also issued in 1919 ). The Germania Germania shipyard had the yacht built in 1908 for Krupp also stainless steel, but it should still take a few years until the material could also be produced inexpensively on an industrial scale. The global stainless steel production in 2012 was 35 million tonnes.

Synonyms

Are synonyms for stainless ( " stainless " ) steel

• Stainless steel, commonly called just stainless steel; fachsprachlich the term stainless steel is used only for steels with very high purity, which need not necessarily highly alloyed and stainless.
• Inox (formed by the French inoxydable, which means " not oxidized " or " stainless " )
• Cromargan, trade name of WMF
• Stainless steel, brand name of Outokumpu stainless steel (formerly "ThyssenKrupp Stainless steel " ) is rarely written or Nieroster Niro Rochester, or shortly Niro. The name derives from the shortening of the name Stainless steel.
• Remanit, brand name stainless steel Witten- Krefeld. Remanit is also used as a material for art in public spaces, such as by Friedrich Becker for his fountain Remanit for the exhibition halls of the city of Hanover. Remanit is DPMA since 1926 under the number 360 467 registered as a trademark.
• Stainless steel

In Germany special designations are used for the following two types of stainless steel, but they are not synonyms for stainless steel, but denote a certain type of steel:

• V2A ( experimental melt 2 austenite was 1912 for Alloy Type X12CrNi18 -8 or also called 1.4300 ), is no longer manufactured today. (The designation V2A is now used for the successor 1.4301 ( X5CrN18 -10) ).
• V4A (similar V2A, but with the addition of 2% molybdenum ( Mo) alloy, which makes this steel resistant to corrosion by chloride media - salt water, swimming pools, chemical industry, etc. ) is the general term for V4A 1.4401 ( X5CrNiMo17 -12-2 )

Also VA steel derived from these terms. Are used less steel grades V1A, V3A and V5A.

Description

Stainless ( " stainless " ) steel is characterized by a content of more than 10.5 percent chromium, which must be solved in the austenitic or ferritic solid solution. Due to this high chromium content, a protective passive layer of chromium oxide and dense forms on the surface of the material. This passive layer can be simultaneously used for coloring the steel surface after special treatment. Other alloying elements such as nickel, molybdenum, manganese and niobium lead to an even better corrosion resistance and better mechanical properties. Since chromium is less expensive than nickel as an alloying element, a higher chromium content is the nickel content ( to same corrosion resistance ) are preferred.

Properties

The stainless steels are characterized mainly by the following common characteristics:

• Increased resistance to corrosion and acids
• High toughness and machinability so bad (for example, drilling, turning ) and increased tendency to " seizure " for threads (the latter especially when they are heated, for example, in containers of the chemical and pharmaceutical industry)
• Poor electrical and thermal conductivity

Steels without addition of nickel ferrite crystals form and have the following properties:

• Magnetic ( but less saturation polarization than unalloyed steels )
• Coefficient of thermal expansion rather deeper than mild steel ( 10.0 ... 10.5 × 10-6 K-1)
• Most varieties are curable

Steels with higher nickel contents (about 70 % of production) form austenitic structure and have the following properties:

• Higher corrosion resistance than low-nickel chromium steels (particularly resistance to stress corrosion cracking )
• De-energized largely non-magnetic
• Comparatively low yield strength ( 200-300 N / mm ²) at a relatively high tensile strength ( from 700 to 1300 N / mm ²)
• High thermal expansion coefficient ( for example, a value of 16.0 x 10-6 K -1 for the material 1.4301 as compared to a value of 10.5 x 10-6 K-1 for carbon steel)
• Density 1.4301 (V2A): 7.9 g/cm3, 1.4401 (V4A): 8.0 g/cm3
• Not by heat treatment followed by quenching curable
• Surface hardening possible only by plasma nitriding, Kolsterising or cold working
• Well to be joined by welding

Use

Because of the good formability of sheet metal stainless steel components made of this material at an increasing rate in the industry, at home or even in medical devices. Although most stainless steels can be very bad chipping, their use has mainly advantages. Here are, for example, in addition to hygienic aspects ( in the brewing industry, the food industry and pharmacy carried the steam sterilization ) to name the longevity of the parts and benefits in environmental protection produced. However, a disadvantage compared to other steels is mostly low tensile strength and often missing hardenability (see further text ). Also noteworthy is the lower compared to copper and its alloys antibacterial effect, also known as oligodynamic effect ( Oligodynamie ).

Without stainless steel cryostat many would not be feasible. The poor thermal conductivity and thin walls (for example, tubes with wall thickness less than 0.3 mm ) provide good insulation between the cryogenic fluid and room temperature. Other advantages are UHV - tightness of welded joints and low magnetism.

Since the grant of a patent on steels with " high resistance to corrosion" in 1912 to the company Friedrich Krupp AG in Essen the production of stainless steel has taken a huge boost. Trigger for the development of such steel was the emerging chemical industry in the German Reich. The former synthesis methods with superheated steam, acidic media and high temperatures were conventional steels brittle ( hydrogen embrittlement ) and are cracked. Many former reactors were up to that of granite made ​​to circumvent these drawbacks. The 1913 incipient ammonia synthesis (Haber -Bosch process ) could only be through the use of austenitic stainless steels as they had a year earlier Krupp developed, can be realized. The parallel development of the steel and chemical industries, in particular these two events, therefore, was no accident.

Heat-resistant stainless steels are marketed as heat-resistant steels and can partially be used at temperatures up to 900 ° C.

Applied Materials and composition

Under the generic term stainless steel, there are a variety of alloys, which differ in their alloy components, properties, and applications. To clearly distinguish the individual alloys material numbers are assigned.

Austenitic steels

Steels with more than 8 % nickel have austenitic crystal structure and exhibit a particularly favorable combination of processability, mechanical properties and corrosion resistance.

Material number 1.4301 - X5CrN18 -10

The most common type of alloy of stainless steel, which we encounter in everyday life, is the alloy X5CrN18 -10 ( material number 1.4301, inscription 18/10, also known as V2A). 33 % of the production of stainless steels account for this type of alloy, an additional 20 % to the similar steel 1.4307 ( X2CrNi18 -9). At 1.4301 there is a relatively soft, nickel-containing, non-ferromagnetic austenite steel for example, pots, cutlery (except cutter blade ), sink. The term 18/10 refers to the percentage of chromium / nickel.

The material is not stable in the presence of chlorine-containing media, such as salt water or chlorine-containing atmosphere in indoor swimming pools. The alloy is extremely tough and has a tendency to extreme hardening during cold deformation, making them ( ... drilling, punching, machining ) is poorly machinable.

1.4571 - X6CrNiMoTi17 -12-2 ( historically ) or 1.4404 - X2CrNiMo17 -12-2

For use in the presence of chloride-containing media of 1.4571 or 1.4404 ( colloquially V4A) is often used. He has, in contrast to 1.4301 by its share of 2% molybdenum increased resistance to chlorides. Applications include all areas that constantly come into contact with salt water, such as fittings for shipbuilding. In addition, he is for the rehabilitation of chimneys, in indoor pools ( wetted with safety-related components that can not be cleaned regularly or water, higher molybdenum-containing grades must be used, eg 1.4529 ) used and the chemical industry.

Other steel grades

However curable martensitic- ferritic steels are used for tools and knives, which are in addition to chromium often contain vanadium and molybdenum and magnetizable. Typical steel grades for this are X30Cr13 and the higher quality alloy X50CrMoV15 ( cf. knife steel).

In the offshore sector is also duplex steel, eg 1.4462 ( X2CrNiMoN22 -5-3 ) application. Instead of nickel, for austenitic steels may also be used as the manganese cheaper alloying element, but the overall quality of these steels is low.

The resistance to corrosion decreases with increasing carbon content, since chromium has a high carbon affinity and hard, brittle chromium carbide forms predominantly at the grain boundaries, at the expense of protective chromium oxide. In addition, then the steels are prone to intergranular corrosion. To counteract this effect and thereby improve the weldability, the carbon content is kept low, and the corresponding steel grades or by addition of niobium and / or titanium (which has a higher affinity to carbon than chromium have ) stabilized. Stabilized in this way, purely ferritic steels containing 12 to 18 % chromium content as X2CrTi12 ( 1.4512 ), X2CrTiNb18 ( 1.4509 ) and X3CrTi17 ( 1.4510 ) today provide the main material for the construction of exhausts in the automotive industry dar. Approximately 10% of global production of stainless steels not applicable to this application. The cost-saving absence of nickel, as well as the lower thermal expansion coefficient of the ferritic crystal lattice are the specific advantages of these steels. The additional alloying with molybdenum improve the corrosion resistance.

Importance of material numbers

Abbreviations:

• Cr = chromium
• Cu = copper
• Mo = molybdenum
• Nb = niobium
• Ni = Nickel
• Ti = titanium

Overview of material designations

Material no.

Short name

Designation

The table shows the general stainless and acid resistant steels are listed.

Screw

On bolts of stainless steels is often the designation A2 -70. Here is A2 for the steel grade (A for austenitic, two for variety ), 70 for the tensile strength in kgf / mm ² (obsolete) corresponding to 1/ 10 of tensile strength 700 MPa. Components from the special material X2CrNiMoN17 - 13-5 (material number 1.4439/Alloy 317 LN) are preferable necessarily for the offshore sector and for systems for seawater desalination. Bolting materials are made ​​of stainless steels and their names are standardized in EN ISO 3506. When replacing screws with conventional material by stainless steel screws should be noted that the material properties (tensile strength, elongation at break, yield strength, etc. ) of this stainless steel screws usually below those of conventional screws with property class is greater than or equal 5.6. A simple replacement for the 1:1 principle is to look closely at just security-related compounds. Furthermore, in contact between stainless steel and normal steel may occur additional corrosion electrochemical reasons.

In the three-part European standard EN 10088 specifies the technical delivery conditions for stainless steels are generally regulated.

To estimate the corrosion resistance of stainless steel, the pitting resistance equivalent ( PRE value also ) serve. The higher this is, the more resistant the alloy to pitting or crevice corrosion. Alloys with an effective sum of 33 are considered to be resistant to seawater.

Other corrosion-resistant alloys

Are no longer counted to steels Cr -Ni alloys containing less than 50 % iron and even better properties have regarding corrosion and heat resistance. These so-called super alloys are among the most high-temperature alloys, and are based on a method described in 1906 for the first time alloy type NiCr8020. By additions of aluminum and titanium, these are curable and the greatly increased strength at high temperatures. Modern trade names are eg Inconel, Incoloy, Hastelloy, Cronifer, Nicrofer. The latter is a highly corrosion resistant nickel - chromium -molybdenum alloy, which is still divided into various alloys, depending on the additive ( Nicrofer 3127 hMo, Nicrofer 5923 hMo, H- C4 - C22 or H ).

Find application such alloys mainly in jet engines, power plant industry (gas turbines ), oil and gas industry, environmental technology ( REA), as well as chemical engineering, so wherever high strength must be guaranteed at very high temperatures or highly corrosive conditions in the long term.