Rusts tungsten carbide

Materials

metallic materials


Steel:

Steels are iron materials, the carbon content of which is generally less than 2%. From a chemical point of view, steel is an alloy of iron and iron carbide. In order to influence the chemical and mechanical properties of steels, other metals are added (e.g. chromium and manganese). There are different ways of producing steel, but we will not go into them here. The following steels are used for RETSCH devices and accessories:

  1. Stainless steel
  2. Hardened steel ("special steel")
  3. Steel for heavy metal-free size reduction

Stainless steel

“Stainless steel” is the name given to corrosion-resistant steel. The corrosion resistance of these steels is based on an extremely thin, invisible oxidic protective layer, the so-called passive layer. This passive layer forms with chromium contents> 12%. The corrosion resistance increases with the chromium content of the steel. However, if the chromium content is> 20%, the hardenability and thus the wear resistance of the steel is lost. Constant mechanical stress, such as B. in grinding tools, can lead to the destruction of the passive layer but also to the roughening of the tool surface. In addition to cross-contamination, corrosion can also occur. These are not caused by poor material quality, but are due to wear and tear. Commercially available metal cleaners and the use of a brush are suitable for removing corrosion, soiling or external rust from the grinding tools. After wet cleaning, we recommend wiping the parts with isopropanol or acetone and drying them thoroughly.

Hardened steel ("special steel")

Hardened steel, like stainless steel, belongs to the chromium steels, but has a lower chromium content of 12%. Accordingly, the special steel is not characterized by corrosion resistance but by its greater hardness.

Manganese steel ("Manganese precision casting")

Manganese steel is also called Hadfield steel after its inventor. The manganese content is between 12% - 14%, the carbon content between 1% - 1.2%. After quenching from the molten metal at 1100 ° C, the steel still shows an extremely ductile, deformable structure. Part of the structure only changes when exposed to shock loads, so that hardness values ​​of over 600 HV (approx. 55 HRC) are achieved. This process is known as work hardening, in technical parlance manganese steel is therefore also referred to as work hardening agent.


Tungsten carbide (hard metal):

Tungsten carbide is one of the hard metals. Pure tungsten carbide, a mixed crystal made of tungsten and carbon, is very brittle. Therefore, a cobalt content of 6-10% is added to the material tungsten carbide. This increases the toughness of the material and minimizes abrasion. The correct material composition of the carbide depends on the application and the technical requirements. Tungsten carbide is made by sintering tungsten and carbon. When heated under high pressure, the carbon is embedded in the crystal structure of the tungsten. The resulting carbide increases the melting point and hardness of the metal. Tungsten carbide is characterized by its very high hardness (even at high working temperatures) and wear resistance. Depending on its composition, it can have a hardness comparable to that of diamond. Tungsten carbide is therefore widely used, e.g. B. in the fitting of tools for machining and non-cutting shaping. In addition, it is used for tools that are subject to frictional wear. The various grinding tools that are used in RETSCH devices belong to this group.


Cast iron / malleable iron:

Cast iron is an iron material with a carbon content> 2% and a silicon content> 1.5%. The carbon is in the so-called gray cast iron or lamellar cast iron in the form of fine, irregularly distributed graphite flakes. The term cast iron refers to the manufacture of this material, the pouring of molten metal into molds. In contrast to cast iron, malleable cast iron is tempered after it has solidified. This means an annealing process lasting several days at approx. 1000 ° C in an oxidizing atmosphere.


Titanium:

Titanium (Ti) is a chemical element with the atomic number 22. It belongs to the transition metals and has a high strength with a relatively low density. Due to the formation of an oxidic protective layer, titanium is corrosion-resistant. Since high-purity titanium is ductile, small amounts of other elements, such as oxygen and nitrogen, can be added to improve the mechanical properties (embrittlement). Titanium occurs relatively frequently in the earth's crust, but mostly in low concentrations in the form of oxides. Titanium is primarily extracted from the minerals rutile (TiO2) and ilmenite (FeTiO3).



Ceramic materials

Zirconia:

The main raw material for the production of zirconium oxide ceramics (ZrO2) is the mineral zircon (ZrSiO4). ZrO2 is obtained from it on an industrial scale by melting it with coke and lime (reduction of SiO2). The resulting starting powder is mixed with additives and brought into the desired shape by dry pressing. The molded part is sintered into the raw product and then ground and polished depending on the application. The sintering process can take place both at atmospheric pressure and under high pressure. The molded parts only acquire their actual properties during the sintering process. Zirconium dioxide occurs in different crystal modifications depending on the temperature. These take up different volumes. The addition of yttrium oxide prevents the zirconium oxide from being converted into the modification that is more stable at room temperature during cooling after the sintering process: it is forced to remain in the partially stabilized form. If a crack occurs, the zirconium oxide spontaneously changes into the more stable modification at this point with volume expansion. This effect closes the crack again immediately. Zirconia is traditionally used as a refractory ceramic. Due to the material properties and the good biocompatibility, zirconium oxide has recently been used more and more as ceramic in the field of prostheses and dentures. In addition, partially stabilized zirconium oxide has many technical applications. It is very stable against thermal, chemical and mechanical influences, so that the material is very well suited for grinding tools.


Sintered corundum

orund is a pure aluminum oxide (Al2O3). It occurs relatively frequently in nature and is considered the second hardest mineral after diamond. To the group of corundums belong z. B. also the gemstones ruby ​​(red) and sapphire (blue). Synthetic corundum was first used in the refractory industry around 1900. Due to their greater chemical and physical homogeneity, these are often preferred to natural ones for further industrial processing. In the production of synthetic sintered corundum, pressed aluminum oxide powder is heated to 1,300 ° C. The aluminum oxide powder is an intermediate product in the process of aluminum extraction from bauxite. The use of sintered corundum in grinding tools is primarily due to the high hardness and purity, but also the low price. Due to the relatively low density of the sintered corundum and the resulting low energy input, the thermal load on the material to be ground is low during grinding. This is often an advantage with sensitive grist (e.g. plants).


Hard-paste porcelain:

Hard-paste porcelain is a silicate ceramic material made from the raw materials kaolin (iron-free clay), potassium feldspar and quartz. It consists of 25 - 70% Al2O3 and 30 - 75% SiO2. It is fired at temperatures around 1400 ° C. In the middle of the 19th century, porcelain began to be used as technical ceramics, especially in the electrical industry, where it is still an important material today.


Silicon nitride:

Silicon nitride is a non-oxidic ceramic, which is characterized by extreme fracture toughness and wear resistance. Ceramic does not occur naturally and is produced in different ways. One of the best-known manufacturing processes is reaction bonding, in which the pure silicon forms the nitride in a nitrogen atmosphere. The reaction starts at 1200 ° C. Since silicon nitride decomposes under normal pressure at temperatures above 1700 ° C, the powder must be sintered at very high nitrogen pressures. Silicon nitrides are characterized by a homogeneous gray color. The addition of sintering additives (AL2O3 and Y2O3) increases the hardness while maintaining the same tensile strength, which is also called sialons (SiAlON). The high wear resistance of silicon nitride makes it ideal for use in roller bearing technology and as a cutting tool.



Other materials

Agate (natural stone):

Agate is a quartz mineral (silicon dioxide, SiO2) and is one of the so-called semi-precious stones. It is created in the cavities of volcanic rocks by the precipitation of SiO2 from silica solutions. The name agate can be traced back to its first place of discovery, which is said to be on the river Achates in Sicily. Today's sites for agate are z. B. Bohemia, Brazil, Sicily or Uruguay. It was not until the beginning of the 20th century that the agate was used as a technical material. Pharmacists and homeopaths quickly recognized that hand mortars made of agate, due to their pure SiO2 composition and their abrasion resistance, offered significant advantages when grinding powders and pastes. No disruptive abrasion, easy cleaning thanks to smooth surfaces and thus avoidance of cross-contamination, distinguish the agate from the brass and porcelain mortars that were common up to that time. After the process of processing agate with diamond tools had been improved over time, it was now possible to equip a wide variety of mill types with grinding sets made of agate. As a result, porcelain is increasingly losing its status in mechanical processing technology.


Glass:

Glass is amorphous, i.e. H. not crystalline substance. It is usually made from a melt. This is cooled down relatively quickly, so that there is not enough time for crystals to form. Glass consists largely of SiO2. The properties of the glass can be changed by using various additives. Borosilicate glass, e.g. B. borosilicate glass 3.3, used because it has good chemical resistance and is relatively temperature stable. The chemical resistance is v. a. determined by the boron oxide content, which is around 13%. Borosilicate glass is also known under the names Jenaer Glas and Duran.


PTFE (plastic):

PTFE or Teflon is a thermoplastic plastic (polymer). The abbreviation PTFE stands for polytetrafluoroethylene, a fluorocarbon compound. The term Teflon is a trade name of the DuPont company. PTFE is a practically inert material; H. it is resistant to many chemicals, such as B. acids, bases, alcohols and benzines. Accordingly, a coating made of PTFE often serves as protection against corrosion against aggressive substances. PTFE is also used as a non-stick coating (e.g. on pots and pans), in functional textiles (Gore-Tex), as a sealing material and in chemical plant construction and medical technology (implants). PTFE was developed in the 1930s in search of a new refrigerant for refrigerators.


Polypropylene (plastic):

Polypropylene (PP) is a thermoplastic material (polymer) which is produced from the hydrocarbon compound propene by polymerisation. Polypropylene is often used in laboratories. It is resistant to almost all organic solvents and fats, as well as most acids and alkalis.


Polycarbonate (plastic):

Polycarbonate is a thermoplastic plastic (polymer) that is produced by esterifying carbonic acids with diols. Polycarbonate is used, for example, to protect the data layers in CDs and DVDs because it is very scratch and shock resistant. It is resistant to mineral acids and most non-polar solvents, but not e.g. to acetone.