Stainless steel is an umbrella term for all steel grades that are the product of a special smelting process, possess a high level of purity and react uniformly to designated thermal treatments. According to this definition, stainless steels do not necessarily have to be alloy or high-alloyed steels. The description from here on will, however, be confined to high-alloy stainless steels with a chromium content of at least 10.5%.
Categorisation of high-alloy stainless steels
Based on their structure, high-alloy stainless steels can be grouped into the following categories:
- ferritic stainless steels
- martensitic stainless steels
- austenitic stainless steels
- ferritic-austenitic stainless steels (duplex steels)
Ferritic stainless steels
Ferritic stainless steels fall, again, into two groups:
- containing approx. 11% to 13% of chromium
- containing approx. 17% of chromium (Cr)
Stainless steels with a chromium content of 10.5% to 13% are categorised as inert to corrosion" due to their low chromium content. They are used wherever service life, safety and low maintenance requirements are of paramount importance and no specific requirements are made on appearance. Common fields of applications are container, carriage and vehicle construction.
Martensitic stainless steels
Martensitic stainless steels with a chromium content of 12%-18% and a carbon content exceeding 0.1% turn austenitic at temperatures above 950-1,050°C. Rapid chilling (quenching) leads to the formation of a martensitic structure. This structure, especially when hardened and tempered, offers outstanding strength, which increases even further with a rising carbon concentration. Martensitic stainless steels are used, for example, for the production of razor blades, knives or scissors. The prerequisite for a sufficient level of corrosion resistance is a suitable surface finish, such as can be achieved by grinding.
Austenitic stainless steels
Austenitic stainless steels (also known as: chromium-nickel steels) with nickel concentrations above 8% represent the most favourable combination for practical use when in comes to workability, corrosion resistance and mechanical properties. The foremost characteristic of this grade of stainless steel is its high resistance to corrosion. For this reason, austenitic stainless steels are used in areas with exposure to abrasive media, e.g. contact with seawater containing chloride and in the chemical and food industry.
Austenitic-ferritic stainless steel
Austenitic-ferritic stainless steels are often referred to as duplex steels on account of the two structures of which they are composed. As they offer exceptional ductility and improve corrosion resistance at the same time, these steels are particularly well suited for use in off-shore engineering.
Grinding of stainless steels
Many components made of stainless steel are ground at the end of the machining process. Klingspor's product range offers a host of abrasives that are specifically designed for processing this material. Nonetheless, the machining of stainless steel requires that peculiarities specific to the material be observed in order to achieve consistently satisfying results.
The surface finish produced at the factory, i.e. the rolled, stained and/or subsequently heat-treated material, should be selected in such a way that the original surface resembles the desired surface at the end of machining as closely as possible.
The corrosion resistance of stainless steel work pieces requires that the material be properly stored and transported before it is even processed.
Things that need to be avoided in particular:
- Any kind of contact with other types of steel (steel brushes, wire cables).
- Damage to surfaces and edges as well as chafe marks.
- The storage of materials in processing areas or storage together with other types of steel (e.g. rolled steel).
Another rule that applies to corrosion resistance: the finer the surface the greater the resistance to corrosion
There is a multitude of parameters that affect the roughness and visual appearance of ground surfaces:
- The grinder including the contact elements fitted on it and the machining parameters (cutting speeds and feed rates)
- The use of grinding fluids (oils and emulsions)
- The quality of the grinding fluid
Due to the framework conditions dictated by the grinding process, it is not possible to make a statement about the finished surface and the used grinding fluid that applies in all cases. To avoid any misinterpretations regarding the agreed finished surfaces, it is advisable to define a boundary sample and roughness averages (Ra) before machining commences.
Grinding and corrosion
Regardless of the stainless steel used, it is recommended to strictly observe the measures listed below prior to grinding non-corroding steel:
- Never use abrasive tools on normal steel first before using the same tools on stainless steel!
- Clean all surfaces thoroughly of grinding dust!
- Never spray stainless steel surfaces with fountains of red hot sparks!
- The machining temperatures must be low enough to prevent the formation of chromium carbides and of subsequent intercrystalline corrosion. If the metal tarnishes it has be to reworked!
This is the only way to ensure that the passive layer in the processed areas rebuilds and the other surfaces are prevented from sustaining damage from pitting corrosion or intercrystalline corrosion.
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