Carburizing

Carburizing ( carburizing ) is a method of heat treatment of steel.

The carburizing or insertion should steels which are difficult or impossible to cure because of its low carbon content, in so far as carbon accumulate that a cure is possible. Usually only the surface layer is enriched with carbon, so that there is more than martensite forms in the core and a hard surface layer is formed. The core is intended to remain mostly chewy and soft.

Recarburizers

Basically, there are three different Recarburizers:

• Festival The carburized parts to be used in cases with Kohlungspulver and held for a defined time at temperatures of about 930 ° C. Carburizing in Kohlungsgranulat is rarely performed because of the lack of reproducibility. When using a new Carburizing could arise in the first Aufkohlungschargen structural defects in the carburized surface layer by Überkohlung ( grain boundary cementite ). To avoid this, never only new pellets should be used, but are mixed with old. Due to the rather not given reproducibility carburizing is offered in only a few granules hardening shops and carried out. In many operating hardening plants, this process is still being carried out.

Carburization of the gas phase according to Boudouard equilibrium. ( Special, but rather outdated method. )

• Liquid The carburized parts are treated in a molten salt at a carburizing temperature 860-950 ° C. The Aufkohlungssalz used is cyanide. The advantages of Salzbadaufkohlung is the lower form and dimensions with continuous charging and shorter process times during the carburization especially at low depths up to 0.4 mm. Through constant monitoring of potassium cyanide and potassium cyanate content in the carburizing process reliability and reproducibility are ensured. The determination of potassium cyanide and potassium cyanate content can be performed by simple titration. The Salzbadaufkohlen was formerly widely used in hardening shops, but is today because of the lack of environmental and economical little application.
• Gaseous The carburized parts are used in hydrocarbon- containing gaseous media. A carburizing atmosphere is produced, which consists of a light reducing the carrier gas and a regulated supplied enrichment gas. The carrier gas is typically from 20% carbon monoxide, 40% hydrogen and 40% nitrogen. The enrichment gas is a hydrocarbon such as Methane or propane. This represents the presently most widely used method, the
• Low pressure The carburized parts to be inserted into a vacuum furnace. Due to the very high carbon transition the low pressure carburizing carburizing is controlled by a change of carburizing and diffusion cycles. In the carburising pure hydrocarbons such as methane, propane or acetylene are used as the furnace carburizing atmosphere and at pressures 2-25 millibar. During the diffusion phase, no carbon donor of the furnace atmosphere is supplied: typically, the pressure is lowered by means of vacuum or purged with inert gases at low pressures.

Carburization is mainly used as a first step in case hardening.

Carburization of the molten iron

In the production of cast iron and cast iron with spheroidal graphite is generally used as a feedstock in electric furnaces during melting steel scrap. The carbon content must then be corrected by adding either cast iron or carburizing agent. For economic reasons, usually the correction is performed with carburizing agents. As Recarburizers either graphite or cheaper, calcined petroleum coke is used. On the market there are many varieties of Carburizers. Quality and price are ash content ( from 0.10 to 27% ), nitrogen ( 0.1- 9000ppm ) determined sulfur ( 0.1-2 % ) and carbon ( 75 to 99 %).

• Change material properties