Carburizing

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Carburizing (also called carburizing or cementation) describes a process from heat treatment , primarily for steel , in which carbon diffuses into metals at high temperatures . For this purpose, iron / steel is heated to temperatures of 850 to 950 ° C , other metals up to 2000 ° C, in a carbon-containing environment . The carburization or insertion is intended to enrich steels that are difficult or impossible to harden because of their low carbon content to such an extent that hardening is possible. Usually only the surface layer is enriched with carbon so that more martensite is formed there than in the core and a hard surface layer is created. The core should usually remain tough and soft. Many metal carbides , such as tungsten carbide or tantalum carbide, can be produced in a targeted manner by carburizing .

Carburizing agents

There are basically three different carburizing agents:

  • Fixed
    The carburized parts are in boxes with Kohlungspulver inserted and held for a defined time at temperatures of about 930 ° C (after Kregelin). The carburization in carbon granulate is rarely carried out because of the poor reproducibility. When using new carburizing granulate, structural defects in the carburized surface layer due to overcarburization (grain boundary cementite) can occur in the first carburization batches. To avoid this, new granules should never be used, but should be mixed with old ones. Due to the lack of reproducibility, the carburizing in the granulate is only offered and carried out by a few contract hardening shops. This process is still carried out in many hardening shops.

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(Special, but rather outdated procedure.)

  • Liquid
    The parts to be carburized are treated in a molten salt at a carburizing temperature of 860 to 950 ° C. The carburizing salt used contains cyanide. The advantages of salt bath carburization are the smaller changes in shape and dimensions with hanging charging and shorter process times for carburizing, especially with low carburization depths of up to 0.4 mm. Constant monitoring of the potassium cyanide and potassium cyanate content in the carburizing bath ensures process reliability and reproducibility. The determination of the potassium cyanide and potassium cyanate content can be carried out by simple titration. Salt bath carburization used to be widespread in contract hardening shops, but is rarely used today due to its lack of environmental compatibility and economic efficiency.
  • Gaseous
    The parts to be carburized are used in gaseous media containing hydrocarbons . A carburizing atmosphere is created, which consists of a slightly reducing carrier gas and a regulated enrichment gas. The carrier gas usually consists of 20% carbon monoxide , 40% hydrogen and 40% nitrogen . The enrichment gas is a hydrocarbon, e.g. B. methane or propane .
    This is currently the most widely used method.
  • Low pressure
    The parts to be carburized are placed in a vacuum furnace. Due to the very high carbon transfer in low-pressure carburizing, the carburizing is controlled by alternating the carburizing and diffusion cycles. In the carburizing phases, pure hydrocarbons such as methane , propane or acetylene are used as the furnace or carburizing atmosphere at pressures of 2 to 25 millibars. During the diffusion phase, no carbon donor is added to the furnace atmosphere: typically, the pressure is lowered using vacuum pumps or flushed with inert gases at low pressures.

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

Carburization of the molten iron

In the manufacture of cast iron and spheroidal graphite cast iron, scrap steel is generally used as the feedstock for smelting in electric furnaces. The carbon content must then be corrected either by adding pig iron or by using carburizing agents. For economic reasons, the correction is usually carried out with carburizing agents. Either graphite or cheaper, calcined petroleum coke is used as the carburizing agent. There are many types of carburizing agents on the market. Quality and price are determined by the ash content (from 0.10 to 27%), nitrogen (0.1–9000ppm), sulfur (0.1–2%) and carbon (75–99%).

literature

  • Eberhard Roos, Karl Maile: Materials science for engineers. 5th edition, Springer Verlag Berlin Heidelberg, Berlin 2015, ISBN 978-3-642-54988-5 .
  • Wolfgang Weißbach: Materials science. 18th edition, Vieweg Teubner Verlag, Wiesbaden 2012, ISBN 978-3-8348-1587-3 .

Web links

Individual evidence

  1. ↑ Entry on carburization. In: Römpp Online . Georg Thieme Verlag, accessed on January 7, 2018.