If a material is exposed to gases containing carbon, e.g. in the form of CO, CO2 or CH4, it can pick up carbon.
The degree of carburisation is governed by the levels of carbon and oxygen in the gas, also the temperature and
steel composition. The carbon which is picked up by the steel will largely form carbides, primarily chromium
carbides.
Carbon pick-up causes embrittlement of stainless steel because carbides, or even a network of carbides, form in
the grain boundaries as well as within the grains. The formation of a large amount of chromium carbides causes
chromium depletion and thus a reduced resistance to oxidization and sulphidation. The resistance to thermal
cycling is reduced and, since carburization leads to an increase in volume, there is a danger of cracks developing in
the material.
Carbon pick-up can occur even at relatively low temperatures (400-800°C) in purely reducing - carburizing
atmospheres and gives rise to catastrophic carburisation or metal dusting. Attack is severe and characterized by
"powdering" of the steel surface due to the breakdown of the protective oxide layer and inward diffusion of
carbon which forms grain boundary carbides. The increase in volume on carbide formation means that grains are
rapidly broken away from the steel surface, giving rapid and serious attack.
Chromium, nickel and silicon are the alloying elements which most improve resistance to carburization. Table 7
shows carburization of some stainless steels in carburizing atmospheres. Note the beneficial effect of silicon,
apparent from a comparison of Type 304 and 302B. Also note the high level of carburization in Type 316. In
materials selection it is however necessary to consider both carburization and the effect of an increased carbon
content on mechanical properties. In general, austenitic stainless steels can tolerate an increased carbon content
better than other types of stainless steel.
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