Expert: Javad Mola - 10/15/2009

Question
what is carbide? How can be avoided in castings?

Answer
Carbides are compounds formed as a result of combination of carbon with other elements. In ferrous alloys, whether cast or wrought, formation of iron carbide Fe3C is very common. In the case of alloyed steels and alloyed cast irons, carbides other than Fe3C are also likely to form. For stainless steels for instance, where Cr is present in amounts exceeding 10 mass%, chromium carbides Cr23C6 and Cr7C3 are readily formed since the affinity of C to Cr is higher than its affinity to Fe.

Depending on their type, carbides can form in a variety of temperatures. Iron carbides can form at very low temperatures like, let's say, 300 degree Celsius since its formation only requires diffusion of C which, due to its small atomic radius, can easily diffuse at low temperatures to form iron carbide. In the case of carbides other than iron carbide (in steels indeed) however, higher temperatures are required since in addition to carbon, the carbide former element (e.g. Cr) must also diffuse in the Fe matrix for the carbide (e.g. Cr23C6) to form. Carbides like Nb and Ti carbides can form at still higher temperatures. Now, if carbides are to be avoided, the ferrous alloys must not be exposed to the temperatures at which carbides can form. Therefore, controlled cooling is the answer to your question.

By the way, controlled carbide formation is and has been a big concern to metallurgist: they sometimes want carbides to form while in some other instances their inhibition is desirable. For this purpose, depending on the alloying elements, property requirements etc., different tricks are being used.