(57) A method of manufacturing a rotary drill bit at least partly from cast iron comprises
forming a hollow mould in the configuration of at least a portion of the bit body,
and casting said portion of the bit body in the mould by a casting process which includes
the steps of pouring molten cast iron into the mould and then cooling in the mould
so that the cast iron solidifies to form said portion of the bit body. The method
includes the further step of subjecting the cast iron to treatment, during the casting
process, which results in at least one surface portion of the cast iron bit body becoming
hardened. The hardening treatment may comprise adding an iron carbide forming additive,
such as boron or tellurium, to the cast iron before it is poured, or by chill hardening
during solidification, or by coating the interior of the mould, before pouring, with
a wash of iron carbide forming material, such as tellurium or by controlling the rate
of cooling after solidification so as to effect the formation of martensite.
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