The International Gearing Standard IS0 6336, in part 5, addresses the metallurgical requirements to achieve the rated
service performance.
For high quality carburise case hardened gears the heat treater is required to achieve, in conjunction with other properties,
a surface carbon level of Eutectoid Carbon + 0.20% to – 0.10%.
This creates practical difficulties because, at present, there is no method to calculate eutectoid carbon from chemical
analysis and the eutectoid carbon is not necessarily the best value to base the surface carbon requirement on.
The UKrepresentatives on the international committee preparing the next issue of the standard agreed to attempt to solve
this problem.
As a result this paper defines the conditions to calculate an optimum carbon level to minimise the possibilities of
producing undesirable metallurgical phases including retained austenite, cementite and bainite.
A method is proposed to determine the optimum carbon level from regression equations which calculate CCT diagram
bainite, ferrite, pearlite and cementite nose times plus martensite transformation temperatures from the alloy content and
austenitising conditions.

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