For high quality carburise case hardened gears
achievement of close case carbon control is
essen tial.
Whilst tight carbon control is possible, views on
what optimum carbon level to target can be wider
than the tolerance.
The IS0 standard IS0 6336 in part 5 makes an
attempt to specify a target and the tolerance for the
highest quality grade as Eutectoid Carbon
Percentage plus 0.20%, minus 0.1 0%.
This implies that either a method exists to calculate
Eutectoid Carbon Content from alloy content or the
values have been determined for a wide range of
steels and are widely available. Unfortunately
neither exist. Also implicit is that the Eutectoid
Carbon is the optimum. But no rationale is given.
A simplistic interpretation is to use the Eutectoid
Carbon Content from the Iron -Carbon Phase
Diagram - see Fig 1. This value is 0.77% which
seems reasonable at a first glance. However,
experienced heat treaters realise that the higher
alloy steels would develop excessive retained
austenite if targeting 0.77% with the above
tolerance.phases including retained austenite, carbides,
bainite and pearlite.
The conclusion is that any calculation of optimum
carbon content must reflect this requirement.
The objective of this paper is to define a readily
available methodology to calculate optimum carbon
content from alloy content and austenitising
temperature at the hardening stage.