There is a continual need to apply heat treatment processes in
innovative ways to optimize material performance. One such
application studied in this research is carburizing followed by
austempering of low carbon alloy steels, AISI 8620, AISI 8822 and
AISI 4320, to produce components with high strength and toughness.
This heat treatment process was applied in two steps; first,
carburization of the surface of the parts, second, the samples were
quenched from austenitic temperature at a rate fast enough to avoid
the formation of ferrite or pearlite and then held at a temperature
just above the martensite starting temperature to partially or
fully form bainite. Any austenite which was not transformed during
austempering, upon further cooling formed martensite or was present
as retained austenite. The final result was a high-carbon surface
layer of bainite or martensite which is expected to have good wear
and fatigue resistance and a low carbon bainite and/or martensite
core to provide toughness.
The effect of the austempering temperature and time was studied
for the three types of steel. A combination of three austempering
temperatures, 260, 288 and 304°C and three times 1, 2 and 4 hours
provided seven different heat treatments that were applied to the
three types of alloy steel selected for the tests. Charpy tests
were done on all the heat-treated steels to determine the
differences in toughness due to the material and heat treatment
conditions. The surface hardness and the geometric distortion were
measured as well. The results obtained for the carburize/austemper
process were compared to results obtained for the more traditional
carburize/quench/temper process. One of the important conclusions
was that the distortion produced by the carburize/austemper process
was significantly less than the distortion produced by the
carburize/quench/temper process. The results can be used to help
select the best heat treatment conditions and materials for
applications that include transmission shafts, output shafts, pump
shafts, differential pinions, splines and parking pawls.