The mechanical properties and microstructure evolution
of AISI 4340 steel under different tempering conditions
have been studied. The results from the tensile tests
indicate that tempering temperature and the holding time
have obvious effects on the mechanical properties and the
microstructure features, but the former effect is more
pronounced than the latter. Under the tested tempering
conditions, the strength, hardness and strain-hardening
exponent decrease with an increase in tempering temperature
and holding time. It is also found that the area
reduction and elongation increase as the tempering
temperature and holding time are increased, however,
there is a loss in toughness after tempering at 300°C due
to retained interlath austenite and the formation of
interlath carbide films. TEM observations reveal that the
quenched structure within the martensite laths shows
extremely high dislocation densities and some microtwins.
When tempering at different temperatures, various
types of carbide precipitates in different shapes and sizes