| Sumario: | AISI 5160 steel is used for the production of coil and leaf springs. Springs
are usually oil-quenched and tempered. In this work, a brine solution is
used as a quenchant. The brine promotes a more severe and uniform
quenching. The severity of the quenching was analysed, and the heat
transfer characteristics determined. A brine interrupted quenching was
developed to produce a martensite case with a bainite core. The effect of
various conditions after the interruption of the cooling is addressed.
Tempering temperatures of 150, 250, 350 , 450 and 550 °C were
employed. In addition, a partial decarburisation was promoted in the
samples prior to the quenching and tempering, this in order to modify the
local martensite start temperature and promote the martensite
transformation to take place at a higher temperature, while reducing the
amount of retained austenite. To determine the fatigue properties of heat
treatments, a rotating bending fatigue tester with constant deflection was
developed. Fatigue results showed that the interrupted brine quenching
with partial decarburisation has a higher fatigue limit that the case with no
decarburisation and also that the case of conventional oil quenching and
tempering. Fractography and load data from experiments allowed to
model the cracking stages. The model was useful to establish the
presence of cracks and their length. It was found that when a martensite
case and a pearlite core was produced, nucleation was retarded up to 5
times, however after a crack nucleates it propagates much rapid than in
bainite.
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