Y. Garces et al., Fatigue behavior of a quenched and tempered AISI 4340 steel coated with anelectroless Ni-P deposit, THIN SOL FI, 356, 1999, pp. 487-493
The fatigue hfe of a quenched and tempered AISI 4340 steel has been evaluat
ed in three different conditions: (a) uncoated, (b) coated with an electrol
ess Ni-P (EN) deposit of a P content of approximately 12-14wt.%, as-deposit
ed and (c) as-deposited, followed by a two-step post-heat treatment (PHT):
473 K for 1 h plus 673 K for 1 h, The results indicate that plating the bas
e steel with this kind of deposit leads to a significant reduction of the f
atigue life of the material, particularly if the deposit is subjected to a
subsequent PHT. Such a reduction has been quantified by determining the Bas
quin parameters from the fatigue life curves obtained for the uncoated, coa
ted, coated and PHT substrates. It has been shown that the fatigue life of
the base steel can be reduced by 78% in the as-deposited condition and a 92
% after a subsequent PHT. The microscopic observation of the fracture surfa
ces of the samples indicate that the fatigue process is initiated at the su
rface of the deposit and, subsequently, transferred to the substrate, with
the assistance of the metallic bonding established at the deposit-substrate
interface. This belief is supported by the observation of some continuity
of the fracture features between the coating and the substrate under low al
ternating stresses. In the present study, the bonding between the EN deposi
t and the base steel was observed to be rather poor. Extensive secondary cr
acking along the coating-substrate interface after fatigue testing as well
as the complete separation of the deposit from the substrate during tensile
testing support this view. Such a behavior is believed to be related to th
e significant difference that exists between the elastic and plastic proper
ties of the EN deposit and the base steel, Nonetheless, the slight degree o
f metallic bonding that remains after the first stage of fatigue testing se
ems to be enough to allow the passage of the fatigue cracks, prior nucleate
d in the deposit, into the substrate. It is therefore concluded that, in th
e present case, the EN deposit acts as a surface crack source or surface no
tch which decreases the fatigue life of the coated material by reducing the
crack nucleation stage. (C) 1999 Published by Elsevier Science Ltd. All ri
ghts reserved.