Fatigue behavior of a quenched and tempered AISI 4340 steel coated with anelectroless Ni-P deposit

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.