Fatigue properties of a SAE 4340 steel coated with a Nimet HP autocatalytic nickel deposit

The influence of a commercial electroless Ni–P (EN) deposit known as Nimetʹs HP autocatalytic nickel®, on the fatigue properties of a quenched and tempered SAE 4340 steel, has been investigated. Such an EN deposit had a thickness of approximately 10 μm, a P content of approximately 12 wt.% and it was evaluated in two different conditions: (a) as-deposited; and (b) deposited and post-heat treated (PHT) at 723 K for 1 h, the condition in which the deposit showed its maximum hardness. It has been determined that the application of such a coating to the steel substrate gives rise to a significant reduction of the fatigue life and fatigue limit, in comparison with the uncoated material, which is more marked in the PHT condition. The reductions in fatigue life have been quantified in terms of the computed values of the Basquin parameters of the materials tested under different conditions. Thus, it has been shown that the fatigue life of the steel can be reduced up to 93% in the as-deposited condition and up to 97% in the as-deposited and PHT condition. The fatigue limit can also be reduced between 12 and 23% depending upon the condition of the coating. From the microscopic point of view, it has been observed that the fatigue fracture of the substrate-coating composite initiates in the deposit and that it remains well adhered to the substrate during fatigue testing since interfacial cracks have been very rarely observed. Such adhesion characteristics enhance the transference of the early cracks nucleated towards the substrate steel, a belief that is supported by the analysis of the fracture surfaces of the samples tested at different stress levels.