Fatigue behavior of a 4340 steel coated with an electroless Ni-P deposit

The fatigue properties of a quenched and tempered (Q&T) 4340 (NiCrMo) steel coated with a commercial electroless Ni-P (EN) deposit, has been investigated. Such an EN deposit had a P content of approximately 10 wt.% and a thickness of approximately 18 μm. The evaluation of the fatigue properties of the coated material was conducted in two different conditions: (a) as-deposited and (b) deposited and post-heat treated (PHT) at 698 K for 1 h, a condition in which the maximum hardness of the deposit was observed. It has been determined that both the fatigue life and fatigue limit of the coated substrate are substantially reduced, in comparison with the uncoated material, and that such a reduction is significantly higher if the deposit is in the PHT condition. The Basquin parameters of the materials tested under different conditions have been determined and used to compute the reduction in fatigue life. Thus, it has been shown that the fatigue life of the steel substrate can be reduced up to 94% in the as-deposited condition and up to 99.1% in the as-deposited and PHT condition. Also, the fatigue limit can be reduced between 28.3–50.5% 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.