Estimation of damage parameters for ductile fracture of Zn-Ni coated ultra-high strength 300M steel using a finite element model

Zinc-Nickel alloy coatings are being researched as a possible replacement for cadmium-based coatings because of the health and environmental concerns associated with cadmium coatings. The Zn-Ni coating process is accompanied by the production of hydrogen. The hydrogen produced is primarily stored at the metal-coating interface and during the application of the part, this hydrogen may diffuse inwards into the metal and cause hydrogen embrittlement. Hydrogen embrittlement in ultra-high strength steels (UHSS) is detrimental to their application, as it leads to degeneration of properties and delayed cracking. Although outgassing steps are employed to remove the hydrogen from the steel, residual hydrogen might still be present in the steel and over the lifetime of the part, ingress of hydrogen in the part can be expected, which can lead to hydrogen embrittlement as well.

In this work, a numerical model is proposed to investigate the development of stresses and hydrogen concentration in a Zn-Ni coated UHSS part. The proposed model, which includes the transport of hydrogen in the coating and metal substrate, couples the hydrogen distribution with a continuum damage mechanics model. The geometry of the simulation domains for the coating were directly developed from micrographs of the coating, and this facilitated the study of the effects of the morphology of the coating on the behavior of the metal-coating structure. The metal-coating interface aspects are explored as well. Different coating conditions lead to contrasting morphologies, and the amount of hydrogen generated is varied as well. The model will compensate by implementing distinct initial conditions. The simulation results are compared against experimental results from tensile test and charpy impact test, and the parameters of the damage model are calibrated.