Abstract
The micro ablation characteristics of steel inflicted by the lightning currents and the influence of anti-corrosion coating on these characteristics are seldom investigated, even though fundamental to applications of steel. In this work, ablation tests on uncoated and coated mild steel plates were conducted with the 2 ms rectangular current simulating the component B of lightning currents. The macro-morphology, microstructure, and Vickers hardness of ablation zones were investigated systematically. The ablation characteristics and mechanisms of coated and uncoated steel were analyzed comparatively. It was found that the energy density of the 2 kA 2 ms rectangular current arc exceeded 106 W/cm2 causing the steel near the arc root melted within 2 ms and ablation formed. The width-depth-ratio of the ablated zone was 43.7 for uncoated plate, and only 7.5 for coated plate, since coating constricted the splashing effect of steel melt occurring on uncoated plate and confined the arc root. The metallographic microstructure and EBSD results showed the ablated zones of uncoated and coated steel both consisted of quenched martensite mixed with a handful of ferrite. There were fined equiaxed grains both in the fusion zone (FZ) and heat affected zone (HAZ) of uncoated plates whereas coarser columnar grains and fined equiaxed grains in the FZ and HAZ of coated plates, respectively. In the HAZ of uncoated plate, the average Vickers hardness of steel increased by 113%, while, in the HAZ and FZ of the coated plate, it increased by 209% and 136%, respectively.
Highlights
Lightning is the most hazardous natural events for industrial facilities, since lightning-triggered accidents of these facilities are of the highest frequency among all types of natural-caused accidents [1,2].Storage tanks, such as the oil-storage tanks, are the most vulnerable industrial facilities that are threatened by the lightning [1,2]
In the steel substrate area that is close to the ablated zone, the Vickers hardness of the steel is slightly higher than the Vickers hardness in the area that is distant from the ablated zone
In the heat affect zone (HAZ) of the uncoated plate, the average Vickers hardness of steel increased by 113%, while, in the HAZ and fusion zone (FZ) of the coated plate, it increased by 209% and 136%, respectively
Summary
Lightning is the most hazardous natural events for industrial facilities, since lightning-triggered accidents of these facilities are of the highest frequency among all types of natural-caused accidents [1,2] Storage tanks, such as the oil-storage tanks, are the most vulnerable industrial facilities that are threatened by the lightning [1,2]. The ablation characteristics of steel under direct lightning strikes are fundamental to the lightning protection of oil-storage tanks and other steel-made industrial facilities It has been investigated for a long time in the field of lightning protection [5,6,7,8,9,10,11]
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