Ablation Damage Characteristic And Mechanism Of Steel Sheets Coated With Organic Coating Under Simulated Lightning Current

The ablation damage of metal materials caused by the arc of direct lightning current is one of the major cause of numerous lightning-triggered accidents.Insights into the characteristic and mechanism of lightning-induced ablation damage of steel is fundamental to the design of direct lightning protection.Organic protective coating on steel surface has significant effects on the ablation damage characteristics of steel,and the ablation damage characteristic and mechanism of steel painted with organic protection coating is still obscure and need further investigation.In this paper,systematic investigation was conducted on the ablation damage characteristic and mechanism of the steel coated with organic coatings inflicted by simulated lightning currents.The investigation and its conclusions are mainly as follows:(1)Ablation damage tests have been conducted on the anticorrosion-coated and antistaticcoated steel sheets with coating thickness of 100 ?m,200 ?m,300 ?m and 400 ?m and on the uncoated steel sheets,with the simulated lightning continuing current(C component),first return stroke current(A component)and intermediate current(B component),respectively.Ablation damage characteristics of the tested steel sheets were characterized and analyzed from aspects of surface morphology and cross-sectional geometric characteristic of the ablation damage zone,the flowing characteristic of melted steel,the microstructure and micro-hardness of the steel,and the distribution of chemical elements in the ablation damage zone.According to comprehensive analysis on the characterization results of ablation damage characteristics,the rear-face temperature of tested sample sheets,the characteristics of the simulated lightning current arc and the thermal decomposition characteristics of the organic coatings,the ablation mechanism of tested steel sheets was studied.(2)Based on the characterization results of the ablation damage inflicted by the C component current,it was analyzed that the ablation mechanism of tested sheets and influences of coating materials type,coating thickness,current amplitude and current duration on the ablation characteristics.It's been found that there are martensite mixed with little proeutectoid ferrite both inside the melting zones of anticorrosion and antistatic coated steel sheets.The micro-hardness inside the two melting zones increased 143% to 150% than the steel substrate.Inside the heataffected zones of anticorrosion and antistatic coated steel sheets,the content of martensite decreases with the distance from the melting zone increasing,and the micro-hardness inside the two heat-affected zones increased 42.0%(anticorrosion coated steel sheet)and 20.4%(antistatic coated steel sheet)than the steel substrate,respectively.The anticorrosion coating block more heat of the arc from being injected into steel than the antistatic coating,and the inorganic filler in anticorrosion coating would remain on the melted steel after the base material of coating being combusted,which consequently decreased the surface coefficient tension and the driving force of melted steel.With the coating thickness increasing,the ratio between the diameter and the depth of melting pool decreases,and the distribution of energy injection from the arc into the steel surface coarctates.It's been found as well that,the steel sheet does not necessarily penetrate when the temperate of steel rear-face reaches melting points,since the melted steel would re-solidified if it remained in the molten pool.The steel would penetrate only when the melted steel flowed or splashed out from the molten pool.(3)Based on the characterization results of the ablation damage inflicted by the A component current,further insights about the ablaiton mechanism of tested steel sheets,and influence factors on the ablation characteristics were obtained.The fitting relation curves between the diameter of ablation damage area on coated and uncoated steel sheet surface with the amplitude of 30/80 ?s impulse current were calculated,respectively.Investigation result shows that the splashing of melted steel on uncoated steel sheet is caused by the temperature-induced explosive force of the current arc,and the diameter of splashing area is in quadratic function with the current amplitude.The ablation pit on coated steel sheet is formed since coating was stripped form steel surface due to the thermal effect and explosice force of the current arc,and the its diameter is in linear relation with the current amplitude.According to the microstructure and micro-hardness,it can be known that the organic coating relieves the ablation damage of steel.The surface resistance and thickness of organic coating influence the breakthrough strength of coating film and further influence the size and shape of the ablation pit on coated steel sheet surface.Comparing with conditions that the coating thickness is 100 ?m?300?m or 400 ?m,when the coating thickness is 200 ?m,the ablation area of the anticorrosion and antistatic coated steel is both the smallest.(4)The ablation damage inflicted by the B component current was characterized,and the ablation mechanism and influence factors on ablation characteristic was analyzed.It's been found that the organic coating has the effect of blocking the heat from the current arc,limiting the movement of arc root and increasing the energy injection density of the current arc.For uncoated steel,the explosive damage effect is more obvious and the damage area is in splashing morphology,while the thermal damage effect is more obvious for coated steel sheet.Organic coating decomposed,carbonized,combusted and converted into gas,which caused the formation of ablation pit in coated steel sheet,and melted steel aggregated inside the ablation pit.The metallographic microstructure,crystal microstructure and micro-hardness of steel show that the ablation damage of steel in coated steel sheet is severer than that in uncoated steel sheet.Size of the ablation area on coated sheets increases with the coating thickness increasing and with the ampulitude of current component B increasing.(5)Last but not least,distinctions between the ablation damage characteristics induced by the A,B and C current components were analyzed.Analysing results show that the explosive force of current component A and B is higher than that of current component C,which causes the splashing effect on tested sheet surface and reduces the energy injection from the current arc to the steel sheet.Thus,the energy injection of the three components of lightning current is not linear with the charge transfer of them.Diffenerces between the duration of the three current components is the main cause of the distinctions between the influences of organic coating on the ablation damage characterics under the three components of lightning currents.