Damage Characteristics and Microstructure Response of Steel Alloy Q235B Subjected to Simulated Lightning Currents

Abstract

This paper elucidates the damage characteristics, microstructure response and temperature rise of steel alloy Q235B suffered from lightning currents, which is the basis for lightning protection of oil tanks. Damage is inflicted in the designed Q235B specimens by using the multi-waveform multi-pulse impulse current generator to characterize the damage response induced by four typical lightning current components. The changes in the element composition and micro-hardness of Q235B are documented in the experiment. The microstructure changes in response to the temperature rise are analyzed by the proposed lightning-metal temperature rise model. The results reveal that, first, return stroke current with an amplitude of 201.2 kA leads to the largest damaged area of 3523.8 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Continuing current in the stroke intervals with a charge transfer of 12.1 C results in the damaged area of 12.6 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and the damaged depth of 0.5 mm. Long continuing current after stroke with a charge transfer of 210.1 C leads to the deepest damaged depth of 3.0 mm. The demixing phenomenon occurs on the cross-section, forming the damage zone, transition zone, and origin zone. The damage zone mainly consists of martensite transferred from pearlite and ferrite at temperature 990 °C. The transition zone is mainly martensite and ferrite forming at temperature 900 °C. The hardness of the three zones is 450, 310, and 180, respectively. The damaged depth is only 0.001 mm caused by the subsequent return stroke current with an amplitude of 102.2 kA.