Hot-dip galvanizing of high and ultra-high strength thin-walled CHS steel tubes: Mechanical performance and coating characteristics

Abstract

Hot-dip galvanizing is one of the most common corrosion protection techniques for steel. This widely available method increases the lifetime of steel structures by up to 50 years. Although there is a well-established method for hot-dip galvanizing of structural mild-steel, the effects of this method on mechanical properties of high and ultra-high strength steels (HSS and UHSS), with nominal yield strength greater than 700MPa, has not been thoroughly studied. The study of galvanized thin-walled HSS and UHSS tubes is important since (a) galvanizing heat can easily penetrate the small thickness of thin-walled tubes and change their mechanical behaviour, (b) these materials are made through special heat treatment process which might render them sensitive to heat exposure during hot-dip galvanizing, and (c) these materials are high in alloy especially Si equivalent (Si+P) content which categorizes them as reactive steel with rapid reaction with molten zinc. Therefore, to deeply study the performance of galvanized high-grade steels, HSS and UHSS thin-walled tubes were dipped in zinc bath for various durations. For each bath time, a tube with intact and one with sand-blasted surface were used so that the roughness effect of the steel surface on coating performance could be explored. Dog-bone, tube shape, and small beam-shaped specimens were cut out of the larger tubes for tensile, compressive, impact, microscopy, coating thickness, and hardness studies. A mild steel tube was also galvanized and studied as a control specimen for comparison purposes. These studies showed that after galvanizing, HSS and UHSS materials lost their ultimate stress and recovered some of their ductility. On the other hand, galvanized mild steel specimens lost a small portion of their ductility and gained higher yield and ultimate stresses. In addition, it was found that galvanizing did not affect the hardness of HSS materials but decreased the hardness of UHSS materials up to 10%. Coating studies were conducted through thickness measurements, impact tests, and microscopic studies. The measurements showed that the coating thickness increased with respect to bath time and that a thicker coating formed on sand-blasted surfaces. Moreover, impact tests and microscopic studies on plastically bent HSS and UHSS specimens demonstrated that the favourable coating in terms of cracking under severe plastic deformation was formed on non-sand-blasted steel surfaces which were experienced a zinc bath time of 1.5 min. The outcomes of this study have the potential to be incorporated into the codes of practice and design guidelines for suitable hot-dip galvanizing and design of structural components made of galvanized HSS and UHSS.