Buckling behaviour of high-strength retrofitted steel sections manufactured through post heat-treatment processes

• Production of ultra-high strength steel sections via induction heat (IH) treatment. • Impact of IH treatment on material properties, imperfections and residual stresses. • Tests showed higher compression capacity for IH sections than conventional ones. • IH sections with double imperfection of conventional had same buckling performance. • Eurocode 3 buckling curve ‘a’ is proposed for IH sections with L/500 imperfections. High-strength steel is increasingly popular in construction for its strength-to-weight ratio, which lowers the self-weight of structures and reduces transportation, erection, and foundation costs. Induction hardening (IH) process which involves rapid heating and cooling of the material leads to microstructural changes which enhance the hardness and strength of conventional steel, elevating it to the level of High Strength (HS) steel. This study reports an extensive investigation on the buckling response and design of IH post-treated structural steel Circular Hollow Sections (CHS). It includes tensile coupon tests, microstructural analyses, initial geometric imperfection measurements, and residual stress evaluations to determine the effects of IH treatment on CHS. Column buckling tests were conducted, and a numerical model was developed and validated against the experimental results which was utilised to study systematically the effect of imperfections. The findings suggest that the magnitude of the imperfections in IH steel sections doubled compared to the non-treated condition, whilst there was a minimal impact on the residual stresses. A comprehensive parametric study was performed using finite element models to study the structural response of IH steel CHS columns over a large range of global slendernesses and allow the assessment of the buckling curves specified in EN 1993–1–1. It was concluded that the buckling curve a (imperfection factor, α=0.21) specified in EN 1993–1–1 provides the best buckling load predictions for the IH steel CHS columns, the response of which is superior to that of their virgin counterparts, despite the increased imperfections caused by the heat-treating process.