Experimental Investigation of Structural Steels with Through‐Thickness Strength Variation

Abstract

Efficient steel use in modern structures requires targeted strengthening to reduce weight and enhance performance. This study investigates partial thickness induction hardening (IH) with optional tempering as a method to locally increase strength in S355 circular hollow sections (CHS) while limiting ductility loss compared to through thickness hardening. Partial IH heats the CHS via induction but only the outer surface is quenched, forming a martensitic layer while retaining a ferritic core, creating a beneficial property gradient across the thickness. Scanning Electron Microscopy (SEM) quantified martensitic transformation through thickness. CHS with a diameter of 139.7 mm and wall thicknesses of 5, 8, and 10 mm were treated using controlled IH parameters, with selected specimens subsequently tempered at 200 °C and 250 °C for two hours. Tensile tests were conducted to determine yield strength, ultimate strength and strain, with corresponding Vickers hardness profiles. The results demonstrate that partial IH raises yield strength by ~40% and ultimate strength by ~30% relative to untreated material; notably, in some cases this enhancement occurs with no reduction in ductility. Overall, the ductility penalty remains far lower than that associated with full hardening. Tempering at 200 °C and 250 °C slightly lowered strength but had no notable impact. Findings highlight partial IH as a promising approach for locally strengthened CHS.