Effect of weld on the mechanical properties of high strength and ultra-high strength steel tubes in fabricated hybrid sections

• Mechanical properties of welded tubes in fabricated sections are investigated.
• Tensile tests and microstructure examination are conducted.
• Weld heat decreases tensile strength of ultra-high strength steel tubes up to 30%.
• HSS and UHSS microstructure softens a zone width close to thickness of specimen.
• Fast post-weld cooling by argon creates hardening in immediate vicinity of weld.

The effect of welding on the mechanical properties of high and ultra-high strength tubes in hybrid sections is a critical factor which influences the global performance of fabricated members and should be considered in the design and incorporation of these materials. To investigate and quantify these effects on the material of welded steel, a specific type of hybrid section composed of plates and tubes is considered in which two scales of experimental analysis are conducted at different positions from weld. In the macro scale, standard tensile tests are performed at various distances from weld to obtain the mechanical properties of heat affected steel in the axial direction parallel to the weld. Comparisons are made among three different welded materials: mild steel, high strength steel and ultra-high strength steel. The original and after-weld alteration in the ultimate tensile strength (UTS) and the strain corresponding to the UTS are investigated giving particular attention to the effect of yield strength and strain hardening rate. At the micro level, the microstructure of three above-mentioned materials in addition to microhardness profiles have been examined providing extensive data of the material behavior after welding. It is shown that the welding and post welding process, such as heat input and cooling rate affect the local material properties at different distances from weld leading to an overall heterogeneity in the material influencing the mechanical behavior.