Mechanical Properties of Thick Plates After Welding of Q690 High Strength Steel

A comprehensive investigation into the mechanical properties of 50 and 70 mm thick plates of Q690 high strength steel and their butt-welded sections under tension was presented in this paper, and a total of 40 tensile tests were conducted. Firstly, a total of 18 proportional coupons with circular cross-sections were extracted at three different layers within the plate thicknesses. Tensile tests on all of these coupons were carried out to obtain their mechanical properties, and their variations across the plate thicknesses were also examined. Secondly, submerged arc welding was adopted to prepare butt-welded sections between these thick steel plates with various heat input energy. Micrograpic examinations on typical heat-affected zones of the welded sections were also performed. Standard tensile tests on a total of 22 proportional coupons with rectangular cross-sections were carried out to obtain their mechanical properties, and the full range deformation characteristics of these coupons were assessed and compared, in particular, their tensile strengths and elongations at fracture. Based on the tests on the base metal, it was found that there was only a negligible difference in the mechanical properties of the coupons from different layers of the Q690 50 mm thick steel plate, and it can be concluded that the mechanical properties of the 50 mm plate are homogeneous along the thickness direction. The yield strength and tensile strength of the coupons from the middle layer of the 70 mm plate, compared with the specimens in the upper and lower layers, possess a 7% and 6% reduction, while no reduction of elongation after fracture is found. The tests of welded sections show that, compared with these 16 mm thick plates, the influence of heat input energy on the mechanical properties of these 50 and 70 mm thick plates is small: with the increase of heat input energy from 1.0 to 2.0 kJ/ mm, the reduction of the tensile strength of coupons from 16 mm thick welded sections increased from 0% to 8%; While for the 50 and 70 mm plates, with the increase of heat input energy from 2.4 to 5.0 kJ/ mm, the reductions of tensile strengths maintained to be 4% and 1%. It is obvious that for the thick plate, the effect of increasing heat input energy on the reduction of the mechanical properties is smaller. Therefore, a larger heat input energy is allowed to improve the welding efficiency.