Mechanical Properties of Welded Joints of 780 MPa Grade Ultra-fine Grained Steels

Abstract

Three kinds of 780 MPa grade ultra-fine grained steels with different chemical composition were produced by warm rolling. The steels were characterized by ultra-fine ferrite grains (less than 1 mm). With the steels strength-overmatching welded joints were prepared, and their mechanical properties were investigated. It is found that softening occurred in the heat-affected zone (HAZ) because of the coarsening of ferrite grains due to welding heat input. However, by using low welding heat input and strength-overmatching weld metal, the detrimental effect of softening on strength was restrained, and welded joints with strength equivalent to that of base metal were obtained. The deformability of welded joints was found to be related to the yield ratio (yield strength / tensile strength) of base metal. Low yield ratio is desirable to the deformability of welded joints. The HAZs in the welded joints of low welding heat input of 10 kJ/cm have good impact toughness for all the steels. Except for 0.14C-0.30Si-1.46Mn steel, the HAZs in the welded joints formed from the other two steels also have good impact toughness for welding heat input of 20 kJ/cm, and their fracture appearance transition temperature (vTrs) of the HAZs is lower than Ϫ40°C, and their Charpy impact energy at Ϫ40°C exceeds 200 J.KEY WORDS: ultra-fine grained steel; welded joint; mechanical property; tensile property; Charpy impact test. Table 1. Chemical composition of steels examined (mass%).

Relative effect of C and Mn on strength-toughness of medium Mn steels

“…The tensile tests were carried out using a Instron-8862 R universal testing machine at room temperature and 0.5 mm min −1 cross-head velocity. The sub-sized Charpy V-Notch specimens (10 mm × 5 mm × 55 mm) were fabricated following ASTM-E23 [27] and tested using a Instron R 400J pendulum-impact testing machine at −40°C which is an crucial temperature for fine-grained highstrength steel in quenched and tempered condition steel typically used in welded joints, shock absorbing components and different structural parts [28,29]. The sample temperature was achieved by soaking the samples for 30 minutes inside a mixture of methanol and liquid nitrogen.…”

Technological Properties and Applications of High-Carbon Nanobainitic Steels

“…The use of the thermo-mechanical control process when making plates in microalloyed steels enables to achieve a ferrite grain having a minimum size restricted within the range of 2 μm to 3 μm [10]. Further grain refinement could be obtained through high deformation within the ferritic range or even using interoperational cold rolling [11,12]. The use of complicated deformation operations and heat treatment cycles enable to obtain a grain sized below 1 μm, yet such technologies are unfit for industrial applications [7].…”