PSI - Issue 36

Odarka Prokhorenko et al. / Procedia Structural Integrity 36 (2022) 254–261 2 Odarka Prokhorenko , Serhii Hainutdinov, Volodymyr Prokhorenko et al. / Structural Integrity Procedia 00 (2021) 000 – 000 1. Introduction A lot of welded structures in shipbuilding, industrial and civil engineering, transport and other industries consist of a flat butt welded structural elements of various shapes and sizes. Such are deck and side sections of welded ship hulls, flat bottom sections of sheet welded structures in the form of storage tanks for various liquids, gas holders, flat structural elements of other welded structures. In order to increase the productivity for joining flat structural elements an automatic arc welding with a melting electrode in shielding gas or submerged arc welding is used, as a process of a high heat input which provides a full penetration of the welded elements in one pass. It is known that the heating during welding leads to a change in the mechanical properties of steel, which is depending on the parent material can cause a decrease in the strength characteristics in the heat-affected zone (HAZ) of the welded joint, as it was shown in the work of Slyvinskyy et al. (2019) for high-strength steels, and the formation of quenching structures, for example, during the welding of the low-carbon steels at low temperatures. In addition, as shown in the works of Makhnenko (1976), Prokhorenko et al. (2018, 2019), a high-gradient heating of metal by the welding arc leads to a complex kinetics of the thermal deformation processes and the formation of stresses and strains which are changing as the metal is heated and cooled, and which are gradually transform into residual stresses and strains, which are remain unchanged throughout the entire period of the structure operation. During the welding thermal cycle structural transformations of the metal occur in HAZ, accompanied by volumetric changes, which may be the cause of welding strains and stresses. The residual stress state in the welded structure is an undesirable force factor that complements the standard loads. In some cases, it can contribute to the destruction of both a single element and the entire welded structure. Then it is desirable to provide a full range of activities, as considered in the paper by Haievskyi et al. (2020), aimed at reducing the probability of defects, softening or embrittlement of weld metal and welded joint metal, to ensure that the level of residual stresses is reduced by various technological methods. One of the ways to reduce residual stresses and strains during welding of the long welds and to prevent or reduce warping of work pieces from overheating during welding is to weld by back step welding scheme. When it is applied, the entire weld is divided into sections of equal length, from 100 to 300 mm, and then each of them is welded in turn in the direction opposite to the main welding direction of the entire weld. The end of a particular welded section coincides with the beginning of the previous welded section. The technological strength research development has raised the interest to the questions of the welding strain and stress kinetics in order to assess the deformation conditions preceding the formation of hot and cold cracks, since the strength characteristics of the welded joint depend on the phase composition of the weld and HAZ. Therefore, the question about the influence of different technological welding schemes on the residual phase composition of the weld and HAZ is relevant. 2. Finite element model, material properties and heat source Finite element model of the welded joint – a 600x600x10 mm plate made from steel DC04 which is divided into four identical 2.5mm thick layers and consists of the cubic and rectangular-prismatic 3D elements with square bases of 2.5x2.5 mm, 5x5 mm and 10x10 mm, and rectangular-trapezoidal bases of 10 mm x 5 mm, and 5 mm x 2.5 mm, respectively. The third type boundary conditions of heat transfer (thermal convection and radiation) were set by surface of heat transfer modeled with 2D elements, and the welds - with 1D elements. The technological clamping of the welded joint during the welded process and cooling was modeled by setting the possibility of free displacement in three nodes selected on the plate ends outside the zone of high-temperature influence. The plate was melted along the middle section in the direction of the X-axis for the entire thickness in one pass. The origin of the right rectangular coordinate system XYZ of the finite element model of the welded butt joint is placed in the middle plane of the plate in the central node of the mesh model, so the X-axis is directed along the weld, the Y-axis in lateral direction to the weld, and the Z-axis in up-direction to the face of the plate. The technological welding schemes, considered in the work, are denoted by the number of frames needed to achieve the required accuracy of the problem to be solved: for the back-step welding scheme with three 200 mm long sections - 72 frames are required (Fr_72), for the back-step welding scheme with six 100 mm long sections - 82 frames are required (Fr_82). Welding time for each technological scheme is 120 s. Total heating and cooling time of the welded joint is 1200 s. Using the SYSWELD (2015) software package solver, a joint FEM solution of the thermal and metallurgical problems for a 255