PSI - Issue 36

Odarka Prokhorenko et al. / Procedia Structural Integrity 36 (2022) 290–297 2 Odarka Prokhorenko, Serhii Hainutdinov, Volodymyr Prokhorenko et al. / Structural Integrity Procedia 00 (2021) 000 – 000 1. Introduction Welding is the most technologically advanced and economical process for the manufacture of various metal structures in industries such as mechanical engineering, shipbuilding, construction, railway transport and others. Most welded structures consist of flat structural members of various shapes and sizes with butt welds made by automatic arc welding. An example of such structures are deck and side sections of welded hulls of ships, flat bottom sections of sheet welded structures in the form of tanks for storing various liquids, gas holders, flat structural elements of other welded structures. Among arc welding methods, automatic submerged arc welding is a highly productive progressive method that allows welding sheet material in a single pass to obtain high-quality welded joints with high mechanical properties. At the same time, automatic submerged arc welding is a process with high heat generation, which leads to the formation of various structures in the weld and heat affected zone (HAZ), continuously changing in the direction of the temperature gradient from the weld to the base metal. The structural state of steel determines its ability to resist plastic deformation and fracture. Depending on the initial composition of the material, this can cause a decrease in the mechanical properties of the weld and HAZ of the welded joint, which is shown in the work of Slyvinskyy et al. (2019) for high-strength steels, or the formation of hardening structures when welding low-carbon steels at low temperatures. High heating and cooling rates of the metal of the weld and HAZ affect the kinetics of phase processes in the HAZ of the welded joint with accounting the non-equilibrium of their thermodynamics. In addition, as shown in the works of Makhnenko (1976), Prokhorenko et al. (2018, 2019) a high-gradient heating of the metal by the welding arc leads to the complex kinetics of thermal deformation processes, which leads to the formation of temporary stress and strain fields in the welded joint, which, upon cooling, gradually turn into residual, existing without changes during the entire period of the structure operation. In the works of Lobanov et al. (2016), Nikolaev et al. (1990, 1982), Sagalevich (1974), Talypov (1973), Vinokurov (1968), Trochun (1964) a negative influence of the residual stress state on the strength of the welded structures was shown, which can be reduced by using a set of measures aimed at reducing the level of residual stresses and improving the mechanical properties of the weld metal and HAZ by various technological methods, as discussed in the work of Haievskyi et al. (2020). Since, during the automatic single-pass submerged arc welding the metal of the weld and HAZ is subjected to the high heating rates of ~1000 o C/s, this causes volumetric structural changes of the metal, which can be an additional reason for the formation of high internal residual stresses due to the local high-temperature effect of the welding thermal deformation cycle, therefore study of the effect of the kinetics of the temperature on the residual phase composition of the weld and HAZ of the welded joint remains a relevant task. 2. Finite element modelling and material properties A finite element model of the butt welded joint made from steel DC04 is a 600x600x10 mm plate, which consists of four identical layers of 2.5 mm height 3D prismatic elements. In the zone of intense heating and phase transformations, the size of the base of 3D prismatic elements is 2.5x2.5 mm. Outside the highly heated zone, the bases of the finite elements are increased to 5x5 mm and 10x10 mm to reduce the calculation time. The transition between zones modeled by elements with different base sizes is provided by the finite elements with rectangular trapezoidal bases of the following dimensions: 2.5x5 mm and 5x10 mm. The third type boundary conditions, which determine the heat transfer between the body surface and the environment, are set by 2D elements in the form of a heat transfer surface that simulates convection and radiant heat transfer during welding. The weld path is defined by 1D elements. The technological clamping of the joint during welding and subsequent 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 of the heat source. The plate was melted along the middle section in the direction of the X-axis for the entire thickness in single pass. The origin of the right rectangular coordinate system XYZ of the finite element model of the butt welded 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. Heating and cooling time of the welded joint is 1200 s. The change in material properties on temperature was taken into account both during heating and cooling (Fig. 1). 291