Thermal imaging investigations of the temperature fields on the surface of a welded item during manual arc welding with coated electrodes

Abstract

724Most welding processes provide for heating of the units to be joined using different heat sources. In this case, the quality of the welded joint is mainly determined by the conditions of heating and cooling. The character of the thermal processes specifies the efficiency of melting of the parent metal and welding sup plies, the conditions for the formation of the weld metal structure and the thermal action zone, and the operating abilities of welded joints.Control over the thermal cycle in the material to be joined is one of the main problems in welding engi neering in the process of manufacturing critical parts.Modern theories and studies [1-4] devoted to thermal processes that occur during welding and formu lation of the key concepts and laws of temperature field propagation do not take a number of significant factors (heat exchange with the environment, the method of welding, etc.) into account. The recommended design equations, as a rule, contain complex functions, thus leading to an increase in the computational burden and the need for a sophisticated mathematical apparatus and computing machinery. Nevertheless, the data obtained during calculations do not often coincide with the actual values faced in practice. Thermal processes can be described most accurately and adequately on the basis of the results of exper imental investigations of the temperature fields. Traditionally, temperature fields in practice are recorded using stickers, thermocouples, and pyrometers. However, all these methods for evaluating the distribution of the heat energy in an item have their own drawbacks related to the local and contact (except for pyrom eters) character of measurements. The most complete and exact pattern of the temperature fields can be obtained using modern thermal imaging equipment.To record the temperature fields in items during fusion welding, investigations performed in accor dance with the technique described in [5] were carried out. A bead was fused using manual arc welding (MAW) with LB 52U coated electrodes onto a 100 × 150 mm steel 09Г2С plate that was 6 mm thick. It was accomplished using the most widely used ВД 306 power source and a Nebular 315 inverter power source of the new generation. The temperature fields were recorded with a ThermaCAM P65HS thermal imager (FLIR, USA) (Fig. 1). The shooting lasted one run of welding at a 5 kHz frequency of thermal image repetition. The adequacy of the obtained temperature fields was controlled with a C 500 infrared pyrometer. The isotherm images on the plate surface were obtained in the form of a set of m files after pro cessing the source thermograms with the ThermaCAM Researcher application and the MATLAB math ematical package (Fig. 2).The analysis of the experimental results showed that the type of the power source used for welding sub stantially affects the temperature distribution over the surface of the welded item during MAW. It was revealed by analyzing the obtained thermograms (Fig. 2) that the use of the inverter leads to temperature field lines that ...

Effect of Protective Coatings in Welded Constructions on Welding Stability and Electrode Metal Transfer into a Weld Pool in MMA

“…Oscillograms of current in a welding circuit, voltage between an electrode and an item, recorded when MMA with protective coatings (Figure 2) were processed statistically and presented in Table 1. As seen in Table 1, there is 5-11% increase in time of a droplet transfer and 10-12 % rise of cycle duration (period of its formation and transfer), it suggests that electrode metal is transferred in bigger drops [9][10][11][12][13][14][15][16][17][18]. This regularity is possible since protective coatings deposited on edges to be welded form a film, which is a layer between an electrode and an item, being important for welding stability.…”

“…Figure 3 demonstrates heat patterns of welding processed in a mathematical package MATLAB presented as a set of m-files [9]. The difference in heat field distribution in welding with protective coatings (Figure 3) is caused by a large drop transfer of electrode metal (Table 1).…”

“…The difference in heat field distribution in welding with protective coatings (Figure 3) is caused by a large drop transfer of electrode metal (Table 1). Studies [6,9,19,20] suggest that dimensions of transferred electrode metal drops are relevant for a grain size in the weld structure. Furthermore, while drying up of a protective coating a film is formed on the surface of an item to be welded -«heat cover», preventing from heat removal from the surface of an item to be welded and furthers its propagation along the entire surface.…”

“…For the purpose of research we selected coated electrodes LВ-52U (KOBELCO) broadly applied in oil-refining and gas industries and a specially developed protective coating [7] and spray [8]. 4 -spots to be examined Heat content of weld pool metal can be determined according to patterns of heat field distribution (as stated in [9]). When fusion welding with protective coatings, a bead was formed manually by arc welding with coated electrodes LB 52U on 6 mm thick steel St37-2 plate of 100х150 mm.…”

Calculation of the Heat Content of the Electrode Metal Droplet When Applying Power Supplies for Manual Arc Welding with Different Volt-Ampere Characteristic

“…The calculations [6] completed according to the analytical method show that when inverterpowered welding the average energy spent on melting electrode metal per second equals Qк = 1.1325х107 J and when diode rectifier-powered welding the average energy equals Qк = 1.2503х107 J.…”