OBRABOTKAMETALLOV technology Vol. 26 No. 2 2024 Introduction New energy policy has greatly contributed to the rapid increase in the share of renewable, clean energy such as wind, solar and hydropower. At the same time, thermal power plants still remain an important element in the production of electricity and heat. From year to year, regulatory requirements for the characteristics of steels, for welding and repair procedures for various machine parts and mechanisms of thermal power plants are growing. Traditionally, the main process of welding and repair at thermal power plants is manual arc welding (MAW) with coated electrodes, mechanized welding in shielding gases. The main consumables in accordance with the guidelines (GL) [1] for manual arc welding are electrodes of the basic type: UONI-13/45, UONI-13/55, UONI-13/55S, LEZUONI-13/55, TMU-21U etc. Control of the moisture content in the electrode coating is crucial for obtaining defect-free, high-quality welds when arc welding of steels in a protective environment [2]. The welding industry has long been faced with the problem of high sensitivity of basic type electrodes to moisture absorption [3, 4]. Moisture is the main source of hydrogen entering the weld pool. The presence of hydrogen in the melting zone when welding steels can be dangerous because it causes the formation of cold cracks, both in the heat-affected zone and in the melting zone, which cause catastrophic failure of the welded steel structure. Cold cracks caused by hydrogen are a serious problem in the weldability of lowalloy high-strength steels [2–7]. Cold cracks occur with the simultaneous existence of three factors: residual stresses after welding, brittle structures in the heat-affected zone (HAZ) and a high diffusive hydrogen content in the weld overlaid metal [2]. During welding, hydrogen absorbed in the weld zone has a high tendency to diffuse into the HAZ. The parameters that influence the diffusion of hydrogen from the weld zone into the welded product are temperature, metal microstructure, solubility, residual stresses and the effect of accumulation in metal defects. It has been established that the main source of hydrogen in the weld metal during MAW is the electrode coating decomposition products [5, 8]. Before the hydrogen atoms dissolve in the liquid weld pool, H2O and H2 dissociate. The dissolution of molecular hydrogen in the weld pool increases with increasing partial pressure of the components of the gas mixture according to Sieverts law. One of the mechanisms of diffusion reduction of hydrogen is a decrease in the partial pressure of hydrogen in the atmosphere of the welding arc, for example, due to the dissociation of carbonates and fluorides, namely Na2CO3, NaF, CaCO3, CaF2, MgCO3 and MgF2. Carbonates dissociate to form CO2 and CO, which reduces the partial pressure of hydrogen above the weld pool [5–11]. Decomposition of the basic type electrode coating, containing CaCO3 as the main component (45–50 %), leads to the formation of gas protection with low hydrogen content. The second important component of the main type electrode coating is calcium fluoride CaF2. The introduction of fluoride compounds into the composition of welding materials is one of the effective ways to reduce the absorption of hydrogen by liquid metal [5, 8, 9]. Fluorine atoms, combining with electrons, transform into ions with low mobility [10, 11]. This leads to a decrease in the conductivity of the arc gap and deterioration in the stability of the arc. However, fluorine atoms are able to bind hydrogen into HF molecules that do not dissolve in the weld pool, reducing the saturation of the weld metal with hydrogen [5]. Therefore, the use of a basic electrode coating is a key approach to reducing the risk of cold cracking when welding high-strength steels [12–14]. Although the electrode base coating is a low-hydrogen welding material, it is susceptible to moisture absorption when exposed to the atmosphere [5, 8, 14]. In Europe, the measurement of diffusive hydrogen in arc welded metal is regulated by the ISO 3690 standard [15]. This standard is similar to the American standard AWS A4.3-93 [16] and the Japanese JIS Z 3113 [17]. There are differences in details; however, with respect to the methods described, the standards are largely equivalent. Welding electrodes are classified into groups depending on the diffusible hydrogen content electrodes can introduce into the weld metal by various national and international standards. The International Institute of Welding (IIW) uses a linear scale increment to measure hydrogen levels in units of 5 (5–10–15 ml/100 g), as well as a logarithmic scale (4–8–16 ml/100 g) used by AWS, based on the correlation of lower critical voltage, lower preheating temperature with diffusing hydrogen levels to avoid hydrogen cracking. The content of diffusion hydrogen in the weld overlaid metal depends not only on the
RkJQdWJsaXNoZXIy MTk0ODM1