ON THE QUESTION OF USE OF AGGLOMERED WELDING FLUX IN THE FORMATION OF WELD METAL DURING ARC WELDING OF LOW- ALLOYED HIGH -STRENGTH STEELS

ABSTRACT

Based on the analysis of the results of scientific research in the field of metallurgy of welding high-strength low-alloy steels, the article shows a change in the view on the role of welding flux in ensuring the quality indicators of weld metal. Modern welding fluxes should take the most effective part in the processes of refining the weld pool, controlling the metallurgical processes of the formation of non-metallic inclusions of a certain composition, morphology and distribution in the solid solution in order to ensure the required structural composition of the weld metal and a complex of its mechanical properties when welding steels.

АННОТАЦИЯ

В статье показано на основании анализа результатов научных исследований в области металлургии сварки высокопрочных низколеги­рованных (ВПНЛ) сталей изменение взгляда на роль сварочного флюса в обеспечении показателей качества металла швов. Современные сварочные флюсы должны принимать самое эффективное участие в процессах рафинирования сварочной ванны, управления металлургическими процессами образования неметаллических включений определенного состава, морфологии и характера распределения в твердом растворе с целью обеспечения требуемого структурного состава металла шва и комплекса его механических свойств при сварке ВПНЛ сталей.

Keywords: high-strength low-alloy steel, welding, welding flux, non-metallic inclusions, microstructure, mechanical properties

Ключевые слова: высокопрочная низколегированная сталь, сварка, сварочный флюс, неметаллические включения, микроструктура, механические свойства

Introduction. Despite numerous forecasts of rapid growth in the use of polymeric materials ­in construction, mechanical engineering, and power engineering, today steels remain the most common structural material. It is likely that this situation will continue in the coming decades. Welding firmly occupies a leading position among the methods of joining steel products, and arc welding remains the main technology in this area. An analysis of the situation with the consumption of welding materials for arc welding methods over the past decade shows that submerged arc welding occupies about 10% of the total volume of arc methods and observations show that there is no reason to assume significant changes in this position.

Submerged arc welding began to be used in the 30s. XX century. Until now, this welding method has gone through a stage of intensive development, during which deep fundamental ­studies of metallurgical, electrical, and physicochemical processes were carried out, which served as the basis for widespread introduction by the mid-70s. automated welding in various industries. The studies carried out over the years, combined with the accumulation of practical experience in the use of fluxes and the improvement of the technology for ­the production of high quality steels, caused changes in approaches to determining the role of the flux itself ­in the process of weld formation. If at the initial stage of development, the flux was assigned the role of passive protection of the weld pool from the environment­, and the working personnel - from the impact of the arc, then in subsequent years, the flux began to be considered as an active participant in metallurgical ­processes occurring in the arc burning zone and in the liquid bath.

Objects and methods of research.The requirements for the operation of welded structures ­determine the need to guarantee the service properties of welded joints at the level ­of modern high strength steels, so the flux in combination with the electrode wire should provide alloying, microalloying, modification and refining of the weld metal. At the same time, high welding and technological properties of the flux should be ensured in ­order to obtain high-quality welds in a wide range of welding modes and technologies.

Rolled sheets of low-alloy steels currently used for the manufacture of ­welded structures are characterized by a combination of high strength, ductility, and toughness due to the formation of a fine-grained ferritic-bainitic or bainitic-martensitic microstructure. Welded joints of such steels must have a ­complex of mechanical properties at the level of values of the base metal. with welding fluxes, in this case, the leading role is played in obtaining the necessary microstructure of the weld metal and the mechanical properties of the welded joint.

A large number of works have been devoted to the study of the conditions for the formation of the microstructure ­of the weld metal of HSLA steels ­, as a result of which it was found that one of the factors that have a decisive influence on the structure is non-metallic inclusions ­(NI) [1]. Modern fluxes should not only protect the weld pool from the ambient ­atmosphere, but also set the required level of oxygen potential of the slag phase , together with low alloyed wire, contribute ­to the formation of NI of the predictable amount, composition and size [2]. The weld metal obtained by submerged arc welding of modern HSLAsteels contains 0.02 - 0.04% oxygen ­and less than 0.01% sulfur. Using the well-known expression V_NI= 5.5 [% O + % S ], it can be established that the given content of oxygen and sulfur ­corresponds to 0.15 - 0.30 vol. % there are nometallic inclusions. However, to provideonly about 30% of them can actively influence the nucleation of the ferrite phase.

The most effective in this regard are inclusions­with sizes from 0.3 to 1.0 µm, which have a specific morphology. Refractory oxides (for example, Al₂O₃), which are present as crystals in the liquid metal of the weld pool. When titanium oxide precipitates on the surface of a refractory inclusion, zones with a reduced content of alloying elements ­having a high mobility in the γ phase can form in the adjacent regions of the solid solution. Inclusions of this type are ­the most efficient centers for the nucleation of a bainitic microstructure.

For the purpose of refining iron-based alloys ­in metallurgy, such deoxidizing elements as aluminum, silicon, titanium, and manganese are widely used. The manufacturing technology of agglomerated fluxes makes it possible to control their oxygen potential over a wide range [3]. Controlling the oxygen content in the weld metal by changing the oxidizing capacity of the slag phase in combination with the introduction of active deoxidizers into the flux composition makes it possible to use agglomerated fluxes not only to reduce the volume fraction of NI in the weld, but also to form inclusions of ­a certain size and composition (Fig. 1).

Figure 1. Influence of the oxygen content in the weld metal on the average size of the NI and the content of titanium in them

The use of fluxes of this type in arc ­welding significantly expands the scope of methods for the predictable influence on the formation of NIs of a certain composition and morphology in the weld metal. Welded joints in this case acquire a complex of mechanical properties at the level of the values of the HSLA steels [4].

Figure 2. Influence of Titanium/Oxygen Ratio in NV on the Content of Structural Components and Fracture Resistance of Weld Metal

With a decrease in the oxygen content in the welds alloyed with titanium, not only does the average size of the NI decrease, but the ­precipitation of titanium compounds on the surface of refractory inclusions of the Al₂O₃ type also increases. An analysis of the chemical ­composition of NIs of this morphology and the metal matrix surrounding them, performed using a microprobe for X ray ­analysis, showed that those inclusions with a thin film of titanium compounds on their surface have an increased concentration of manganese in the outer layer and a decreased concentration of manganese. content of manganese in the zones of the solid solution adjacent to the inclusion. Inclusions of such a morphology contribute to the formation of a ferrite phase with increased viscosity during ­the γ → α transformation (Fig. 2).

The complex of mechanical properties of a weld is determined ­by the combination of the components of its structure. An increase in the proportion of microstructural fractions with increased hardness leads to an increase in ­the strength of the metal (Fig.3), and a high content of microstructures formed in the low-temperature zone of the γ → α- transformation ­is characterized by increased resistance to brittle fracture at low climatic temperatures (Fig.4).The optimal combination of ­strength, toughness, and plasticity indicators for each individual case is determined by a set of these structural components.

As can be seen from the data shown in Fig. 3 and 4, an increase to 60% of the content in the microstructure ­of such viscous components as acicular and grain-boundary ferrite, granular ­and lower bainite, provides an increase in the impact strength of the weld metal, while the yield strength ­does not exceed the level of 500 - 550 MPa, ­typical for seams with a ferritic structure. An increase in the proportion of upper bainite in the microstructure of the weld contributes to an increase in its strength indicators, but reduces the impact strength at low temperatures.

Conclusion. Scientific research and extensive ­practical experience in the field of welding of HSLA steels have led to an obvious change in the view on the role of welding flux in ensuring the quality of the weld metal. Modern welding fluxes should take the most effective part in the refining of the weld pool, control of metallurgical processes of the formation of NI of a certain composition, morphology and nature of distribution in the solid solution in order to ensure the required structural composition of the weld metal and a complex of its mechanical properties in the welding of HSLA steels. Production experience has ­shown that agglomerated fluxes, which are characterized by high technological efficiency, have a significant advantage in this regard,flexibility by controlling their oxidative capacity. Welded joints obtained with the use of this type of fluxes have a complex of mechanical properties at the level of the ­HSLA values of steels.

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