FEATURES OF USING WELDING FLUXES FOR WELDING HIGH-STRENGTH STEEL 28Х3СНМВФА

ABSTRACT

The article presents comparative data on the mechanical characteristics of welded joints made of high-strength steel 28Х3СНМВФА, made under fluxes АН-15 and АВ-5 with various filler wires. It is shown that the use of non-oxidizing flux АВ-5 in combination with wire 28Х3СНМВФА practically ensures equal strength of the welded joint to the base metal.

АННОТАЦИЯ

В статье приведены сравнительные данные механических характеристик сварных соединений из высокопрочной стали 28Х3СНМВФА, выполненных под флюсами АН-15 и АВ-5 с различными присадочными проволоками. Показано использование безокислительного флюса АВ-5 в комплексе с проволокой 28Х3СНМВФА практически обеспечивает равнопрочность сварного соединения основному металлу.

Keywords: welding flux, high-strength steel, silicon and manganese oxides, weld clogging, non-oxidizing flux

Ключевые слова: сварочный флюс, высокопрочная сталь, окислы кремния и марганца, засорение шва, безокислительный флюс

Introduction. At present, there are still no sufficiently clear recommendations for choosing a welding method for high-strength medium-alloy steels with a thickness of more than 6 mm (with σ_в = 160 kg/mm² for critical welded joints).

When welding under fluxes containing silicon and manganese oxides, due to reactions associated with silicon and manganese reduction processes, the weld becomes clogged with dispersed endogenous oxide inclusions and, consequently, the total oxygen content increases [1, 2].

When welding high-strength steels, the main task of the flux is to protect the melting zone from the effects of atmospheric components. The required degree of alloying is achieved by using an electrode wire of the appropriate composition.

Materials and methods of the study. In the works carried out on welding high-strength steel 25ХСНВФА using welding wire 20ХСНВФА and fluxes with different degrees of acidity, as well as a composition of fluxes AH-348A, AH-15, AВ-4, the advantages of non-oxidizing flux AВ-4 in comparison with the others were revealed [1]. Taking into account the results of work [1], the non-oxidizing flux AВ-5 (like the previously studied flux AВ-4) was tested in welding stronger, compared to 25ХСНВФА steel, 28Х3СНМВФА steel with a thickness of 8 mm in the form of a forging in order to obtain high mechanical properties of the weld joint and weld metal.

Submerged-arc welding of critical-purpose products (high-pressure cylinders, thick-walled single-use structural elements, etc.) made of high-strength steels (σ_в = 160-180 kg/mm²) with a thickness of more than 6 mm is of great practical importance, since this thickness is the maximum possible for high-quality argon-arc welding using transverse oscillation of the electrode and special fluxes [3].

Table 1 shows the composition of the flux AВ-5 and AН-15 with a lower content of the component СаF₂, which has a negative effect on the stability of the arc.

Table 1.

Flux composition with lower CaF₂ content

Results and discussions. The acidity coefficients of these fluxes are approximately equal to 1. The acidity calculation was made using the formulas given in the works [2,4].

In order to compare the properties of the specified fluxes, multilayer surfacing was carried out using 20ХСНВФА wire with a diameter of 3 mm of the following chemical composition (in %): 0,23 C; 1,18 Si; 0,98 W; 0,022 P; 0,58 Mn; 0,90 Ni; 0,019 S; 1,20 Cr; 0,15 V.

The surfacing was performed on a 28Х3СНМВФА steel plate with a thickness of 20 mm in 8 layers. The first half of the plate (200 mm) was surfaced under flux AН-15, the second under flux - AВ-5. Surfacing mode v_wel = 28 m/h,   І_wel = 420 - 440 A; U_d = 32-34 V. The ВДМ-1601 rectifier was used as a power source. The detachability of the slag crust was the same for both fluxes. The actual chemical composition of the deposited metal and the absorption coefficients of some elements (the ratio of the analytical concentration of a given element in the deposited metal to its initial content in the wire K_у) are given in tables 2 and 3. From the data in tables 2 and 3 it is evident that flux AВ-5 has some advantages over AН-15. It improves the absorption of carbon by the weld metal and promotes a sharp decrease in the oxygen content in the weld. These data are in good agreement with the data of work [1].

Table 2.

Chemical composition of weld metal

Table 3.

Element assimilation coefficient

Preliminary data allowed us to proceed to welding plates from a forging with a thickness of 8 mm. In order to obtain guaranteed penetration of the weld root, it was performed with a non-consumable electrode in helium. After that, the welded plates were subjected to low tempering at 300-320°C for 3 hours. The filling of the plate joints was carried out by welding under flux AB-5 with wire 28ХЗСНМВФА (option 1) and 20ХСНВФА (option 2) and under flux AН-15 with wire 28Х3СНМВФА (option 3) and 20ХСНВФА (option 4) (Fig. 1).

Welding according to the specified options made it possible to select the best option, which should meet the following requirements: obtaining equal strength of the welded joint during tensile testing with the weld reinforcement removed, obtaining a high level of plastic properties of the weld metal (δ₅=8%) and impact toughness (a_n = 4 kgm/cm²). Before welding, the fluxes were calcined in a furnace at 250-300°C to reduce moisture. The filling of the groove was carried out in one pass in the surfacing mode.

Figure 1. Comparative characteristics of mechanical properties of 28X3CHMBФА steel and weld metal for different metallurgical options

The chemical composition of the welding wires used is presented in table 4. The welded plates were subjected to high tempering at 720°C for 2 hours with air cooling, and then sandblasted. The welded joints were inspected using the X-ray method, as a result of which no defects were detected.

Table 4.

Chemical composition of welding wire

When determining the mechanical properties of the welded joint, samples made from plates were subjected to hardening according to the following regime: hardening temperature 950-960°C, holding for 20-25 minutes, cooling in air followed by tempering at 300°C for 3 hours. As a result of mechanical tests of the weld metal, it was established that the best welding option is welding under flux AB-5 using welding wire 28X3CHMBФА (fig. 1). This option ensures equal strength of the welded joint with the base material, high plastic properties of the weld metal: δ₅=10,5%, ψ=46,9%, а_n=6,9 kgm/cm² (data for 5 samples).

Table 5.

Hardness of the deposited layer

The data in table 2 confirm that the improvement in the mechanical properties of the welded joint when using the AB-5 flux occurs mainly due to a reduction in the oxygen content in the weld metal.

Metallographic analysis of unetched welded joints revealed characteristic non-metallic inclusions (SiO₂ and 2MnO SiO₂ globules), which were recorded on a dark field (SiO₂ - brightly glowing, and 2MnO SiO₂ - transparent with a pink or brown tint). This analysis revealed significant contamination of the weld metal made under flux AН-15 with inclusions of silicate origin. This circumstance confirms the fact of a direct dependence of the content of silicate inclusions in the weld metal on the content of SiO₂ in the flux composition, i.e. the silicates found in the weld are of endogenous origin, which also agrees well with the conclusions of work [1].

Conclusion. Thus, the use of non-oxidizing flux AB-5 in combination with wire 28Х3СНМВФА practically ensures equal strength of the welded joint to the base metal. The work carried out confirmed that, with the appropriate composition of the welding wire, the welding flux is of decisive importance when choosing the technology of automatic submerged arc welding of high-strength steels, which should ensure high assimilation of alloying elements, as well as minimal contamination of the weld metal with silicate inclusions.

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