ISSUES OF INFLUENCE OF THE CONTENT OF SULFUR AND PHOSPHORUS IN WIRE ON THE PROPERTIES OF WELDED JOINTS

ABSTRACT

In the article, the influence of the concentration of sulfur and phosphorus in the wire Св-08Г2С on the properties of welds welded in carbon dioxide is studied. Welding in a carbon dioxide environment is usually performed with Св-08Г2С wire. The standard limits the content of harmful impurities in it. The modern technical level of metallurgy makes it possible to produce this wire with less sulfur and phosphorus.

АННОТАЦИЯ

В статье исследовано влияние концентрации серы и фосфора в проволоке Св-08Г2С на свойства швов, сваренных в углекислом газе. Сварка в среде углекислого газа, как правило выполняется проволокой Св-08Г2С. Стандарт ограничивает содержание вредных примесей в ней. Современный технический уровень отечественной металлургии позволяет выпускать эту проволоку с меньшим количеством серы и фосфора.

Keywords: carbon dioxide, harmful impurities, seam durability, crystallization crack, phosphorus and sulfur content, manganese content.

Ключевые слова: углекислый газ, вредные примеси, стойкость швов, кристаллизационная трещина, содержание фосфора и серы, содержание марганца.

Welding in carbon dioxide, as a rule, is performed with Св-08Г2С wire. ГОСТ 2246-60 limits the content of harmful impurities in it to 0.03% S and 0.03% P. The modern technical level of domestic metallurgy makes it possible to produce this wire with less sulfur and phosphorus. In order to clarify the feasibility of such an increase in the purity of this material, in this work, the influence of the concentration of sulfur and phosphorus in the Св-08Г2С wire on the properties of welds welded in carbon dioxide was studied.

Research methodology. The influence of sulfur and phosphorus on the properties of the welds was studied during automatic welding in carbon dioxide of steel Ст 3 using experimental wires of the Св-08Г2С type with different sulfur and phosphorus contents, manufactured at the PWI. E.O. Paton.

To obtain wires with a low concentration of phosphorus, carbonyl iron was used in the smelting of ingots of steel Св-08Г2С. The amount of sulfur was reduced by electroslagmelting of ingots of this steel under AНФ - 7 flux. The content of sulfur and phosphorus was increased by introducing iron sulfide and Ferrophosphorus into the mixture of experimental melts. Seven batches of experimental wires with 0.007÷0.035% S and 0.011÷0.037% P. At the same time, commercially manufactured Св-08Г2С wire was tested. The wires were tested when welding steel Ст 3 δ = 20 mm of the following composition: 0.17% C, 0.44% Mn, 0.16% Si, 0.18% P and 0.033% S.

The weld metal was subjected to chemical analysis, studies on resistance to crystallization cracks and mechanical tests. Welding was carried out with wires with a diameter of 2 mm at direct current of reverse polarity in the mode: Iсв=400÷420 A, U=33÷35 V, Vсв=20 m/h, Qco₂=1200 m3/h.

The resistance of welds against crystallization cracks was determined by a method based on the static bending of butt specimens during welding [1]. Butt specimens were made from plates 160x120x20 mm in size with a beveled edge at an angle of 30⁰ and a bluntness of 4 mm. Samples were collected without a gap using tacks, welding was performed on a copper lining.The maximum strain rate of the sample v_кр at which cracks have not yet appeared was taken as the criterion for the resistance of welds to hot cracks. The presence of cracks was visually detected by the fracture of the seam.

The effect of the phosphorus concentration in the wire on the mechanical properties of the weld metal was determined from the results of tests of tensile and impact specimens. For this purpose, plates made of steel Ст 3 δ =20 mm were butt welded with a V-shaped groove at an angle of 60⁰. Welding was carried out on an automatic machine in four or five passes until the groove was filled. From the middle layers of the weld metal, discontinuous specimens of type II and impact specimens of type IV (ГОСТ 6996-66) were cut out. The notch on the impact specimens was applied along the critical section from the side of the upper rollers.

Research results. The chemical composition of the experimental wires and the results of testing the weld metal made with these wires for resistance to crystallization cracks are given in Table 1.For comparison, the composition and test results of Св-08Г2С wire of industrial production (wire №.8) are also indicated there.

Figure 1 shows the dependence of the resistance of the weld metal against crystallization cracks on the sulfur content in it. This dependence was obtained for welds with 0.12-0.14% C, 0.30-0.40% Si and 0.90-1.10% Mn. It indicates a significant influence of the sulfur concentration in the joints on their resistance to crystallization cracks. So, with a decrease in the amount of sulfur in the wire from 0.034 to 0.007%, the resistance of welds against crystallization cracks more than doubles.

Figure 1. Dependence of the resistance of the weld metal against crystallization cracks on the sulfur content in it

         Table 1.

Chemical composition of experimental wires

The seams were welded on steel with a high sulfur content at (0.032%) in a mode that provides an increased proportion of the base metal in the weld metal. Obviously, the effect of the sulfur concentration in the wire on the resistance of the welds against solidification cracks can be even stronger if the proportion of the base metal in the weld is reduced or steel with alower sulfur content is used. However, it should be noted that all the given data refer to welds with a small amount of carbon (up to 0.14%). It is known that an increase in the ratio of manganese and sulfur concentrations prevents the formation of crystallization cracks only at a certain limiting carbon content in the metal. With a larger amount of carbon, a decrease in the sulfur concentration or an increase in the manganese content in the metal no longer gives a positive effect [2].

In table. 2 shows data on the composition of the metal of multilayer welds welded with experimental wires. Tensile and impact samples were made from these welds to determine the effect of phosphorus content on impact strength at various temperatures. Chips for chemical analysis were taken from the metal of impact samples after their testing.

Table 2.

The chemical composition of the metal of multilayer welds welded with experimental wires

The mechanical properties of the metal of welds made with experimental wires are indicated in tab. 3. As can be seen, the phosphorus content within the studied limits, without significantly affecting the strength and ductility of the weld metal, significantly affects its impact strength. So, when welding with experimental wires №. 3-8, providing the content of  P < 0.020% in the weld metal, the impact strength is much higher than when using wires №. 1 and 2, in which the concentration of this element is more than 0.020%. Phosphorus has a particularly strong effect on the impact strength of the weld metal at low temperatures. So, when welding with wires №. 4 and 7, which guarantee the phosphorus content in the weld metal is less than 0.015%, the impact strength at -70^°C was 8.6 and 7.8 kg•m/cm², respectively. According to this indicator, the seam was superior to the base metal.

Figure 2 shows the dependence of the impact strength of multilayer welds on the concentration of phosphorus. As can be seen, the effect of phosphorus content on impact strength is greatest at low temperatures. So at -70⁰ C and the presence of 0.018% phosphorus in the weld metal, the value of an < 3 kg•m/cm². It should be noted that all data on the effect of phosphorus concentration on impact strength refer to welds with a low carbon content (up to 0.14%).

As follows from, a decrease in the concentration of phosphorus increases the impact strength of the weld metal only when the carbon content is not higher than a certain limit [2] .

Figure 2. Dependence of the impact strength of multilayer welds on the content of phosphorus in them

Table 3.

Mechanical properties of the metal of welds made with experimental wires

Findings. 1. With a relatively low carbon content (up to 0.14%), a decrease in the amount of sulfur in the welding wire significantly increases the resistance of the weld metal against crystallization cracks. Reducing the phosphorus content in the welding wire significantly increases the impact strength of the weld metal at low temperatures.

2. Wire Св-08Г2С with a low content of sulfur and phosphorus (<0.02% S and <0.02% P) can be recommended for welding critical metal structures in carbon dioxide.

References:

1. Podgaetsky V.V. Nemetallicheskiye vklyucheniya v svarnykh shvakh. - M.: Mashgiz, 1962. pp. 99-108.
2. Friedland L.A., Timofeev K.I. Ispytaniye staticheskim izgibom svarnykh shvov na sklonnost' k obrazovaniyu goryachikh treshchin // Avtomaticheskaya svarka. – 1985. № 2. -  P. 56 - 61.