RU2757447C1 - Method for welding large diameter straight-seam pipes - Google Patents

Abstract

FIELD: pipe production.

SUBSTANCE: invention relates to pipe production, in particular to a method for welding a longitudinal seam of welded straight-seam pipes of large diameter. A longitudinal joint of a pipe is assembled in an assembly mill, welding of a connecting technological seam with a melting electrode in a protective gas medium, and subsequent welding of working outer and inner seams are carried out. First, the connecting technological seam is welded with a technological burner with the melting electrode in the protective gas medium. Then, after the crystallization of the technological seam, the first layer of an outer seam is welded directly on the assembly mill with the melting electrode in a mixture of argon-based protective gases by a burner, which is installed with an offset from the technological burner welding the connecting technological seam at a distance of 400÷1000 mm. Then, multi-arc submerged welding is carried out, first for an inner seam and then for the remaining part of the outer seam.

EFFECT: optimizing cooling rates to obtain the required phase composition of metal in the zone of thermal influence of the welded joint, which leads to an increase in the quality of resulting welded pipes.

1 cl, 2 dwg, 2 tbl

Description

The invention relates to pipe production, in particular to electric-welded pipe production, and can be used in the production of longitudinal welded pipes of large diameter.

At present, specialized low-alloy controlled rolling tube steels are used for the construction of trunk gas pipelines. The use of such steels is due to a number of factors, the main one of which is satisfactory weldability with a simultaneous high level of strength properties. Satisfactory weldability is ensured by a moderate content of alloying elements, so that the carbon equivalent, determined by the formula

,

,

where C, Mn, Cr, Mo, Ni, V - the content of the corresponding alloying elements in%, does not exceed the value of 0.45%. A high level of strength properties is ensured, on the one hand, by the presence and the required content of alloying elements capable of forming a carbonitride network, and on the other hand, by the controlled kinetics of deformation and cooling processes during strip rolling. In a simplified way, the technology of manufacturing a sheet of controlled rolling and an increase in the mechanical properties of the metal associated with the manufacturing process can be represented as a combination of work hardening and hardening. Moreover, the higher the requirements for their strength properties (the higher the strength group), the greater the effect of hardening on the properties of the metal. The phase composition of the steel changes accordingly. Table 1 shows the typical minimum strength and elastoplastic requirements for X70, X80, X100 pipe steels (API 5L API).

It should be borne in mind that domestic designers and consumers of pipe products can assign their own requirements to the mechanical properties of pipes, both to the base metal and to the properties of the welded joint. As for the values of impact strength (in the table - the work of impact), it is obvious - and this can be seen from the table, the requirements for this parameter in most cases are underestimated due to the impossibility of obtaining impact strength in the welded joint at the level of the base metal. At the same time, the requirements for the equal strength of the welded joint to the base metal, i.e. compliance of the actual tensile strength of the welded joint with the minimum requirement for the tensile strength of the base metal are preserved and are quite realizable. Impact toughness values indirectly reflect the operability of the metal under conditions of low temperatures and shock loads, which is very important for domestic gas pipelines laid in the polar zones of the territory of Russia, as well as in seismically active regions. Today, pipe consumers - builders and operators of main gas pipelines - in order to reduce the costs of transporting pipes to the installation sites, as well as to reduce the cost of the construction and installation works themselves, by reducing the metal consumption of pipelines, tighten the requirements for the strength properties of pipes, increasing the strength group. At the same time, as mentioned above, as the strength of the pipe metal increases, the role of the thermal component in the formation of the complex of metal properties increases, and the phase composition of the metal also changes. If steels of strength group X70 and below can have a ferrite-pearlite structure [1], then steel of strength group X80 (domestic analogue of K65), as noted in RF patent No. 2 615 667 (C21D 8/02 006.01) C22C 38/00 (2006.01 ), publ. 04/06/2017), has a predominantly bainitic structure, with a dispersed ferrite content of no more than 20%. In this steel, a pearlite content of not more than 5% is allowed. It is obvious that pipe steel of the X100 strength group has a completely bainitic structure. The chemical composition of all domestic pipe steels, except for cases when special properties are required, for example, resistance to hydrogen corrosion cracking, is developed on the basis of steels of grades 10G2FB, 09G2FBYU, 10G2FBYU. At the same time, the variations are due not so much to the need as to the features and technological capabilities of specific industries. Table 2 shows the comparative chemical composition of steel 10G2FBYU in accordance with GOST 19281-2014 and steel specified in the patent of the Russian Federation No. 2 615 667.

As can be seen from the table, in the patented chemical composition of steel, in comparison with the "standard" one, the carbon content is reduced and molybdenum is added, as well as the content of sulfur and phosphorus is significantly toughened (the latter two elements directly affect the impact strength). Considering all of the above, it should be considered the main factor determining the complex of strength and elastoplastic properties of controlled rolling steels of high strength groups (above X70 / K60), the optimally selected cooling rate in the temperature range of completion of the transformation of supercooled austenite, i.e. ~ 800 ÷ 500 ° C. In this case, the value of the cooling rate should be in the range (for steel X80, having up to 20% ferrite and not more than 5% pearlite) 20 ÷ 30 ° C / s, as indicated, for example, in RF patent No. 2 492 250 (C21D 8 / 02 (2006.01) C22C 38/38 (2006.01), C22C 38/42 (2006.01), publ. 09/10/2013).

Considering all of the above, it should be considered that at present the technology for the production of sheet strip of high strength groups (X80 and higher) for the manufacture of longitudinal welded pipes of large diameter with the required level of strength and elastoplastic properties has been sufficiently developed. At the same time, ensuring the required properties of a welded joint of pipes presents certain difficulties due to the impossibility of obtaining values of impact toughness at the level of the base metal. The condition of equal strength, as indicated above, is satisfied. The reason is the impossibility of obtaining the required level of metal cooling rate in the heat-affected zone of the welded joint using the welding technology used.

The current state of the art includes the conventional technology for welding large diameter pipes. This welding technology is built into the overall technological chain of pipe manufacturing along with shaping operations and, having high productivity, is not a "bottleneck". A tubular billet with a gap between the edges in the range of 20 ÷ 100 mm and the edges prepared by milling for double-sided welding are fed to the assembly mill, where assembly (closing of opposite edges) is carried out with simultaneous welding of the connecting (technological or tack weld). Welding of the joint is performed by consumable-electrode arc welding ∅ 2.4 ÷ 5.0 mm in a shielding gas environment (carbon dioxide or in a mixture of gases based on argon) at a speed of 3.0 ÷ 5.5 m / min - depending on the thickness of the metal. The connecting seam (pos. 1 of Fig. 1) is of extremely low quality, since it is carried out on weight without blowing the root of the seam with protective gas, it is saturated with internal defects and is completely remelted with working seams - this requirement is laid down in all regulatory documents. After welding of the connecting seam, an inspection is carried out and, if necessary, its repair by stripping and (or) welding. Then the pipe goes to the execution of the inner seam, (pos. 2 Fig. 1), which is performed by multi-arc submerged-arc welding - from 2 to 4 arcs, depending on the thickness of the metal, working in one weld pool. The diameter of the welding wire is in the range of 3.0 ÷ 5.0 mm. The welding speed, depending on the thickness of the metal, is 1.2 ÷ 3.0 m / min. The outer seam (pos. 3 of Fig. 1) is performed similarly to the inner one, but the number of arcs can be up to 5. The method used for welding working seams - multi-arc submerged-arc welding - has not been studied in detail until today, but due to high welding speeds, and at the same time sufficiently high-quality seam formation, no alternative is used for welding large-diameter pipes. The same welding method at the above speeds and currents required to fill the groove of the seams, as well as reliable penetration, necessary, among other things, and to remove the technological seam, provides a low cooling rate in the most critical temperature range (800 ÷ 500 ° C) when recrystallization is intense enough, thus increasing the weldability to a level where preheating is not required to prevent the formation of cold cracks, which is especially important at a carbon equivalent value of 0.45%. At the same time, the low cooling rate (less than 20 ° C / sec) does not allow obtaining the required values of the impact toughness of the welded joint, in particular, along the fusion line. This technology makes it possible to obtain guaranteed, subject to the correct choice of welding materials, the required set of mechanical properties of the welded joint for pipes of strength class X70 / K60, providing the temporary resistance of the welded joint at the level of 690 MPa, which is ensured by the ferrite-pearlite structure of the heat-affected zone [1], obtained at cooling rates less than 20 ° C / sec. However, for higher grades (X80 / K65 and higher), obtaining the required properties is difficult, since for a higher level of mechanical properties, as indicated above, a ferrite-bainitic or completely bainitic phase composition of the metal is required, which, in turn, with the limitation of the carbon equivalent in the range of 0.45 ÷ 0.47% can be provided at a cooling rate of 20 ÷ 30 ° C / sec. This provision applies equally to the base metal and to the metal in the fusion line with the weld. The existing technical solutions aimed at achieving the required properties relate mainly to the optimization of the chemical composition of the base metal and the proper selection of welding consumables. So, in the patent of the Russian Federation No. 2 434 070 (C22C 38/00 (2006.01) B23K 35/30 (2006.01) B23K 9/23 (2006.01), published on 20.11.2011), a set of measures is proposed to clarify the chemical composition of the base metal (the main difference from the chemical composition given above in Table 2 for the strength group X80 - an increase in the manganese content - to increase the hardenability), and also the alloying of the weld is limited to prevent the formation of cold cracks in the weld. As a result, the ultimate tensile strength is guaranteed at a level of at least 800 MPa (strength group X100) both in the base metal and in the weld, without a guarantee of obtaining the corresponding values of the impact toughness along the fusion line. Also known RF patent No. 2 509 171 (C22C 38/14 (2006.01) B21C 37/08 (2006.01), publ. 03/10/2014), where, along with a slightly different chemical composition from the previous patent (additional alloying with molybdenum in combination with boron to increase hardenability), it is proposed to limit the heat input of welding, calculated for multi-arc welding according to the formula

,

,

where k is the thermal efficiency of the welding arc, taken for submerged-arc welding 0.93 ÷ 0.97; i - current arc number; n is the number of arcs; U _i , I _i - respectively, welding voltage and current on each arc, with an interval from 3.5 kJ / mm to 10.0 kJ / mm. In this case, additional alloying insufficiently solves the issue of the impact toughness of the metal on the fusion line, as well as obtaining the required metal structure. It is well known that the lower the heat input, the higher the cooling rate [2], but not tied to specific pipe wall thicknesses, should be considered negligible. A more effective way to ensure the level of mechanical and elastoplastic properties of the boundary between the base metal and the weld zone is proposed in the RF patent No. 2 639 086 (C21D 9/50 (2006.01) B23K 26/00 (2014.01), publ. 19.12.2017) - the use of laser welding, when the entire thickness of the pipe wall is boiled with a laser beam - an extremely concentrated heat source providing a cooling rate of more than 100 ° C / s, then the seam is cooled down and reheated with a slower cooling. In addition, variants of this method are known, for example, laser-arc welding, proposed in the same patent. It should be borne in mind that all methods of achieving the required microstructure of the heat-affected zone of a welded joint by providing a cooling rate in the range of 800 ÷ 500 ° C in the range of 20 ÷ 30 ° C / sec and more with full or partial use of laser welding entail difficulties associated with with the quality of the welded joint assembly, as well as the direction of the laser beam to the desired point.

The technical problem aimed at optimizing the cooling rates in order to obtain the required phase composition of the metal in the heat-affected zone of the welded joint can be solved by changing the sequence of seams. If, with the traditional scheme, the connecting (technological) seam is performed first of all, after which the metal is cooled to a temperature not higher than 50 ° C (cooling is carried out in air and is combined with other technological operations: inspection and repair of the technological seam, welding of technological strips, if they are were not welded earlier, preparatory operations before welding of working seams), the cooling rate in the range of 800 ÷ 500 ° C is excessive. In the case of combining the joint seam and the first layer of the outer seam with an offset of 400 ÷ 1000 mm in one welding installation, as shown in Fig. 2, where the tubular billet (pos. 6), covered around the entire perimeter by assembly cassettes with rollers (pos. 7) and lying on the rolls of the bed (pos. 8) is welded with a consumable electrode in a protective gas environment, first with a process burner (pos. 9) on exit from the zone of action of the assembly cassettes, then, after crystallization of the technological seam, welding continues with the torch of the first layer of the outer seam (pos. 5). Further welding of working seams is carried out according to the traditional technology: multi-arc submerged-arc welding of the inner seam, then cooling in calm air to a temperature not higher and welding the remaining part of the outer seam is similar to the inner one. As a result of using the proposed sequence instead of the traditional one:

1) after the combined welding of the technological and the first layer of the outer seam:

a) the cooling rate of the metal in the region of the neutral line decreases - in the zone of action of arcs in the range of 800 ÷ 500 ° С from 120 ° С / sec to ~ 75 ° C / sec;

b) heats the edges under the undercooked part of the outer seam - from the melting temperature with cooling rates from 500 ° C / s (from the melting temperature to 1000 ° C) to 40 ° C / s (at 550 ° C);

2) after additional welding of the outer seam, the reheating temperature of the inner seam decreases.

The above data are the result of a comparative finite element thermal calculation performed under the conditions of equality of the total heat input of welding and initial temperatures for each stage of welding.

References to literary sources

1. Livshits LS, Khakimov AN Metallurgy of welding and heat treatment of welded joints. - M. Mechanical Engineering. 1989 331 s.

2. Rykalin NN Calculations of thermal processes in welding. - M. Mashgiz. 1951 291 s.

Claims (1)

A method of welding a longitudinal seam of welded longitudinal seam pipes of large diameter, including the assembly of a longitudinal joint of a pipe in an assembly mill, welding of a connecting technological seam with a consumable electrode in a shielding gas environment and subsequent welding of working external and internal seams, characterized in that, first, with a process torch, the connecting technological seam is welded with a consumable electrode in a shielding gas environment, then, after crystallization of the technological seam, the first layer of the outer seam is welded directly on the assembly mill with a consumable electrode in a mixture of shielding gases based on argon with a torch, which is installed with an offset from the process torch, which welds the connecting technological seam, at a distance of 400 h 1000 mm, then multi-arc submerged arc welding is carried out first of the inner seam and then welding the remaining part of the outer seam.

Images