Method for welding pipelines from high-strength pipes with controllable heat input

Abstract

The invention relates to the field of construction, particularly to welding of above- and underground high-strength pipelines with controlled heat input. Application of the invention will increase the load-bearing capacity of the pipelines made with the use of butt-welded pipes, pipe spools, pipe strings. The method includes joining of two or more cylindrical metal pipes, pipe spools and pipe strings by the welded ring butt with the use of the arc welding for the whole perimeter of the pipe. Criteria for a high-quality welded joint include optimal selection of parameters of the welding thermic cycle. The suggested welding method allows to have the optimal structure, high strength and viscoplastic properties in areas of the welded joint, to provide for the required load-bearing capacity of the pipeline and its reliability during operation.

Claims

1. A method for butt welding of pipes at negative ambient temperatures, comprising: arranging butted ends of pipes to be welded to form a gap of 3-8 mm between the butted ends at their closest point, the gap tapered outward; preheating the butted ends of the pipes to be welded to a temperature in a range of 170-220 C., prior to arc welding; arc welding the pipes together around a perimeter thereof forming a butt welded joint comprised of overlapping rings of weld beads, wherein successive ones of the weld beads that form the butt welded joint are configured such that thickness of each previous bead is maintained in a ratio to thickness of each subsequent bead within a range of 1 to 2; during the arc welding, controlling heat input to the metal in the range of 0.8-1.2 kJ/mm; maintaining an inter-layer temperature between applied weld beads of the butt welded joint within a range 170-220 C. during the arc welding; and controlling cooling of the butt welded joint at a rate within a range of 150-200 C. per hour until cooled to 50 C.

2. The method of claim 1, wherein the arranging the butted ends of the pipes to be welded comprises maintaining a ratio of the width of the gap to thickness of the pipes to be welded within a range of 1.3 to 2.0.

3. The method of claim 1, wherein each run around the perimeter of the arc welding includes parallel application of a bead pair wherein each second bead is for normalizing temperature of each first bead and fully overlaps the each first bead.

4. The method of claim 1, wherein a number of beads making up the butt welded joint is not less than five.

5. The method of claim 1, wherein controlling the cooling is performed using a heat-insulated belt around the butt welded joint.

6. The method of claim 1, wherein the arranging the butted ends of the pipes to be welded comprises configuring each of the tapered walls according to a skew angle in a range of 25 to 30 degrees.

7. The method of claim 1, wherein the arranging the butted ends of the pipes to be welded comprises configuring each of the ends of the pipes to be welded with a bluntness of 1.80.8 mm.

8. The method of claim 1, wherein an ambient air temperature during the arc welding is below 40 C.

9. The method of claim 1, wherein a wall thickness of the pipes to be welded is in a range of 4 to 32 mm.

10. The method of claim 1, wherein a breaking stress of the pipes to be welded is less than a breaking stress of the butt welded joint.

11. The method of claim 1, wherein the gap is configured with tapered walls opening to an increasing width outwardly.

12. The method of claim 1, wherein a breaking stress of the pipes to be welded is 640 MPa, and a breaking stress of the butt welded joint after performance of the method is 680 MPa.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The features, nature, and advantages of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings.

(2) FIG. 1 shows a scheme of bead application according to the method. Beads are applied in parallel, one upon the other, and every next bead overlaps the previous one at 100%.

(3) FIG. 2 is a temperature graph showing that an important microstructural changes of the fused metal and the heat-affected zone (HAZ), as for its mechanical features, happen during the cooling process within the temperature range of 800-500 C. The cooling rate is characterized by the time of the cooling process in this very temperature zone (t.sub.8/5).

(4) FIG. 3 is a graph showing that the high heat input and, subsequently, the longer cooling time (t.sub.8/5) decrease both hardness characteristics (thus, strength indices) and resilience. At that, resilience is more sensitive to the increased heat input value. Optimal values of hardness and resilience correspond to the optimal heat input within the range of 0.8-1.2 kJ/mm.

DETAILED DESCRIPTION

(5) The present method may be applied for pipe welding during construction of above- and underground pipelines at negative ambient temperatures, for example, temperatures below 40 C.

(6) Main parameters that determine the heat input value during welding are: the welding current, the arc voltage and the welding rate. The formula for calculation of the heat input during the welding

(7) $\begin{array}{cc}E=\frac{6IU}{100}& \left(1\right)\end{array}$
where I is the welding current, A; U is the welding rate, mm/min; E=arc energy, kJ/mm.

(8) The value of heat input to the metal is determined by the formula (2):
Q=k.Math.E(2)
where Q is the heat input, kJ/mm.

(9) The research that has been done in the Transneft R&D, LLC and affiliated companies of OJSC JSC Transneft as well as tests during the layout in the Extreme North districts with the air temperature of up to 50 C. below zero have shown that the full-strength welded joint for steel pipes with temporary breaking stress of 590-690 MPa, the wall thickness of 4-32 mm is obtained, the residual welding voltages and prevention of stowing structures with low crack formation strength are provided by the disclosed welding method.

(10) The method may be implemented as follows. Referring to FIG. 1, butted ends 110, 110 of the pipes 100, 100 or elements to be welded are opened up for the welding, such that a ratio of the cumulative width of end openings E to thickness of welded elements S is within the range of 1.3-2.0 and the gap 140 between the butted ends 110, 110 at their closest point is in the range of 3-8 mm. Then, the ends of the pipes 100, 100 or elements to be welded are preheated within the range of 170220 C.

(11) Multiple beads 1-1, 1-2, 2-1, 2-2 . . . n-1, n-2 are laid down between the ends of the pipes 100, 100 or elements to be welded, where a ratio of thickness values of each previous and each next bead is selected within the range of 1.0-2.0.

(12) The inter-layer 120 temperature control between applied beads 1-1, 1-2, 2-1, 2-2 . . . n-1, n-2 is controlled within the range of 170-220 C. Application of a heat-insulating belt (not shown) around the completed welded joint may be used to control cooling of the welded joint with the rate of 150-200 C. per hour until cooled to the temperature of 50 C. Application of the weld beads 130 that form the weld may be with 100% overlapping, as shown in FIG. 1. Welding of butt welds are done by application of not less than 5 main and 5 normalizing beads, at this every subsequent bead 100% overlaps the previous one.

(13) Experiments have shown that:

(14) if the correlation of cumulative width of the gap E between the abutted ends 110, 110 to thickness S of welded elements exceeds 2.0 in upper weld pass 150, polygonizational cracks appear and decrease strength properties of the welded joint and, accordingly, parameters of its reliability and workability;
if the correlation of cumulative width of the qap E between the butted ends 110, 110 to thickness of welded elements S is less than 1.3, welds acquire a negative columnar structure, and the level of residual welded stresses increases in the welded joint that decreases its strength properties and, accordingly, parameters of its reliability and workability;
compliance with the said parameters of preheating, inter-layer temperature control, correlation between thickness values of the previous and the next beads allow to control parameters of the heat input to the metal within the required range, application of a heat-insulating belt allows to control the process of butt weld cooling and to prevent formation of stowing structures with low crack resilience at temperatures below 300 C. and decreased mechanical features of the metal within the area of thermic effect of the welded joint;
a tempering bead run with 100% overlapping provides heat treatment of the previous bead and to stabilize mechanical features of the weld upon thickness of the pipe wall.

(15) Welding is made by electrodes with strength characteristics equal to the welded metal or exceeding them by no more than at 30%.

(16) To determine workability of the construction and its optimal characteristics, the full-scale test works under industrial conditions have been carried out at temperatures of up to minus 50 C. inclusive. Stock that is 36 m long, made out pipes with 1,020 mm diameter, with a wall thickness of 10 mm, out of the pipe of steel grade K65 was welded in the aboveground version on demountable supports. The non-destructive control has shown absence of any defects of the welding origin, as well as of any mechanical damages and cracks.

(17) As a result, parameters of the welding technology listed in Table 1 have been determined.

(18) TABLE-US-00001 TABLE 1 Numeric values of Parameter Designation parameters Temporary breaking stress for the in Ohms 640 pipe metal (main metal), MPa Temporary breaking stress for the in msh 680 metal of the weld, MPa Parameters of edge opening: skew angle, grade a 25-30 bluntness c 1.8 0.8 Degree of the main bead's N 100 overlapping by the normalizing one, % Range of thickness values of S 4-32 welded elements, mm Minimal number of the applied n 5 main and normalizing beads Inter-layer temperature range, C. T 170-220 Maximum rate of post-welding V 200 butt cooling, C./hour Ambient air temperature during T Minus 40, up to welding Minimal temperature at which the T 50 butt can be cooled down at the air without any protective coverage

(19) The experimental research has shown that full-strength welded joints for steel pipes with a temporary breaking stress of 590-690 MPa can be achieved, with a wall thickness of 4-32 mm. Residual welding stress relief and prevention of quenching cracks are provided due to the controlled heat input to the metal, within the range of 0.8-1.2 kJ/mm, by application of the end opening for welding with a ratio of the cumulative width of end opening E to thickness of welded elements S within the range of 1.3-2.0, preheating of the elements to be welded to within a range of 170-200 C., the multi-layer ring butt welded joint of tube stocks with a ratio of thickness values of each previous and next bead within a range of 1.0-2; maintaining inter-layer temperature control between applied beads of the weld within a range of 170-220 C., application of a heat-insulated belt (not shown) to control cooling of the welded joint within a rate of 150-200 C. per hour until cooled to the temperature of 50 C., and application of the welded beads that form the weld with 100% overlapping.

(20) Suggested modes give the opportunity to remove residual welding stresses and to prevent formation of the quenching structures with the low resilience to formation of cracks that appear during cooling of the butt weld.

(21) Application of the suggested method provides for a full-strength weld with a high metallurgical quality and high viscoplastic properties, that increases the weld's resilience to crack formation and increases the load bearing capacity of the pipeline.