Title of Invention

METHOD OF WELDING METALLIC WORKPIECES AND APPARATUS FOR CARRYING OUT THE METHOD

Abstract

In a welding method, in particular for the joint welding of metallic workpieces (2, 4) having a wall thickness (d), the metallic workpieces (2, 4) are cooled during the welding with liquid gas (25), in particular liquid carbon dioxide, for producing a temperature gradient over the wall thickness (d). Consequently, the development of tensile stresses at the surface of a workpiece (2, 4) can be avoided and the risk of stress corrosion cracking can be reduced.

Full Text

Description Method of welding metallic workpieces and apparatus for carrying out the method The invention relates to a method of welding metallic workpieces and an apparatus for carrying out the method in the joint welding of pipes. In a multiplicity of steels, for example austeni- tic steels, corrosion occurs primarily in the range of high tensile stresses upon contact with certain media, for example water. In order to avoid this type of cor- rosion, designated as stress corrosion cracking, it is necessary to reduce the level of tensile stresses in the workpiece. This can be done, for example, by design measures when fashioning a component or by production measures during its manufacture. However, tensile stresses occur in particular in the area of welding points, in particular at joint welding points of austeni- tic pipelines. These tensile stresses, which are residual stresses above the yield point, are caused by the shrink- age of the weld deposit introduced and are unavoidable. To shift such tensile stresses, it is known, for example, from US-A-4,683,014 to apply compressive forces to the periphery of the pipes in the area of a finished pipe weld seam, which compressive forces cause plastic deformation of the pipes and the build-up of axial compressive stresses at the inner surface of the weld seam. US~A-4,678,894 discloses a method of reducing residual stresses in a pipe weld seam, in which the pipe weld seam is subjected to a subsequent thermal treatment. In this known method, the pipe weld seam is inductively heated at its outer surface and is simultaneously cooled at its inner surface with water in order to produce a temperature gradient between the inner and outer surface. Compressive stresses are produced at the inner surface and tensile stresses are produced at the outer surface of the pipe by the different linear expansion on account of the temperature difference between the inner and outer surface. Here, the stresses produced at the outer surface are above the yield point and thus produce a permanent compressive stress at the inner surface after cooling. Methods are also known in which compressive stresses are brought about in the root area of a joint weld seam during the welding by producing a temperature gradient over the wall thickness of the workpiece. In so- called heat sink welding or last pass heat sink welding, this is done by the weld seam being cooled with water from the inside in a similar manner to the aforesaid thermal treatment method after the weld seam has been closed in the root area and during the welding of the outer filler runs. However, in particular during the pipeline assembly and in particular in the case of closing seams the cooling of the inner surface with flowing water can be carried out only at an unjustifiable cost and is therefore of limited suitability for practical applica- tion in situ and is very expensive. The object of the invention, then, is to specify a method of welding metallic workpieces with which a high temperature gradient over the wall thickness of the workpieces can be achieved at little cost in the area of the welding point during the welding. In addition, an apparatus for carrying out the method in the joint welding of pipes is to be specified. The said objects are achieved by the features of Patent Claims 1 and 6, respectively. In a welding method, in particular for the joint welding of metallic workpieces having a wall thickness, the metallic workpieces are cooled with liquid gas during the welding for producing a temperature gradient over their wall thickness. The liquid gas vaporizes upon striking the surface of the workpiece so that the heat of vaporization is additionally available for the cooling. The vaporizing gas flows by itself out of the pipeline into the ambient, so that no further technical measures are necessary for the removal of the gas. The cooling with liquid gas is here preferably effected on one side, on the surface of the workpieees which is opposite the heat input. The cooling with liquid gas is carried out in particular only after the welding groove is closed in the root area. The weld seam can also be partly or completely filled. Even at old weld seams, it is possible to subsequently reduce the tensile stresses by subsequent application of one or more additional welding beads during simultaneous cooling with liquid gas. These welding beads can be made by the stringer technique or by weaving. The heat input is reduced in an advantageous manner by the application of a plurality of small welding beads or additional finish runs. Liquid nitrogen or argon, for example, are suitable as the liquid gas. In an especially preferred development of the invention, liquid carbon dioxide is used as the liquid gas. In the case of this gas, so- called carbon-dioxide snow develops during the spraying, which is deposited on the inner surface of the pipe and results in especially effective cooling. As a result of the sublimation of the carbon dioxide or the vaporization of other chemically inert liquid gases suitable for cooling, the oxygen present is in addition displaced in the ambient of the weld seam and a shielding-gas atmosphere comparable to an inert forming gas is produced. Tarnishing or oxidation of the root area is thereby also prevented during the cooling. A suitable apparatus for the joint welding of two pipes contains a spray device which can be inserted into the interior of the pipes, is connected to a feed line for liquid carbon dioxide and is provided with at least one discharge opening. This spray device preferably contains a cylindrical spray head having a plurality of discharge openings distributed over its periphery. In an especially preferred development of the invention, the spray head is provided with baffle plates between which the discharge openings are arranged. To improve the cooling effect, swirling or a rotational flow of the liquid-gas stream can also be additionally provided by special nozzle arrangements and suitably shaped baffle plates. In a further development of the invention, spray devices can also be provided which have a plurality of spray heads, preferably of identical construction, to increase the cooling capacity. In a further preferred development of the inven- tion, the spray device contains sealing means for forming a chamber which is closed off to both sides of the pipe and in which the spray head is located. In particular, the feed line is connected via a three-way valve to both a liquid-gas supply and a shielding-gas supply. Consequently, before the root area of the weld is closed, forming or shielding gas can be admitted to the chamber formed by the spray device, so that a shielding-gas atmosphere can be produced in the interior of the pxpe in the area of the weld seam. To explain the invention in more detail, refer- ence is made to the exemplary embodiment in the drawing, in which: Figure 1 schematically illustrates in side view an apparatus, inserted into a pipe, according to the invention, Figure 2 shows a sectional detail of the weld seam during the welding. According to Figure 1, two workpieces, in the example of application two pipes 2 and 4 having a wail thickness d, are joined together at their ends 2a and 4a, respectively. Located between the ends 2a and 4a pre- formed before the welding operation is a welding groove 6 which in accordance with the figure is already partly filled with a molten weld metal 8. The heat input takes place, for example, by means of an arc on the outer surface 2b, 4b of the pipes 2 and 4, respectively. In addition, the detail enlargement of the root area 10, already closed and solidified on the inner surface 2c, 4c, of the weld seam can be recognized in Figure 2. Inserted into the interior of the pipes 2 and 4 is a spray device 20 which is connected to a flexible feed line 40. The spray device 20 contains an approxi- mately cylindrical spray head 22 which is provided at its periphery with a plurality of nozzle-shaped discharge openings 24. These discharge openings 24 are arranged between two disc-shaped baffle plates 26 with which the medium flowing out of the discharge openings 24 can be aligned with the root area 10 of the weld seam. In addition, the spray device 20 contains two sealing discs 28 which form a chamber 30 which consists of a pipe section and is used at the start of the weld- ing, before the root 10 is closed, to build up a forming or shielding-gas atmosphere. To avoid excess pressure in this chamber 30, the sealing discs 28 are provided with openings 29a and b. As schematically shown in the figure, the feed line 4 0 is connected to a three-way valve 42, so that alternatively liquid gas 25, in particular liquid carbon dioxide CO2,liq, or shielding gas, for example argon Argas can be fed to the weld seam via the feed line 40. In the exemplary embodiment, one of the sealing discs is provided with openings 29b which are arranged at its periphery. This not only avoids the build-up of excess pressure but also prevents excessive accumulation of CO2-snow. The openings 29b can additionally be pro- vided with a flap in order to prevent backflow of CO2- snow. In the exemplary embodiment, austenitic pipes 2, 4 having an outside diameter of about 160 mm and a wall thickness of 14 mm are welded to one another from outside by a TIG welding process. Forming is carried out on both sides with argon during the welding of the root run 10 and the first backing runs. After the seam is filled to about one quarter, liquid carbon dioxide CO2,liq is injected into the spray device 20 via the three-way valve 42 and the root run is cooled until all finish runs are applied. In the exemplary embodiment, a temperature difference of about 200 °C resulted between the outer surface of the welding bead 8 at about 15 mm distance from the arc and the inner surface 2c, 4c of the pipes 2 and 4, respectively, at about 0.5 mm distance from the welding root 10. The temperature at the inner surface was here always less than 50 °C. The differential stress a produced by this temperature gradient at the inner surface 2c, 4c is about 700 N/mm2 and was clearly higher than the yield point of the austenitic material used, which is about 200 N/mm2, so that permanent compressive stresses could be produced and the build-up of tensile stresses at the inner surface 2c, 4c in the area of the weld seam could be avoided. The welding technique illustrated with reference to joint welding of two pipes 2, 4 can be applied subsequently in the same manner to workpieces already welded to one another. By the application of an addi- tional weld or finish run by the method according to the invention, existing tensile stresses can subsequently be reduced. A plurality of small welding beads or finish runs can also preferably be applied one after the other to reduce the heat input. Patent claims WE CLAIMS 1. Welding method, in particular for the joint welding of metallic workpieces (2, 4) having a wall thickness (d), in which method the metallic workpieces (2, 4) are cooled with liquid gas (25) for producing a temperature gradient over the wall thickness (d) during the welding. 2. Welding method according to Claim 1, character- ized in that the cooling with liquid gas (25) is effected on one side on the surface (2c, 4c) of the workpieces (2, 4) which is opposite the heat input, 3. Welding method according to Claim 2, character- ized in that, during the joint welding at a weld seam (8), the cooling with liquid gas is carried out only after the weld seam (8) is closed. 4. Welding method according to one of the preceding claims, characterized in that liquid carbon dioxide is used as the liquid gas (25). 5. Welding method according to one of the preceding claims, characterized in that, during the joint welding of two pipes (2, 4), a spray device (20) connected to a feed line (40) for liquid gas and provided with at least one discharge opening (24) is inserted into the interior of the pipes (2, 4). 6. Method of reducing tensile stresses at an exist- ing weld seam, in which method one or more additional welding beads or finish runs are applied by a method according to one of Claims 1 to 4. 7. Method according to Claim 6, characterized in that, when reducing the tensile stresses in the area of a weld seam of a pipe, a spray device (20) connected to a feed line (40) for liquid gas and provided with at least one discharge opening (24) is inserted into the interior of the pipe. 8. Apparatus for carrying out the method according to either of Claims 5 or 7, having a spray device (20) which can be inserted into the interior of the pipes (2, 4), is connected to a feed line (40) for liquid gas (25) and is provided with at least one discharge opening (24) 9. Apparatus according to Claim 8, characterized in that the spray device (20) contains at least one cylin- drical spray head (22) having a plurality of discharge openings (24) distributed over its periphery. 10. Apparatus according to Claim 9, characterized in that the spray head (22) is provided with baffle plates (26) between which the discharge openings (24) are arranged. 11. Apparatus according to one of Claims 8 to 10, characterized in that the spray device (20) contains sealing means (28) for forming a chamber (30) which is closed off to both sides of the pipe (2, 4) and in which the spray head (22) is located. 12. Apparatus according to Claim 11, characterized in that the sealing means (28) are provided with an opening (29a, 29b) for reducing the excess pressure xn the chamber (30) and for drawing off the CO2-snow from the chamber (30). 13. Apparatus according to one of Claims 8 to 12, characterized in that the feed line (40) is additionally connected to a shielding-gas supply via a 3-way valve (42). In a welding method, in particular for the joint welding of metallic workpieces (2, 4) having a wall thickness (d), the metallic workpieces (2, 4) are cooled during the welding with liquid gas (25), in particular liquid carbon dioxide, for producing a temperature gradient over the wall thickness (d). Consequently, the development of tensile stresses at the surface of a workpiece (2, 4) can be avoided and the risk of stress corrosion cracking can be reduced.

Documents:

308-cal-1995-abstract.pdf

308-cal-1995-claims.pdf

308-cal-1995-correspondence.pdf

308-cal-1995-description (complete).pdf

308-cal-1995-drawings.pdf

308-cal-1995-examination report.pdf

308-cal-1995-form 1.pdf

308-cal-1995-form 2.pdf

308-cal-1995-full specification.pdf

308-cal-1995-gpa.pdf