GB2455086A

GB2455086A - Weld Cooling

Info

Publication number: GB2455086A
Application number: GB0723186A
Authority: GB
Inventors: Christopher Rand; Walter Veldsman
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 2007-11-27
Filing date: 2007-11-27
Publication date: 2009-06-03
Anticipated expiration: 2027-11-27
Also published as: GB2455086B; GB0723186D0

Abstract

An apparatus for the forced cooling of a heated weld zone 26 in work being melded comprises at least one nozzle 10 for ejecting at least one jet of a cryogenic coolant at the heated weld zone 26 and a first passage 14 for the extraction of vapour of the cryogenic coolant. The first passage 14 has a mouth communicating with a vapour space in the vicinity of the heated weld zone 26. The vapour space is open to the atmosphere outside the apparatus. There is a second passage 18, or arrangement of second passages, surrounding the first passage 14, the second passage 18 having an outwardly facing circumferential outlet 20 from which, in use, a flow of auxiliary gas is able to be ejected so as to have an Aaberg effect and to restrict radially outward escape of the vapour of the cryogenic coolant along the surface of the work being welded.

Description

-1-2455086

WELD COOLING

The present invention relates to apparatus for and a method of forced cooling of a heated weld zone by directing a jet of cryogenic coolant at the weld zone.

During the thermal welding of metallic workpieces a high heat input is required to generate an acceptable weld. However, this high heat input has a disadvantage that the thermal stresses generated by the welding process can cause significant levels of distortion of the workpieces being welded. Sheet workpieces of relatively soft metals and alloys such as mild steel, titanium, titanium alloys and stainless steels are particularly prone to distortion and reduced tensile strength.

It is known to use a jet of cryogen, particularly carbon dioxide, to provide cooling to thermal joining processes (including solid phase welding, for example, friction stir welding), particularly arc welding processes, so as to reduce or eliminate distortion or to modify the microstructure of the weld metal. The cryogenic coolant extracts heat from the workpieces and is vaporised or sublimed as a result. There therefore tends to be a build up of vaporised coolant which, if unchecked, can be hazardous to the welder.

According to the present invention there is provided apparatus for the forced cooling of a heated weld zone in work being welded, the apparatus comprising at least one nozzle for ejecting at least one jet of a cryogenic coolant at the heated weld zone, a first passage for the extraction of vapour of the cryogenic coolant having a mouth communicating with a vapour space in the vicinity of the heated weld zone, the vapour space being open to the atmosphere outside the apparatus, and a second passage, or arrangement of second passages, surrounding the first passage, the second passage, or arrangement of second passages, having an outwardly facing circumferential outlet from which, in use, a flow of auxiliary gas is able to be ejected so as to have an Aaberg effect and to restrict radially outward escape of the vapour of the cryogenic coolant along the surface of the work being welded.

The invention also provides a method of forced cooling of a heated weld zone in work being welded, the method comprising directing at least one jet of cryogenic coolant at the heated weld zone from at least one nozzle, extracting vapour of the cryogenic coolant through a first passage from a vapour space in the vicinity of the heated weld zone, the vapour space being open to the atmosphere, passing an auxiliary gas through a second passage, or arrangement of second passages, surrounding the first passage, and ejecting a flow of the auxiliary gas through an outwardly facing circumferential outlet of the second passage, or arrangement of second passages, so as to have an Aaberg effect and to limit radially outward escape of Ihe vapour of the cryogenic coolant along the surface of the work being welded.

The terms "welding" and "weld" as used herein refer to any thermal joining method.

The term "circumferential" as used herein to describe the outlet of the second passage, or arrangement of second passages, refers not only to an outlet that extends circumferentially but also to an arrangement of orifices that are disposed on the circumference of a notional circle.

The auxiliary gas is normally air. In this case, an advantage cE the method and apparatus according to the invention is that they can be operated without substantial risk of the vapour of the cryogenic coolant concentrating in the atmosphere experienced by the welder and reaching a hazardous level. In particular, theflow of auxiliary gas counteracts the tendency for the vapour of the cryogenic coolant to form a strong flow away from the weld along the surface of the work being welded.

Further, the method and apparatus according to the invention do not employ an extraction hood surrounding the nozzle of a kind in which the hood has sealing surfaces which engage the work and in which the hood completely overlaps the nozzle, thereby interfering unduly with the welder's line of sight. The apparatus according to the invention can be made sufficiently small not to significantly impede the line of sight of the welder. An additional advantage of the method and apparatus according to the invention is that a significant portion of any potentially hazardous fume evolved during a welding procedure can be extracted with the vapour of the cryogenic coolant, thereby further reducing the hazard to the welder.

The Aaberg effect is that extraction of gas through an exhaust passage can be facilitated by injecting a flow of gas radially outwards from a circumfrentially extending aperture surrounding the entrance to the exhaust passage.

In the absence of the radially outward flow of the auxiliary gas, the vapour of the cryogen would escape from the vicinity of the mouth of the extractim passage along the face of the work being welded. Attempts to extract the vapour by applying a suction to the extraction passage would be only partially successful. In the operation of the method and apparatus according to the invention, however, not ory does the radially outward flow of the auxiliary gas have the effect of facilitating extraction of the vapour of the cryogen through the first passage ("the Aaberg effect") it also has the effect of seemingly creating an inward flow of gas along the face of the work in the vicinity of the mouth of the first passage. As a result, improved extraction of the vapour can be achieved.

The configuration of the outlet of the second passage and the exit velocity of the auxiliary gas may be such that a relatively thin "flange" or a thicker, diverging, "wedge" of auxiliary gas radiating outwards is formed.

Typically, the mouth of the first passage has an operating position forward of the outlet of the second passage. If desired, the axial position of the outlet d the second passage may be adjustable relative to the position of the mouth of the first passage, so as to enable an operator to optimise the extraction of the vapour.

The flow rate of the auxiliary gas may be selected in relation to the flow rate of the coolant or to the rate of extraction of vapour. Typically, the flow rate of the auxiliary gas is about 2 to 3 times the rate of vapour extraction.

In use, it is preferred that the axis of the nozzle is perpendicular to the face of the work being welded. Some small deviation from a truly perpendicular arrangement may be tolerated without undue disruption of the Aaberg effect.

The apparatus according to the invention is typically arranged to be moved over the weld relative to the work. The apparatus according to the invention may therefore be mounted on a suitable carriage. The carriage may also carry a welding torch so that the nozzle is moved in tandem with the torch, the torch leading so that the coolant contacts weld metal that has just been deposited.

The cryogenic coolant may be any substance having a temperature of minus 50°C or less which is gaseous or vaporous at ambient temperatures. The cryogenic coolant is preferably carbon dioxide. A jet of particles of solid carbon dioxide can be formed by passing a stream of liquid carbon dioxide at a pressure above its critical pressure through the nozzle. A particular advantage of solid carbon dioxide is that it can collect on the surface of the heated weld zone and take its enthalpy of sublimation from the heated weld zone. The stand-off distance of the nozzle from the heated weld zone is typically in the order of 10mm to 50mm. The nozzle itself typically has a relatively small internal radius, for example, a radius of 1.2mm.

The nozzle is preferably adapted to eject the cryogenic coolant at a supersonic velocity.

The method and apparatus according to the invention is suitable for use in conjunction with conventional arc welding methods, such as MIG welding, TIG welding and plasma arc. They may also be used to provide cooling to any other thermal joining process, including solid phase welding, for example friction stir welding, so as to reduce or eliminate distortion or to modify the microstructure of the weld metal.

The method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic sectional side elevation of the distal end of a first weld cooling apparatus according to the invention; Figure 2 is a section through the line A-A of Figure 1; and Figure 3 is a section through the line B-B of Figure 1.

The drawings are not to scale.

Referring to the drawings, a cooling head 2 is symmetrical about its longitudinal axis 4 and is located above work 6 to be welded. The head 2 is mounted on a carriage (not shown) behind an arc welding torch (not shown). The head 2 comprises a central, axial pipe 8 having at its proximal end (not shown)a connector enabling it to be connected in a fluid type manner to a flexible conduit (not shown) through which a supply of pressurised liquid carbon dioxide flows in use of the apparatus. The distal end of the pipe 8 terminates in a nozzle 10. The nozzle 10 may be integral with the pipe 8 or may be connected in a fluid type manner thereto.

The nozzle 10 may have a uniform bore of defined dimensions or may have a bore that is non-uniform. For example, the bore may have a convergent-divergent configuration so as to enable fluid to be ejected from the nozzle 10 at a sLpersonic velocity. The central pipe 8 is surrounded by a sleeve 12 which is coaxial therewith.

The sleeve 12 defines a first passage 14 for the extraction of carbon dioxide vapour from a vapour space 15 in the vicinity of a heated weld zone 26. The vapr space is open to the external atmosphere. Typically, the nozzle 10 protrudes a little from the mouth of the sleeve 12 so as to enable it to be visible to a welder.

The cooling head 2 also has a outer sleeve 16 which defines with the inner sleeve 12 a second passage 18. The distal end of the outer sleeve 16 terminates short of the distal end of the inner sleeve 12. The passage 18 defined between the sleeves 12 and 16 is of annular cross-section and is adapted to be connected at its proximal end (not shown) to a source of pressurised air. The passage 18 has a circumferential outlet 20 that faces outwards. As shown in Figure 1, the outlet 20 is defined between a bottom edge 22 of the sleeve 16 and a flange 24 integral with the sleeve 12. The outlet 20 is essentially horizontal.

In operation, a stream of liquid carbon dioxide, or other cryogenic coolant, under pressure, (typically in the range of 15 to 25 bar) is passed from a source thereof (not shown) to the cooling head 2. As the liquid carbon dioxide passes through the nozzle 10, so it is converted into a jet of gas carrying particles of solid carbon dioxide.

This jet deposits solid carbon dioxide on the heated weld zone 26 as the cooling head 2 follows the welding torch (not shown). At any instance during a welding operation, the region of the work impinged upon by the carbon dioxide jet may be arranged to be between 10mm and 60mm behind the area impinged upon the welding arc. A distance of, for example, from 20mm to 30mm is typical.

Liquid carbon dioxide is supplied at a pressure such that solid carbon dioxide will not be formed until the pressure is released as a consequence of the ejection of the carbon dioxide through the nozzle 10. The nozzle 10 is preferably arranged to eject the pressurised stream of carbon dioxide at a high, preferably supersonic, velocity.

Accordingly, the nozzle 10 is preferably a Laval nozzle which has a characteristic convergent-divergent bore at its distal end. It is as the liquid carbon dioxide is expanded rapidly in the divergent section of the Laval nozzle that a part of it solidifies. At optimal conditions about 40% is converted to solid and the rest to gas.

The solid particles are accelerated by the gaseous component. One advantage of employing a high velocity jet of cryogenic coolant is that it has the momentum necessary to penetrate any blanket of vapour that forms in operation over the heated weld zone 26. A particular advantage of the use of liquid carbon dioxide as a cryogenic coolant is that at least a part of the solid carbon dioxide that is formed by passage through the nozzle 10 collects on the surface of the work and extracts heat therefrom on change of state. The acceleration of the solid particles in the Laval nozzle facilitates their deposition on the work.

The cooling head 2 is in operation position quite close to the work 6 being welded.

Typically, a stand-off position of from 10mm to 50mm from the heated weld zone 22 is used. An advantage of a relatively short stand-off distance is that the jet of cryogenic coolant issuing from the nozzle 10 diverges and loses momentum only to a limited extent.

We have found that if a jet of cryogen is simply directed at the heated weld zone 22, there tends to be established a flow regime in which there is a strong radially outward flow of vapour of the cryogen along the surface of the work being welded. Because of this radial flow, the mere application of suction around the exterior of the nozzle is insufficient to effect satisfactory extraction of Ihe vapour of the cryogenic coolant.

One solution to this problem outside the scope of the invention would be to provide o the nozzle with a hood which makes a sealing engagement with the surface of the work being welded. In this way escape of the cryogen vapour would be prevented.

However, motion of the weld cooling head relative to the work would make it difficult to achieve satisfactory, durable, sealing of the hood to the work. Further, such a hood would tend to impede the welder's sight of the welding arc.

In accordance with the invention, however, ejection of a jet of air (or other auxiliary gas) from the outlet 20 of the passage 18 has the effect of creating a flow regime in which the air or other auxiliary gas airtails the escape of coolant vapour along the surface of the work 6 to be welded, and helps to create a flow regime in which the spent vapour of the cryogenic coolant tends even without the application of suction to collect in the space around and below the end of the nozzle 10 so that it can be readily extracted by the application of a suitable suction to the proximal end of the extraction passage 14. This effect of an air jet flowing radially outwards in the vicinity of the mouth of an extraction passage is known as the Aaberg effect. As shown in Figure 1 the outlet 20 is located some distance above the mouth of the extraction passage 14. The axial distance therebetween may typically be in the order of 200mm to 750mm. Such an axial separation facilitates, we believe, the formation of a recirculating air flow which helps to confine the carbon dioxide vapour evolving from the heated weld zone 22 to be confined in the vicinity of the mouth of the extraction passage 14. It is also possible, however, to employ an alternative arrangement in which the outlet 20 is at approximately the same level as the mouth 14, or one in which the outlet is at a level up to 200mm above the mouth 14.

Any suitable extraction device, such as an extraction fan (not shown), may be used for the purpose of applying a suction to the proximal end of the extraction passage 14.

Air is desirably chosen as the auxiliary gas because it is of course not harmful to people in the vicinity of the cooling apparatus. The air is typically supplied from a source of compressed air to pressure typically in the range of 2 bar to 10 bar in order to form a suitable jet. In general, the velocity of the jet effects its outward extent of travel before it starts to recirculate.

The flow rate at which the cryogenic coolant is provided is dictated by the rate at which it is desired to cool the weld in order to prevent distortion or to bring about a desired metallurgical change to the microstructure of the resulting weld metals. The flow rate of the auxiliary gas is related to the flow rate of the coolant. Typically the mass flow rate of the auxiliary gas is approximately equal to the mass flow rate of the cryogenic coolant.

Various changes and modifications can be made to the apparatus shown in the drawings. For example, the sleeves 12 and 16 may be integral with each other. In such a construction, instead of having a single auxiliary gas passage 18 surrounding the sleeve 12, there can be a series of individual passages each terminating in the outlet 20. In this construction, the outlet 20 can be formed as a slot, in the upper face of which the auxiliary gas passages terminate.

Further, although, as shown in the drawings, the outlet 20 is disposed horizontally and at an angle of 90° to the axis 4 of the cooling head 2, it is not essential (but is preferred) that the outlet 20 be truly horizontal. Some deviation from the strict horizontal may be tolerated. Thus, the outlet 20 could be at an angle of up to 10° from the horizontal pointing either upwardly or downwardly. -9.-

Claims

1. Apparatus for the forced cooling of a heated weld zone in work being welded, the apparatus comprising at least one nozzle for ejecting at least one jet of a cryogenic coolant at the heated weld zone, a first passage for the extraction of vapour of the cryogenic coolant having a mouth communicating with a vapour space in the vicinity of the heated weld zone, the vapour space being open to the atmosphere outside the apparatus, and a second passage, or arrangement of second passages, surrounding the first passage, the second passage, or arrangement of second passages, having an outwardly facing circumferential outlet from which, in use, a flow of auxiliary gas is able to be ejected so as to have an Aaberg effect and to restrict radially outward escape of the vapour of the cryogenic coolant along the surface of the work being welded.

2. Apparatus according to claim 1, wherein the mouth of the first passage has an operating position forward of the outlet of the second passage.

3. Apparatus according to claim 1 or claim 2, wherein, in use, the axis of the nozzle is perpendicular to the face of the work being welded.

4. Apparatus according to any one of the preceding claims, wherein the apparatus is mounted on a suitable carriage.

5. Apparatus according to claim 4, wherein the carriage carries a welding torch so that the nozzle is moved in tandem with the torch, the torch leading.

6. Apparatus according to any one of the preceding claims, wherein the nozzle is adapted to eject the cryogenic coolant at a supersonic velocity.

7. A method of forced cooling of a heated weld zone in work being welded, the method comprising directing at least one jet of cryogenic coolant at the heated weld zone from at least one nozzle, extracting vapour of the cryogenic coolant through a first passage from a vapour space in the vicinity of the heated weld zone, the vapour space being open to the atmosphere, passing -10-an auxiliary gas through a second passage, or arrangement of second passages, surrounding the first passage, and ejecting a flow of the auxiliary gas through an outwardly facing circumferential outlet of the second passage, or arrangement of second passages, so as to have an Aaberg effect and to limit radially outward escape of the vapour of the cryogenic coolant along the surface of the work being welded.

8. A method according to claim 7, wherein the auxiliary gas is air.

9. A method according to claim 7 or claim 8, wherein the mouth of the first passage has an operating position forward of the outlet of the second passage.

10. A method according to any one of claims 7 to 9, wherein the flow rate of the auxiliary gas is 2 to 3 times the rate of vapour extraction.

11. A method according to any one of claims 7 to 10, wherein the axis of the nozzle is perpendicular to the face of the work being welded.

12. A method according to any one of claims 7 to 11, wherein the cryogenic coolant is carbon dioxide.

13. A method according to claim 12, wherein solid carbon dioxide is deposited and collects on the surface of the heated weld zone and takes enthalpy of sublimation from the heated weld zone.

14. A method according to any one of claims 7 to 13, wherein the cryogenic coolant is ejected at a supersonic velocity from the nozzle.

15. A MIG welding process including the step of cooling a heated weld zone by a method according to any one of claims 7 to 14.

16. A TIG welding process including the step of cooling a heated weld zone by a method according to any one of claims 7 to 14.

17. A plasma arc welding process including the step of cooling a heated weld zone by a method according to any one of claims 7 to 14.

18. A friction stir welding process including the step of cooling a heated weld zone by a method according to any one of claims 7 to 14.