GB2233923A - Repairing defective metal workpiece - Google Patents

Abstract

A method of repairing a defective metal workpiece such as metal plate comprises removing metal from the defective region of the workpiece and friction welding a compatible metal plug 4 in place. The plug 4 or the bore may be tapered to allow the weld to develop progressively through the workpiece. <IMAGE>

Description

REPAIRING DEFECTIVE METAL WORKPIECE The invention relates to methods for repairing a workpiece which is defective in the sense that it requires an inherent defect, a weld, corrosion, or other damage to be repaired.

Certain types of welds, particularly arc welds, can be deemed unsatisfactory for a variety of reasons, such as the occurrence of porosity, undercut or cracks.

Equally, defects in the vicinity of arc welds can develop with service operation, particularly as a result of fatigue stressing which leads to extensive cracking.

In all these cases the conventional practice is to cut out the defective material and to re-weld so as to fill up the missing material. Well-known arc welding techniques are normally utilised, such as manual metal arc welding, tungsten inert gas welding with filler wire, and metal inert gas welding.

There are many situations where the arc welding approach is undesirable or untenable, such as in the presence of hazardous or explosive atmospheres.

Equally, arc welding is not possible in rarified atmospheres such as in the upper atmosphere or in the vacuum of outer space. Conversely, arc welding is difficult to carry out satisfactorily underwater, especially at depths of 50m and greater, such as in excess of 200m, where the ambient pressure affects the arc behaviour even with a hyperbaric chamber. Moreover, providing pressure vessels to enable arc welding to be carried out at near atmospheric pressure is inconvenient and costly.

In accordance with the present invention, a method of repairing a defective metal workpiece comprises removing metal from the defective region of the workpiece; and friction welding a compatible metal plug in place.

We have developed a technique which can be safely applied in situations where the arc welding approach is unusable but which leads to a secure repair.

As mentioned above, the defective nature of the workpiece may be due to a defect, corrosion, or a defective weld and the like.

Typically, the step of removing metal comprises machining or drilling and this will typically leave a bore into which the plug is welded.

In some cases, a bore which is open at each end can be formed into which the plug is bonded. However, in many cases, the step of removing metal may generate a blind bore having a closed end which is ruptured when the plug is bonded to the workpiece. In this case, the friction welding technique will comprise rotating the plug relative to the workpiece at a suitable speed while urging the two together under pressure and as the plug engages the thin, end wall of the blind bore, this will be ruptured before the plug is finally bonded to the workpiece. This has particular advantage where the workpiece forms a barrier the remote side of which is exposed to a hazardous atmosphere, for example water, explosive gases, or a vacuum since the integrity of the barrier can be maintained up until the final stage of the bonding process.

Preferably, one or both of the plug and bore are tapered and in the latter case are preferably tapered relatively to one another. This is important to allow the weld to develop progressively through the thickness of the plate. The necessary action and reaction forces can be generated by the relative tapers. The relative difference in tapers of the plug and bore is preferably not less than 10 so as to avoid the plug tending to pass through the bore, especially with bores of less than 150 total included angle. The relative difference is preferably 2-40C.

In addition, or alternatively, the plug and/or the bore may be formed of sections having more than one diameter.

In a further modification, the leading end of the plug may include a counterbore to improve the charcteristics of the friction weld and material displacement on the far side.

In general, when repairing plate, the diameter of the plug is at its maximum not less than 2/3 that of the plate thickness, and not greater than 1d x plate thickness. Preferably the taper on the plug does not exceed 300 total included angle so as to avoid the necessity for high axial thrust.

In an important extension of the method, relatively long defective regions can be repaired by repeating the method in a series of adjacent and/or overlapping sub-regions to produce a seam.

Some examples of methods in accordance with the invention will now be described with reference to the accompanying drawings, in which: Figures la-ld illustrate different combinations of plugs and bores; Figure 2 is a macrosection showing a plug welded in place; and, Figure 3 illustrates graphically some fatigue test results.

Figures la-lb illustrate two typical configurations in which a defective region 1 in a plate 2 is drilled out to form a tapered bore 3 open at each end.

Subsequently, a tapered plug 4 mounted to a spline drive 5 of a friction welding machine (not shown) is rotated and inserted, while rotating, into the bore 3 such that it tends to bear on the taper of the remote side of the plate to be plugged. As frictioning proceeds, material is plastically extruded and the friction weld completed, preferably with some material being plastically extruded both on the remote and near sides of the tapered hole.

There are many variations in the form of the bore 3 and the plug 4 some of which are shown in Figures la-ld.

Typically, the total included angle on the tapered plug 4 is some 2-4 less than that of the prepared bore.

Thus in Figure la the total included angle defined by the taper of the plug 4 is 80 while that of the bore of 100.

In Figure lb, the plug taper is 180 and the bore taper is 200. Preferably, the minimum diameter of the tapered plug 4 corresponds to the minimum diameter of the bore 3 (Figure la), so that the contact first develops in the vicinity of the remote side of the plate being plugged.

However the minimum plug diamater could be slightly larger than the minimum bore diameter (Figure 1B).

A parallel-sided plug can also be used with respect to a suitably tapered aperture in the plate 2 (Figure Ic), although a tapered shoulder 6 is preferred to ensure complete filling of the tapered bore 3.

A special form of the plug 4 is shown in Figure ld in which a counterbore 8 is provided in the leading face of the plug. This counterbore or recess 8 allows the tip of the plug to collapse more readily in the early stages of making the plug weld before equilibrium temperatures are established. This is particularly suitable for thick plate, for example in excess of 25mm thickness. It should also be noted in Figure ld that the bore 3 is blind and has a thin end wall 7 having a thickness of 2-3mm which will be ruptured towards the end of the welding process. The reasons for this have been outlined above.

Figure 2 illustrates a macrosection of a finished weld formed using a plug of the type shown in Figure Id and a bore 3 of the form shown in Figure Id but without the end wall 7. In this case, the plate 2 is 30mm thick SOD type steel.

Although the method has been exemplified with respect to steel plate, in principle the technique is applicable to all materials which are commonly friction welded, including stainless steel, and other alloys such as aluminium, nickel, and copper alloys.

In some situations, particularly where corrosion is not a specific problem to be avoided, the plug may be of a dissimilar material to the parent plate. Thus a mild steel plug can be utilised in an alloy steel plate without significantly reducing its overall strength characteristics. This is due to the compressive stresses induced by the friction welding process. Also, the fatigue performance is satisfactory even when the plug is not machined flush with the plate surface after welding is complete. An example of the fatigue performance obtained is shown in Figure 3 for a steel plug and plate 30mm thick.

The method can also be applied to repairing a titanium plate using a titanium plug. In addition, bimetal plugs can be used. For example a steel plug with a stainless steel tip which has been found to give better support to the joint.

For friction welding the preferred procedure is initially to bring the rotating plug and work plate into contact under a light load and then to continue to advance the plug into the prepared tapered hole at a controlled rate during which time heat is built up at the interface between the plug and plate. After a sufficient forward movement the rotation of the plug is arrested while the applied force may be increased to consolidate the weld. In general, the forging load is not greatly in excess of the frictioning load, for example 200kN compared with 150kN for a mild steel plug such as illustrated in Figure la. Alternatively the forge load may be substantially equal to the friction load in operation.

For operating underwater it is preferable to shield the immediate vicinity of the friction plug weld from direct contact with the water. Suitable shielding media include polyurethane or foamed plastics. In principle a wide range of materials can be utilised to prevent direct contact between the water and the heated zone of the plug friction weld, which material does not impede significantly the flow of metal from the friction weld zone. The shape of the plug and tapered hole is adapted according to the material being joined, although conforming to the general principles outlined with respect to steel plug welds as described above.

Claims (12)

1. A method of repairing a defective metal workpiece, the method comprising removing metal from the defective region of the workpiece; and friction welding a compatible metal plug in place.

2. A method according to claim 1, wherein the step of removing metal comprises machining or drilling the workpiece to produce a bore into which the plug is welded.

3. A method according to claim 2, wherein the bore comprises a blind bore having a closed end which is ruptured when the plug is bonded to the workpiece.

4. A method according to claim 2 or claim 3, wherein one or both of the plug and bore are tapered.

5. A method according to claim 4, wherein both the plug and the bore are tapered with different taper angles.

6. A method according to claim 5, wherein the difference between the tapers is not less than 10.

7. A method according to claim 6, wherein the difference between the tapers lies in the range 2-4 .

8. A method according to any of the preceding claims, wherein the plug has sections with different diameters.

9. A method according to any of the preceding claims, wherein the leading end of the plug includes a counterbore.

10. A method according to any of the preceding claims, wherein the plug material differs from the workpiece material.

11. A method according to any of the preceding claims, wherein the plug has a body of one material and a tip of a different material.

12. A method of repairing a defective metal workpiece substantially as hereinbefore described with reference to any of the examples shown in the accompanying drawings.