There are many applications in industry where there is a need to have pressure vessels of various types made from rare metals such as titanium, zirconium and tantalum to contain various chemicals. Since rare metals such as titanium (Ti) are very expensive and only the internal contact surfaces of the vessels need to be of such a material, a preferred approach is to use explosive-clad welded (EXW) plate or roll-clad plate. EXW plate & roll-clad plate comprises a titanium plate forced onto a base material e.g. carbon steel, resulting in a sufficiently strong bond to enable the clad plate to be shaped in the manner of a single plate.
The use of EXW or roll-clad plate is especially indicated where, if a titanium-only construction was employed, the titanium wall thickness would have to be greater than say 10 mm. In such a situation it is usually a far more economical solution to use clad plate consisting of a thin, e.g. 2 mm to 16 mm thick titanium plate as the cladding material.
For the purposes of explanation the present invention will be described with reference to clad plate comprising a layer of titanium bonded to a thicker steel layer. It will be realised that the invention encompasses methods for joining clad plate, and apparatus formed by such methods, wherein the clad plate is comprised of other combinations of metals than titanium and carbon steel.
In order to fabricate a vessel of desired shape and size from clad plate a number of pre-shaped pieces of clad plate must be joined. Thus there are various seams which need to be welded in order to join all the individual plates together. Whilst welding the base material, e.g. carbon steel, is usually straightforward, in order to do so it is necessary to firstly remove a margin of the titanium covering along the opposing edges of each of the clad metal pieces that are to be joined. A problem then arises in that subsequent to welding the base metal the titanium layer must then be repaired in order to cover the welded base metal. This problem is compounded because, due to metallurgical reasons, titanium, for example cannot be successfully welded to the base metal but rather must be welded to other titanium.
A prior art approach to joining clad metal pieces will now be described with reference to FIG. 1.
FIG. 1 is a cross section through a typical prior art join 26 between a first piece 2 and a second piece 4 of clad plate in the form of titanium clad steel. The join makes use of a batten strap technique. Initially titanium cladding is removed from the area around all edges where steel welds 6 and 8 are to be made, typically 12 mm inward from the steel weld preparation edge. The steel base metal pieces 10 and 12 are then prepared and welds 6 and 8 applied using conventional steel fabrication procedures. The join is then cleaned-up and prepared for titanium welding. In the conventional batten strap technique, a filler-metal strip 7 is inserted into the space where the titanium has been removed. The choice of filler is dependent upon fabrication preferences; commonly used materials include copper, steel, aluminium and titanium. A wider strip of titanium 16, comprising the batten strap, is then placed over the weld area. The batten strap is welded along its edges with fillet welds 18 and 20 to adjacent titanium cladding portions 11 and 13 respectively.
FIG. 2 depicts a reaction crucible formed by joining a number of pieces of clad plate together using the batten strap technique described in relation to FIG. 1. It will be noted that the batten strap joins 26A-26D protrude into the interior of the crucible. Often agitators are used to swirl the contents of vessels made from clad plate in order to enable mixing of the contents of the vessel and to encourage reaction. The contents of the vessels are often erosive to metals. A problem has been found to occur in that the batten strap joins 26A-26D have been found to be susceptible to erosion. The batten strips therefore become the first point of failure in the titanium cladding which then exposes the base material to corrosion and pressure vessel failure if undetected.
Titanium has a substantially lower coefficient of thermal expansion than the base material, which is usually of steel. As a result, since the contents of the vessel are often at high temperatures, differential expansion and contraction of the titanium and base material can occur which lowers the fatigue life of a titanium envelope of the vessel.
In light of the above, it is an object of the present invention to provide a method for joining clad metals which can be used to fabricate vessels and which addresses one or more of the above-described problems.
It is a further object of the present invention to provide a clad plate vessel that is not as susceptible to the above-described problems as has hitherto been the case. The clad plate vessel may include a titanium envelope.