Clad pipes

This is a QA session answers from our senior member Mr MRC Nagarajan, that I thought worth recording from group posting.

1. What are the different technologies usually use for producing inconel clad pipes?
2. Which cladding process is most preferred and how the particular cladding process is selected?
3. From the design experience for inconel clad pipelines, any
4. particular problem encountered?
5. Finally, what is metallurgically bonded clad pipe?

A clad material, in particular flat products are produced by different methods.

Roll Cladding: This is formed by two sandwiched combination of carbon steel slabs on the top side and two corrosion resistant metal plates in the center. The CRA layer will have will have to be thick enough to result in about 3 mm layer when the backing material is drawn to the required thickness. This sandwich is welded and sealed and the heated combination is heated and rolled to the require thickness. Luckily the reduction in hot rolling is same both for carbon steel and CRA, with the result when the plates are reduced to the desired thickness the clad plates are finished. In this roll bonding process there is no metallurgical bond, except in case some buffer layer is deposited on the carbon steel plate for special effects, when there would be better continuity. There would be areas of ‘ dis-bonding patches here and there. That is the reason the ultrasonic testing is an integral part of manufacture of clad plates. Such bare areas, if they are large and the end service is hazardous, such areas are gouged out and weld repaired.

Kawasaki came up with a special process. They placed a twin slabs of CRA in the centre of mould and cast a slab of carbon steel and CRA embedded inside. This blob is rolled as above to the required dimensions. This material would have better bonding than plain rolled clad product as in case 1, above.

The third method, which came into existence after the second world war, employs a carbon steel base plate placed on the ground, and the CRA layer held above with some calculated ‘ stand off’. On the top of the CRA plate. a layer of pre-selected explosive is evenly spread. A detonator is placed at one end and explosion effected from safe distance. The trick is in the top flyer plate reaching the needed critical velocity so that the bond is developed. At the junction of flier plate and base carbon plate a high speed jet forms, producing a ripple effect on the carbon steel plate. This ripple and the jet mixes locks up and produces a ‘ stitching effect’. This is an interesting and very strong ‘ mechanical hold’. The resulting clad plate is the best the man has produced till now. The process now is fully understood and computer program would give all the critical parameters like the ‘ stand off distance, the type and thickness of explosive layer required, such that the selected material reaches the desired critical velocity for cladding. With this method it has been possible to have any combination of backing and cladding material. Roll bonding can not be adopted for materials which have distant melting temperatures and/or widely different malleability, like steel and aluminum ( used in Aluminum cell anodes), copper and aluminum (used in the bimetal strips). More useful combination of Ti-clad steel has been possible, which finds use in Urea reactors.

When the clad plate is rolled into tubular and longitudinally welded, it becomes a pipe. Such pipes are further mandrel drawn or cold extruded to different sizes. Of course there is a limit for such reduction, which can be achieved to a limited extent.

It has also been possible to form explosion bonded seamless tubular which are further reduced to pipes. The basis of formation is similar to what has been explained for flat product, but the explosives used and placed in the backing tubular is much different. Hence this process is secretly guarded by the few who have developed this process.

Of course, there is the CRA sleeve placement and welding which is in still in vogue as in nozzles of pressure vessels, is purely for low pressures and for corrosion resistance only.

There is also the process of weld overlay, which, of course, is slow, but for small requirements is an ideal process. The deposited layer is bit higher than the final required thickness and the formed tubular is bored to the required dimension. This is the only process which can be called a process giving metallurgical bonding. In this there is a continuum between the parent material and the CRA and between the two there is a mixed layer which has 60% weld material + 40% parent material, does not resemble both in any properties. This is the reason that CRA welding consumable is selected to be of better material than the CRA layer. Just go back and study the overlay qualification in ASME section IX and try to understand why they call for chemical composition to be equal or better than the clad layer at a depth of 2.5 mm from surface. This metallurgical bond would lead to migration of elements like carbon into low carbon CRA layer. To limit this migration a buffer layer is sometimes placed before laying the CRA layer. Similarly even for better bond in Explosive forming a buffer layer is sometimes added. There has been additional explosive bonded layer over the two is special circumstances.

I hope you are aware that no code permits use of CRA layer as material thickness to withstand the pressure/ temperature of service. This layer is excluded in wall thickness calculations.

The welding of such clad steels is itself a special subject. This aspect is not touched here.