The big bang

Steel plate with a thin layer of explosion-bonded corrosion resistant alloy or metal provides the corrosion resistance of the more expensive metal in combination with the strength and low cost of steel.

Corrosion resistant alloys, or metals such as titanium and zirconium, are essential components of chemical plants. Cladding the corrosion resistant metal onto steel provides a cheap solution for many equipment applications, particularly large vessels operating at high pressures. For many combinations of metal type, thickness and technical requirements, explosion-clad is the most cost effective and technically superior option.

The explosion bonding or cladding process was commercially developed in the early 1960s by DuPont, and has been used in industrial applications extensively since 1970. Many of the pressure vessels and heat exchangers commissioned in the '60s are still in service today.

The unique nature of the bond formed by the explosion process has special characteristics not achieved by any other technique. The explosive force creates an electron-sharing metallurgical bond between two metal components. Even though the explosion generates considerable heat, there is no time for heat transfer to the components, so the metals' temperature does not increase. The microstructure, mechanical properties and corrosion resistance of the components are all unaltered by the explosion. Therefore, explosion-clad products do not exhibit any of the deleterious metallurgical characteristics typical of metal clads manufactured at high temperature.

In most applications, the explosion-clad bond is stronger than the weaker component of the sheet. Bond zone shear strength is typically in the 75,000psi range for stainless steels and nickel alloys and in the 40,000 psi range for titanium and zirconium. Ultrasonic inspection data demonstrate that explosion-bonded clad has higher than 99.95per cent sound bond reliability.

To make an explosion-bonded sheet, the two plates to be bonded are prepared and positioned parallel to each other, with a small gap between them. A layer of blasting powder with a specific recipe depending on the metals and thickness is then spread over the surface. The explosion is then detonated at one point and travels across the surface, forcing the two plates together.

Because of the way the plates are set up and the detonation point chosen, a high pressure jet accelerates across the plates, removing debris or surface oxidation before the plates meet. The ultraclean surfaces are forced together under pressures of several million pounds per square inch. This forms a bond zone with a characteristic wavy shape, evidence of a strong, ductile bond.

Most metals can be bonded successfully, the only requirement being that they must have adequate fracture toughness to absorb the impact energy. A minimum of 20 ft-lb Charpy V-Notch toughness is needed to ensure the material does not crack during cladding.

Explosion-clad components have been used since the process became commercially available in the 1960s. The clad materials have been mainly used for tubesheets in shell and tube heat exchangers and for pressure vessel construction. Equipment fabricated using explosion-clad components have also been used extensively in separator vessels and in refinery equipment.

Explosion-clad plates can be readily formed into cylinders, heads and cones for pressure vessels. The table below shows the most commonly used cladding and backing metals for pressure vessel fabrication. It is also used for tubesheets in shell and tube heat exchangers., which can be as much as 75 per cent cheaper than solid alloy tubesheets.

There are only a small number of high-energy metal working companies in the world. The largest is Dynamic Materials Corporation (DMC) of the US, who in 1996 acquired the Detaclad division of DuPont. DMC is now expanding its international business and has recently appointed a European agent, A&N Steel.

Alan Dibbo, a former director of the Chartered Institute of Marketing, is currently working with A&N Steel to develop the use of explosion-clad and explosion-formed products

{{Which metal?

Cladding Metals Base Metals300 Stainless Carbon Steel400 Stainless ASTM A516Titanium EN 10028Zirconium JIS G 3103 x 3126Copper Cr-Mo SteelAluminium ASTM A387Copper-Nickel Steel & Alloy ForgingsIncoloy 800 & 825 ASTM A105 & A18Z & A350Inconel 600 & 625}}