Production of Large Diameter Clad Bends using Explosion Welded Alloy 625 Clad Plates

Abstract

The goal of this development was to manufacture an induction bent pipe from explosion clad plate to qualify the explosion welding process for induction bent pipe applications and to investigate the economic, corrosion and mechanical advantages of explosion welded material for an induction bending application.

The clad plate was welded using the explosive welding process. The backing carbon steel plate was 19 mm (3/4”) thick API 5L-X70M-PSL2 and was prepared by grinding the surface to remove any mill scale and rust. The corrosion resistant alloy (CRA) plate was 4.8 mm thick Alloy 625 and was prepared in two steps. The first step was to seam weld smaller plates to achieve the full width and length needed for the pipe. The second step was to surface grind the mating surfaces prior to assembly. The plates were assembled with a calculated gap between the base plate and CRA plate. The explosives were blended to match the metal properties and loaded on top of the CRA plate. The explosives were then ignited to bond the plates together. The welded plate was then flattened and trimmed to final size and delivered to the pipe manufacturer.

Subsequently the plate was formed by press bending using the JCO-process without any prior heat treatment. After forming, the pipe was submerged arc welded from inside and outside following standard DSAW pipe production. In addition, the inner submerged arc weld seam was clad with Alloy 625 by utilizing the resistant electro-slag (RES) welding process using special strip cladding nozzles. The mother pipe of OD 508 mm (20”) x WT 19+5 mm (3/4”+1/5”) x L 11,278 m (37’) was finished calibrated to out-of-roundness below 2 mm (< 0.4%) over the entire length using external compression.

The bends were made by the hot induction bending process.  With a pipe bending machine, a narrow section of the pipe is heated by using the electric induction process and subsequently bent while controlling the radius, heating temperature, cooling rate and bending speed. Only the bent area is exposed to the induction coil.

Each bend received a specific heat treatment to restore the mechanical properties (Carbon). The bends fully comply with the latest edition of ISO 15590-1, DNV-OS F101 and ASME B16.49 specification for hot induction bends.

In this study, mechanical testing, metallographic investigations and corrosion tests show that the discussed manufacturing route provides a reliable and economic way to produce clad bends.  This can be accomplished without any risk of disbonding or cracking of the CRA layer while achieving good mechanical properties of the backing steel.

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