Overview

Chemical attack on steel pipes can cause corrosion of the pipe material and lead to pipe failure.  To overcome this, pipelines often use Inconel or stainless steel cladding/lining, i.e. there is Inconel inside and steel e.g. X65 outside.  This type of pipe structure can exhibit an unusual magnetism problem.

The problem:

Remenant magnetism is concentrated in the steel part of the pipe and little if any magnetic flux is in the inconel lining.  The result of this is that the root pass is often completed without serious trouble and it is only when the welding advances to the Inconel/steel interface that the trouble starts.  See document at the Nickel Insitute pages 20-21 for more details.

Fill material:

What happens next depends on the weld metal.  If the filler is a non-ferritic material then magnetism problems will persist right to the capping pass.  With even slightly ferritic filler then as the passes go in, the magnetic problem is progressively reduced as more of the flux is diverted through the filler.

Solutions:

It is easy to produce poor welds, but the time taken to grind out porous material and the danger of inclusions and cracking make weld tacking a poor solution.  Often this will allow a bridge for the magnetic field allowing unhindered welding on subsequent passes.  But with non-ferritic filler the welder is faced with magnetism problems all the way through the joint and this make-do solution is completely invalid.

Zeromag supplies the complete solution, either by balancing the magnetic field to zero or by joint degaussing using Zeromag and the ZM150. Using this equipment it does not matter what the weld metal is – the field is forced to zero regardless.

In practice:

Pipe laying, whether on land or off a lay barge, is an expensive process and the cost of the pipeline will soar if the process is stalled because of magnetism. The best insurance for this scenario is afforded by having a Demagnetizer’s Kit for Pipe Welding (DKPW) available for immediate deployment.