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News Release from: Arc Energy Resources | Subject: Weld overlay cladding
Edited by the Engineeringtalk Editorial
Team on 03 March 2005
Welding corrosion resistant alloys
Alan Brown of weld overlay cladding and fabrication specialist Arc Energy Resources explains what to expect when choosing a fabricator to weave corrosion resistant alloys.
A first-time specifier of fabrications or protective weld overlay cladding using corrosion resistant alloys could be forgiven for thinking of it as a black art - a modern day version of the alchemist's attempts to weave gold from flax In this article Alan Brown of weld overlay cladding and fabrication specialist Arc Energy Resources explains what to expect when choosing a fabricator to weave corrosion resistant alloys
This article was originally published on Engineeringtalk on 29 Mar 2004 at 8.00am (UK)
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When selecting a grade of conventional carbon or low alloy steel for weld overlay cladding or a fabrication, mechanical properties such as strength, ductility or optimum working temperature, are the controlling factors.
These properties are produced principally by heat treatment during production rather than by chemical composition and the welding process and consumables are chosen to ensure that the properties of the steel are not compromised during welding.
Corrosion resistant alloys (CRAs) however, are chosen on an entirely different basis.
The numerous grades of stainless steel and nickel based highly alloyed materials have different levels of resistance to different media.
Knowing the properties of your corrosive medium and choosing the right alloy to combat it is, although not exactly a black art, certainly a matter of considerable experience.
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Selecting a fabricator who understands these implications and is conversant with the materials is equally important.
For those involved with specifying plant and equipment produced from or clad with corrosion resistant alloys, a fundamental understanding of the properties of the materials and the factors affecting their fabrication is therefore, desirable if not essential.
Let's look first at the materials.
A few years ago most were considered difficult to fabricate but now, advances in production techniques means this is no longer the case.
However, there are basic rules that should be followed.
The chemistry of corrosion resistant alloys is the overriding factor in their ability to resist corrosion, so it follows that consumables used to join them must display the same ability.
The general rule is to weld CRAs with a "matching" consumable - 316L is welded with 316L, 70/30 copper nickel with 70/30 copper nickel and so on.
In fact, consumables are normally slightly overalloyed to allow for small losses caused by remelting during welding.
As with all rules, there are exceptions for specific materials and specific environments, but experienced materials engineers and reputable fabricators and consumable suppliers will be able to advise on these.
Having decided on the material specification the next stage is to examine the welding environment.
It is vital that CRAs are fabricated in a segregated area with no possible contamination by carbon steels.
Workbenches and associated tools (even the humble wire brush) should be made from stainless steel.
Cleanliness is essential in any welding operation, but none more so than the welding of CRAs.
Materials produced with a finely balanced mix of elements don't take kindly to oil, grease or metal oxides being added.
Areas to be welded, and adjacent areas, must be thoroughly cleaned with a residue free chemical agent.
If grinding wheels are used they must be specially produced for CRAs and must never be used on carbon steels.
Failure to adhere to these guidelines can lead to (and has been implicated in) weld failures in service.
Because the introduction of atmospheric gases into the weld will also have a significant effect on its performance, the shrouding gas supplies and welding torches should be checked regularly for leaks.
Moisture in the coating of MMA electrodes can add hydrogen and oxygen to the weld, which will also adversely affect quality.
Therefore, if coated electrodes are used the manufacturers' guidelines should be followed, particularly in terms of storage and baking (if applicable).
Corrosion resistant alloys in general, and stainless steels in particular, have poor thermal conductivity, which causes heat to build up quickly in the weld area.
Fabricators with a good working knowledge of materials should, as a matter of course, recommend the use of welding techniques that reduce heat input and spread it evenly over the joint area.
However, heat buildup can cause some distortion when using thin materials and we recommend that tacks are made at much more regular intervals than would be the case with carbon steel.
All residues from the welding process, including slag, scale or discolouration must be removed.
Chemical cleaning such as pickling and passivation is ideal, but does require the safe disposal of the toxic agents after use.
Alternatively, residues can be removed by thorough mechanical cleaning with appropriate equipment although it should be stressed that wire brushing is not effective.
I hope these notes have convinced you that welding corrosion resistant alloys is not a black art, it's a skill.
And like most skills it requires a combination of knowledge and experience.
So before you specify fabrications or weld overlay cladding using corrosion resistant alloys, talk to people who know the materials and have experience in the techniques.
You should also expect the cost of fabrication with CRAs to be higher than with carbon steels.
The consumables are more expensive and the care required when welding means additional production time.
Consider the environment for which the equipment is intended and the service that's expected of it - and don't be tempted to take a commercial decision when a sound technical one is what's required.
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