weld failure in monel and inconel

I have a few querys about welding 3/8 monel and inconel
In recent events I have been seeing welds from contractors
splitting down the seams of our cones and vapor lines. I also am seeing failure at what seems to be the heat affected
weld area. These materials are holding raw brine for a salt evaporation process and was wondering if buttering the root
joint would help or what about pre and post heat treatment.
I should mention that mig and stick welding are the foremost
process used , but what about tig welding? This is a highly
corrosive enviroment and it seems to me that a number of things need to change here. I would appreciate your opinions.

 

[Seldom]

Hi gozzi,
There are a few questions I need to ask here. By splitting, do you mean a crack-like indication that develops and propagates to joint failure? Or are you seeing a severe corrosive degradation more or less in the center to the weld and traveling the length of the joint?

Also you mention “I also am seeing failure at what seems to be the heat affected
weld area.” What specific condition are you seeing that is attributed to the HAZ failure? Are there crack-like indications or corrosive deterioration?

Which welding process seems more prone to be involved with failure and what were the indications? Also, which of the Inconels are you using?

The answers to the questions I’ve asked will certainly aid in getting you help in finding a more specific solution to your problem.

Speaking off hand, there are a few general insights I’ve gained from past experiences with those materials (non specific Inconels) that may offer help toward a solution.

1. If the proper base material (active galvanic) is being used and the proper filler material (noble galvanic) as well, look to interpass control as the weak link. Very often I have found the lack of interpass temperature control, either during the initial fabrication process or during repair/modification scenarios to be a leading cause of in-service weld failure in nickel base alloys due to carbide precipitation (Cr depletion). This can occur in the form of intergranular corrosion of the weld specifically or SCC (stress corrosion cracking) in the HAZ.

2. If any slag (SMAW) or overlap (GMAW) that’s left along the edge of the cap pass or root bead will provide an excellent environment for crevice corrosion to initiate and because of the size of the occluded space, will accelerate the attack.

3. Avoid single sided V-groove welds, opting for double-V grooves where possible and avoid excessive weaving by increasing travel speed. (Less filler metal applied either totally or at any one time, the less heat applied and allowed to build up)

 

Thanks for your thoughts, they seem to hit the mark. I have found that the base materials are monel 400 and inconel 625.
The cracks do indead travel the length of the seams and appear at the toes of the weld. In addition to the weld failures, severe wearing occurs the base material such as
pitting and thining of the interior walls.It seems that stick welding is the main process used because of the on site conditions(leaking salt,water,rust). I have also mentioned hastelloy, but it seems to make management cringe, any other suggestions?

 

[Seldom]

Hi gozzi,
Lord I hated to weld in our old Evaporator buildings! Gloves, clothing, and tools became soaked with brine and everything you touched was slimy. The very worst aspect was you were continually getting shocks when you were welding! Jeez, I hated that!

Concerning the weld related problems, I still stand on my original assumption (first reply and not seeing it firsthand) that there has been little thought to interpass control either during initial shop fabrication or definitely during field applications. It should be in the neighborhood of between 200-300 degrees max.

In addition to and before interpass temperatures are addressed, there needs to be careful consideration given to joint and adjacent base metal Cl contamination. Cleanliness is next to godliness when welding nickel base alloys! This is a major concern especially when ANY heat (plasma cutting, gouging, grinding, welding, etc.) is applied to these alloys once they’ve been in a Cl service. I always recommend using an acetone wash/swab anywhere in and adjacent the work area before and after any hot work be initiated.

As far as the pitting and uniform corrosion, both can be controlled or accounted for by using the proper alloy, designing for good drainage and eliminating hotspots, and even giving consideration to coating. I’d check with the Material Engineering Forum on this site for better answers then I’m experienced enough to offer.