Has anyone else had problems welding on a saltwater exposed 316 SS casting? Generally 300 series are easy to work with, but we I am seeing extensive porosity and general poor weld quality. I would appreciate anyone's explantion and suggestions for what could be done to combat the problem. I am using 316L SMAW.
Austenitic stainless steels of the 300 series are not suitable for use in a salty medium. Salty medium, in this case, means a medium containing sodium chloride (table salt), as saltwater does. Sodium chloride produces on austenitic stainless steels a type of corrosion called "stress corrosion" which is particularly severe when the ss is subjected to stress, like in a piping, pressure vessel or equipment.
You should ask yourself the following questions:
Is a 300 type ss really necessary? If so, why?
Couldn't it be replaced by carbon steel protected against corrosion by a suitable paint or lining (rubber, plastic, ceramic)?
If not, couldn't a ferritic stainless steel (400 series) be used? Ferritic ss's aren't prone to stress corrosion.
If not, is there any other type of material which could be used (graphite, for example)?
After having answered these questions, and other that may show up, your good engineering judgement will suggest the proper solution.
Giovanni S. Crisi
Sao Paulo - Brazil
Castings do have porosity after fabrication. So during exposion the seawater can enter into these porosities. When you're welding, you will suck all entered dirt/salt/water/... into your weld pool. These components will decompose and vaporise, resukting in weld metal porosity.
Try a basic electrode E-307 ; the basic slag will clean up your weld pool and a 307 type is very resistant to cracks. It is a very good "repair" electrode. But you will have to live with a small amount of porosities, due to the casting.
Your problem could be comming from:
1) A "dirty" casting - Often castings are full of contaminants that make welding a big problem. To reduce these problems, use a filler that will give a high ferrite content in the weld. (e.g. E312)
2) A material that has been degraded (e.g. stress corrosion cracked) due to its service environment. - In this case, I would discard the component and replace with a new one. Duplex S/Steels are good for chloride containing environments. (To check if the material is cracked, perform a dye penetrant test, but ensure a very long penetration time because these cracks can be very small.)
3) Or your electrodes are not dry. If the welds are looking OK at some places, but suddenly have these large pores in, then moisture is a probable problem. - Bake your electrodes and try again.
Hope this helps
Regards
Niekie Jooste
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