repair welding

Date 01-18-2008 07:58 Edited 01-18-2008 08:21

Hello Tracy, it is very likely that the saltwater use that you spoke of will have had an effect on the contamination that you are seeing in the form of the ooze and surface conditions to some degree. Cavitation and possibly some electrolytic action could be responsible for some of the additional conditions and surface imperfections. If you have access to a hot-water industrial wash system such as that used by certain automotive operations you may want to consider using this to help in the cleaning of the item prior to attempting repairs. The hot water wash system would likely open up and promote bleeding off trapped oils, salts, and other contaminants that might be present at the surface or in the microscopic pores and other surface discontinuities. As far as the welding end of this project is concerned, size and scope of repairs could possibly determine for you whether you should consider using GTAW, GMAW, or possibly even FCAW gas-shielded stainless steel wire to make the necessary repair and build-up. IMHO if the item isn't overly large and the build-ups aren't overly extensive I would opt for the GTAW method. Otherwise if the necessary repairs and such are fairly extensive the wire approach might provide a better choice. In any event also consider the operating conditions of this impeller. If speed is an issue then balancing the impeller upon completion will need to be done. To promote this particular requirement, try to come up with a pattern of some sort that you can use to check on the symmetry and consistency of the build-ups that you are applying to the various impeller blades. If possible, avoid over-welding of the various portions of the casting as this may further impede the ease of re-balancing. Consider that welding of single sides only of the various blades could cause bending and deflection of these surfaces and affect the performance of the finished part and require additional attention and possible correction to make the impeller perform properly. If single side welding is required you may have to utilize some experimentation with heat sinks and possible back-bowing of the blades to allow for contraction and shrinkage forces induced from the welding process. Just a few thoughts for your consideration. I imagine others will have terrific suggestions for you as well. Best regards, Allan

By alumtig (**)

Date 01-18-2008 15:02

Good morning Allan, and thank you for your response. I have enclosed a pic in my previous post as I didn't completely understand where or how to add the link. I believe that I have it now.LOL As you can see the impeller is of considerable size and the most likely weld repair would be wire, we have rebuilt many impellers for multiple applications and generally have good success with the repairs. We actually repaired a sister pump to this unit last year and there was a failure on that impeller after our repair. The welding was done with a flux cored process-316L using a CO2 shielding gas. A failure anaylisis was done and the conclusion was the failure mechanism was intergranular and interdendritic stress corrosion cracking. The report states that the material was severly sensitized. My welding procedure holds the interepass temp. to 300 deg. This unit had a previous repair by others that I cannot vouch for the weld procedure or interpass temp. The report also stated that the carbon dioxide shielding gas is not recommended to use for welding austenitic stainless steels because of a high potential for carburizing the stainless steel,thereby increasing the sensitization effect in the weld and heat affected zones. I tend to dissagree with this however I have qualified a procedure using a 75/25 argon/co2 shielding gas. I would like to weld this impeller successfully to a reliable operational condition. If there are any magic formulas or anyone with experience in this area I would love to hear from them. My boss has ask me to research and locate a welding engineer that might be able to add some input into this scenario. He believes that the investment would be worth it to maintain our credibility as a repair facility as well as satisfy our customer.

By GRoberts (***)

Date 01-18-2008 17:53

Stress corrosion cracking on welded 316L in seawater (especialy warm Florida seawater) is expected. It is generally not a good choice of materials for seawater service. It can do marginally ok if the water is always moving, such as in propellers. If the pump is not continually on, it will experience more problems with pitting and SCC than if it is run all the time to eliminate stagnation. Was the earlier weld repair performed at the foundry, or in a later operation? If it was at the foundry, there is a good chance that the weld could have been solution annealed, thus reducing the residual stress from welding and ehnahcing the stress corrosion cracking resistance of the welds. If you want better SCC resistance with CF3/CF8/316L, the best way to reduce the residual stress is to solution anneal, but it will ruin all the machined tolerances. However, there will always be a chance of SCC because of the material selection. High ferrite content in the weld and casting can only help with the SCC resistance as well. You can't control the casting, but you can probably procure 316L filler that has higher ferrite than normal.

As far as sensitization, was it in the weld or HAZ? If the impeller is CF8, you might expect some in the HAZ, but if it was CF3, senstitization can be avoided. Also, with FCAW wire, the slag covers the metal being transfered in the arc, so carbon pickup is not an issue, and CO2 is commonly used without any problem. If your customer doubts this, get a chemical anyalysis from your FCAW wire manufacturer that was welded with CO2 gas and show them that the carbon will be low. If the wire is metal cored or solid as in GMAW, then the report is correct and CO2 is detremental to the weld. In addition to your maximum interpass temperature, you want to keep your welding heat input low. Don't go above 50 kJ/inch for sure. In the thinner areas you will probably want to keep it down even a little lower.

As far as your repair sequence, now that the part is sand blasted, do a liquid penetrant tests and see where all the pitting is, as 316L is well known for pitting in seawater, and it appears from you picture that you have some. You need to eliminate that so that it will not start immediately again once it is put back in the water. Once the impeller is PT clear, you should have eliminated all the spots holding contamination (your oozing).

It is too bad you are in Florida, or I would be glad to come by and help you out, as I am a welding engineer and used to work in a foundry repairing CF3/CF8 on a daily basis.