Hello CK;
Are you experiencing vibration during welding or is the system subject to vibration once placed into service?
A weld that is subject to vibration can experience two different phenomena. Depending on the frequency and amplitude, the weld can crack as it passes through the mushy state during solidification. The weld is neither completely liquid, nor is it completely solid at this stage of solidification. Solidification initiates along the existing grain boundaries of the base metal in the HAZ. As the weld solidification proceeds toward the center-line of the weld it can reach a point where the center portion of the weld is mushy and it has very little strength. Vibration with high amplitude can cause cracking to occur.
Some studies suggest that if the vibration and amplitude is "just right" the vibrations can relax the stresses that occur during solidification when the phase change from liquid to solid causes the weld to shrink. The shrinkage is different from contraction. Shrinkage is due to the phase change, contraction is due to the cooling of the solidified weld from high temperature to ambient temperature.
If the system is subject to vibration during service, the welds, the HAZ, and the base metal will be subject to fatigue cracking as our good friend Giovanni suggests.
Best regards - Al
Al,
CK Welding says that the vibration happens when the water injection pumps are running. So, in my opinion, vibration occurs when the equipment is put in service.
CK Welding,
Check with a flexible joint manufacturer in your country (USA). In the meantime, I'll check into my catalogs and with local (Brazilian) manufacturers.
Giovanni S. Crisi
This is an extention to a manifold in wich is shut in at the time of the weld so the vibration is lower but, there are other pumps running that are connected to the manifold but the valves are shut so there isn't fluid running though it on the end where the weld is preformed. There is a vibration on the pipe during the weld process, and even more vibration after the process. Thanks for your responses. P.S "So far the weld has held for about a week."
Hello CK;
After responding to your inquiry this morning I happened upon Bonal Technologies' booth here at FabTech. They have a web site that might provide some additional insight on how the weld puddle responds to vibration while solidifying. I haven't checked it out yet, I just got back to the hotel. I must have walked twenty miles today. OK, so that might be a slight exaggeration, it was only nineteen miles!
Here's Bonal's website: www.bonal.com
Giovanni, it sounds like we have a combination question that involves both of our assumptions. Good to hear from you!
Best regards - Al
Hi CK , We sometimes have the same problem with vibration from water pumps around here. Some things I've found to help out are : going to heavy wall pipe ( schedule 160 or XXH ), and using E- 7018 for all passes after the bead. Also having the spools stress relieved in an oven, but this is sometimes not readily available and of course it costs money. Installing extra pipe supports also helps in my opinion as they can reduce vibration. A lot of the pumps here also have had a pulsation dampner bottle on top of a tee bolted to the discharge head of the pump. I would try to avoid welding anything under vibration if at all possible. I spent a few days of my honeymoon in wyoming - It was beutiful there. - WS
In a wohler diagram you are able to see the "fatigue limit" below that limit the material will not crack, regardless of number of cycles.
Materials such as Aluminum and its alloys has not limit.
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