alloy 20 tube to tube sheet welding 1

[tickfaw]

Im having cracking problems on the root pass. Im welding 3/4" alloy 20 tubes (.065 wall) to an alloy20 tubesheet (2" thick). We are making a 2 pass strengh weld, saving the tube end. We are maintaining a 250 degree preheat and slow cooling, using ER320 filler on both passes (.045 on the first and 1/16 on the 2nd). we are tig welding using 100% argon. any suggestions would be helpful. Thanks

 

[GRoberts]

Two common problems resulting in root pass cracks that may apply to your situation are, too small of a root pass for the given restraint. This can cause cracking, usually centerline, because the root pass does not have enough strength to resist the forces imposed upon it by the thick base metal. The second one, also centerline cracks has to do with a bead shape that is too narrow for its width. You usually want to get a weld that is twice as wide as it is deep. This type of cracking is common on root passes, but can take place on any pass that has the wrong combination of depth to width. Also, I see you are using ER320 filler. If you are getting a fully austenitic deposit (check on Schaffler diagram or WRC diagram), then you could also try switching to 320LR type filler metal. The LR is for low residuals (P & S) which can segregate and cause hot cracking (again centerline) in the weld.

 

[tickfaw]

I`m sorry I did not give all the imformation required to answer my request. Iam using 320lr wire and using pulse arc welding,that does spread the bead to the width you are refering to. the company we are doing this job for has just changed there specs from a 3 tube mock up to a 12 tube mock up.we have in the past made the 3 tube test.I am now on my fifthn test the last one had one tube that had a crack that was only .015" long . it seems to be at a start or stop. I am at my wits end and open for any sugestions .I am a welding engineer with 30 years experince. thanks tickfawq

 

[mcprofusion]

You are using the recommended filler 320LR.

Start and stops are critical on this material, craters must properly filled; and make sure yor getting enough gas Pre and Post-flow.

Do you see any benefit from using pusle ? why not straight DCEN tig ? Weld heat input must be keep to miniumum, to limit residuals stresses. Also, ACHF is sometimes used for thin material.

 

[amexhts]

Explosivew welding will provide the greatest joint integrety possible with alloy 20 or almost any other tubesheet to tube material combination. It is somewhat restrictive in its' design and use to specialty companies but has no heat treatment requirements preheat.

 

[tickfaw]

Because of the timing of this problem (re the holidays ) I was unable to contact some of the people I would normaly contact with this problem. Since then I have contacted the manufactor that supplied the tubes (welded not seamless),I was told that because of the joint design it could not be welded with alloy 20 but we had to use alloy 625 filler metal. I am still not sold on this but I have fowarded this imformation to my customer and they seem to except it.I do thank all of you for all the suggestions and information Thanks Tickfaw