Problem on welding St-52

By Charlie Date 04-20-2001 21:24

Hi everybody! I am new user of this forum and I dont know the rules of board ,but anyway I suppose here is proper location to discuss about welding.
I have encountered a problem in st-52 welding and I hope find good direction to solve it.
I have welded a st-52 25mm thk. pipe to st-44 40mm thk. plate, weld is multipass fillet and fillerwire is Er70s-6 (SG-2) 1.2 mm thk. in mig process,shielding gas is Argon82-Co218, current 200-220 and votage 20-21. I have preheated the pipe and plate to 120 (centigrade)with more concentration on pipe(120 for pipe and 80 for plate).After completing NDT I found a small crack on bead and after grinding many cracks in weld deposit and also in st-52 (parent metal) appeared.Cracks are 1-1.5 cm long!
I have checked metal and wire analysis but i'm note sure about gas and also my preheat torch is air type(without oxygen).
Is the preheat reason of cracks? what should I do?
Appreciate in advanced for replies.

By - Date 04-22-2001 19:28

Are your cracks predominantly in the wled metal? What do they look like. Are they very linear, or are they "branched".

The reason I am asking, is because from your description, it appears that the major "cracking" is in the weld metal. If this is the case, and the cracks are "isolated" and very linear, it is possible that these are actually not cracks but rather indications where there have been lack of inter-run fusion.

This would be a typical problem with dip-transfer mig welding. If you are welding at 20 - 21 volts, you will definately be in dip transfer. (also called Short Sircuit transfer.)

Let us know what the cracks look like.

Regards
Niekie

By Charlie Date 04-26-2001 07:12

Dear Niekie,yes most of cracks are in weld metal and also as I noted I have found these cracks in base metal just under weld metal (after grinding I checked by PT).
Other parts of base metal are sound.
Thanks for reply.
Charlie

By - Date 04-26-2001 21:11

Tell us a bit more about what the cracks look like. Are they isolated? Are they branched? Are they "step - like"?

I believe your gas is OK. I can not believe that your pre-heat could possibly lead to the cracking. MIG is a low hydrogen process, so hydrogen cracking should not be a problem. (Unless wire, parent metal or gas is contaminated.) Generally hydrogen cracking is more severe in the HAZ of the parent metal, and would not normally lead to short 1cm isolated cracks.

Have you considdered the lack of fusion theory that I suggested in my previous reply?

Give us more info regarding what the cracks look like.

Regards
Niekie

By dong won oh (*)

Date 04-27-2001 01:29

You have a cold crack due to high hardness or strength of weld metal.
The combination of mixed gas and ER70S-6(high Mn type) will produce weld metal with HIGH HARDNESS because of high Mn at weld metal.
The purpose of Mn is deoxidizer with 100% CO2 shielding.
With mixed gas, supplus Mn remains at weld metal and increases the hardness.
I have experienced this matter.
Solve the problem.
You have to change the wire from ER70S-6 to ER70S-3(midium Mn).
If you have no crack you can reduce the preheating temperature.
Dong Won Oh
South Korea

By - Date 04-28-2001 19:01

Dear Dong Won Ho

I read your reply with interest, because in South Africa ER 70S-6 is the default wire used to weld carbon steels with the GMAW process. Almost always we (in the pressure vessel industry) use mixed gases for the shielding.

I have been involved in the welding of a number of 40mm thick test plates with this filler, and mixed gasses, and to date I have never experienced hard, brittle weld deposits. The welds are always below 200 HB in hardness.

Perhaps you can tell me a little bit more about the conditions under which you experienced the hardened weld deposits.

I also doubt that the cracking is hydrogen induced, because usually one would find that the visible cracks are substantially longer than the 1,5cm that Charlie mentioned. Perhaps Charlie can do a hardness test on the weld deposit and tell us how hard this is.

Regards
Niekie

By Charlie Date 05-02-2001 21:54

Thanks Nikie and Dong,
Finally I welded it by 7018.There was no time,unfortunately I cant measure hardness.
Cracks were linear and small (1-1.5 cm), and really were crack not LOF.
But as I stated even in base metal surface (Under the weld deposit) I found these cracks.
May it confirm Dong's theory about Mn hardening?
Regards
Charlie