Longitudinal Weld Metal Cracking 2

[PatE]

We have been MIG welding mild steel for some time with no problems. However, we just purchased a new TIG unit and are experiencing severe cracking problems. We are new to the TIG process. We are welding short sections (1.5 inchs long) of 3.5 inch pipe to 2 inch thick plate. Both are carbon steel. As soon as we finish the weld bead a longitudinal crack begins to form right down the center of the bead from one end to the other. Preheating does not seem to help. Or maybe we are not preheating enough. The cracking can be heard as well (a brittle tinkeling sound). Multiple passes seem to help. Adding filer rod (mild steel & stainless) seems to help. But nothing works well. Any ideas?

 

[GRoberts]

My first thought is that you may be making the weld too small for the large components you are joining together. Especially if you are just making a small autogeneous weld (i.e. without filler metal). The 2" plate is a very large heat sink, and is very stiff. If the pipe is very thick at all, it could just be the residual stresses tearing the weld apart. How large are the welds you are making? (leg size and length?) This might also explain why multi-pass welds and adding filler metal would help, as both would make the weld larger.

 

[metengr]

Totally agree with GRoberts. There is an interesting article published by Lincoln Arc Welding titled "Weld Cracking". Longitudinal cracking can be caused by three problems-segregation induced cracking, bead shape cracking or surface profile conditions.

This sounds like it would fall under bead shape cracking. The remedy here is to modify your bead shape using a width to depth profile of 1:1 or 1.4:1

 

[pipewelder1999]

More than likely this is related to the depth to width ratio as indicated by metengr. I have a page that breifly explains some of the theory at

http://www.weldinginspectionsvcs.com/COncaveWeld.htm

.

One of the reasons that you may have this with GTAW is that the bead profile is more concave and therefore has a thinner equivelent section during cooling.



Gerald Austin
Iuka, Mississippi

http://www.weldinginspectionsvcs.com

 

[attilon]

hi,
All the others are right, but:

What is the main difference between Mig and Tig?
The difference is the Linear Energy Introduced in the assembly.

Now, if you had no problems with Mig, it means that the Energy (heat) input could be supported by the plate and the tube. (also it is possible, that it was just under the limit, and you were luky)

E(linear)= (U x I)/weldingspeed

If you make an equasion of the Mig result to the Tig parameters, you will get the necessary speed for the Tig welding speed to introduce the same Linear Energy.

I'm sure, that this high speed you can not obtain at Tig.

Also, that's why welding Tig with multiple layers help (you weld with higher speed). You introduce lower Linear Energy, resulting lower Internal Stresses, also the second layer gives you a heat treatment to the first one.

All the other measures help a lot, but they are not the heart of the problem. (i.e. stainless steel has higher maleability and elongation properties, thus having higher Rp02, so cracking apears at higher internal stresses)

I wonder, why did you have to change from Mig to Tig, if it was OK? Was it a request of the client to have a perfect root? In this case it is still better, if you weld the root Tig, and fill the weld Mig. It is faster than all just Tig, and you have higher quality than all Mig.