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cracking of 625 overlayed 8620/ 4130 during GTAW welding
2

cracking of 625 overlayed 8620/ 4130 during GTAW welding

cracking of 625 overlayed 8620/ 4130 during GTAW welding

(OP)
I am observing cracking of 625 overlayed 8620-4130 butt welds ("hot cracking type occurrence). The butt welding is done after the 625 overlay (which has a PWHT) is in place. Has anyone experienced similar?

RE: cracking of 625 overlayed 8620/ 4130 during GTAW welding

KimberleyS;
What is the exact sequence for joining and where exactly is the hot cracking or cracking located?

RE: cracking of 625 overlayed 8620/ 4130 during GTAW welding

(OP)
The sequence is:

overlay
pwht (stress relieve)
butt weld

cracking along centreline of the weld.

RE: cracking of 625 overlayed 8620/ 4130 during GTAW welding

What welding consumable are you using for the butt weld?  If you are welding any part of the 625 overlay, you need to be using a nickel-based consumable.  If you are trying to do the butt weld with low alloy or carbon steel, the overlay needs to be stripped back, or held short from the joint and then the joint can be overlayed afterwards.  

RE: cracking of 625 overlayed 8620/ 4130 during GTAW welding

(OP)
The butt weld consumable is ER70S2 for the root and E10018-D2 for the fill passes.

RE: cracking of 625 overlayed 8620/ 4130 during GTAW welding

Well, if you are using the ER70S-2 or E11018-D2 and the weld comes into contact with the overlay (i.e. gets diluted), then hot cracking as you describe is not uncommon.  If you have to weld the joint in contact with the overlay, use nickel based filler like alloy 625, either that, or strip back the overlay so that the carbon steel filler doesn't come into contact with it.

RE: cracking of 625 overlayed 8620/ 4130 during GTAW welding

GRoberts is spot on. One can deposit 625 filler metal to a carbon steel or low alloy steel base metal, but the opposite should not be done because the carbon or low alloy steel weld deposit will form a mixture of lower chromium and nickel compositions that will create a range of microstructures susceptible to cracking hot or cold.

RE: cracking of 625 overlayed 8620/ 4130 during GTAW welding

If you are stripping back the overlay it is a good idea to verify that you got it all with a copper sulfate solution. Spray or swab on the weld area and remove any traces of the overlay revealed by the etchant.

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