Lage Casting, Several Weld Heating Cycles.

[tc7]

For a large casting that must be weld repaired (4130,CE~.695)I am wondering if I should exercise some cautions as we will have several interuptions before we can send it out to final PWHT.

A large steam valve casting consists of a couple of adjent chambers with a very heavy wall (~3" thk) in between and the outside wall thickness is also ~3" thk. Some of the cracks to be weld repaired may require excavation up to 3/4" deep and are in/near the corners of the chambers. We will preheat the entire body upto 400 deg F before any welding takes place. Before all repairs are finished, we will have to stop and cool to room temps several times for inspections as well as manual repositioning to reach other crack repair locations.

In between each weld cycle would it be advisable to cool this casting down to just below the martensite finish temperature and hold until temperature equilibrium is reached, then increase temp to accomplish local stress relieving temperature and THEN cool to room temperauture?

This was suggested to me by a young University professor who admits not having more than academic welding experience. But it makses sense. The method will be cumbersome to me - Is it all necessary in your point of view? Comments/thoughts welcomed.

By the way, temper beading was discussed and rejected by our engineer folks.

 

[EdStainless]

Why not temper bead to get you through the repair with the plant to fully PWHT afterward?

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[unclesyd]

I go along with EdStainless on this on. We have made sevral repairs on large cross sections in castings, forgings, and rolled plate using this procedure.

We rebuild fairly intricate 4140 castings using this procedure and have never seen any problem.

 

[metengr]

tc7;
If your folks won't agree to temper bead weld repairs as you stated, you can indeed follow the what was recommended, although it is cumbersome. I would add that you should use low hydrogen electrodes.

 

[tc7]

Ed, Unclesyd & Met-

Agreed on the low hydrogen rod, we rarely will use anything else when 70xx, 80xx or 90xx is called for.

I've discussed temper beading with this particular crowd a couple of times now and they won't budge. The concern is that the temper bead is not predictable in it's ability to anneal the weld induced hardened areas. Considering the geometry and the wall thicknesses involved, there are many heat sink path's to quench the weld area, even with a high preheat and interpass. They probably have a good point.


Speaking of high preheat, I was of the idea that high preheat would typically be sufficient to cause reduced cooling rates to the point that martensite formation would not be likely. If this has merit, how would you know that you have arrived at the "proper" cooling rate to avoid martensite formation?

 

[metengr]

tc7;
This material will form a local band of martensite along the weld fusion zone. The preheat will reduce cooling AND thermal gradient effects to reduce the risk of delayed cracking in the base metal HAZ. You will not eliminate the formation of martensite, only control the amount based on preheat requirements. Low hydrogen electrodes will reduce the susceptibility to delayed cracking, despite the martensite formation.