Welding 1.5" thick 4140, preheat and other issues

[brianmiller8002]

We are currently welding 1.5" 4140 material together with a preheat of only 150F. The part is stress relieved (often not until the next day or even longer). Then the inside diameter is slightly machined and the unit is sent to NDT (Mag Particle). We were having failures of roughly 1 out of 11 at NDT where we simply machined out the bad area and repeated the welding procedure. Well, most recently we had a batch sit after the weld for over a week before it was stress relieved and we experienced a 90% failure rate without changing any other variables as far as we can tell. Can snyone tell us if the amount of time before stress relief and after weld is a factor? What other variables are key factors? What are other things to consider in this application? Are we preheating enough?
Thanks!

 

[WPSGuy]

Preheat should be min of 450F for "4140" and should be kept @ min until PWHT.

 

[metengr]

As equally as important as preheat mentioned above is to use low hydrogen electrodes. I cannot stress how important it is to minimize hydrogen ingress during welding. Preheat is one method and so is electrode conditioning, storage and use. Use low hydrogen electrodes.

 

[vesselfab]

hold it until PWHT?

Overkill for 1 CR don't you think?

 

[WPSGuy]

No it's not over kill. If like crack parts go right ahead!

 

[metengr]

For 4140 steel at this thickness, you really don't need to maintain minimum preheat after welding. Instead, I would conduct a post weld bake to remove hydrogen at 500 deg F for 2 hours only IF PWHT will be delayed.

 

[racookpe1978]

Electrode storage (are you REALLY keeping it at temperature for the whole time the electrodes are exposed to air?) sounds suspicious here.

But allowing the welded steel to cool down between welding and PWHT means that the accumulated stresses from are immediately applied to the two welded sections. And prevention of problems from internal stressses is is the "hole" reason why PWHT is all that it is cracked up to be on even relatively thin (1-1/2") sections. 8<)

 

[WPSGuy]

If you are trying to protect against the potential of delayed hydrogen cracking when the part is allowed to cool down after welding. Then you would need an “Intermediate Stress Relief” But you would need to be around 700F to defuse the hydrogen. But then why not just PWHT?

Also I would hate to know what the HAZ would punch with a 150F preheat and cool down before PWHT.

 

[vesselfab]

sa-387 gr 11 class 2 1.25 Cr 1.5" nominal thick
preheated 300*F slow cool down after welding (insulation)
welded w/ e8018 B2 LA 92 wire 888 flux

2 months between welding and PWHT
100% rt after pwht
100% mt after pwht
100% ut corner joints after ut

200-210 bhn in HAZ
190-200 bhn in weld
(krautkramer mic 10)

67/40 ft/lbs @ -20*F lcvn in weld
94/112 ft lbs @ -20*f lcvn in haz



and no cracks

 

[WPSGuy]

LOL Vesselfab.

 

[metengr]

Electrode storage (are you REALLY keeping it at temperature for the whole time the electrodes are exposed to air?) sounds suspicious here.

What??? Haven't you ever heard of portable weld rod heaters?

But you would need to be around 700F to defuse the hydrogen.

Incorrect statement. Hydrogen diffuses at 450 deg F and increases in diffusivity as temperature increases up to a maximum. Post weld bakes can be effective at 500 deg F, and do not require 700 deg F.

 

[WPSGuy]

Metengr you are right about the portable rod ovens. That’s why I did not address that issue. Any Engineer should know that.
Sorry but I still disagree with the 500F @ 2hr “Intermediate Stress Relief” to completely defuse hydrogen even at 1.5”. I have spent allot years testing PWHT in labs. All times and all temps and have the reports to prove that not everything you read in the books and codes about PWHT is true.

 

[racookpe1978]

Sure - I've heard of portable rod heaters. 8<)

I've even seen them plugged in, and some are even kept at the right temperature even. 8<)

I've also seen them not used, seen rods dropped on the floor and dropped in water, rods rained on, rods kept in unplugged heaters, rods not kept in ovens back at the tool room at the shop, ......

 

[metengr]

WPSGuy;
My only final comment is that the post weld bake is NOT a stress relief, period. The post weld bake is only intended to assure diffusivity of entrapped hydrogen after a weld is completed.

PWHT is a completely different thermal treatment at higher temperature that provides two benefits - reduction in residual stress from welding and tempering of hardened weld metal and base metal in the heat affected zone of the weld.