Heat Treatment of 2.25Cr-1Mo-0.25V Steel Welding

[PAN]

In my understanding the V-modified steels require ISR (Intermediate Stress Relief) heat treatment. However, some fabricators propose DHT (dehydrogenation heat treatment) for minor parts such as piping, flange neck, external attachments etc.).

I'm not familiar with vanadium modified materials, if DHT is unacceptable, please advise me about the supporting reason.

 

[metengr]

Can you explain what is dehydrogenation heat treatment? If this is simply to allow hydrogen to diffuse out of the weldment, I would stick with the intermediate stress relief.

The vanadium-bearing low alloy steels are notorious for reheat cracking in heavily restrained weld joints. The reheat cracking has nothing to do with hydrogen.

 

[PAN]

metengr,
The proposed DHT condition is 350 to 400 Deg. C. for minimum 0.4 hr. The fabricators inform that they have sufficient experience to apply DHT to Cr-Mo-V materials successfully.

I agree that ISR is required for vanadium modified steels to avoid cracking of the welds. However, at least 2 famous fabricators propose DHT instead of ISR for some parts (not the same parts and different minimum holding time). Please advise me.
- What should be the parts to be accepted DHT instead of ISR?
- What should be the acceptable minimum holding time for DHT of V-modified steels?

 

[metengr]

PAN;
I don't believe that the DHT is high enough in temperature to provide any significant reduction in residual stresses for only 0.4 hrs for restrained weld joints (what are the part thicknesses at the weld joint and what type of weld joint ?). The vanadium bearing steels develop intragranular precipitates that result in increased strength thus weakening the grain boundaries. If the parts do not have any additional restraint that could increase residual stresses after welding, you might be ok with the proposed DHT.

If it turns out you do have heavy restraint at the weld joints, I would elevate the DHT temperature to at least 500 deg C for 0.4 hrs to provide a reduction in residual stresses. Increasing the time at 350 deg C will not help.

 

[ijzer]

We accept DHT only for circumferential and longitudinal welds. 300-350 2hrs minimum. For branch welds it is better to do ISR because residual stresses are more severe here.