Stress Relief and ASME Code 1

[geesamand]

I'm looking for information relating to when stress relief is required/specified to be used based on the ASME Section VIII/IX. So far I find how stress relief is to be conducted and documented, but no direction / recommendation for which applications require it. I'm aware the principal issue for stress relief is dimensional stability. This application has no such issues during machining and is not subjected to pressure - only mechanical loads. (It's an A-36 structure mounted above the tank head). The question is whether the strength of the weld is compromised, and whether ASME code provides any such guidance.

Thanks for reading

 

[metengr]

geesamand;
If you are referring to stress relief, and not post weld heat treatment, you will not find any guidance in the ASME B&PV Code. There is guidance for thermally induced post weld heat treatment, which provides tempering AND residual stress reduction.

If you are looking for residual stress reduction, you are on your own for this.

 

[geesamand]

Thanks for the response.

The post-weld heat treatment in Section 8/9: isn't that the same as annealing (aka thermal stress relieving)? The issue is the fatigue strength of weld.

I hadn't compared the ASME PWHT procedures in great detail yet - if it's talking about a different procedure then my question is fully answered.

 

[metengr]

Post weld heat treatment as intended by the ASME B&PV Code means a subcritical thermal treatment. Annealing or normalizing are performed at temperatures above the lower critical transformation temperature, and are not subcritical.

Stress relief can also be performed as a subcritical thermal treatment OR using vibratory methods to reduce residual stress. This is why I like to distinguish between stress relief AND post weld heat treatment.

 

[vesselfab]

ASME Sect VIII only addresses when IT requires Post Weld Heat treatment and that is a function of thickness vs material used vs MDMT vs special service. There are so many other things that enter in to it.

And, I don't really think dimensional stability is really relevant here as much as refining grain structure in the base metal and welds.

As far as your question, are you adding this ""structure"" attaching to an existing vessel, or are you putting on as new construction?

 

[geesamand]

Thanks again for your inputs.

This device is an existing tank-mounted support structure outside of the mechanical seal. So while it's not required to be ASME Section VIII/IX compliant, our welding practices are based on it. Vesselfab, could you help point me to where to find when Post Weld Heat Treatment is required in the code?

The concern is reduced strength of the welds due to the fact they had no thermal stress in manufacturing. While the subsequent machining, transportation, and months of use have provided some vibratory stress relief, the question today is whether a thermal stress relief was called for.

I suppose it's going off topic, but I would appreciate any comments w.r.t. when to use sub-critical and supercritical stress relief, for the purposes of maximizing allowable weld stress. (In the future the welds will not be the highest stress regions)

 

[stanweld]

Reduced strength of the welds without PWHT should not be of consideration unless the welds are subject to cyclc (fatigue) service. PWHT may also be needed if the weld is highly restrained and/or hydrogen embrittlement or a stress orrosion cracking environment is anticipated.

 

[metengr]

Most codes and standards do not mandate subcritical PWHT over a reheat treatment (where you re-normalize or anneal) unless you are dealing with low temperature service(ie., impact test qualification).

I will give you my advice for what it is worth; for cyclic fatigue, elevated temperature and low temperature service, re-heat treatment versus subcritical PWHT eliminates the base metal heat affected zone (HAZ). The elimination of the base metal HAZ will result in reduced risk of creep cracking along the base metal HAZ of the weld, increased resistance to fatigue crack initiation and propagation, and the avoidance of brittle fracture in low temperature service.

If the above items are not a concern, stick with a subcritical PWHT as mentioned in ASME Section VIII, Div 1 for carbon steel (UCS-56) to reduce residual stress and to soften the base metal HAZ.

 

[vesselfab]

OK...this is base purely on code requirements on not towards any users standards or cyclic design criteria.

As long as the vessel was not PWHT and you are not attaching anything with over 1.25" weld thickness (based on p-1 materials) you will not have to PWHT attachments.

If the original vessel was PWHT there are two different requirements that need to be considered.

PWHT per code for thickness...if this is the case...over 1.25" thick, then you are allowed, with the consent from user & AI to do attachment welds up to 1/2" thick without PWHT as long as you preheat to higher than normal temps and follow ASME rules.

PWHT for service....this could be low temp, lethal, HIC, HF or several other things. Could be thin or could be thick. This normally requires a pwht. Some services User & AI will let you follow the rules posted above and not PWHT, but normally not.

There are so many different reasons to PWHT that it is really impossible to cover everything here. First and foremost, I would look at the existing vessel's stamping and if it was PWHT, then go from there and look at rules in sect VIII, div. 1 para UCS-56

 

[geesamand]

The support in question is not welded directly to the tank. It's bolted to a stand that is bolted to the nozzle of the tank. Vesselfab, yes, the references to the code are in absence of any overriding customer or company criteria.

Low temp is not an issue. The post-mortem stress analysis shows the max stress right in the center of a fillet weld, which should be free of HAZ effects. The observed cracking is also along the center of the weld bead. While the vast majority of the stress is uniform, there is some cyclic loading due to vibration.

Interestingly, Blodgett's "Design of Weldments" states that stress relieving the weld has no appreciable effect upon fatigue strength. It makes some sense, since fatigue strength follows ultimate strength, and the weld bead itself is usually stronger than the HAZ/base metal. I think weld surface finish (these were unground 3-pass fillets) would be a far more influential issue, really.

Thanks again - it would appear that I haven't overlooked anything large in the code, and I've learned a good bit about the PWHT options out there.

 

[stanweld]

geeseamand,
Fillet welds are well known to have vastly reduced fatigue life over that of a full penetration groove weld. Flaws at the root of the fillet are even more detrimental. Life can be extended by properly profiling the weld surface as you have stated. Did the crack initiate at the surface or the root of the fillet weld?

 

[geesamand]

Crack origin is unclear at this point.

 

[RORCULLO123]

Busted steam check valve of pressure boiler drum....

Our company has 4 coal fired boiler and two fuel fired boiler. We are required to maintain 1 MPa to maintain the production operate in a good condition.we operate 2 boler at a time to avail these requirement.the connection of the two boiler are in a parallel manner.

at time that we operate the bunker and the other boiler the cheked valve head of bunker side was explode...

the question is, what is the most probable cause of explosion.Is the connection of the boilers are critical if they run not simulteneiously? What are the tips to avoid this kind of sequeces if there is ?Is pressure hammer exist, that is why the pressure at the bunker exceed to the check valve design pressure limit?

thanks....RORCULLO123

 

[metengr]

RORCULLO123;
1) Please do not hijack existing threads. Your post has absolutely nothing to do with the original post. So, please start a new one.

2). It is impossible to tell you the root cause of why the steam check valve failed. I would suggest you seek some assistance by putting together a small team of engineers to help you in determining the root cause of the valve failure. PS; have an operations engineer along with a materials engineer to determine the RCA of the failure.

 

[RORCULLO123]

metengr

Sory, this is not i meant to be...it was my fault to connect this thread at the original post....i suppose to post my thread but accidentally posted at this forum. to tell you franckly, i am just a new member here.
Anyway, thank you for valuable response/advice.....