Weld joint strength reduction factor at elevated temperature

[Spoonful]

ASME B31.3 Table 302.3.5 listed a series of weld joint strength reduction factor at elevated temperature. So at elevated temperature, the parent material strength is already reduced by some factor, should the allowable strength be further reduced by this weld joint reduction factor? For example if parent material strength is only 50% of the strength at room temperature(Sa) and weld joint reduction factor is 0.5. so when determine the wall thickness , the weakest point strength would be Sa x 50% x 50% => 25% of its room temperature strength? Should this be too conservative and doubling up the strength reduction? This significantly reduce the pressure capacity or result a much more thicker wall vessel.


Dose ASME BPV code VIII DIv I or other division has similar requirement?

Also B31.3 table is only limited to about 816C (1500 F). what happen after that temperature? Any reference for higher temperature?

Thanks in advance.

 

[XL83NL]

It's not a matter of safety (well, actually, it is, but in a different to your statement). The background of W is application of the creep regime.
In basis the W-factor is applicable to long and spiral seams, but may (at discretion of the designer) also be applied to circ. seams.
(Last week I read a similar topic on this matter here @ eng-tips, just cant seem to find it back atm).
At these temperature, failure is controlled by creep. Therefores stress allowables are controlled by other factors, review B31.3 para 302.3.2 (d).

To answer your first question; it depends on the seam type, and your design. Strictly; no.

2rd question; no, it's not too conservative, it may be conservative, but it depends on your design.

This article may be worthwhile purchasing on this subject.

Beyond 1500 F, refer to table 302.3.5 note (a). Also read the other notes of the table, they're useful, even though they may not apply to (i.e. specific notes).
basically, there's nothing. You might question if

Also review this topic I once made on this similar subject.

Im not sure on requirements in BPV Sec VIII-1 or similar. BPV Sec III-1 sub NH also has these factors (see table 302.3.5 note (7))

What is your application? You mention B31.3, but also 'a much more thicker wall vessel'.

 

[davefitz]

The weld creep strength reduction factor plays a role in fabricated els, wyes and tees and also in longitudinally welded pipe , spiral wound pipe , and any butt weld that is carrying an axial primary load ( eg, a gravity load) . This latter item is the one that most piping designers , yet it has a high safety impact.

"Whom the gods would destroy, they first make mad "

 

[metengr]

The weld strength reduction factor was first introduced for B31.3 by Becht Engineering to account for weld joints which by experience had shown a reduction in creep life than base material, for those materials operating in the creep range only. Probably the long seam piping failures in the Power Generation industry brought this to light.

Prior requirements for weldments were simply bend and tensile testing with no requirements for elevated temperature service. The W (weld strength reduction factor) was suggested to account for reduced creep life of weldments and heat affected zone material. This factor was brought forward for acceptance by ASME Section I and B31.1 standards committees.

 

[Spoonful]

Hi Metengr,

Thanks for the information on the background of this, it seems Charles Becht VI are quite famous in this field. I have see his name in many references.



Hi XL83NL,

I did read your previous post on this subject for above 1500F, and however the conclsion seems to be still as noted in Tabel 302.3.5 (a) "...factors at temperatures above the upper temperature limit ... or outside of the applicable range... are the responsibility of the designer". And 2 year on since that post, any luck of any further finding? or still testing remain as last resort.

I am looking at design of expansion joint, in the EJMA code it use the W factor, where calculated stress, is compared to allowable strength at temperature x W. Where the W factor said to be taken from applicable design code. I though a expansion joint is more of a piping component rather a vessel? hence looked into B31.3.

In B31.3 Appenix X. X302.2.1.2 (a) and (b), calculated stress is only compare to given allowable strength from table A-1. Where no W factor is mentioned here.

My main question was as allowable material strength is already reduced taken into consideration of high temperature and creep. Is it necessary to double it up with further reduce of the weld strength by using factor W. For example Alloy 800H at 1500F, W is as low as 0.5. Allowable strength of 800H is 1.89 ksi, where at room temperature is at 16.7 ksi, there is already a reduction factor of 0.11, if further reduce it with W of 0.5, the final ratio of allowable strength is only 0.056. Should be sounds over conservative?


And we are talking about the long seam of the rolled barrel, welded by autogenous process without filler metal, but not annealed after welding.

And thanks for the reference to Becht's paper, I will read into it.

And if weld strength reduction is of a additional concern(besides consideration already taken into account when come up with the allowable values) at high temperature, why dose it not included in ASME VIII Div 1?


Regards

 

[Spoonful]

After reading Becht's background article, it seems answers my main question. in my case, I do need to further reduce the allowable strength by W. As the definition of W is the ratio of the creep rupture strength of weldment to that of the base material.

But as mentioned above, shouldn't this be same case for a vessel which have long seam welds? Or is there a temperature limit in ASME VIII Div I scope, that I am not aware of? So high temperature applicable is not dealt with ASME VIII Div 1?

As noted in note 6 of table 302.3.5, if we anneal the weld after autogenous weld, it seems W=1 all the way upto 1500F. Shall we understand it as that autogenous weld + anneal = no reduction of weld strength compare to its parent material (as weldment itself is the parent material, only heat effected, but annealing eliminate the effects from heat,). And hence, see it as no weld, or seamless, and say beyond 1500F W will still remain as 1 ??

 

[XL83NL]

I'm not sure if an exp joint at these temperatures is a good thing, unless the whole things is atmospheric.

In a recent project which fortunately did not go all the way up to 1500 F (but only 1350 F), Ive found that API 530 / ISO 13704 could be an alternative (or API 560 / ISO 13705).
At such high temperatures one could question if the rules for process piping still apply. At higher temperatures, you're going towards to regime of SMR's.
I believe SMR tubes are readily designed acc. these API/ISO standards. I don't think they cover expansion joints.