true cost of intermittent welds 1

[Tmoose]

We are reproducing a simple erosion wear shield bolted inside a coal pulverizer.
Estimated quantities are 3 or maybe 6 of these shields.

The nicely made, but worn out, examples used 1.5" long staggered 1/4" intermittent welds to attach a 3/8" thick 18"Ø steel half pipe to a 1/2" thick flat shield.
I'm thinking the hassle of laying out the stagger pattern, and stopping and starting 16 to 20 times might take more time than just clamping, tacking, and applying a continuous weld.

As a practical matter does the cost of 2X more filler material in the continuous weld outweigh the labor for the staggered welds ?

thanks,

Dan T

 

[dvd]

It is harder to grow cracks through intermittent welds.

 

[kingnero]

and half the deformation.

 

[david339933]

Try it out both ways on a small plate....I think you will be surprised how much quicker stitch welding is compared to continuous.

 

[meltedEng]

You will get significant savings. The labour cost to set-up a stitch weld is very low. The labour to do the actually welding is directly proportional to the % stitched. Stops and starts on most alloys aren't a big issue at all.

Other good reasons it's usually used include: expansion, flexibility, draining, full surface wetting, heat input, or distortion.

 

[gtaw]

What the heck is a stick weld? I get so confused!

Best regards - Al

 

Your stick stuck because you didn't stick with training (or the learning didn't stick).
That clarify things?

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[BridgeSmith]

What the heck is a stick weld?

Standard arc welding, with a "stick" of welding rod.

A stitch weld is another term for an intermittent weld.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[jdonville]

@ironic_metallurgist

SMAW.

 

[gtaw]

Sorry, that should have been "What is a stitch weld?"

Depending on the application and the code restrictions, a continuous weld, half the size of the intermittent fillet weld can be half as expensive if only 50% of the weld length needs to be welded. For example:

A 3/8-inch weld, two inches long on 4-inch centers with a joint length of 40-inches has a total volume of about 1.4 cu. inches. A 3/16-inch weld that is continuous for the full 40-inches has a total volume of about 0.7 cu. inch. Both would have the same load carrying capacity. The small weld would take about 1/2 as long to deposit, would result in less buckling if one is welding sheet steel. Buckling is inversely proportional to the square of the thickness and inversely proportional to the square of the distance between welds. The fly in the ointment is if the governing standard is the structural welding code which has a minimum weld size based on the thickness of the base metal being welded. The designer would have to go with the larger weld size, i.e., based on load or base metal thickness if the structural code applies. In that case, the intermittent fillet weld is one solution to the quandary.

Best regards - Al

 

[MintJulep]

It is harder to grow cracks through intermittent welds.

True. But on the other hand there are more starts and stops to serve as potential crack initiation sites.

 

[Agent666]

... Plus on the same hand, higher stresses for the same loading on intermittent welds.

 

[gtaw]

Isn't that what engineering is all about, compromise? Balancing the pros and cons to derive the best solution considering the constrains involved?

Best regards - Al

 

[phamENG]

Don't forget the labor side of it. In your 3/8" stitch vs. 3/16 continuous, you need to consider the time it takes to accomplish each. It takes about 3 passes to deposit enough filler to achieve a 3/8" weld, whereas the 3/16" can be put down in a single pass. So for a labor equivalency, the 3/16 continuous is 40" of single pass welding, but the 3/8" is (40/4)*2=20" at 3 passes => 20*3= 60" of single pass welds. Just in welding, the 3/8" stitch will take 50% longer to finish.

(Source is AISC Table 8-12)

 

[meltedEng]

phamENG,

You made the key point. I did not consider Tmoose' scenario, and only considered what I normal do (already minimum size welds).

The proper way to compare weld savings is, as you already said, is comparing the total length individual passes. Which can be extended to point out the extra effort needed to stack staggered welds and all the small details.

I guess the only thing I had to really add, was that setting up the intermittent weld pattern is negligible, but cost isn't the only reason for choosing this weld type.

 

[dhengr]

Tmoose:
Gtaw and PhamENG are right on the money. Now, if the shop is working under a serious structural environment with serious inspections, the difference becomes even greater. There is the weld layout time and inspection time. The welds are invariably laid out a little longer so there is no questions about that latter. Then, the welder makes them a little longer to be sure he’s covered, and he spends a little more time on the starts and stops so as not to leave craters, etc. Welders really hate to be called back to do rework, something about pride of workmanship, with the good ones. Then, you usually don’t count some short amount of these weld lengths, at the start and stops, in your strength calcs. Also, each start and stop is, in itself, a potential crack starter. And, I think that Gtaw would agree that a .25” fillet still uses less weld volume (1.25 cu.in.) and may solve the min. weld size problem; and secondly, any short multi pass weld is double nasty weld to do well.

 

[dvd]

Comments -

1. There are code minimum weld size considerations that may govern.
2. Skip welding is often incorporated to avoid distortion - thus starts and stops and welding procedure would not be continuous.
3. Fillet weld throat area is typically used for strength calculation, not volume.
4. Depending on the type of steel used in a wear shield, there may be a pre-heat requirement that works well with the intermittent weld pattern.
5. Ask for the welder's opinion.

 

[gtaw]

Stitch welds, skip welds, they are nonstandard terms. The correct term is intermittent welds.

Best regards - Al

 

[DekDee]

Al,
We have discussed this previously.
Eng-tips is an international forum so terminology may differ between countries, regions, hemispheres.
Stitch welding is a non - standard term commonly used in the Southern Hemisphere.
Stick welding (which you mentioned with your typo) whilst not recognised by any codes/standards I am aware of is a regular way of describing SMAW for many thousands of welders in the Southern Hemisphere.
Cheers,
Shane

 

[Tmoose]

Hi DekDee,

Do you happen to know what codes are commonly specified or at least referenced in the "Southern Hemisphere?" Similar to AWS D1.1 etc ?

When you say Southern Hemisphere, is that all encompassing, or were you further restricting that to the Eastern or Western Hemisphere?

thanks,

Dan T