Selection of consumable to weld dissimilar material

[purode333]

Dear sir,

In welding of dissimilar material, as per code we used the consumable of lower grade to weld the metal.

My question is that can strength of lower grade consumable will not affect the strength of higher grade material.

In general sense, Higher grade will be preferable. (which will take the strength of lower grade material also.

Want to know the reason for selection of lower grade.

Regards
Prasad rode

 

[dhengr]

Purode333:
Of course, the two metals must be weldable and compatible in that respect. I would select welding consumables and a process to match the higher strength steel. You generally slightly over match the higher strength material with the welding wire. Then, I would actually design the weld joint using the allowables for the low strength material. The reason for this is the following: when you melt the two materials and the welding rod/wire, you get a mixing in the weld puddle, which is something of the sum of the three, but you don’t know exactly what percentage of each. At the fusion area of the higher strength matr’l. you have a mixing of that matr’l. and the welding wire, and the higher matr’l. allowables reasonably apply; while in the puddle you will have a mixing of the three; and at the fusion area of the weaker matr’l. you will have something stronger than, but predominantly the lower strength.

Then, when you design the weld you usually use the throat area (least area/dimension, middle of the weld puddle) as the critical area of weld metal, and use the lower allowables because you don’t know otherwise. There are instances where you should also check the weld stress at the fusion line/area, because while this is usually a slightly larger dimension than the throat, the allowable stresses at that location are/may be lower. But, by the above thinking process, you do not have to worry about this, you have it covered. I believe this is what your code is saying, not that you must select a lower matching welding rod/wire. There are a few instances when you might want to select a lower strength welding wire, but these are special situations and need special engineering attention. Otherwise, in the fab. shop, you do not normally change the welding rod/wire/process for every new weld. You select a welding wire which, within reason, over matches most of the matr’ls. being welded, so you are not having to screw around changing all the time.

 

[metengr]

The selection of filler metal for a dissimilar metal (DMW) joint requires review of service conditions to select either over or lower matching filler metal in terms of strength and corrosion resistance. The strength consideration can be ambient temperature or elevated temperature strength. The strength consideration should consider weld type - fillet or full penetration and function, non-pressure part attachment weld or a weld between pressure parts.

I have seen both lower or over-matching strength filler metal used in DMW's to match one of the two lower or higher strength base materials. The selection of a lower strength was purposely done to reduce the risk of failure in the higher strength base metal because of service application. For one example in power boilers tube attachments that fail. You try to avoid having failure in the tube wall by using a lower matching filler metal that is fillet welded to the tube.

My point is there is considerable thought that should go into selection of a filler metal between dissimilar materials and no one size fits all in terms of selecting a high strength filler metal each time. Most of the time the filler metal matches the higher strength between the two base material, but as I pointed out not always.

 

[weldstan]

Agree with metengr and we also use intermediate strength filler metals as well depending on service conditions and need for PWHT.