Dissimilar metal welding

[DK44]

1. Is welding between dissimilar metals a technically sound and safe practice.
Ex : In a Heat Exchanger.
a) SS 304 + SA 516 Gr 70 Shell courses with Circumferential Butt joint
b) SS 304 Channel + SA 516 Gr 70 Tube Sheet making a corner joint.

2. Is the subject addressed in the Code ASME Sec VIII Div 1.

 

[mk3223]

The welding between two dissimilar metals is performed frequently and technically acceptable. However, the possible concerns may need to be addressed in terms of the process application, operation conditions, potential corrosion issue, etc.

The ASME Code doesn't address the specific concern regarding the fabrication method of bimetallic welding, or the safety related to the process application.

 

[weldstan]

Structurally sound welds are often made to dissimilar metals in Code pressure parts. Operational parameters define the appropriate designs and welding parameters used.

 

[davefitz]

As indicated above, such welds are routinely made, but using techniques and weld geometries learned after decades of trial and error. In addition to the metalurgical intracacies at the weld HAZ, there also exists stress issues when the joint undergoes thermal cycling, as the coeficient of thermal expansion is different between the 2 alloys, and stress raisers caused by the componetn geometry and the "metallurgical notch effect" of the HAZ can cause fatigue failure after a small number of thermal cycles. If your component will be theremally cycled and its failure would cause a serious impact on the end user then the weld joint should be tested in a labe or analyzed with modern software.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

Dissimilar metal welding is a perfectly acceptable practice.
The issues arrive in service with thermal cycling to high temperatures, which can lead to a class of failures known as DMW. Fossil boiler plants are particularly afflicted at transition joints between low alloy and stainless steel tubes.
Mitigation is available through improved design and the substitution of more thermal expansion compatible nickel alloy filler metals. But 309 class fillers remain the standard choice for almost all steel to SS DMW welds.

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

 

[Dhurjati Sen]

Dear DK44,

Check for mockups in Sec IX.

Regards.

DHURJATI SEN

https://www.nace.org/people/dhurjatisen