Joint Design for Tube to Sheetmetal Intersection

[Wicsteve]

We are having some difficulty in developing a good, durable welded joint for thin walled tube to sheet metal intersections. In this design, the tube passes through a hole (or extruded hole) which is fabricated in a sheetmetal muffler wall. After time, we are seeing cracks develop along the edges of the welded joint. I'm asking for some help in selecting a proper welded joint design that could reduce the stress concentrations at the intersection.

Tubes are generally 1.6 mm thick and range from 1 to 1.25 inches in diameter. The sheet metal which is 1.2 mm thick may be CRS or it might be aluminized steel (alumininum coated steel).
The application is typical of many mufflers and exhaust systems and is subjected to high temperatures (1200 F) and heavy vibration. If there is a failure (crack), it is generally found along the edges of the weld on the sheet metal side of the weld.

 

[Adrian2]

Dera Wicsteve;

I am going to guess here that the extruded hole is more of a problem than the weld. When the extruded hole is made the punch that creates it drags through the material tending to stretch it causing irregularites in material thickness.

Carefully check the extruded hole in a brand new piece in the area that cracking occurs. Perhaps the wall thickness in these areas is less than you would expect and may be out spec.

Regards

Adrian D.

 

[Wicsteve]

Thanks! The extruded hole is indeed a problem. Extruded holes have often been used in many muffler/manifold designs even though there is good evidence that the practice weakens the structure. We are in some cases using reinforcement plates to strengthen the surrounding area. In such cases the reinforment plate, the original extruded sheetmetal lip and the tube are all welded together. but this ends up with a lot of welding and is still, often inefective in many situations. In almost all cases the joint fails along the edges of this weld.

We are going to try to gently roll the edges of the pass through hole by about 30 degrees, then weld the exposed edges of the hole to the tube.

Are there good and accepted welding practices used in boiler making, or heat exchanger designs that might be applicable?