Selecting Filler materials for differing strength materials

[mat211743]

We are planning to weld A106 Gr.B (30.96 mm) to CSA Gr.448 (12.7mm). We will be using transition piece to weld these varying thicknesses. My welding engineer says that when we weld two materials of differing strengths if we choose filler material that matches the TS of weaker of the two that is sufficient. My question is, would that joint will have adequate machanical properties to withstand the pressure during service. The joint may have TS somewhere between 414 MPa and 448Mpa.is that OK? do we have to match the strength of Gr.448. The second question is do we need Elevated Temperature tensile test to prove the high temperature properties of the weldment? Typically CSA Z662 Annex I requires this ETT for unlisted materials. But we are welding a listed material with unlisted material. What do we do in such circumtances?

 

[BigInch]

No. The weld must be thicker then the higher strength pipe's wall thickness whenever the filler is comprised of a material of lower strength. How can you do that without making severe stress concentration points. You'd have to internally clad-weld to the internal wall of the higher strength pipe and internal clad layers don't count.

 

[weldstan]

Totally agree with BigInch.

 

[dhengr]

Mat211743:
You need a filler metal and welding process which is compatible with both pipes. I’m not disagreeing with BigInch, but I’d answer the question differently. Whichever pipe is stronger (Fy or Ft) and considering their wall thicknesses and other details too, only one of them is critical or controls the design at that joint. The stronger pipe must have been designed/selected because of considerations away from that joint, which required the need for the added strength. And, in that respect, your welding engineer is correct in saying that your weld must match the weaker, controlling, pipe or part of the detail, and that at that location there is less need to match the stronger pipe. Otherwise, the weaker pipe would be over stressed immediately adjacent to the weld. However, there is usually no harm done in providing filler metal which matches the stronger pipe. An analogy would be two 1" sq. bars, butt welded together (CJP) and under tension, one of the bars is 50ksi stl. and the other is 30ksi; what good does it do to insist on a 50ksi weld? The 30ksi bar will break immediately next to the weld. I do suppose some detail about your joint could alter this thinking, so you have to study the whole joint to answer your question.

 

[weldstan]

My full agreement with BigInch is based on the major strength/wall thickness variation described.