Guest Column | November 7, 2000

Bending vs. welding in stainless steel piping installations

By Nick Erdei
Therma Corp.

The current practice for stainless steel piping fabrication and installation in biopharmaceutical facilities is welding (most of it orbital ). That is, welding fittings and valves to pipe in order to achieve the desired configurations as per process needs. The material of choice is 316L stainless steel, an austenitic steel with carbon content kept under .035%. This is an important detail because welding can sensitize stainless steel thus lowering its corrosion resistance.

The most common sensitization is chromium carbide ( Cr23C6 ) formation at the grain boundaries within the heat-affected zone of the weld. This can lead to corrosion cracking and eventually to catastrophic failure of the material. By keeping the carbon content as low as .035% there is no carbon available for the carbide precipitation process whereby most welds in stainless steel display no grain boundary sensitization.

Now, what happens if instead of welding fittings and pipe we employ bending? The outer section of the bend will undergo tensile stress while the inner section will experience compression stresses. Structurally speaking this is not a problem. Austenitic stainless steel belongs in the base metal P-No 8, which is sanctioned in the ASME Code as cold bendable without post-bend heat treatment. The only possible problem is stress induced corrosion susceptibility, especially on the outer, tensile stressed section. Yet independent laboratory testing of a 90 degree elbow formed by cold bending at 2D radius showed no signs of intergranular corrosion.

Specimens removed from an orbital weld on the same piece of pipe yielded the same results. The only undesirable effect of bending is interior surface "orange peel", which might elevate Ra readings by a few units. Choosing a lower Ra material, say 20Ra, would however result in a 25Ra finish even after bending. Considering the substantial savings entailed by bending instead of welding the expense of buying 20Ra material is negligible. In conclusion, we say the time is here for bending piping configurations for biopharmaceutical process installations.

For more information: Nick Erdei, QC Manager, Therma Corp., 1601 Las Plumas Ave., San Jose, CA 95133. Tel: 408-347-3400, ext. 1382.