Accounting for HAZ in 6061-T6 Pipe

[dozer]

In CAESAR II, how do you account for the heat affected zones (HAZ) when welding 6061-T6 aluminum pipe together?

 

[XL83NL]

Im not a CII user (we use AUtoPIPE), but Im pretty sure such effects are not accounted for in pipe stress analysis but rather in your WPS/PQR, and welding inspection plan (ITP).
Can you elaborate on the background of your question? Why did you think it would have influence in pipe stress analysis?

 

[dozer]

The 6000 series aluminum alloys are heat-treatable to increase their strength. 6061-T6 is one such alloy that has had it's strength increased by heat treating. When it is welded it looses strength in the HAZ. The Aluminum Association specifies this to be 1" either side of the centerline of the weld. According to Aluminum Association Design Manual, the minimum ultimate strength for 6061-T6 is 38 ksi. When welded it is 24 ksi. The minimum yield strength is 35 ksi, and 15 ksi when welded. B31.3 has similar numbers.

I suppose this could be a code question because I couldn't find anything specific about this in B31.3 other than Table A-1 (Basic Allowable Stresses) does acknowledge this difference in properties. You could take the conservative route and use the welded properties for the entire length of pipe. I just assumed (there I go again), that there would be some way to differentiate between stresses in welded and unwelded portions of pipe.

Just to be clear, I'm talking about circumferential welds where pipes are joined together (or pipe to elbow or whatever). Aluminum pipe is extruded so it doesn't have a longitudinal weld.

 

[XL83NL]

dozer, never thought of it that way, though we normally dont use aluminum piping. However, Im pretty sure 'strength losses' are accounted for in the applicable ASTM A or B specification, and as such, are accounted for in the vlaues of B31.3. To be honest, Im just guessing based on my experience with the B31.3 code; aluminum isnt the only material in B31.3 for which material properties are affected by heat treatment, so Im pretty sure the Coide has some way to deal with this. Hopefully others can help you better.