Toughness required PQR, technical justification

[Baron20]

Can someone explain the technical justification for limiting the wall thickness for ASME IX procedures when toughness testing is required. I understand it being a supplementary variable. With that, why does API 1104 not require this restriction.
Thanks

 

[BigInch]

Not sure, but I would expect that it is to maintain some geometric similitude. Too wide a dimension in one direction could limit the maximum developed stress, cause unusual stress distributions, or require unreasonably large impact forces. API 1104 is for pipelines, so wall thicknesses would naturally tend to be relatively thin.

 

[XL83NL]

Id expert something with cooling rate, heat input and associated loss of ductility (we call it t8/5)

 

[SJones]

The restriction is based on the need to maintain a reasonable relationship between the thickness of the weld and the dimensions of the Charpy test piece, since toughness is a function of material constraint which is a function of thickness. API Std 1104 does make a half hearted attempt at creating a thickness based essential variable by "suggested groupings" in 6.2.2 (e). It is down to end users to determine whether this suggestion is acceptable or not when taken with toughness requirements.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[weldstan]

Note that B31.3 also provides further thickness limits. Steve has given you the rational in his initial sentence.

In terms of the 5/8" minimum limit, it was instituted in the 1974 Edition. There are conflicting theories behind its imposition; one was that Section III (also fully revamped in 1974) only required impact testing above 5/8", so it was driven by the nuclear industry. The other is based on transference from 2 dimensional cooling to three dimensional cooling during normal welding conditions, so it was linked to heat input.