Comparison on PWHT Requirements of ASME Codes.pdf

COMPARISON OF PWHT REQUIREMENTS OF ASME CODES Note: This information has all been taken from a white paper intended to support revision to the PWHT rules in several ASME Codes. It provided a comparison to the current rules and demonstrates some of the differences between the Codes. While a couple of the Codes referenced have limited application in the nuclear industry, the effort is targeted at providing more consistency between the ASME Codes for the PWHT rules. It should be noted that the B31 Standards Committee has recently authorized the development of a B31 Standard for Preheat and PWHT. General The construction Codes all address PWHT with varying rules for exemptions based on materials, thicknesses, strengths, alloy contents, and the P Numbers as assigned by ASME Section IX. The rules are not consistent between Codes for the thicknesses that require PWHT, the definition of the thicknesses that require PWHT, the material properties that turn on the PWHT requirements, the holding temperatures, the holding times, the heating and cooling rates, and the terminology used to describe the requirements. The general approach is to require PWHT unless exempted. As will be explained in this paper, this in some cases results in degraded weldments as a result of the PWHT. While this may be true for all of the materials being addressed (P Nos. 1, 3, 4, & 5A), further work is needed to make major changes to the low alloy materials. The repair oriented codes (ASME Section XI and the National Board Inspection Code (NBIC)) have allowed the temper bead or half bead alternatives to PWHT. There are related Code proposals which may address the use of temperbead techniques within the B31 Codes (csconnect item 04180 for B31.1) but this is not being addressed in this white paper. The following is a brief description of the various PWHT rules for P Nos. 1, 3, 4, and 5A.

ASME B31.1- Power Piping Code – 2010 Edition ASME B31.1 currently requires PWHT for materials greater than the following thicknesses: P No. 1 (Groups 1,2,3) - > ¾ inch* P No. 3 (Groups 1,2) - > ⅝ inch* P No. 4 (Groups 1,2) - > ½ inch** P No. 5A (Group 1) - > ½ inch** * The thickness may be the weld thickness for some weld configurations but is the greater material thickness otherwise. (Note: B31.1 allows an exemption for P No. 1 materials up to 1½ inch in material thickness with additional criteria of a limited CE and an increased preheat.) ** The thickness is the greater material thickness. Additional limitations exist for carbon content and preheat. Exemptions are offered for non-load-carrying attachments, for seal welds, and for socket welds on P Nos. 4 & 5A materials.

ASME B31.3- Process Piping Code – 2010 Edition ASME B31.3 currently requires PWHT for materials greater than the following thicknesses. Additional criteria include the specified tensile strength of the material and a Brinell hardness after PWHT is specified for some materials. P No. 1 - > ¾ inch* P No. 3 - > ¾ inch** P No. 4 - > ½ inch** P No. 5A - > ½ inch** *The thickness may be the weld thickness for some weld configurations but is the greater material thickness otherwise. ** The thickness may be the weld thickness (any plane through the weld) for some weld configurations but is the greater material thickness otherwise.

ASME Section I - Power Boilers – 2010 Edition with 2011 Addenda ASME Section I currently requires PWHT for materials greater than the following thicknesses: P No. 1 (Groups 1,2,3) - > ¾ inch* P No. 3 (Groups 1,2) - > ⅝ inch* P No. 4 (Groups 1,2) - > ⅝ inch* P No. 5A (Group 1) - >⅝ inch* *The thickness may be the weld thickness for some weld configurations but is the lesser material thickness otherwise. Additional limitations exist for carbon content and preheat. Exemptions are offered for non-load-carrying attachments. An exemption is also offered for seal welds on P No. 4 & 5A materials. ASME Section III, Div 1 – Rules for Construction of Nuclear Facility Components – Subsection NB (Class 1) – 2010 Edition with 2011 Addenda ASME Section III currently requires PWHT for materials greater than the following thicknesses: P No. 1 - >1½ inch* P No. 3 - >⅝ inch* P No. 4 - >½ inch* P No. 5A - >½ inch* *The thickness may be the weld thickness for some weld configurations but is the lesser material thickness otherwise. Additional exemptions are allowed for fillet welds, socket welds, and attachment welds.

ASME Section VIII, Div 1 – Rules for Construction of Pressure Vessels – 2010 Edition with 2011 Addenda ASME Section VIII currently requires PWHT for materials greater than the following thicknesses: P No. 1 (Groups 1,2,3) - > 1½ inch* P No. 3 (Groups 1,2) - > ⅝ inch* P No. 4 (Groups 1,2) - > ⅝ inch* P No. 5A (Group 1) - >⅝ inch* *The thickness may be the weld thickness for some weld configurations but is the lesser material thickness otherwise. Additional exemptions are allowed for fillet welds, socket welds, and attachment welds.