There are circumstances where welded repairs to a pressure vessel and associated fittings may be carried out without a subsequent PWHT, provided that the procedure complies with the requirements of relevant specifications. Several parts of the ASME code allow such repair, provided that certain requirements, such as on preheating or on the maximum depth or area of repair, are met. In PD 5500, such repair is restricted to lightly-loaded and non-pressure parts, and requires agreement between the purchaser, manufacturer and regulatory authorities, as appropriate, as well as suitable tests and controls to establish that the material will not be adversely affected.
The use of a half bead technique, in which approximately half the depth of a first layer of weld metal is removed by grinding, followed by deposition of subsequent layers, using slightly larger electrodes with a maximum diameter of 4mm instead of the 3mm, used for the first layer is permitted in ASME codes (e.g.[2,3] ) and in API 510. The objective is to assure tempering of the prior weld beads and their heat affected zones. A problem with this approach is the difficulty of determining precisely the depth of metal removed, once the surface features of the weld bead have disappeared. Consequently, the temper bead technique[5,6], involving the deposition of two or more layers of weld metal without significant grinding of the first layer, is a much more reliable approach.
The MMA temper bead procedure[5,6] involves depositing a first layer with 2.5 mm diameter electrodes, removing the crown of the first layer by grinding, and depositing a second layer using 3mm diameter electrodes. Subsequent layers are deposited with electrodes which are no larger than 4mm diameter. At least one layer of re-enforcement shall be added and then ground to achieve a finished surface substantially flush with the surface of the vessel surrounding the weld. Upon completion of the repair, the weld area must be held at a temperature of 230 to 290°C for a minimum of 2 hours for an ASME P1 material and 4 hours for an ASME P3 material. Provision is also made for controlled deposition welding in ASME IX (Ref); with certain specified exceptions, manual and semi-automatic TIG and plasma welding are prohibited (Paragraph QW290.2). Paragraph QW-290.3 lists the essential and non-essential variables that apply when temper bead qualification is required.
In the National Board Inspection Code [NBIC] in the USA, covering repair after service exposure, the 1977 issue was the first to include weld repair procedures for which PWHT could be omitted for C-, C-Mn, C-Mn-Si, C-0.5Mo and 0.5Cr-0.5Mo steels. As discussed by Doty, in the light of data provided by several research programmes, the 1995 issue extended the list of steels to include ASME P4 and P5 (Cr-Mo) steels; the 2007 issue also includes some P9, P10 and P11 steels. In view of the high cost, and in some cases impracticability, of carrying out PWHT after repair, it is likely that similar changes will be adopted more widely, as adequate supporting data are generated.
- PD 5500: 2009 'Specification for unfired fusion welded pressure vessels', British Standards Institution, 2009, Section 188.8.131.52.
- ASME Boiler and pressure vessel code, Section I 'Rules for construction of power boilers', Paragraph PW-40.3.4(f), American Society of Mechanical Engineers, New York, July 2004, confirmed in 2010.
- ASME Boiler and pressure vessel code, Section VIII 'Rules for construction of pressure vessels', Paragraph UCS-56 4 (c) Requirements for postweld heat treatment, American Society of Mechanical Engineers, New York, July 2004,confirmed in 2010.
- API 510 'Pressure vessel inspection code: Maintenance inspection, rating, repair and alteration' eighth edition, June 2006.
- ASME Boiler and pressure vessel code, Section III 'Rules for construction of nuclear facility components', Paragraph NB-4622.9 Temper bead weld repair, American Society of Mechanical Engineers, New York, July 2004, confirmed in 2010.
- ASME Boiler and pressure vessel code, Section XI 'Rules for service inspection of nuclear power plant components', Paragraph IWA-4623.1 Temper bead welding of similar materials - Shielded metal arc welding, American Society of Mechanical Engineers, New York, July 2004, confirmed in 2010.
- ASME Boiler and pressure vessel code, Section IX Welding and brazing qualifications, Paragraph QW-290 Temper bead welding, American Society of Mechanical Engineers, New York, July 2004,confirmed in 2010.
- National Board Inspection Code - A manual for boiler and pressure vessel inspectors, National Board of Pressure Vessel Inspectors, Columbus, Ohio, USA, 2007, with 2009 Addendum: Part 3, Section2.5.3.
- Doty W D: 'History and need behind the new NBIC rules on weld repair without PWHT', Welding Research Council Bulletin 412, June 1996, 3-8.