U.S. NUCLEAR REGULATORY COMMISSION March 2011

Revision 1

REGULATORY GUIDE

OFFICE OF NUCLEAR REGULATORY RESEARCH

The NRC issues regulatory guides to describe and make available to the public methods that the NRC staff considers acceptable 

for use in implementing specific parts of the agency=s regulations, techniques that the staff uses in evaluating specific problems or 

postulated accidents, and data that the staff needs in reviewing applications for permits and licenses. Regulatory guides are not 

substitutes for regulations, and compliance with them is not required. Methods and solutions that differ from those set forth in 

regulatory guides will be deemed acceptable if they provide a basis for the findings required for the issuance or continuance of a 

permit or license by the Commission. 

This guide was issued after consideration of comments received from the public. 

Regulatory guides are issued in 10 broad divisionsC1, Power Reactors; 2, Research and Test Reactors; 3, Fuels and Materials 

Facilities; 4, Environmental and Siting; 5, Materials and Plant Protection; 6, Products; 7, Transportation; 8, Occupational Health; 

9, Antitrust and Financial Review; and 10, General. 

Electronic copies of this guide and other recently issued guides are available through the NRC’s public Web site under the 

Regulatory Guides document collection of the NRC’s Electronic Reading Room at 

http://www.nrc.gov/reading-rm/doc-collections/ and through the NRC’s Agencywide Documents Access and Management 

System (ADAMS) at http://www.nrc.gov/reading-rm/adams.html, under Accession No. ML101870612. The regulatory analysis 

may be found in ADAMS under Accession No. ML101870625. 

REGULATORY GUIDE 1.50 

(Draft was issued as DG-1222, dated June 2009)

CONTROL OF PREHEAT TEMPERATURE FOR WELDING OF 

LOW-ALLOY STEEL 

A. INTRODUCTION

This guide describes a method that the staff of the U.S. Nuclear Regulatory Commission (NRC) 

considers acceptable for implementing regulatory requirements related to the control of welding for lowalloy steel components during initial fabrication. This guide applies to light-water-cooled reactors. 

General Design Criterion 1, “Quality Standards and Records,” of Appendix A, “General Design 

Criteria for Nuclear Power Plants,” to Title 10, of the Code of Federal Regulations, Part 50, “Domestic 

Licensing of Production and Utilization Facilities” (10 CFR Part 50) (Ref. 1), requires that structures, 

systems, and components important to safety be designed, fabricated, erected, and tested to quality 

standards commensurate with the importance of the safety function to be performed. Appendix B, 

“Quality Assurance Criteria for Nuclear Power Plants and Fuel Reprocessing Plants,” to 10 CFR Part 50 

requires that measures be established to ensure control of materials and of special processes such as 

welding and that proper process monitoring be performed. 

This regulatory guide contains information collection requirements covered by 10 CFR Part 50 that the 

Office of Management and Budget (OMB) approved under OMB control number 3150-0011. The NRC 

may neither conduct nor sponsor, and a person is not required to respond to, an information collection 

request or requirement unless the requesting document displays a currently valid OMB control number. 

This Regulatory Guide is a rule as designated in the Congressional Review Act (5 U.S.C. 801–808). 

However, the NRC has determined this Regulatory Guide is not a major rule as designated by the 

Congressional Review Act and has verified this determination with the OMB.
Rev. 1 of RG 1.50, Page 2 

B. DISCUSSION

Section III, “Nuclear Power Plant Components,” of the American Society of Mechanical 

Engineers Boiler and Pressure Vessel Code (ASME B&PV Code) (Ref. 2) specifies certain requirements 

associated with manufacturing Code Class 1, 2, and 3 components. 

Procedure Qualification 

Section III requires adherence to Section IX, “Welding and Brazing Qualifications,” of the 

ASME B&PV Code, including the requirements governing procedure qualifications for welds. Review of 

the requirements of Section IX for procedure qualifications and the fabrication requirements of Section III 

indicates the desirability of supplementary requirements to ensure adequate control of welding variables 

in the production welding of low-alloy steels. The assurance of satisfactory welds in low-alloy steels can 

be increased significantly, and, in particular, the propensity for cracks (cold cracks) or reheat cracks 

forming in underbead areas and heat-affected zones can be minimized, by maintaining proper preheat 

temperatures on the base metals concurrent with controls on other welding variables. 

 Cold cracking can occur when the steel is hardened (i.e., it undergoes a phase transformation to 

martensite in the heat-affected zone, weld metal, or both). The martensite exhibits brittle fracture 

tendencies, and it may not be able to withstand rapid cooling and the volume change associated with the 

phase transformation without the occurrence of local cracking. This susceptibility to cracking increases 

with higher stresses, such as those experienced with increased thickness of the part being welded, and also 

increases with a decrease in welding energy input. To avoid or minimize the effects of hardening 

associated with phase transformation, a longer cooling time is needed for the weld; in other words, the 

preheat temperature should be maintained high enough to achieve an acceptable condition of the phase 

transformation. 

 It is generally recognized that atomic hydrogen absorption and diffusion into and through the 

region being welded have an important influence on the tendency to form cracks. The level of hydrogen 

in weld filler metal may be low enough to preclude adverse effects in the welds, but greater quantities of 

hydrogen can be present in the weld region from the dissociation of moisture in hygroscopic welding 

fluxes or from adsorption on metal surfaces if the welding fluxes and surfaces have not been properly 

dried before weld deposition. Embrittlement of metal in the weld area as the result of the presence of 

hydrogen generally occurs at lower temperatures and may be prevented by prolonging the time the 

weldment is maintained at preheating temperature or by performing a postweld heat treatment. 

Preheating at the recommended temperatures can reduce local hardening by reducing the cooling rate 

during welding. Prolonged time at the preheating temperature can assist in reducing the adverse effects of 

a potential hydrogen gradient. This gradient would disappear by means of diffusion of the hydrogen 

before the weldment is returned to room temperature. Therefore, the minimum preheat temperature 

should be established to ensure a desirable cooling rate for the weld, and this temperature should be 

maintained until the achievement of a postweld heat treatment or a postweld hydrogen bakeout. 

In addition to the minimum preheat temperature, a maximum interpass temperature must be 

specified in accordance with the requirements of ASME B&PV Code Section IX if toughness is a 

requirement of the construction code or the design. For low-alloy steel welds not requiring impact 

testing, a maximum interpass temperature should be selected on the basis of such influencing factors as 

the chemical composition of the steel. 
Rev. 1 of RG 1.50, Page 3 

Production Welds

The procedure qualification by itself does not ensure that the production welds will be made 

within the specified preheat temperature range. To ensure that the welds will be acceptable, the metal 

temperature should be monitored during the welding process and through postweld heat treatment or 

postweld hydrogen bakeout. 

C. REGULATORY POSITION 

Weld fabrication1

 for low-alloy steel (P Nos. 3, 4, and 5A) components should comply with the 

fabrication requirements specified in Sections III and IX of the ASME B&PV Code, supplemented by the 

regulatory positions below. 

1. The Welding Procedure Specification should specify a maximum interpass temperature and a 

minimum preheat temperature that are equal to the maximum interpass temperature and minimum preheat 

temperature used during procedure qualification. 

2. For production welds, the preheat temperature should be maintained until final postweld 

heat treatment or a post weld hydrogen bakeout is performed between 200 and 400oC (400 and 

750oF) for a minimum of four hours after which the component may be slowly cooled to ambient 

temperature prior to the performance of the final post weld heat treatment. . The postweld 

hydrogen bakeout temperature and soak time should be based on the materials being welded, the 

geometry, and the welding process used. 

3. Production welding should be monitored to verify that the limits on preheat and interpass 

temperatures are maintained. 

4. If Regulatory Positions 1, 2, and 3 above are not met, the weld is subject to rejection. However, 

the soundness of the weld may be demonstrated by an acceptable examination procedure meeting the 

acceptance criteria specified in Section III of the ASME B&PV Code. 

D. IMPLEMENTATION

The purpose of this section is to provide information to applicants and licensees regarding 

the NRC’s plans for using this regulatory guide. The NRC does not intend or approve any imposition or 

backfit in connection with its issuance. 

In some cases, applicants or licensees may propose or use a previously established acceptable 

alternative method for complying with specified portions of the NRC’s regulations. Otherwise, 

the methods described in this guide will be used in evaluating compliance with the applicable regulations 

for license applications, license amendment applications, and amendment requests.

 

1

 This position does not apply to weld repairs after initial fabrication.
Rev. 1 of RG 1.50, Page 4 

REFERENCES 

1. 10 CFR Part 50, “Domestic Licensing of Production and Utilization Facilities,” 

U.S. Nuclear Regulatory Commission, Washington, DC.2

2. ASME Boiler and Pressure Vessel Code, American Society of Mechanical Engineers, New York, 

NY.3

 

2 All NRC regulations listed herein are available electronically through the Electronic Reading Room on the NRC=s 

public Web site, at http://www.nrc.gov/reading-rm/doc-collections/cfr/. Copies are also available for inspection 

or copying for a fee from the NRC=s Public Document Room (PDR) at 11555 Rockville Pike, Rockville, MD; 

the mailing address is USNRC PDR, Washington, DC 20555; telephone (301) 415-4737 or (800) 397-4209; 

fax (301) 415-3548; and e-mail pdr.resource@nrc.gov. 

3 Copies of American Society of Mechanical Engineers (ASME) standards may be purchased from ASME, Three Park 

Avenue, New York, NY 10016-5990; telephone (800) 843-2763. Purchase information is available through the ASME 

Web-based store at http://www.asme.org/Codes/Publications/.