1、STP-PT-033DEGRADATION OF NOTCH TOUGHNESS BY A POST WELD HEAT TREATMENT (PWHT)STP-PT-033 DEGRADATION OF NOTCH TOUGHNESS BY A POST WELD HEAT TREATMENT (PWHT) Prepared by: C. E. Spaeder, W. D. Doty, and W. J. ODonnell ODonnell Consulting Engineers, Inc. Date of Issuance: June 30, 2011 This report was p
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9、ree Park Avenue, New York, NY 10016-5990 ISBN No. 978-0-7918-3380-3 Copyright 2011 by ASME Standards Technology, LLC All Rights Reserved Degradation of Notch Toughness by a Post Weld Heat Treatment (PWHT) STP-PT-033 iii TABLE OF CONTENTS Foreword . iv Abstract v 1 INTRODUCTION . 1 2 APPLICATION OF I
10、MPROVEMENTS IN TECHNOLOGY . 3 3 BASICS OF NOTCH TOUGHNESS 8 6 4 STOUT WRC 302 1 8 5 SPAEDER-DOTY WRC 407 2 . 9 6 TWI STUDIES 19 . 10 7 EPRI STUDIES 21 11 8 STUDIES ON STEELS USED IN THE CREEP RANGE 12 9 KEY OBSERVATIONS 13 10 RECOMMENDATIONS FOR ASME CONSIDERATION . 14 References 15 Appendix A 18 Ap
11、pendix B. 21 Acknowledgments 23 LIST OF TABLES Table 1 - Proceedings - Forty-Third General Meeting . 4 LIST OF FIGURES Figure 1 - Flaw Size-Fatigue Life Relations for Various Critical Crack Sizes 6 STP-PT-033 Degradation on Notch Toughness by a Post Weld Heat Treatment (PWHT) iv FOREWORD This report
12、 is a natural follow-up to three major studies sponsored by the ASME that address a number of Post Weld Heat Treatment (PWHT) issues. It should be noted that early publications identified a PWHT as a highly desirable treatment for weldments. As will become evident from the review that follows and th
13、e summary of key observations, this recommendation is suitable for some steels and unsuitable for others. The purpose of the report is to provide information and recommendations for consideration by the ASME Code writing committees. The observations made in the various documents reviewed, summarized
14、 as “Key Observations,” are the bases for the recommendations that are made for possible revisions in ASME Code rules for PWHT practices. Established in 1880, the American Society of Mechanical Engineers (ASME) is a professional not-for-profit organization with more than 127,000 members promoting th
15、e art, science and practice of mechanical and multidisciplinary engineering and allied sciences. ASME develops codes and standards that enhance public safety, and provides lifelong learning and technical exchange opportunities benefiting the engineering and technology community. Visit www.asme.org f
16、or more information. The ASME Standards Technology, LLC (ASME ST-LLC) is a not-for-profit Limited Liability Company, with ASME as the sole member, formed in 2004 to carry out work related to newly commercialized technology. The ASME ST-LLC mission includes meeting the needs of industry and governmen
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18、c.asme.org for more information. Degradation of Notch Toughness by a Post Weld Heat Treatment (PWHT) STP-PT-033 v ABSTRACT This report is a review of the literature on post weld heat treatment of steels used in ASME Code construction. Based on this review, recommendations are made for use by the ASM
19、E Code writing committees on issues that these committees should consider. Examples of changes include the elimination of a mandatory PWHT for steels used in all lethal service and the use of fracture mechanics studies to justify departures from present ASME Code rules when a PWHT is not needed to a
20、ddress issues such as dimensional control and/or stress corrosion cracking. STP-PT-033 Degradation on Notch Toughness by a Post Weld Heat Treatment (PWHT) vi INTENTIONALLY LEFT BLANKDegradation of Notch Toughness by a Post Weld Heat Treatment (PWHT) STP-PT-033 1 1 INTRODUCTION This report is a natur
21、al follow-up to three major studies sponsored by the ASME. In the first, Stout 1 outlined the metallurgical factors concerning PWHT. The second Spaeder-Doty report 2 focused on ASME Code-related issues. The Uptis-Mokhtarian 3 studies on design margins for the Section VIII of the ASME Code are releva
22、nt to this report. This study includes a comparison of ASME PWHT rules with major European Codes. It is a “stand alone” document that is useful for assessing recommendations that are presented in the report. The Appendix A contains a comprehensive review of ASME Code PWHT issues related to quenched
23、and tempered steels including early references (1955) to what was referred to as “reheat cracking” or “stress relief cracking.” Note that it also reports on full size testing of pressure vessels fabricated from 100 Ksi yield strength steels and subjected to impact loading at cryogenic temperatures.
24、It should be noted that these full scale tests were conducted on vessels that were fabricated from -inch thick plate; the relatively thin plate is beneficial in terms of ductile behavior. The information provided in the Appendix A is part of the database used for developing recommendations contained
25、 herein. It should be noted that early publications 4identified a PWHT as a highly desirable treatment for weldments. As will be evident from the review that follows and the summary of key observations, this recommendation is suitable for some steels but unsuitable for others. There are a number of
26、other reports 5 through 30 that address key technical issues related to a PWHT. A review of the authors of these references points to large amounts of research conducted on the PWHT issues over about 60 years. There are two primary reasons for a PWHT of weldments: (1) The reduction of residual stres
27、ses and (2) the reduction of hardness (usually by a tempering reaction) of the microstructure developed by welding. These two benefits are especially applicable to the early steels (often relatively high carbon in comparison to modern steels) used when welding replaced riveting as the preferred meth
28、od of fabricating steel structures. There is no exact date for this replacement but the developments associated with the building of World War II Liberty Ships resulted in many developments including understanding the role of residual stresses, hydrogen cracking, and brittle fracture as related to f
29、racture mechanics. Some engineers have taken the position that a PWHT should be applied to all/most weldments. These views may reflect “poor teaching” by the academic community. However, the present understanding of the interrelationships among key factors such as design, notch toughness, and stress
30、 level including residual stresses makes it clear that the decision to require a PWHT should be based on the specific of component characteristics in relation to the service conditions. A reduction in residual stresses can be important in preventing distortion following machining of weldments and in
31、 the prevention of stress corrosion cracking under specific conditions 5. A PWHT can have the desirable effect of also out gassing hydrogen, but in certain steels, it may introduce reheat cracking and/or a reduction in notch toughness. As noted above, hardness is sometimes used as an index of suscep
32、tibility to stress corrosion in certain environments and may require that an otherwise satisfactory weldment be PWHT to reduce hardness below a critical level, often about 20 Rockwell C. In general, propensity for stress corrosion cracking increases with hardness level. Other properties that are rel
33、ated to hardness include the creep-rupture properties. At high hardness levels, some steels exhibit notch weakening in stress rupture tests. Unpublished studies by the senior author of this report indicate that certain quenched and tempered high strength steels should not be used at service temperat
34、ures exceeding about 700q F. Studies by Swift 6 identified notch STP-PT-033 Degradation of Notch Toughness by a Post Weld Heat Treatment (PWHT) 2 weakening in stress rupture tests as being associated with stress relief cracking. The present report does not focus on the effect of PWHT on service perf
35、ormance at temperatures where design is based on creep and creep-rupture properties. However, given the importance of this issue, a general overview is presented for steels specifically designed for enhanced creep and creep-rupture properties. The heat treating practices and steel chemical compositi
36、ons that provide the optimum creep-rupture properties are often not compatible with good notch toughness. As will be covered in a subsequent section, PWHT may be needed to prevent brittle behavior and when there is susceptibility to stress corrosion cracking. The undesirable effects of introducing r
37、eheat cracks is of special concern because the nature of these micro cracks is that they may not be detected by routine nondestructive testing such as radiography. Moreover, the reheat cracks are typically found in the coarse-grained region of the heat affected zone (HAZ) of a weldment. The properti
38、es (including notch toughness) and microstructure of the HAZ are strongly dependent on welding parameters such as preheat and heat inputs. Thus, the ASME Code should recognize the importance of a careful consideration of both the desirable and the potential harmful effects before mandating a PWHT. T
39、he PWHT practice cannot only introduce cracks that may go undetected but also degrade notch toughness thereby reducing the tolerance for cracks. It is also important to recognize that the microstructure and residual stress characteristics developed in the weldment are dependent on the specific chemi
40、cal composition of both the base metal and weld metal and the welding parameters. In effect, there are an infinite number of combinations of HAZ characteristics; thus, it is easy to identify steels that are prone to reheat cracking but difficult to rule out with certainty that a grade of steel is fu
41、lly immune to reheat cracking. As noted above, there are steels, modern or not, that require a PWHT to prevent susceptibility to brittle fracture and stress corrosion cracking. Thus, it is clear that a PWHT is neither universally good nor bad. The purpose of the present report is to sort out these i
42、ssues to provide information and recommendations for consideration by the ASME Code writing committees. The observations made in the various documents reviewed, summarized as “Key Observations,” are the bases for the recommendations that are made for possible revisions in ASME Code rules for PWHT pr
43、actices. In 2006 7, work from The Welding Institute (TWI) was presented reviewing some of the topics covered in this report. The Electric Power Research Institute (EPRI) has also conducted proprietary studies on PWHT issues. Degradation of Notch Toughness by a Post Weld Heat Treatment (PWHT) STP-PT-
44、033 3 2 APPLICATION OF IMPROVEMENTS IN TECHNOLOGY Improvements in production practices for materials used in ASME Code applications, especially in terms of notch toughness, make a periodic review of ASME Code requirements highly desirable. It is especially important to keep the ASME Code practices c
45、ost-effective and consistent with the properties of modern steels. The writings of Barsom 8 9 provide clear practical guidelines on a methodology 9 to integrate notch toughness considerations into welding Codes. This methodology is summarized as a Fracture Control Plan (FCP). The Barsom writings are
46、 especially useful in that they serve as a teaching tool to bring engineers without a detailed background in fracture mechanics to a satisfactory level of understanding this complex field. One historical note is that the 1954 studies 10 conducted after the catastrophic failure of Liberty ships durin
47、g World War II found that residual stresses were not nearly as important as had been expected. The following is a direct quote 11 from the British Admiralty: “Residual stresses were originally thought to be an important contributory cause of the failure of welded ships, but extensive investigations
48、have not confirmed this. It was inferred that in order to produce brittle failure in a ductile material, the residual stress would have to be triaxialbiaxiality would not be sufficient; but triaxiality could occur only locally, whereas the fractures were extensive. It was therefore concluded that re
49、sidual stresses do not impair the strength provided the material is in a notch ductile condition, but may lead to brittle fracture if the material is in a notch brittle condition. Material in the latter condition would, however, be liable to brittle fracture even if there were no residual stresses.” It follows that residual stresses can be tolerated from a brittle fracture point of view if the steels exhibit sufficient notch toughness. Pellini studies 12 related brittle fracture and residual stresses to the Nil Ductility Temperature
