1、An Experimental Study of Causes and Repair of Cracking of 11/4Cr-1/2Mo Steel EquipmentAPI PUBLICATION 938-A MAY 1996API PUBL*938 96 m 0732290 0560345 973 = SPECIAL NOTES (1) API publications necessarily address problems of a general nature. With respect to particular circumstances, local, state, and
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7、API, 1220 L Street, N.W., Washington, D.C. 20005. Copyright 1996 Welding Research Council Inc./American Petroleum Institute API PUBLJ3B 96 0732290 0560346 BOT FINAL REPORTTO THE AMERICAN PETROLEUM INSTITUTE OF RESEARCH UNDER THE GUIDANCE OF THE TASK GROUP ON MATERIALS AND CORROSION RESEARCH OF THE C
8、OMMIlTEE ON CORROSION AND MATERIALS AN EXPERIMENTAL STUDY OF CAUSES AND REPAIR OF CRACKING OF 1 l/qCr=%Mo STEEL EQUIPMENT MAY 1996 Contractor: The Materials Properties Council, Inc. Investigator: M. Prager Subcontractor: Department of Materials Science University of Tennessee Investigators: C. D. Lu
9、ndin, P. Liu, C. Y. P. Qiao, G. Zhou and K. K. Khan API PUBL*738 76 = 0732270 O560347 74b FOREWORD The origin of this project was an in-depth study for API of numerous reported incidents of cracking of equipment of lCr-%Mo and l%Cr-/zMo-Si steels, The report to MI is an explanation of the problem an
10、d the basis for further work to help prevent and repair such cracking. Metallurgical reports, fabrication records and service histories were reviewed. Worldwide research on the subject by steelmakers and studies of these alloys and similar materials in related applications were considered. In many c
11、ases, the cracking was major and cracks propagated in service. Emphasis in the report was placed on the causes of crack initiation during fabrication or of their appearance after only a short time in service. It was concluded that major contributions to the cracking were from poor design, fabricatio
12、n and operating practices which should be corrected using reasonable precautions and well known technology. Such action would pre- vent future vessels from entering service with preexisting cracks or initiating cracks in service. However, there was strong evidence that some of the plates and forging
13、s used for vessel construction were more prone to cracking than others or have disturbingly low toughness. This study was intended to recommend ways to eliminate detrimental fabrication practices and materials. Fabrication and repair operations must be upgraded because subsurface cracks which cannot
14、 be readily detected may occur and then emerge in service. Repairs have been troublesome. Specifically, the study was developed to address the materials, fabrication and repair issues of greatest concern. Under API and MPC Task Groups, Chaired by J. McLaughlin. The objec- tives of the Phase II Study
15、 were established as follows: 1) Develop an understanding of the fabrication/welding factors that affect cracking of Cr-Mo equipment, including the effects of PWHT and pre- heat temperature. 2) Develop an understanding of the inherent material properties that affect cracking of Cr-Mo equipment. This
16、 was to include the effects of impuri- ties in the steel and initial condition of the steel (i.e., annealed vs. normalized and tempered). 3) Define a controlled deposition (temper bead) procedure for repair and initial fabrication that will produce a fine grain, more damage tolerant, microstructure
17、in the weld heat affected zone (HAZ). 4) Determine the effect of using lower carbon, lower strength fillers for repair welds. Experience suggests that depending on conditions, the use of a low carbon filler can either improve or impair the performance of a repaired weld. Appreciation is expressed to
18、 API for support. Portions of the work were cost shared by MPC, PVRC and WRC. This work resulted in important new physical simulation, weldability and notched bar test methods. Fresh insight was gained into the heat affected zone metallurgy of this important class of materials. Dr. M. Prager Executi
19、ve Director, Materials Properties Council, Inc. 11 API PUBL*938 96 0732290 0560348 682 CONTENTS Executive Summary 1 For Applications of 11/ evaluate controlled deposition repair tech- niques; determine the suitability of low carbon filler materials; and understand the role of fabrication and welding
20、 practices on susceptibility to cracking. The program succeeded in all objectives. It was found that fabrication, repair- and service-related cracking often have the same roots and are responsive to the same remedial action. Problems arise where there is low heat affected zone ductility. The followi
21、ng conclusions and recommendations are therefore pro- vided. For Applications of 11/4Cr-1/2Mo Steel at 825F and Higher (a) It is strongly recommended that Class 1 (60/35 ksi tensile, yield strength) materials be specified in Final report to API on Prevention and Repair ofcracking zn Chrome Moly Equi
22、pment, MPC, September, 1990. preference to Class 2 (75/45 ksi tensile, yield strength), accelerated cooled materials. (b) High PWHT temperatures were found to be necessary to improve heat affected zone ductility. PWHT requirements are related to welding variables and material composition. Fabricatio
23、n guidelines are provided herein with specific PWHT recommenda- tions depending on composition, desired strength and welding variables. (c) High PWHT temperatures may be used with- out undesirably impairing creep strength and charpy impact values provided carbon content is not too high. (d) Certain
24、materials display a high sensitivity to cracking. Materials Composition Factors (MPC-5, MPC-7) have been identified and can be used to screen the materials. All of the elements included in the factors may not normally be included in the specification requirements and thus the range of elements contr
25、olled must be especially requested with the accuracy defined in Appendix B. (e) Design, fabrication and materials specifications may now be prepared to assure freedom from cracking. (f Controlled deposition welding techniques and low carbon filler metals may be implemented in repair strategies when
26、performance objectives and materials are identified. (g) A number of screening tests have been demon- strated as suitable for determining material sensitiv- ity to fabrication-related cracking. These include (Gleeble) simulated heat afected zone cracking, spi- ral notch rupture and large scale (PREV
27、EW) weldabil- ity tests. These tests are not intended as require- ments for material purchase. However, if the composition suggests that the material may be sensi- tive to reheat and in-service cracking, it may be wise to consider these tests to define the extent of antici- pated problems. (h) Studi
28、es of smooth and notch bar stress rupture behavior of simulated CGHAZ specimens provided insight into the effect of PWHT, heat input and microstructure on creep rate, ductility and cracking tendency. (i) The results of this work have shown that the term “creep embrittlement” when applied to the low
29、Causes and Repair of Cracking 1 API PUBL*93 9b = 0732290 05b0351 177 ductility in-service cracking in the low Cr-Mo materi- als is inappropriate. The low ductility behavior is essentially inherent from the initiation of service and is a combined result of the material factors in terms of chemistry,
30、resistance to tempering and the degree of thermal treatment provided prior to service. No embrittlement was found to be caused by service in the creep range and, therefore, the use of the term creep embrittlement to describe service behavior of l%Cr-l/zMo HAZ is not appropriate. Report Overview and
31、Conclusions The attached report documents a comprehensive and complex study of cracking associated with an alloy for use at elevated temperatures. The Research Plan was developed and reported in an MPC docu- ment Final Report to API-Prevention and Repair of Cracking in Chrome-Moly Equipment. It was
32、pre- sented originally as a two year plan for Phase II to be conducted under the guidance of the API Task Group on Corrosion and Materials Research which priori- tized the program tasks as follows: 1. Effects of Fabrication and Welding Develop an understanding of the fabrication/ welding factors tha
33、t affect cracking of Cr-Mo equipment. This will include the effects of PWHT and preheat temperature Develop an understanding of the inherent mate- rial properties that affect cracking of Cr-Mo equipment. This will include the effects of impu- rities in the steel and initial condition of the steel (i
34、.e., annealed vs. normalized and tem- pered). Define a Controlled deposition (temper bead) procedure for repair and initial fabrication that will produce a fine grain microstructure in the heat affected zone (HAZ). Determine the effect of using lower carbon, lower strength fillers for repair welds.
35、Experi- ence suggests that depending on conditions, the use of a lower carbon filler can either improve or impair the performance of a repaired weld. Additional work on hydrogen effects originally sug- gested by MPC received a low priority and was not pursued. While the objectives of the tasks are d
36、efined sepa- rately, the work was performed in a testing plan that most efficiently explored the various interrelated issues. Appendix J indicates the relationships of the various tasks as originally described. The various studies in Phase II that are docu- mented and attached here are: (a) Update o
37、f the Literature Survey (Appendix A) (b) Compositional and Microstructural Studies, Heat Affected Zone Transformation and Metallurgi- cal Characteristics (Appendixes B and c) 2. Materials Variables 3. Controlled Deposition Repair Procedures 4. Filler Metals (c) Assessment of Reheat Cracking Suscepti
38、bility (Appendix D) (d) Predicting Reheat Cracking Susceptibility Based on Chemical Composition (Appendix E) (e) Toughness Study (Appendix F) (f) Microstructural and Fractographic Evalua- (g) Notch Bar and Smooth Bar Stress-Rupture (h) Repair Welding (Appendix I) (i) Original Phase II Plan (Appendix
39、 J) The overall logic of the program was as follows: 1. obtain a broad range of materials; 2. select small scale notch tests to screen material variables for susceptibility to elevated tempera- ture cracking; 3. screen and rank materials on the basis of HAZ behavior; 4. validate ranking and test pre
40、dictions by large scale tests; 5. systematically evaluate material variables using small scale tests; 6. examine repair procedures on sensitive heats with large scale tests; 7. use a notched bar rupture test for simulation of cracking in-service; 8. examine the effects of materials and fabrication v
41、ariables on in-service cracking probability; and 9. rank materials and heat treatments for in- service cracking tendency. tions (Appendix G) Studies (Appendix H) A total of seventeen commercial heats were ob- tained and information on others was utilized. Based on analyses of the behavior of the mor
42、e than twenty heats it has been concluded that the hardest areas in the weld heat affected zones of l%Cr-i/Mo steel respond relatively slowly to PWHT and may display low ductility at elevated temperatures. Ductility de- pends on material composition, weld heat input and PWHT conditions. While these
43、qualitative character- istics were not surprising, the quantitative details which emerged from the study were. For example: (a) heat affected zone ductilities among the materi- als varied by a factor of ten; (b) coarse grained heat affected zones of high car- bon materials tended to display low duct
44、ility, perhaps only a fraction of 1% to failure, even after PWHT; (c) for a given heat input and hardness, creep rates of coarse grained heat affected zones varied by as much as a factor of 10 depending on composition (transformation microstructure); (d) the ductilities of some heats were improved s
45、ignificantly by heat treatment while others reached a plateau and remained relatively notch sensitive; (e) smooth bar and notched bar stress-rupture lives of the materials were found to vary by as much as a factor of ten; (f) there is no evidence that the materials become brittle in time (creep embr
46、ittlement). Instead, it is concluded that brittleness is a consequence of the 2 Causes and Repair of Cracking API PUBLX938 96 0732290 0560352 003 as-tempered microstructure which must be softened significantly before ductility can be observed; (g) creep rates and smooth bar rupture lives of simulate
47、d HAZs were only slightly affected by temper- ing temperatures from 125OoF-1350“F. However, the ratios of notch to smooth bar lives and ductilities tended to improve; (h) the PWHT temperature necessary to reduce significantly notch sensitivity in the heat affected zones varied among the heats by as
48、much as 100F. Fabrication and repair procedures should take this into account; (i) controlled deposition techniques and low car- bon filler metals may be used to reduce the tendency for cracking during heat treatment and service; u) materials that were found to display PWHT cracking susceptibility t
49、ended to rate poorer expecta- tions for service; (k) as a result of this work the compositional factors identified as useful are shown below. All quantities are expressed in wt %. (See Appendix E for a more detailed description of these factors); MPC Factor-5 = Cfn(Tramp + Sin)Alfnl - 1 Cfn = (5C + 1000Nb + 1OOV + 50Ti - 0.5) + 1 Tramp = 24.3(Sn + As) + 150Sb + Cu + 50(P - 0.01)l Sfn = 1 + 306 - 0.02 Tramp); For Sfn 825F). The fabrication guidelines recommend that the users first establish the composi- tion of the material of construction and consider the strength level (Class) t
