1、STP-PT-027EXTENDED LOW CHROME STEEL FATIGUE RULESSTP-PT-027 EXTEND LOW CHROME STEEL FATIGUE RULES Prepared by: Martin Prager Pressure Vessel Research Council Date of Issuance: January 29, 2009 This report was prepared as an account of work sponsored by ASME Pressure Technologies Codes and Standards
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9、pyright 2009 by ASME Standards Technology, LLC All Rights Reserved Extend Low Chrome Steel Fatigue Rules STP-PT-027 TABLE OF CONTENTS Foreword v Abstract . vi 1 INTRODUCTION . 1 2 MATERIALS. 2 3 CREEP-FATIGUE DATA. 4 4 CREEP-FATIGUE INTERACTION. 5 5 A MODEL FOR CREEP-FATIGUE IN PRESSURE VESSEL APPLI
10、CATIONS 6 6 CREEP-FATIGUE DAMAGE AND EVALUATING 9 7 THE DESIGN CURVE 12 8 COMMENT ON MARGINS. 14 9 PROPOSAL FOR TEST PROGRAM . 16 References 21 Acknowledgments 22 Abbreviations and Acronyms. 23 LIST OF TABLES Table 1 - Creep-fatigue Test Matrix (hours) 16 Table 2 - Creep-fatigue Test Matrix (cycles)
11、 . 16 LIST OF FIGURES Figure 1 - The Effect of Tensile Strength on the 105Hour Stress Rupture Strength at 850F for 2 Cr-1Mo-V Alloy. 3 . 2 Figure 2 - Cyclic Softening and Hardening Behavior are Illustrated. High Strength Cr-Mo-V Alloys of Interest Here Display the Softening Behavior in the Upper Plo
12、t 3 Figure 3 - Assembled Creep-fatigue Data for Strain Softening Alloys Showing Similarity of Behavior. 4 Figure 4 - Creep-fatigue Interaction Diagram of Type Used by International Codes for the Strain Softening Alloy 91. 5 Figure 5 - Cyclic Straining in a Creep-fatigue Test Will Accelerate the Cree
13、p Strain Rate and Thereby Shorten Creep Life. 7 Figure 6 - Interaction Diagram Indicates Strong Creep- Fatigue Interaction for Endos Data Shown in Figure 3. . 9 Figure 7 - Krempls Study of the Effect of Hold Time on High and Low Ductility Materials. . 10 Figure 8 - Predictions of Hold Time Effects o
14、n Cyclic Life for 3 Plastic Strain Amplitudes 11 Figure 9 - Total Life Increases With Fewer Cycles, i.e., Longer Hold Time. 11 iii STP-PT-027 Extend Low Chrome Steel Fatigue Rules Figure 10 - Fatigue Cycles Dependent on Creep Life With Comparison to No Hold Time Fatigue Tests. .12 Figure 11 - Compar
15、ison of Design Line and Experiments. 13 Figure 12 - Hold Time Creep-fatigue Data as Compared to Design Lines Indexed to Stress Rupture Life Absent Fatigue. Only the Very High Strain Results on Brittle Material Approach the Design Curves. .14 Figure 13 - Reduction in Life Associated With Increase in
16、Pseudoelastically Calculated Stress Amplitude. 14 Figure 14 - Comparison of Life With and Without Fatigue Cycling for Various Pseudoelastically Calculated Stresses15 Figure 15 - Comparison of Test Results (top) With Model Prediction (bottom) of Tertiary Creep Strain Accumulation With and Without Str
17、ain Cycling. With Strain Cycling, Tertiary Creep Strain Rises Rapidly as Compared to Constant Stress18 Figure 16 - Comparison of Test Results (top) with Model Prediction (bottom) of Creep Rate Acceleration With Tertiary Creep Strain Accumulation. Test Results With and Without Strain Cycling Disclose
18、 Cyclic Strain Softening (top). With Strain Cycling, Tertiary Creep Rate Rises Rapidly as Compared to Constant Stress as Predicted by Model (bottom). 19 Figure 17 - Comparison of Test Results (top) With Model Prediction (bottom) of Total Strain Accumulation With Strain Cycling Plus Steady Load Creep
19、 for Indicated Cycles. With Strain Cycling, Strain Rises Rapidly as Compared to Constant Load, see Figure 15.20 iv Extend Low Chrome Steel Fatigue Rules STP-PT-027 FOREWORD This document was developed under a research and development project which resulted from ASME Pressure Technology Codes therefo
20、re, this development work is of high interest to the petrochemical industry. vi Extend Low Chrome Steel Fatigue Rules STP-PT-027 1 INTRODUCTION The impetus for this activity arises because the new ASME B&PV Code, Section VIII, Division 2 rules permit high strength materials of the type enumerated to
21、 be used to temperatures above 700F and into their respective creep ranges. A life limiting failure mode is potentially the phenomenon of “creep-fatigue.” We shall define a “creep-fatigue” failure as one in which life is shorter than that expected due to either creep or fatigue acting on a structure
22、 independently. This occurs in those regimes of stress, strain-rate, time and temperature where the damage mechanisms due to creep and fatigue can be expected to damage the same microstructure and property characteristics. Creep-fatigue is of concern especially where there may be time-dependent stra
23、ining and where varying stresses (loads, including start-up and shut down) are among the design conditions. Comprehensive and correct creep-fatigue design rules are needed now for the aforementioned alloys because, under the new Section VIII, Division 2 rules, as the respective creep ranges of the m
24、aterials are approached, in many cases the allowable stresses are significantly higher than those for which there is applicable service experience that would permit exempting design details from fatigue analysis based on documented “years of relevant experience.” The same must be said for any new al
25、loys and applications for which there is literally no relevant service experience. In summary then, the combination of new materials and applications for advanced energy systems with higher allowable stresses and increased design temperatures requires an understanding of creep-fatigue not now availa
26、ble, analytical models to explain and express damage accumulation and relevant test data in order that new, justifiable and correct rules may be developed. 1 STP-PT-027 Extend Low Chrome Steel Fatigue Rules 2 MATERIALS Relatively high strength alloys such as the very popular 2 Cr-1Mo-V (22V) and mod
27、ified 9 Cr- 1Mo-V-Cb-N (91) achieve their superior properties through accelerated cooling of these hardenable alloy steel compositions from high (normalizing) temperatures, transformation of the microstructure to martensite or bainite followed by tempering. For these materials, the specified minimum
28、 ambient temperature yield and tensile strengths are 60 and 85 ksi, respectively. Corresponding maximum respective yield and tensile strength values may range up to about 85 and 110 ksi. Typical values of strengths in finished pressure vessels are likely to be about 70 ksi yield and 92 ksi tensile.
29、For the ranges of room temperature strengths usually expected, the time-dependent stress-rupture and creep properties increase directly as shown in Figure 1 for the 100,000 hour stress-rupture strength at 850F for the 22V material. Figure 1 - The Effect of Tensile Strength on the 105Hour Stress Rupt
30、ure Strength at 850F for 2 Cr-1Mo-V Alloy. 3 Elevated temperature straining of the alloys under consideration during creep exposure or cyclic stressing will lower the tensile strength and hardness, alter the optimal microstructure from that obtained by proper heat treatment and reduce the creep life
31、. This behavior is well known and has been reported for decades in studies of 1Cr-1Mo-V turbine rotor steels and, more recently, in studies of the modified 9Cr-1Mo-V alloy used in many power piping and similar applications. Figure 2 below contrasts strain softening behavior of a high strength Cr-Mo-
32、V alloy with that of a strain hardening material such as a low tensile strength austenitic stainless steel or a conventional low tensile strength ferritic steel. Data on the latter types of materials are not useful in developing the approach to creep-fatigue design sought in this ASME project for th
33、e strain softening materials such as the accelerated cooled and enhanced 1-1/4, 2-1/4 and 9 to 12 Cr alloys. 2 Extend Low Chrome Steel Fatigue Rules STP-PT-027 Figure 2 - Cyclic Softening and Hardening Behavior are Illustrated. High Strength Cr-Mo-V Alloys of Interest Here Display the Softening Beha
34、vior in the Upper Plot. 3 STP-PT-027 Extend Low Chrome Steel Fatigue Rules 3 CREEP-FATIGUE DATA Creep-fatigue data have been developed in tests utilizing many combinations of strain range, hold time, temperature and load measurement. For the most part, creep-fatigue tests are run with loads that cyc
35、le between compressive and tensile and with tensile hold periods ranging from seconds to times exceeding a few minutes, but rarely more than an hour. Plastic strain amplitudes typically do not exceed 1-2 percent and are usually only a fraction of 1 percent. The total number of cycles applied before
36、failure or a specific load reduction is reached may extend into the thousands, but because of the high cyclic frequency, the total time of exposure may be only tens or, at most, a few hundreds of hours. For the purpose of this study, creep-fatigue data on several of the strain softening alloys were
37、gathered from many sources. The data from which the plastic strain range may be estimated are shown in Figure 3. Included in the plot are some data from tests that show the effects of tensile hold times. Most of the data are from relatively high frequency tests where the accumulated time at creep te
38、mperature is very short. It appears that longer hold time tests result in fewer numbers of cycles, i.e. there is a creep-fatigue interaction. A line provided by a producer of 2 Cr-1Mo-V alloy, shown in Figure 3, did not include significant hold time effects. The most widely scattered points in the f
39、igure are for hold time tests of one brittle heat of an alloy for which the “no hold time” tests were also mainly outside the scatter band. It is not expected that pressure vessel alloys of interest in this project will behave in a creep brittle manner when tested in uniaxial tension. 0.00010.0010.0
40、10.11E+01 1E+02 1E+03 1E+04 1E+05FATIGUE CYCLESPLASTIC STRAIN AMPLITUDE225V STEEL PRODUCER NIL HOLD TIMEIGCA R 9 1EP RI - CRIEPI 91CEA -CF 91JNC- CF 91HT 303 94 91ORNL JAPC 9 1CEA CONTR 9 1NIMS 1CR 1MO V 500 CNIMS 1CR 1MO V 550 CNIMS AL LOY 92 50 0CNIMS AL LOY 92 60 0CKREMPL BRITTLE1 CR MO- VKREMPL
41、DUCTILE 1 CRMO-VMHI ENDO 1 Cr -Mo -V BRITT LEMHI ENDO 1 Cr -Mo -V DUCTILEMHI ENDO 1 Cr -Mo -V BRITT LEMHI ENDO 1 Cr -Mo -V DUCTILEFigure 3 - Assembled Creep-fatigue Data for Strain Softening Alloys Showing Similarity of Behavior. 4 Extend Low Chrome Steel Fatigue Rules STP-PT-027 4 CREEP-FATIGUE INT
42、ERACTION Creep-fatigue test results are often plotted on an “interaction diagram” of the type shown in Figure 4 for the 91 alloy. The data supporting the ASME NH line for the 91 alloy lie close to the horizontal axis and suggest that short hold time fatigue loading severely negatively influenced cre
43、ep life. However, such test data emphasize fatigue loading whereas high-temperature pressure vessel service would normally be expected to be creep dominated, i.e. representative data would be more closely aligned with the Y axis. Little data of that type is available because of the long test duratio
44、ns required and the corresponding increase in cost of data acquisition. Specifically, pressure vessel service can be better simulated by tests to demonstrate how a relatively small number of cyclic loads would shorten creep life (i.e test results would plot close to the Y axis). The severe effect of
45、 fatigue damage on creep life indicated by the NH lines in Figure 4 might be attributed in large part to a high level of strain softening that occurs with these alloys in hundreds or thousands of strain cycles which reduce the tensile strength and thereby degrade the creep properties. Data reported
46、from creep-fatigue tests of the type shown in Figure 3 seldom, if ever, include post mortem information on the material properties or microstructural changes due to cycling. However, acceleration of softening behavior associated with creep straining and the life reduction that goes along with it are
47、 well known from interrupted stress-rupture tests of the subject alloys. What is needed is modeling and quantification of the effects. Figure 4 - Creep-fatigue Interaction Diagram of Type Used by International Codes for the Strain Softening Alloy 91. 5 STP-PT-027 Extend Low Chrome Steel Fatigue Rule
48、s 5 A MODEL FOR CREEP-FATIGUE IN PRESSURE VESSEL APPLICATIONS We can start to develop a model for creep-fatigue interaction beginning with any expression for creep strain rate that may be modified to describe the increase of strain and strain rate with time (creep damage) and, eventually, with cycli
49、ng. It is immaterial what model is chosen for the strain computation as long as it includes an explicit term for strain rate that we can propose will be accelerated by cycling or other damage related to fatigue. It is also important that the function can be integrated to obtain stress rupture life. Then, we can start with strain rate, , defined as follows: /ddT = (1) For usual applications in which primary creep due to steady loading at allowable stresses is small, we can chose to only express tertiary creep behavior as follows: coe= (2) where: accumulated
