Asme stp nu 035 2012 (american society of mechanical engineers)

STP-NU-035 EXTEND ALLOWABLE STRESS VALUES FOR ALLOY 800H STP-NU-035 EXTEND ALLOWABLE STRESS VALUES FOR ALLOY 800H Prepared by: Robert W Swindeman Cromtech Inc Douglas L Marriott Stress Engineering Services Inc Jude R Foulds Clarus Consulting, LLC Date of Issuance: November 20, 2012 This report was prepared as an account of work sponsored by the U.S Department of Energy (DOE) and the ASME Standards Technology, LLC (ASME ST-LLC) This report was prepared as an account of work sponsored by an agency of the United States Government Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights Reference herein to any specific commercial product, process or service by trade name, trademark, manufacturer or otherwise does not necessarily constitute or imply its endorsement, recommendation or favoring by the United States Government or any agency thereof The views and opinions of authors expressed herein not necessarily state or reflect those of the United States Government or any agency thereof Neither ASME, ASME ST-LLC, the authors, nor others involved in the preparation or review of this report, nor any of their respective employees, members or persons acting on their behalf, make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe upon privately owned rights Reference herein to any specific commercial product, process or service by trade name, trademark, manufacturer or otherwise does not necessarily constitute or imply its endorsement, recommendation or favoring by ASME ST-LLC or others involved in the preparation or review of this report, or any agency thereof The views and opinions of the authors, contributors and reviewers of the report expressed herein not necessarily reflect those of ASME ST-LLC or others involved in the preparation or review of this report, or any agency thereof ASME ST-LLC does not take any position with respect to the validity of any patent rights asserted in connection with any items mentioned in this document, and does not undertake to insure anyone utilizing a publication against liability for infringement of any applicable Letters Patent, nor assumes any such liability Users of a publication are expressly advised that determination of the validity of any such patent rights, and the risk of infringement of such rights, is entirely their own responsibility Participation by federal agency representative(s) or person(s) affiliated with industry is not to be interpreted as government or industry endorsement of this publication ASME is the registered trademark of the American Society of Mechanical Engineers No part of this document may be reproduced in any form, in an electronic retrieval system or otherwise, without the prior written permission of the publisher ASME Standards Technology, LLC Three Park Avenue, New York, NY 10016-5990 ISBN No 978-0-7918-6859-1 Copyright © 2012 by ASME Standards Technology, LLC All Rights Reserved Extend Allowable Stress Values for Alloy 800H STP-NU-035 TABLE OF CONTENTS Foreword vi Abstract vii INTRODUCTION THE DATABASES 2.1 The Assembly of the Database for Calculation of the SY1 and SU Values 2.2 The Assembly of the Database for Calculation of the Sr, St and Smt Values 3 ANALYSIS OF TENSILE DATA 3.1 Analysis of Yield and Ultimate Tensile Strength Data for Estimating the SY1 and SU Values ANALYSIS OF STRESS-RUPTURE DATA 4.1 Analysis Stress-Rupture Data for Minimum Stress-to-Rupture, Sr ANALYSIS OF CREEP DATA 14 5.1 Analysis of Tensile Curves 14 5.2 Development of an Alternate Plasticity Model for Alloy 800H 15 5.3 Analysis of the Time to Initiate Tertiary Creep 19 RECOMMENDATIONS OF ALLOWABLE STRESS INTENSITY VALUES 28 6.1 Values for SY1 and SU 28 6.2 Values for Sm 29 6.3 Values for St 30 6.4 Values for Smt 33 6.5 Values for Sr 36 DISCUSSION 39 SUMMARY AND RECOMMENDATIONS 40 References 41 Appendix - Estimation of the Time to Tertiary Creep 44 Appendix - Time-Temperature Parametric Analysis 48 Appendix - Notes on the Hot Tensile Curve 51 Acknowledgments 57 LIST OF TABLES Table - Comparison of Chemistries for Variants of Alloy 800 Table - Typical Minimum Stress-to-Rupture Values in MPa 12 Table - Modulus Values for Alloy 800H 14 Table - Summary of the Curve-Fitting Results 19 Table - Fit of Three Time-Temperatures to Alloy 800H Stress-Rupture Data and Comparison of Predicted Average Rupture Strengths (ksi) for 100,000 h at 1650°F (900°C) 49 iii STP-NU-035 Extend Allowable Stress Values for Alloy 800H Table - Fit of Three Time-Temperatures to Alloy 800H Tertiary Creep Data and Comparison of Predicted Average Strengths (ksi) for 100,000 h at 1650°F (900°C) 50 Table - Coefficients for the GA Hot Tensile Equation 51 Table - Values for the Cefficients in a New Hot Tensile Model for Alloy 800H and the Calculated Stress for 1% Strain 53 Table - Coefficients for the Ramberg-Osgood Model for the Hot Tensile Curve 54 LIST OF FIGURES Figure - Definition for the Components of Creep Used in ASME Section III, Subsection NH Figure - Various Shapes for Creep Curves Observed in Alloy 800H Figure - Coefficients and R-squared Values for Polynomial Fit to RY and RT Customary Units Figure - Fit of the the Polynomial to the RY1 Values for Alloy 800H Figure - Fit of the Polynomial to the RT Values for Alloy 800H Figure - Distribution of the Yield Strengths Relative to the SYRY Trend Curve Figure - Distribution of Ultimate Tensile Strengths Relative to STRT Figure - Stress Versus the Larson-Miller Parameter for the Stress-Rupture of 106 Lots of Alloy 800H (Stress in MPa, Parameter Based on Temperature in Kelvin) 10 Figure - Residuals in Log Time Versus Temperature for Larson-Miller Parameter for the StressRupture of Alloy 800H 10 Figure 10 - Histogram of Residuals in Log Time for the Fit of the Larson-Miller Parameter 11 Figure 11 - Cumulative Percent of Residuals in Log Time for the Fit of the Larson-Miller Parameter to Stress-Rupture Data for Alloy 800H 11 Figure 12 - Calculated Rupture Life Versus Observed Rupture Life for the Fit of the LarsonMiller Parameter to Stress-Rupture Data for Alloy 800H 12 Figure 13 - Minimum Stress-to-Rupture Versus Time for Alloy 800H 13 Figure 14 - Comparison of Minimum Stress-to-Rupture Values at 100,000 h for the Current ASME III-NH, the German Standard KTA-3221 and the New Analysis Reported Here 13 Figure 15 - Modulus Data for Alloy 800H 15 Figure 16 - Comparison of Hot Tensile Curves for 1400°F (760°C) 17 Figure 17 - Calculated Hot Tensile Curves for Selected Temperatures (deg F) 17 Figure 18 - Stress Versus the Larson Miller Parameter for 1% Strain and a Parametric Constant, C, of 21.21 19 Figure 19 - Leyda-Rowe Plot for Classical Creep (left) and Tertiary-Type Creep (right) of Alloy 800H 20 Figure 20 - Larson Miller Parameter for Time to Initiate Tertiary Creep for Classical-Type 21 Figure 21 - Larson Miller Parameter for Time to Initiate Tertiary Creep for “Tertiary-Type” Creep 22 Figure 22 - Larson Miller Parameter for Time to Initiate Tertiary Creep for all Data 22 iv Extend Allowable Stress Values for Alloy 800H STP-NU-035 Figure 23 - Residuals in Log Time Versus Temperature for Larson-Miller Parameter for the Tertiary Creep of Alloy 800H Including Combined Classical and Tertiary-Type Data 23 Figure 24 - Histogram of Residuals in Log Time for the Fit of the Larson-Miller Parameter for the Tertiary Creep of Alloy 800H Including Combined Classical and Tertiary-Type Data 24 Figure 25 - Cumulative Percent of Residuals in Log Time for the Fit of the Larson-Miller Parameter to All Tertiary Creep Data for Alloy 800H 24 Figure 26 - Calculated Time to Tertiary Creep vs Observed Time to Tertiary for the Fit of the Larson-Miller Parameter to All Tertiary Creep Data for Alloy 800H 25 Figure 27 - Isothermal Curves for Tertiary Creep Covering 450 to 900°C in 50°C Increments 25 Figure 28 - Isothermal Curves for Tertiary Creep Covering 850 to 1650°F in 100°F increments 26 Figure 29 - Stress Versus Time to Initiate Tertiary Creep at 1650°F and 900°C 26 Figure 30 - Stress Versus Time Rupture for Alloy 800H at 1650°F and 900°C 27 Figure 31 - St Versus Temperature 30 Figure 32 - Smt Versus Time 33 Figure 33 - Minimum Stress-to-Rupture 36 Figure 34 - Typical Plot of Creep Data Supplied by HAI for Alloy 800HT (1.7 ksi at 1800°F) 44 Figure 35 - Examples of the Etimation of the Time to Tertiary Creep, t3, for Tertiary-Type Creep Curves Plotted from the HAI Data for 1500 and 1600°F (816 and 871°C) 45 Figure 36 - Examples of the Estimation of the Time to Tertiary Creep, t3, for Tertiary-Type Creep Curves Plotted from the Petten Database 46 Figure 37 - A Creep Curve for 1472°F (800°C) Extracted from a VAMAS Report Showing Sigmoidal Character 47 Figure 38 - Examples of the Estimation of the Time to Tertiary Creep, t3, for Tertiary-Type Creep Curves Plotted from the NIMS Data for 1650°F (900°C) 47 Figure 39 - The Effect of Extension Rate Changes on the Flow Stress of Alloy 800H at 1650°F (900°C) 55 Figure 40 - The Effect of Extension Rate on the Yield Strength (left) and Ultimate Tensile Strength (right) at 900°C (1650°F) 55 Figure 41 - Estimated Hot Tensile Curves for Alloy 800H Based on the Consideration of the Effect of Slow Extension Rate (0.005/minute) on the Ultimate Tensile Strength 56 v STP-NU-035 Extend Allowable Stress Values for Alloy 800H FOREWORD This document is the result of work resulting from Cooperative Agreement DE-NE0000288 between the U.S Department of Energy (DOE) and ASME Standards Technology, LLC (ASME ST-LLC) for the Generation IV (Gen IV) Reactor Materials Project The objective of the project is to provide technical information necessary to update and expand appropriate ASME materials, construction and design codes for application in future Gen IV nuclear reactor systems that operate at elevated temperatures The scope of work is divided into specific areas that are tied to the Generation IV Reactors Integrated Materials Technology Program Plan This report is the result of work performed under Task 13 titled “Extend Allowable Stress Values for Alloy 800H.” ASME ST-LLC has introduced the results of the project into the ASME volunteer standards committees developing new code rules for Generation IV nuclear reactors The project deliverables are expected to become vital references for the committees and serve as important technical bases for new rules These new rules will be developed under ASME’s voluntary consensus process, which requires balance of interest, openness, consensus and due process Through the course of the project, ASME ST-LLC has involved key stakeholders from industry and government to help ensure that the technical direction of the research supports the anticipated codes and standards needs This directed approach and early stakeholder involvement is expected to result in consensus building that will ultimately expedite the standards development process as well as commercialization of the technology ASME has been involved in nuclear codes and standards since 1956 The Society created Section III of the Boiler and Pressure Vessel Code, which addresses nuclear reactor technology, in 1963 ASME Standards promote safety, reliability and component interchangeability in mechanical systems Established in 1880, the American Society of Mechanical Engineers (ASME) is a professional notfor-profit organization with more than 127,000 members promoting the 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 for 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 government by providing new standards-related products and services, which advance the application of emerging and newly commercialized science and technology and providing the research and technology development needed to establish and maintain the technical relevance of codes and standards Visit www.stllc.asme.org for more information vi Extend Allowable Stress Values for Alloy 800H STP-NU-035 ABSTRACT The tensile, creep and stress-rupture databases for alloy 800H (UNS N08810) were assembled and analyzed with the intent of extending the allowable stresses in ASME Section III, Subsection NH for service to 500,000 h at 1400°F (760°C) and below and recommending new allowable stresses for limited service times to temperatures as high as 1650°F (900°C) Values for SY1 and SU were produced for the temperature range of 800 to 1650°F (425 to 900°C) These include a revision of existing values to 1500°F (800°C) Values for Sm were produced for the same temperature range Values for the minimum stress-to-rupture, Sr, were produced for the temperature range of 800 to 1650°F (425 to 900°C), including a revision of existing values to 1400°F (750°C) Development of values for St and Smt required the construction of tensile curves to 1% strain, the estimation of the stresses to produce 1% creep strains to 500,000 h and the estimation of the minimum stress to initiate tertiary creep for times to 500,000 h Because of the shortage of very longtime data for all categories, extensive use was made of time-temperature parametric models based on Larson-Miller The observation of “non-classical” creep behavior in many of the creep curves for alloy 800H greatly reduced the confidence in the extrapolations needed to estimate stresses corresponding to the criteria on which the St and Smt values were based As a result, restrictions on the scope of the St values were recommended These restrictions limited values to less than 500,000 h for temperatures of 1550°F (or 850°C) and above vii STP-NU-035 Extend Allowable Stress Values for Alloy 800H INTENTIONALLY LEFT BLANK viii Extend Allowable Stress Values for Alloy 800H STP-NU-035 INTRODUCTION This work was undertaken in support of the ASME/DOE Generation IV Reactor Materials Program [1] Most of the advanced nuclear reactor concepts being considered in the Generation IV effort will require that the structural materials operate at temperatures where time-dependent allowable stresses control In the United States, the design and construction rules for the Class nuclear components operating in the time-dependent temperature regime are provided in ASME Section III, Subsection NH (III-NH) Currently, only five materials are permitted in this construction code and one of the materials, namely alloy 800H (UNS N08810), is limited to service temperatures of 1400°F (760°C) To meet the design goals for some reactor concepts that require the use of alloy 800H, it has become desirable to extend the coverage for alloy 800H to higher temperatures and longer times [2] Among the several tables and figures that need extension in III-NH are the following (1) the yield strength, SY1, provided in Table I-14.5, (2) the ultimate strength, SU, provided in Table NH-3225-1, (3) the minimum-stress-to-rupture, Sr, provided in Table I-14.6C, (4) the limiting time-independent strength, Sm, as represented by the lowest of the timeindependent strength quantities defined in Section II, Part D, (5) the allowable limit of general primary membrane stress intensity, Smt, provided in Table I14.3C, (6) the maximum allowable value of the general primary stress intensity, So, provided in Table I14.2, and (7) the limiting temperature and time-dependent stress intensity, St, provided in Table I-14.4C Other data are needed that are identified in a recent review of the requirements of III-NH [3] However, the activity reported here is restricted to the estimation of the stress intensity values listed above The goal is to extend time to 500,000 h, if possible, and extend temperature to 1650°F (900°C), if possible STP-NU-035 Extend Allowable Stress Values for Alloy 800H APPENDIX - ESTIMATION OF THE TIME TO TERTIARY CREEP The database of creep curves for alloy 800H was substantial The data from US sources, largely from HAI, were in the form of hand-plotted creep curves, as captured in Figure 34 The data from Petten (European) came in the form of tables of time versus creep strain (and creep rate) and an Excel format that permitted plotting Finally, data from NIMS were in the form of tables that listed the time to specific strains, the time to tertiary creep, and the mcr Figure 34 - Typical Plot of Creep Data Supplied by HAI for Alloy 800HT (1.7 ksi at 1800°F) As stated earlier, the creep curves were separated into “classical” and “tertiary-type” behavior, based on the time to tertiary creep relative to the time to 1% strain Curves corresponding to a ratio of and greater were judged to be “classical” curves and the identification of the time to tertiary was straightforward Curves with early tertiary creep presented a problem In Figure 34, a linear creep stage appeared at the start of the test but was followed by an upturn to a higher, almost linear new rate Above 10% strain, the curve turned downward to a lower, almost linear rate Finally, the curve turned upward around 18% and rupture occurred at a high strain This sigmoidal behavior was very characteristic of alloy 800H over the temperature range of 1100 to 1800°F (600 to 1000°C), especially around 1472°F (800°C) A decision was made to estimate the time to tertiary creep on a 0.2% offset strain based on what was judged to be the first linear rate near the start of the test Examples are shown in Figure 35 In one case, at the top, the curve turned upward leading to rupture In the other case, at the bottom, there was a tendency for the curve to exhibit sigmoidal behavior 44 Extend Allowable Stress Values for Alloy 800H STP-NU-035 800H Heat 8416 1500F ksi 10730 h t3 2000 4000 6000 8000 104 1.2 10 Time (h) 800H heat 8808 1600F ksi t3 200 400 600 800 1000 1200 1400 Time (h) Figure 35 - Examples of the Etimation of the Time to Tertiary Creep, t3, for Tertiary-Type Creep Curves Plotted from the HAI Data for 1500 and 1600°F (816 and 871°C) Curves constructed from the “tertiary-type” Petten data are shown in Figure 36 Here, the right curve for each pair is a “blow-up” of the left curve at low strain It is clear that some judgment was needed in estimating the initial creep rate and the intersection of the 0.2% off set strain from the initial rate with the creep curve Many tests were dicontinued before rupture and exhibited only the early tertiary character 45 STP-NU-035 Extend Allowable Stress Values for Alloy 800H 800H Heat ADU 900C 13 MPa 50 800H Heat ADU 900C 13 MPa B B 40 1.5 37597 h 37597 h 30 20 t3 0.5 10 0 5000 104 1.5 10 104 2.5 10 104 3.5 10 4 104 500 1000 t (hr) 2500 3000 3500 4000 800H Heat BAK 900C 17.1 MPa B B 1.5 15 20434 hr 20434 hr 10 t3 2000 t (hr) 800H Heat BAK 900C 17.1 MPa 20 1500 0.5 t3 5000 104 1.5 10 t (hr) 104 2.5 10 800h-adu-950c-6.96MPAt 40 35 1000 2000 25 5000 6000 7000 B 30 4000 t (hr) 800h-adu-950c-6.96MPAt 2.5 B 3000 1.5 20 15 10 t3 0.5 0 5000 104 1.5 10 104 2.5 10 104 3.5 10 1000 2000 3000 4000 5000 t (hr) t (hr) Figure 36 - Examples of the Estimation of the Time to Tertiary Creep, t3, for Tertiary-Type Creep Curves Plotted from the Petten Database 46 Extend Allowable Stress Values for Alloy 800H STP-NU-035 One full creep curve was found for the NIMS tests This curve is shown in Figure 37 and is probably for heat fCD at 33 MPa and 800°C (1472°F) The curve has a sigmoidal character with a reported time to 1% strain as 4290 h and time to tertiary as 3680 h with an mcr near 3.5x10-5 %/h and rupture near 11645 h Two curves constructed from the NIMS data are shown in Figure 38 Times for 0.5, 1,2 and 5% strain and rupture time and elongation were fitted by a cubic spline Reported values for the mcr and time to tertiary were also helpful in constructing creep curves Figure 37 - A Creep Curve for 1472°F (800°C) Extracted from a VAMAS Report Showing Sigmoidal Character 15 fCF 900C 14 MPa 11331 h (constructed curve) fCC 900C 10 MPa > 188,000 h (constructed) 10 t3 t3 0 2000 4000 6000 8000 10 1.2 10 104 105 1.5 10 105 Time (h) Time (h) Figure 38 - Examples of the Estimation of the Time to Tertiary Creep, t3, for Tertiary-Type Creep Curves Plotted from the NIMS Data for 1650°F (900°C) Notice that the curve on the right exhibits a small primary stage but enters into “tertiary creep” below 1% strain and subsequently continues on to more than 188,000 h 47 STP-NU-035 Extend Allowable Stress Values for Alloy 800H APPENDIX - TIME-TEMPERATURE PARAMETRIC ANALYSIS Three parameters were examined: The Larson-Miller parameter: LMP = T (C + log tr); (A2-1) The Orr-Sherby-Dorn parameter: OSD = log tr - (Q/R)/T; (A2-2) And The Manson-Haferd parameter: MH = (T – Ta)/(log tr – log ta), (A2-3) where T is absolute temperature, tr is the rupture life or the time to a specific event and the parametric constants are C, Q/R, Ta and ta For the OSD parameter the parametric constant is usually given as Q/R, (sometimes given as D) where Q is an activation energy and R is the universal gas constant For the MH parameter, the temperature parametric constant is usually written as Ta and the time parametric constant as log ta For computation purposes, the MH parameter is usually inverted: 1/MH = (log tr – log ta)/(T – Ta) (A2-4) For the three parameters: log tr = f(S)/T – C, (A2-5) log tr = f(S) + (Q/R)/T (A2-6) log tr = f(S) (T – Ta) + log ta Where the stress function, f(S), is written as a polynomial in log stress: f(S) = a0 + a1 log S + a2(log S)2 + a3(log S)3 + … (A2-7) The coefficients, ai, and the parametric constants are found by regression analysis under the assumption that log tr is the dependent variable and the errors are normally distributed In the “global” analysis, the independent variables, log S and T, are entered along with the observed log tr values in the regression analysis In the Lot-Centered analysis, the average log Savg, Tav, and log travg values are obtained for each lot These averages are then subtracted from each datum in the lot (log Sn-log Savg), (Tn-Tavg) and (log trn-log travg) file In the process of setting up the regression analysis, the parametric constants drop out and only the optimized coefficients in the stress function are obtained Then, the calculated coefficients are used to evaluate the parametric constant for each lot The weighted average of the lot constants produces the parametric constant representing the “average” lot constant This average lot constant is then used to calculate the log tc for each test The reported “rms” represents the deviation and scatter of individual lots about the calculated average log tc The variance of the observed data about the calculated values is given by: Variance = [∑(log tr – log tc)2]/(nd – k -1- p) 48 (A2-8) Extend Allowable Stress Values for Alloy 800H STP-NU-035 where nd is the number of data, k is the order of the polynomial, p is the number of parametric constants and the Standard Error of Estimate (SEE) is as the square root of the variance: SEE = (Variance)0.5 (A2-9) In a stricter sense, a rigorous procedure (Mendel-Pauli) should be used when the number of samples in each lot differs greatly, but such a procedure requires sufficient data for each lot to obtain a “valid” within-lot variance [1], [2] The minimum strength is determined from the stress-time curve in log time that is displaced 1.65 SEE less than the average log time (tc) In Table 5, comparisons are made with respect to the fit of the parameters to stress-rupture and data for alloy 800H The Larson-Miller parameter includes three f(S) representations for both LotCentered and Global The Orr-Sherby-Dorn parameter includes three f(S) representations for the Lot –Centered option and two for the Global option The Manson-Haferd parameter includes two f(S) representations for the Global fit and two for the Lot-Centered fit The parametric constants and standard error of estimate are tabulated The average and minimum strengths are tabulated for 100,000 hours at 1650°F (900°C) Generally, the second and third order stress functions provided the best fits The strengths produced by the Larson-Miller parameter were the greatest and those produced by the Manson-Haferd parameter were the least For the Manson-Haferd parameter, the first number is the log ta and the second is the Ta The parameter selected for representing the rupture strength of alloy 800H was the Larson-Miller with the Lot-Centering option and the second order stress function This choice was considered to be consistent with other efforts to represent the strength of alloy 800H for stress allowables in ASME Section II-D Table - Fit of Three Time-Temperatures to Alloy 800H Stress-Rupture Data and Comparison of Predicted Average Rupture Strengths (ksi) for 100,000 h at 1650°F (900°C) PARAMETER OPTION ORDER OF POLYNOMIAL CONSTANT STD ERROR AVERAGE STRENGTH MINIMUM STRENGTH Larson-Miller L-C 15.36 0.382 1.78 1.33 Larson-Miller L-C 15.48 0.377 1.74 1.28 Larson-Miller L-C 15.61 0.374 1.76 1.27 Larson-Miller Global 14.00 0.378 1.68 1.23 Larson-Miller Global 14.16 0.372 1.62 1.16 Larson-Miller Global 14.38 0.369 1.63 1.12 Orr-Sherby-Dorn L-C 17223 0.480 1.58 1.07 Orr-Sherby-Dorn L-C 20104 0.375 1.57 1.01 Orr-Sherby-Dorn L-C 20101 0.375 1.57 1.02 Orr-Sherby-Dorn Global 18420 0.368 1.39 0.81 Orr-Sherby-Dorn Global 18412 0.368 1.39 0.81 Manson-Haferd Global 19.99, 45 0.366 1.47 0.83 Manson-Haferd Global 15.32, 323 0.364 1.36 0.79 Manson-Haferd L-C 17.47, 250 0.474 1.58 0.89 Manson-Haferd L-C 14.90, 396 0.370 1.47 0.89 The selection of a cut-off for allowable stress intensity when based on the tertiary creep criterion was influenced by the characteristics of the stress function and its extrapolation outside of the stress range 49 STP-NU-035 Extend Allowable Stress Values for Alloy 800H of the data Emphasis was placed on the behavior at 1650°F (900°C) In Table 6, the results from the fit of three time-temperature parameters are provided The conditions are for 100,000 hours at 1650°F (900°C) For this exercise, the Larson-Miller, Orr-Sherby-Dorn and Manson-Haferd parameters were used with the first, second, and third order of the polynomial stress function, f(S) The Lot-Centered option was included for the Larson-Miller and Orr-Sherby-Dorn parameters The standard error of estimate changed very little for the Larson-Miller parameter but tended to decrease with the increasing order of the polynomial for the other two parameters The Lot-Centered option of the Larson-Miller parameter produced the greatest strengths for both the average and minimum strength comparisons Minimum strength values for 100,000 hours could not be obtained for the second order polynomial stress function of the Global option for the Orr-Sherby-Dorn parameter because the predicted minimum strength curve turned back to shorter time before reaching 100,000 hours The same is true for the second order Manson-Haferd parameter minimum strength curve The first order polynomial for the Manson-Haferd parameter did not converge, which means that the isostress lines are parallel Nevertheless, values for the SEE, average strength and minimum strength were produced and are indicated in the table Table - Fit of Three Time-Temperatures to Alloy 800H Tertiary Creep Data and Comparison of Predicted Average Strengths (ksi) for 100,000 h at 1650°F (900°C) PARAMETER OPTION ORDER OF POLYNOMIAL CONSTANT STD ERROR AVERAGE STRENGTH MINIMUM STRENGTH Larson-Miller L-C 14.45 0.463 1.06 0.72 Larson-Miller L-C 14.86 0.453 0.99 0.62 Larson-Miller L-C 14.88 0.454 0.99 0.63 Larson-Miller Global 13.42 0.461 1.01 0.66 Larson-Miller Global 14.19 0.451 0.88 0.50 Larson-Miller Global 14.24 0.456 0.89 0.54 Orr-Sherby-Dorn L-C 17890 0.497 0.99 0.63 Orr-Sherby-Dorn L-C 19374 0.448 0.80 0.32 Orr-Sherby-Dorn L-C 19819 0.444 0.91 0.58 Orr-Sherby-Dorn Global 16343 0.493 0.97 0.61 Orr-Sherby-Dorn Global 18800 0.444 0.59 NA Orr-Sherby-Dorn Global 19191 0.442 0.81 0.50 Manson-Haferd Global NA 0.470 1.02 0.67 Manson-Haferd Global 17.32, 163 0.447 0.47 NA Manson-Haferd Global 9.957, 655 0.432 0.76 0.54 Manson-Haferd L-C NA 0.472 1.08 0.73 Manson-Haferd L-C 17.09, 203 449 0.70 NA Manson-Haferd L-C 9,979, 658 0.434 0.76 0.52 50 Extend Allowable Stress Values for Alloy 800H STP-NU-035 APPENDIX - NOTES ON THE HOT TENSILE CURVE The GA model: The current model for the hot tensile curve in Section III Subsection NH (III-NH) was based on the work of Smith at the General Atomic Company [A3-1] and was developed from a collection of 38 yield curves supplied by Huntington Alloys Data covered the temperature range from 800 to 1400°F (427 to 760°C) The curves were normalized to the average yield strengths for each temperature and then fitted to third order polynomial in natural log stress (S) and natural log strain (e) for a series of strains up to 2%: ln S = B1 + B2 ln e + B3 (ln e)2 + B4 (ln e)3 (A3-1) where B1, B2, B3 and B4 were coefficients which were determined for each temperature The coefficient values covering temperatures from 800 to 1400°F at 50°F increments are provided below The yield strength determined from the fit showed good agreement with the yield strength based on the ratio trend curve which was established as the basis for setting the SY values in ASME Section II, Part D Table - Coefficients for the GA Hot Tensile Equation Temp °F 800 850 900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400 B1 B2 3.1782 3.1655 3.1529 3.1387 3.1234 3.1099 3.0964 3.0804 3.0662 3.0505 3.0336 3.0162 3.0002 0.17904 0.17683 0.17612 0.17604 0.17361 0.17102 0.16813 0.16749 0.16693 0.16417 0.16348 0.16232 0.15802 B3 B4 4.0125x10-2 3.7435x10-2 3.7435x10-2 3.8449x10-2 3.9557x10-2 4.0358x10-2 3.8213x10-2 3.7636x10-2 3.6402x10-2 3.6017x10-2 3.5773x10-2 3.7208x10-2 3.8120x10-2 5.2622x10-3 5.2748x10-3 5.3437x10-3 7.4307x10-3 9.0871x10-3 1.0809x10-3 1.0851x10-3 1.1463x10-3 1.1953x10-3 1.3210x10-3 1.4016x10-3 1.5759x10-3 1.8052x10-3 A rational polynomial mode based on SY and SU: One alternative model for the hot tensile curve that was investigated was based on a modified rational polynomial that made use of SY and SU The yield curve assumed power-law hardening, once the proportional limit was exceeded, but a limit stress was introduced that was approached according to the expression: (S - Spl) = (Su – Spl)[h epm/(1 + h epm)], (A3-2) where S is a stress between Spl and Su, Spl is the proportional limit, Su may be taken as the value in Table U of ASME Section II-D or Table NH-3225-1 of ASME Section III-NH, h is a hardening constant, ep is plastic strain and m is a hardening exponent which was chosen to be 0.5 for alloy 800H The value for Spl was chosen to be 0.73 SY The values of h may be estimated by setting ep to the 0.2% off set yield strain and S to be the yield strength, Sy, taken from Table Y-1 in II-D or Table I14.5 in III-NH Then: 51 STP-NU-035 Extend Allowable Stress Values for Alloy 800H h = [ (Spl –Sy)/(Sy-Su)] (0.2)-m (A3-3) Substituting the right hand side for h in eq (A3-2) and solving for ep gives: ep = [(Spl–S)(Su-Sy)(0.2)m/(Sy-Spl)/(S-Su)]1/m, (A3-4) and from an examination of the hardening around the yield strength m = 0.5: ep = 0.2 [(Spl–S)(Su-Sy) /(Sy-Spl)/(S-Su)]2 (A3-5) When the elastic component of strain was added, this simple model was a reasonable representation of the hot tensile curve at low strains but tended under-estimate the flow stress at strains of 1% and higher One problem was that the SU drops significantly relative to the yield strength above 1000°F (540°C) and this produces a “fan” of curves whose slopes at large strains decrease as the temperature for the curves increase The pattern for alloy 800H to at least 760°C (1400°F) is for the curves to show similar slopes until the ultimate strength is reached at uniform strains that decrease with increasing temperature A new model based on parabolic and linear hardening: To better represent the yield curve at strains of 1% and greater, a linear hardening component was added to the modified rational polynomial A similar approach was used by Sikka and Booker [A3-2] and Hammond and Sikka [A3-3] on austenitic stainless steels Here, the stress above the proportional limit, (S-Spl), is represented by: S – Spl = δS h1ep1/2/(1 + h1ep1/2) + h2 ep (A3-6) where δS is the limit of parabolic hardening similar to (Su – Spl) in eq (3A-2), h1 is a “parabolic” hardening factor, ep is plastic strain, and h2 is a linear hardening factor The first term in the equation is the rational polynomial (RP) and characterizes the high rate of initial hardening that dominates the stress increase for the first few percent strain The second term characterizes the almost linear increase in the flow stress with strain that is typical of alloy 800H and austenitic stainless steels from one percent to greater than five percent strain The drawback of the model is that the coefficients in the model must be derived from the actual hot tensile curves To accomplish this task, hot tensile curves from the GA collection, along with data from Idaho National Laboratory and Oak Ridge National Laboratory, were examined and estimates of the coefficients were made Values are provided in Table Also included in the table are values for 1.25 SY which is considered to be the average yield strength The flow stress at 1% strain is provided for temperatures from 800 to 1650°F (425 to 900°C) For temperatures above 1472°F (800°C), the tensile curve is highly strain-rate dependent The coefficients in parentheses represent values for strain rates that exceed 0.05/minute 52 Extend Allowable Stress Values for Alloy 800H STP-NU-035 Table - Values for the Cefficients in a New Hot Tensile Model for Alloy 800H and the Calculated Stress for 1% Strain Temp (°F) Spl (ksi) δS (ksi) h1 h2 (ksi/%) S1% (ksi) 1.25Sy (ksi) 800 15.8 10 1.5 23.8 20.9 850 15.5 10 1.5 23.4 20.5 900 15.1 10 1.5 23.0 20.1 950 14.8 10 1.5 22.7 19.8 1000 14.3 10 1.5 22.3 19.4 1050 14.3 10 1.5 22.3 19.4 1100 14.3 10 1.5 22.3 19.4 1150 14.3 10 1.5 22.3 19.4 1200 14.3 10 1.5 22.3 19.4 1250 14.0 10 1.5 21.9 19.0 1300 13.5 10 1.5 21.4 18.5 1350 12.7 10 1.5 20.6 17.8 1400 12.0 10 1.5 19.9 17.0 1450 11.5 9.3 1.5 18.9 16.1 1500 11.2 (9) (1.5) (17.8) 15.3 1550 10.5 (7) (1.5) (16.5) 14.1 1600 9.27 (7) (1.5) (15.3) 12.9 1650 8.06 (7) (1.5) (14.1) 11.6 Note: Values in parentheses are extension rate dependent The Ramberg-Osgood model: A strong alternative model for the plastic component of the hot tensile curve is the Ramberg-Osgood equation: (S - Spl) = a epm (A3-7) Again, requiring that the curve pass through the yield strength (taken as 1.25 SY) and the stress at 1% strain obtained from the new model allows a and m to be calculated for temperatures from 800 to 1650°F (425 to 900°C) In Table 9, the values for a and m are provided 53 STP-NU-035 Extend Allowable Stress Values for Alloy 800H Table - Coefficients for the Ramberg-Osgood Model for the Hot Tensile Curve Temp (°F) Spl (ksi) 1.25 SY (ksi) S1% (ksi) a m 800 15.8 20.9 23.8 8.1571 0.30040 850 15.5 20.5 23.4 8.1562 0.30347 900 15.1 20.1 23.0 8.1568 0.30629 950 14.8 19.8 22.7 8.0548 0.30629 1000 14.3 19.4 22.3 8.1500 0.29924 1050 14.3 19.4 22.3 8.1532 0.29948 1100 14.3 19.4 22.3 8.1566 0.29974 1150 14.3 19.4 22.3 8.1501 0.29924 1200 14.3 19.4 22.3 8.1524 0.29942 1250 14.0 19.0 21.9 8.1643 0.30266 1300 13.5 18.5 21.4 8.1516 0.30245 1350 12.7 17.8 20.6 8.1543 0.29895 1400 12.0 17.0 19.9 8.1528 0.30379 1450 11.5 16.1 18.9 7.6492 0.30276 1500 11.2 15.3 17.8 6.8050 0.31481 1550 10.5 14.1 16.5 6.1440 0.32783 1600 9.3 12.9 15.3 6.1573 0.33261 1650 8.1 11.6 14.1 6.1540 0.33851 Strain rate effects: Data on which to establish the significance of strain rate effects are scarce Figure 39 shows the results of extension rate changes for a test at 1650°F (900°C) The initial straining at an extension rate of 0.005/minute produced plastic flow when the stress was around 11.5 ksi (79 MPa) which is close to the 11.6 ksi listed as the average yield strength in Table for this temperature However, there was very little hardening whereas Table indicates a flow stress at 1% strain to be 14.1 ksi (97 MPa) An increase in the extension rate to 0.05/minute produced an increase in elastic strain followed by plastic flow at 15 ksi (103 MPa) Here, there was evidence of hardening With a decrease in extension rate to the original value, the flow stress decreased to the original value A further increase in the extension rate to 0.5/minute produced instant elastic strain again and an increase in the flow stress to near 20 ksi (140 MPa) Hardening at the high extension rate was not retained when the rate was returned 0.005/minute rate, and so on The behavior at the higher extension was typical of the behavior seen in cyclic testing which is often performed at rate of 0.06/minute and higher 54 Extend Allowable Stress Values for Alloy 800H STP-NU-035 1000 25 S ksi 20 100 RATE 10 15 0.1 10 0.01 0.001 alloy 800H 1650 F 0 0.2 0.4 0.6 0.8 0.0001 Nominal strain (e %) Figure 39 - The Effect of Extension Rate Changes on the Flow Stress of Alloy 800H at 1650°F (900°C) The data in Figure 39 are re-plotted in Figure 40 where they may be compared to other yield and ultimate strength data on the basis of stress versus extension rate These other data were taken from the HAI and NIMS test listings It is assumed that the HAI data were collected in accordance with the standard ASTM E-21 practice which requires an extension rate of 0.005/minute to beyond yield followed by some higher rate to the ultimate strength An increased rate of 0.05 or even 0.5/minute is typical The NIMS data were produced according to JIS G 0567 at a rate of 0.003/minute to 1% strain followed by a rate of 0.075% to the ultimate strength With respect to the yield strengths, plotted in Figure 40 left, an extension rate effect based on the ORNL data is clearly present and the HAI and NIMS data fall on the trend The extrapolated strength at 0.0001/minute is near 50 MPa (7 ksi) 1000 1000 Ultimate Tensile Strength Yield Strength ORNL NIMS HAI ORNL UTS NIMS UTS HAI UTS 100 100 alloy 800H 10 0.0001 0.001 0.01 0.1 alloy 800H 10 0.0001 Extension Rate (1/minute) 0.001 0.01 0.1 Extension rate (1/minute) Figure 40 - The Effect of Extension Rate on the Yield Strength (left) and Ultimate Tensile Strength (right) at 900°C (1650°F) 55 STP-NU-035 Extend Allowable Stress Values for Alloy 800H With respect to the ultimate tensile strength, only the two points from ORNL are available to estimate the extension rate effect The points extrapolate to 50 MPA (7 ksi) at 0.0001/ minute If so, then it appears that little or no hardening should be expected and the stress at 1% strain would be the same as the yield strength The difference between the yield strength and the ultimate strength was estimated from Figure 40 for the extension rate of 0.005/minute This difference was about ksi (7MPa) or less and suggested that the coefficient δS in equation A3-6 should be ksi (7 MPa) or less at 1650°F (900°C) Further The hardening coefficient, h2 should be close to zero for this temperature For temperatures between 1472°F (800°C) and 1650°F (900°C), the coefficients h2 and δS in equation A3-6 can be estimated from a linear interpolation assuming no significant strain rate effect at 1472°F (800°C) Hot tensile curves based on these assumptions are plotted in Figure 41 The curve for 1500°F (816°C) is virtually the same as plotted in Figure 17 but the curves for higher temperatures show much reduced hardening The stress corresponding to 1% strain in the hot tensile curve was unchanged at 1500°F (816°C) but decreased significantly with increasing temperature The St in Table I-14.4C was not affected, however, since the stress at hour at 900°C (1650°F) was controlled by the time-dependent criteria 25 150 20 125 15 100 10 1500F 1550F 1600F 1650F 75 50 25 0 0.5 1.5 2.5 Strain (%) Figure 41 - Estimated Hot Tensile Curves for Alloy 800H Based on the Consideration of the Effect of Slow Extension Rate (0.005/minute) on the Ultimate Tensile Strength References for Appendix [A3-1] A Smith, “Revised Incoloy 800H Isochronous Stress Strain Curves for Code Case 1592,” attachment to communication from D Roberts , General Atomic Co., San Diego, CA to R Jetter, Atomic International, Canoga Park, CA (November 1976) [A3-2] V Sikka and M Booker, “Influence of Laboratory Annealing on Tensile Properties and Design Stress Intensity Limits for Type 304 Stainless Steel,” Effects of Melting and Processing Variables on the Mechanical Properties of Steel, MPC-6, American Society of Mechanical Engineers, New York, NY, 1977, pp 255-272 [A3-3] J Hammond and V Sikka, Predicted Strains in Austenitic Stainless Steels at Stresses above Yield, Effects of Melting and Processing Variables on the Mechanical Properties of Steel, MPC-6, American Society of Mechanical Engineers, New York, NY, 1977, pp 309-322 56 Extend Allowable Stress Values for Alloy 800H STP-NU-035 ACKNOWLEDGMENTS The software used for data analysis was developed by Michael Swindeman The authors acknowledge, with deep appreciation, the following individuals for their technical and editorial peer review of this document: • • Robert I Jetter Dana K Morton The authors further acknowledge, with deep appreciation, the activities of ASME ST-LLC and ASME staff and volunteers who have provided valuable technical input, advice and assistance with review of, commenting on, and editing of, this document 57 A2321Q