JAT'L INST. OF STAND & TECH R.I.C HIST PUBLICATIONS A111D5 flTiaiE National institute of Standards and Technology Technology Administration, United States Department of Commerce N/ST Technical Note 1500-8 Materials Reliability Series NIST Technical Note 1500-8 Materials Reliability Series Recommended Practice: Installing, Maintaining, and Verifying Your Charpy Impact Machine D.P. Vigliotti T.A. Siewert C.N. McCowan Materials Reliability Division Materials Science and Engineering Laboratory National Institute of Standards and Technology 325 Broadway Boulder, Colorado 80303-3328 March 2000 •^^4TES O* U.S. DEPARTMENT OF COMMERCE, William M. Daley, Secretary TECHNOLOGY ADMINISTRATION, Dr. Cher/I L. Shavers, Under Secretary of Commerce for Technology NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Raymond G. Kammer, Director National Institute of Standards and Technology Technical Note Natl. Inst. Stand. Technol., Tech. Note 1500-8, 20 pages (March 2000) CODEN:NTNOEF U.S. GOVERNMENT PRINTING OFFICE WASHINGTON: 2000 For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325 1 Contents 1 . Introduction 1 2. Overview of the NIST Program 2 2.1 Materials 2 2.2 Acceptance Criteria 3 2.2. Dimensional Tolerances 3 2.2.2 Impact Energy Requirements 3 2.2.3 The Direction in Which the Specimens Leave the Machine During Impact Tests 4 3. Machine Installation 4 4. Direct Verification 6 5. Indirect Verification 7 5.1 Post-Fracture Examination 8 5.1.1 Worn Anvils 8 5.1.2 Off-Center Specimen 10 5.1.3 Off-Center Striker 10 5.1.4 Uneven Anvil Marks 10 5.1.5 Chipped Anvils 12 5.1.6 Anvil Relief 12 5.1.7 Damaged Anvils 13 5.1.8 Bent Pendulum 13 6. Summary 15 7. Bibliography 15 ui Foreword The Materials Reliability Series of NIST Technical Notes are reports covering significant research accomplishments of the Materials Reliability Division. The Division develops measurement technologies that enable the producers and users of materials to improve the quality and reliability of their products. Measurement technologies are developed for process control to improve the quality and consistency of materials, for nondestructive evaluation to assure quality of finished materials and products, and for materials evaluation to assure reliable performance. This report is the eighth in the series. It documents recommended practices for installing, maintaining, and verifying a Charpy impact machine. Previous reports in this series are: Technical Note 1500-1 Tensile Testing of Thin Films: Techniques and Results, by D.T. Read, 1997 Technical Note 1 500-2 Procedures for the Electron-Beam Moire Technique, byE.S. Drexler, 1998 Technical Note 1500-3 High-Energy, Transmission X-ray Diffraction for Monitoring Turbine-Blade Solidification, by D.W. Fitting, W.P. Dube, and T.A. Siewert, 1998 Technical Note 1 500-4 Nondestructive Characterization of Reactor Pressure Vessel Steels: A Feasibility Study, by H.I. McHenry and G.A. Alers, 1998 Technical Note 1500-5 Electron-Beam Moire Technique: Advances, Verification, Application, byE.L. Drexler, 1998 Technical Note 1 500-6 Constitutive Behavior Modeling of Steels Under Hot-Rolling Conditions, by Y.W. Cheng, 1999 Technical Note 1500-7 Structure-Property Relationships in Steel Produced in Hot-Strip Mills, by P.T. Purtscher, Y.W. Cheng, and C.N. McCowan, 1999 IV Recommended Practice: Installing, Maintaining, and Verifying Your Charpy Impact Machine D.P. Vigliotti, T.A. Siewert, and C.N. McCowan Materials Reliability Division Materials Science and Engineering Laboratory National Institute of Standards and Technology Boulder, CO 80303-3328 The quality of the data developed by pendulum impact machines depends on how well the machines are installed, maintained, and verified. This is the reason that ASTM Standard E 23 Standard Test Methods for Notched Bar Impact Testing of Metallic Materials specifies armual direct and indirect verification tests. Each year, NIST provides reference specimens for indirect verification of over 1000 machines around the world. From evaluation of the absorbed energies and the fractured specimens, we attempt to deduce the origin of energies that are outside the ranges permitted by Standard E 23, and report these observations back to the machine owners. This recommended practice summarizes the bases for these observations, and hopefully will allow machines to be maintained at higher levels of accuracy. In addition, we provide details of the NIST verification program procedures and the production of the specimens. Key words: absorbed energy, Charpy V-notch, impact test, machine repair, misalignment, mounting, pendulum impact test, verification testing, worn anvils 1. Introduction The low cost and simple configuration of the Charpy impact test have made it a common requirement in codes and standards for critical structures such as pressure vessels and bridges. For many years, engineers and designers have recognized that materials behave differently when loaded statically than when loaded dynamically, and that a number of materials have a brittle-to- ductile transition temperature. The Charpy impact test is a very cost-effective method of evaluating the behavior of materials for applications where these attributes are important. The history of the pendulum impact test extends back about 100 years. Over the years, procedure improvements, such as the addition of shrouds to prevent specimen jamming and the addition of indirect verification to the verification requirements, have resulted in a simple yet robust test method. The attached bibliography shows how NIST has contributed to the understanding of tlie test method, and for those interested, points to a brief history of the test method. 1 Accurate impact results can be obtained only from machines that are installed correctly, then remain in good working condition, such as within the tolerances specified by Standard E 23. Our indirect verification program is referenced in Standard E 23, and supplements the direct verification requirements found there. Our examination of over 2300 sets of these specimens each year allows us to identify problems that are often not recognized during routine measurement of machine dimensions or routine check procedures (such as the fi-ee-swing test). We have learned to recognize which marks on the broken verification specimens indicate factors that could be affecting the results. We can then advise our customers to recheck or replace the anvils or the striker, tighten bolts, check bearings, check machine alignment or level, check cooling bath or thermometer, or review testing procedures. This recommended practice describes the most common problems that we detect, and gives advice on how to avoid or correct most of them. We have divided the description of impact test problems into four major sections: Overview of the NIST Program, Machine Installation, Direct Verification (evaluation of the machine alone), and Indirect Verification (evaluation of the machine by the testing of specimens). While the following sections give suggestions to improve the accuracy of your impact machine, the machine manufacturer and Standard E 23 also are important sources of information. 2. Overview of the NIST Program The NIST program focuses on evaluating the performance of pendulum impact test machines through indirect verification (the production and evaluation of standardized verification specimens that we distribute to customers of our verification service), as described in ASTM Standard E 23. In chronological order, the NIST program involves obtaining steel that can be made into verification specimens, heat treatment and machining of batches of verification specimens, inspection of representative specimens from each batch to check quality, assignment of a reference value to each batch, packaging of sets of specimens for shipment to customers, evaluation of the fractured specimens and customer test data, and preparation of a certificate of compliance for the customers or suggestions on how to correct any problems. Further details on these tasks are found in the following sections. The basic materials and procedures currently used by NIST have remained unchanged for the past 10 years, and date back to the procedures maintained by the U.S. Army at their arsenal in Watertown, Massachusetts (AMMRC). 2.1 Materials Two materials are currently used to make the specimens for indirect verification of Charpy impact machines to E 23 specifications. A 4340 steel is used to make specimens for the low- and high-energy levels. A type T-200 maraging steel is used to make specimens for the super-high-energy level. The steels are purchased as square bar. The bar stock is supplied to subcontractors that machine and heat-treat the impact specimens to meet the NIST specification. In these steels, the hardness, impact energy, and strength are interrelated. Since hardness correlates to impact energy and is a more convenient property to measure during processing, it is used as the initial process control. The low-energy specimens are typically heat-treated to attain a room-temperature hardness (HRC) of 45, which corresponds to a Charpy impact energy near 16 J (12 ft-lbf) at -40° C (-40° F). The high-energy specimens are typically heat-treated to attain a room temperature HRC of 32, which corresponds to a Charpy impact energy near 100 J (65 ft-lbf) at -40° C (-40° F). The super-high-energy specimens are typically heat-treated to attain a room-temperature HRC of 30, which corresponds to a Charpy impact energy near 220 J (163 ft-lbf) at room temperature. Note that the two different steels have different responses to heat treatment, and are tested at different temperatures. 2.2 Acceptance Criteria Acceptance of a batch of verification specimens is based on the data obtained from a pilot lot of 75 specimens, taken from a heat-treatment batch of 1000 to 1200 specimens. Although impact energy is the desired output, three criteria are combined to determine the acceptability of verification specimens: (1) dimensional tolerances of specimens, (2) mean and standard deviation of impact energy, and (3) the direction in which the specimens leave the machine during impact , testing.