NATL INST. OF STAND & TECH RIX r MIST PUBLICATIONS AlllOS flb512D Installing Maintaining, STTR Your Verifying Charpy Impact Machine D.P. Vigliotti T.A. Siewert McCowan C.N. Nisr Special National Instituteof StandardsandTechnology Publication 10.960-1 TUe.Sc.hnDoelpoagrytAmdemnitniosftrCaotmimonerce 960-4 flOO NIST Recommended Practice Guide Special Publication 960-4 Installing, Maintaining, and Verifying Your Charpy Impact Machine D.P. Vigliotti T.A. Siewert McCowan C.N. Materials Science and Engineering Laboratory October 2000 Previously issued as NIST Technical Note 1500-8 U.S. Department of Commerce Norman Y. Mineta, Secretary Technology Administration Dr. Cheryl L. Shavers, Under Secretary of Commerce forTechnology National Institute of Standards and Technology Raymond G. Kammer, Director Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the entities, materials, or equipment are necessarily the best avail- able for the purpose. National Institute of Standards and Technology Special Publication 960-4 Natl. Inst. Stand. Technol. Spec. Publ. 960-4 22 pages (October 2000) CODEN: NSPUE2 U.S. GOVERNMENT PRINTING OFFICE WASHINGTON: 2000 For sale by the Superintendent ofDocuments U.S. Government Printing Office Washington, DC 20402-9325 Foreword This Special Publication is a reprint ofNIST Technical Note 1500-8, a TN series started by the Materials Reliability Division. This TN series describes their division's significant research accomplishments in measurement technol- ogy, reported so that producers and users ofmaterials can improve the quality and reliability oftheir products. This particular Practice Guide provides practical advice on how to correct problems discovered during the testing of Standard Reference Materials 2092, 2096, and 2098 on Charpy impact machines. Although only a small percent- age ofmachines fail to meet the requirements during their annual performance tests, the failure ofa machine can have large economic implications to the machine's owners, and we try to assist in correcting the problems. From the study ofthe fractured specimens and test data that are returned to NIST for evaluation, we have learned how to identify many ofthe common problems. Also, over the years, we have had discussions with thousands ofengineers and technicians at these companies, and have learned the most efficient sequences for identifying the sources ofthe problem and for correcting them. Now, we have collected and organized the various problems and solutions in this Special Publication. We hope that you will find it useful for installing a new machine correctly, and then for preparing for the annual verification tests. More information on the SP 960 series can be found on the Internet at http://www.nist.gov/practiceguides. This web site includes a complete list ofNIST Practice Guides and ordering information. Abstract: The quality ofthe data developed by pendulum impact machines depends on how well the machines are installed, maintained, and verified. This is the rea- son thatASTM Standard E 23 Standard Test Methods for Notched Bar Impact Testing ofMetallic Materials specifies annual direct and indirect verification tests. Each year, NIST provides reference specimens for indirect verification of over 1000 machines around the world. From evaluation ofthe absorbed ener- gies and the fractured specimens, we attempt to deduce the origin ofenergies that are outside the ranges permitted by Standard E 23, and report these obser- vations 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 ofaccuracy. In addition, we provide details ofthe NIST verification program procedures and the production ofthe specimens. Key words: absorbed energy, Charpy V-notch, impact test, machine repair, misalignment, mounting, pendulum impact test, verification testing, worn anvils 1. Introduction .1 ............ 2. Overviev/ of the NIST Program .2 2.1 Materials .2 2.2 Acceptance Criteria .3 2.2.1 Dimensional Tolerances .3 2.2.2 Impact Energy Requirements .3 2.2.3 The Direction in Which the Specimens A Leave the Machine During Impact Testing ....... 3. Machine Installation .5 4. Direct Verification .7 5. Indirect Verification ,9 . . 5.1 Post-Fracture Examination 9 5.1.1 WornAnvils 10 5.1.2 Off-Center Specimen .11 5.1.3 Off-CenterStriker .12 5.1.4 Uneven Anvil Marks ...................... .12 5.1.5 ChippedAnvils .13 5.1.6 AnvilRelief ..13 5.1.7 DamagedAnvils ........ .14 5.1.8 Bent Pendulum .15 6. Summary ................................ .16 Bibliography .17 7. Introduction 1. Introduction The low cost and simple configuration ofthe 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 ofmaterials have a brittle-to-ductile transition temperature. The Charpy impact test is a very cost-effective method ofevaluating the behavior ofmaterials for applications where these attributes are important. The history ofthe pendulum impact test extends back about 100 years. Over the years, procedure improvements, such as the addition of shrouds to prevent specimenjamming and the addition ofindirect 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 ofthe test method, and for those interested, points to a briefhistory ofthe test method. 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 ofthese 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 free-swing test). We have learned to recognize which marks on the broken verification speci- mens 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 ther- mometer, 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 ofthem. We have divided the description ofimpact test prob- lems into four major sections: Overview ofthe NIST Program, Machine Installation, Direct Verification (evaluation ofthe machine alone), and Indirect Verification (evaluation ofthe machine by the testing of specimens). While the following sections give suggestions to improve the accuracy ofyour impact machine, the machine manufacturer and Standard E 23 also are impor- tant sources ofinformation. 1 Overview 2. Overview of the NIST Program The NIST program focuses on evaluating the performance ofpendulum impact test machines through indirect verification (the production and evaluation of standardized verification specimens that we distribute to customers ofour veri- fication 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 ofbatches ofverification specimens, inspection ofrepresentative specimens from each batch to check quality, assignment ofa reference value to each batch, packaging of sets of specimens for shipment to customers, evaluation ofthe fractured specimens and customer test data, and preparation of a certificate ofcompliance 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 verifica- tion ofCharpy impact machines to E 23 specifications. A4340 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 subcon- tractors 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 hard- ness (HRC) of45, 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 HRC specimens are typically heat-treated to attain a room-temperature of30, 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