ROCKWELL HARDNESS TESTING by Bernard Levine (c)1997 - for Blade Magazine _________________________________________________________________ Note: the technical information in this article is based on "Fundamentals of Rockwell Hardness Testing," published by Wilson Instruments, division of Instron Corporation (used by permission); and from the Metals Handbook, published by the American Society for Metals. _________________________________________________________________ HARDNESS Knife manufacturers and merchants often advertise that their blades test to certain hardness values on the "Rockwell C scale." One German firm even features an indentation left by its Rockwell testing apparatus on every blade it sells. But what, exactly, does Rockwell hardness mean? What does it tell you about a blade? And what is a good or useful value? Knifemakers seek two qualities in their knife blades: the ability to take a sharp edge, and the ability to hold that edge under all sorts of use and abuse. The main factors that influence these two qualities are hardness, toughness, and flexibility. Material that is not hard can still be very sharp. If you have ever gotten a paper cut, or a splinter from a pine board, you will appreciate this fact. Of course a sharp piece of soft material will quickly be blunted by any material harder than itself -- recall the children's game "Scissors, Paper, Stone." To make a blade whose edge will remain sharp, one must use a material that is either already hard, such as glass, or a material that can be hardened, such as steel. You can see the importance of toughness and flexibility when you compare a glass knife to a steel knife. Glass can easily be made as sharp as the finest steel, perhaps even sharper. All you need to do is break it. But as sharp as it is, glass is also brittle and inflexible. When American Indians made knives out of volcanic glass (called obsidian), they made them in large numbers, and treated them as casually as we now treat disposable razor and utility knife blades. They knew that these sharp but brittle blades would not last long in daily use. And those Indian knifemakers did not need to worry about hardness testing. Once they had learned to recognize obsidian, or flint, they knew that any such stone would be hard enough to make a good blade. Steel is another problem. While different steel alloys do vary slightly in color and texture, there is no way to tell by eye if a given piece of steel has been hardened or not. Indeed, even if you are sure that a piece has been hardened, there is no way to tell by looking just how hard it actually is, nor if it has in fact been uniformly hardened across its length, width, or thickness. TESTING HARDNESS Curiously, hardness is not an inherent property of any material, particularly not the "indentation hardness" measured by modern testing instruments. The Metals Handbook defines hardness as "Resistance of metal to plastic deformation, usually by indentation. However, the term may also refer to stiffness or temper, or to resistance to scratching, abrasion, or cutting." The traditional way to gauge the hardness of a steel blade was to touch its edge with a file or a sharpening stone. In past times sets of files of varying temper were made for testing hardness. Yet this sort of test gives only a subjective impression. Moreover if the blade under test is highly finished, a file test will leave unsightly scratches. This uncertainty about the hardness of steel was equally a problem in other areas besides cutlery -- for example machine tools, rock drills, watch springs, and railroad car wheels. To resolve this uncertainty in a practical manner, manufacturers needed a quick, precise, and repeatable way to measure hardness without damaging the workpiece being tested. An effective solution to this problem was invented in 1919 by Stanley P. Rockwell. He was employed as a metallurgist in a New England plant that manufactured ball bearings. It was not the hardness of the steel balls that concerned Rockwell; it was the hardness and uniformity of the races which they rolled in. What he did was to build an apparatus that would quickly, accurately, and repeatably measure the hardness of a bearing race at any number of points. This Rockwell testing instrument proved equally useful for testing the hardness of all sorts of other steel parts -- indeed for testing other metals, as well, and even for testing non-metallic materials, such as plastics. This wide range of material and hardness which a Rockwell tester can accommodate is the reason why there are a variety of Rockwell scales, and a variety of penetrators and anvils made for use in the testing machines. Just as one does not use a truck scale to weigh a parakeet, or a micrometer to measure the height of a building, one does not use the incorrect Rockwell scale if one expects useful and repeatable results. The ASTM (American Society for Testing & Materials) has standardized a set of scales (ranges) for Rockwell hardness testing. Each scale is designated by a letter. SCALE TYPICAL APPLICATION _________________________________________________________________ A Cemented carbides, thin steel and shallow case hardened steel B Copper alloys, soft steels, aluminum alloys, malleable iron, etc. C Steel, hard cast irons, pearlitic malleable iron, titanium, deep case hardened steel and other materials harder than B 100 D Thin steel and medium case hardened steel and pearlitic malleable iron E Cast iron, aluminum and magnesium alloys, bearing metals F Annealed copper alloys, thin soft sheet metals G Phosphor bronze, beryllium copper, malleable irons H Aluminum, zinc, lead K, L, M, P, R, S, V Bearing metals and other very soft or thin materials, including plastics. _________________________________________________________________ * * * ROCKWELL TESTING The process of Rockwell testing is simplicity itself. On a manual tester the test takes 5 to 10 seconds. An automatic tester, built into an assembly line, can test parts and sort them by hardness in about 1 second per part. After the piece to be tested is placed in the Rockwell tester, the hardness test itself involves four steps. First, the penetrator (either a diamond Brale or a hardened steel ball, depending on the scale) is applied to the spot being tested under a minor load of 10 kgf (kilograms of force). This load can be applied with springs or with compressed air. [BraleR is Wilson Instrument's designation for a diamond penetrator with a conical shape, on included angle of 120*, and a spherical tip with radius of 0.200 mm.] Second, the major load is applied to the penetrator. This major load is either 60, 100, or 150 kgf, again depending on the scale. Third, the major load is removed, leaving the minor load in place. Fourth the penetration under the major load is measured, and this measurement is converted reciprocally and automatically into a Rockwell hardness number. The smaller the penetration, the greater the hardness, and the higher the Rockwell number. One Rockwell number represents a penetration of 0.002 millimeter (0.000080 inch, i.e. 8 one-hundred-thousandths of an inch). Thus the penetrator would make an indentation 0.006 mm larger in a blade of hardness RC 58 than it would in a blade of hardness RC 61. The actual indentation in a hardened steel blade is only a small fraction of a millimeter. The reason for applying the minor load before the actual measurement is to improve precision. The minor load flattens any surface imperfections or distortions at the point of testing, allowing for accuracy and repeatability of measurements. THE C SCALE The scale used for testing the hardness of knife blades and other hardened steel items is the Rockwell C scale. C scale tests use the diamond Brale and the full 150 kgf major load. The theoretical maximum hardness is infinity, which would be 100 on the Rockwell C scale. Infinite hardness is of course impossible, and few substances test very much over RC 70. The range of hardness for functional knife blades is mainly between about RC 50 and RC 63. Most good sport knives test around RC 58 to RC 62. Blades toward the high end of this range tend to be good at edge holding, but very difficult to resharpen. Blades toward the lower end are easier to sharpen, but may not stay sharp as long, especially under demanding use. Because Rockwell hardness testing yields a number, it seems absolute and precise. However the test has important limitations often overlooked by both the marketer and the consumer. First of all, a single test is only valid at the point of testing. Any blade, especially one that has been hardened by hand and eye, should be tested at several points. Second, the Rockwell test is a surface hardness test. It cannot reveal anything about the hardness of the interior of a steel item. Third, to quote from Wilson Instruments, maker of Rockwell hardness testers, "... the Rockwell test is a measure of the resistance of a material to permanent indentation. Indentation hardness is not a fundamental property of a material. However, reliable relationships have been established between the various tests and important properties of materials -- for example, tensile strength and machinability. Furthermore, indentation hardness has become one of the more reliable controls of the heat treatment and quality of manufactured parts." *** END *** For more information about Rockwell hardness testing contact: Mr. John Foley Marketing Manager Wilson Instruments, Inc. Instron Corporation 100 Royall Street Canton MA 02021 American Society for Testing & Materials (ASTM) 1916 Race Street Philadelphia PA 19103 This article (c)1997 Bernard Levine http://www.knife-expert.com/