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Tackling hard jobs: work underway to harmonize international hardness standards. (Hardnesstesting).

To a metallurgist, hardness is a material's resistance to penetration.

Mechanical testing, or destructive testing, probes materials to reveal elastic or inelastic behavior when force is applied.

In general, an indenter is pressed into the surface of the materials to be tested under a specific load for a definite time interval, and a measurement is made of the size or depth of the indentation, says Stefan Frank, worldwide hardness product manager at Krautkramer Branson, Inc., ewiston, PA.

One type of mechanical testing is hardness testing. "Hardness is effective compared to other mechanical test methods because it can be easily and quickly performed and causes only a minimal destructive effect," says Jim Fultz, North American hardness product manager ith Krautkramer Branson. "Hardness testing is the most valuable and widely-used mechanical test used by all industries. This is primarily due to the ease and speed at which it can be performed and its relationship to more difficult methods such as tensile testing."

Properties attributed to hardness include resistance to abrasives, resistance to plastic deformation, high modulus of elasticity, high yield point, high strength, absences of elastic damping, brittleness or lack of ductility.

"Hardness testing is a method of checking a material's integrity," adds Mars Webster, president of Webster Instrument Inc., Los Angeles, CA. "This helps determine material composition and strength."

Everything from drill pipes and screwdrivers to foil and prongs on computer chips can be tested, says Ernie Biddle, vice president of sales nd marketing at King Tester Corp., King of Prussia, PA. "If the prongs on the computer chip break off, it will be hard to assemble the computer."

Standards

Frank points out that hardness has always been a subject of much discussion among technical people, thus resulting in numerous definitions.

Hardness is not a fundamental property of a material, but a response to a particular test method, he says. "Hardness has no quantitative value, except in terms of a given load applied in a specific, reproducible manner and with a specified indenter shape."

North America follows American Society for Testing and Materials (ASTM) standards, which are very close to ISO standards, says Samuel Low, chairman of the ASTM subcommittee on indentation hardness testing and a member of the National Institute for Standards and Technology (NIST) technical staff.

In the past, other parts of the world followed the national standards of the individual countries, but are moving toward ISO standards, Low says.

The ASTM Brinell hardness standard (E10) was first published in 1924. The Rockwell hardness standard (E18) was published in 1932, followed by the Vickers and Knoop microhardness standard (E384) in 1969 and the heavy load Vickers hardness standard (E92) in 1952.

The Rockwell standards are reviewed twice a year, according to Low. A task group is formed and any revisions are voted on by a subcommittee on hardness and the main committee.

Measuring hardness

There is several types of hardness testers--Brinell, Rockwell, Vickers and Knoop. Brinell testers are used for large castings. According to Biddle, 3000Kg is forced into a piece of metal with a 10mm ball. "It really gives an idea of what the core is like," he says. "Rockwell bench-top testers are used on small, finished pieces--1" to 1 1/2" thick."

Biddle adds that Vickers and Knoop are variations of Brinell testers. Vickers testers use pyramidal indenters, while Knoop testers use trapezoidal indenters.

"One advantage of Brinell testers is that they leave 2mm to 4 or 5mm impressions in a test piece. If everyone agrees with the loading, all that is needed is a microscope." Biddle says. "It is a verifiable test."

Once the impression is made, then two diameters at 90 degrees are measured.

Static indentation tests--a ball, cone or pyramid is forced into the surface of the material being tested-are widespread. The relationship of the load to the area or depth of indentation is the measure of hardness.

But the different methods and differently shaped indenters used by Brinell and Rockwell produce dissimilar responses of the material being tested. Frank says tables relating to HRC and HB values are only approximations and there is no mathematical equation to transfer measurements from one scale to another.

"To compare the hardness of two different samples, both must be measured to the same hardness scale, or a scale must be developed to convert from one measurement to the other. Hardness scales are only in relationship to themselves," Frank says.

The most used hardness test is Rockwell, Low says. "The ASTM defines 30 Rockwell scales, but ISO only recognizes 15 of these scales. The scales are differentiated by the type of indenter and force level. Which Rockwell scale is used depends on the type of material that is tested and its thickness."

Letters identify the scale. The Rockwell C scale is designated "HRC" and is used for testing steel and other hard materials.

Portable testers

While conventional hardness testers, like Rockwell, Brinell or Vickers machines, require the test piece to be brought to the testing device, mobile handheld instruments allow measurements to be made on the spot.

"Portable hardness testers can check odd shapes," Webster says. "Hardness values can be checked on location." He says this has been especially helpful to specialty industries like window frame and bicycle manufacturing.

There are several methods of portable hardness testing instruments. The Ultrasonic Contact Impedance (UCI) method measures the frequency shift of a resonating rod with a Vickers-diamond tip which occurs when the diamond penetrates into the test material by applying a specific test load. The frequency shift is evaluated and electronically converted in a LCD readout of the hardness value, Frank says.

Another principle for portable hardness testers is the rebound method. These instruments measure the velocity of a propelled impact body directly before and after the impact onto the test material's surface. The ratio between both velocities indicates the hardness of the material, which can be converted into different scales by using conversion tables stored in the instrument for different materials.

Choosing the method

According to Frank, the UCI method is recommended for testing fine-grained material having any shape and size, especially for material properties with narrow tolerances. Examples include determination of strain hardening on drop-forged parts.

Rebound hardness testing is used on large, coarse-grained materials, forged parts and all types of cast materials because the spherical tip of the impact device produces a rather larger indent than the Vickers diamond and therefore processes the characteristics of the casting structure better, Frank says.

The test task will determine whether the UCI method or the rebound method should be used, Frank says. "It is not always immediately clear about the most suitable method," he says. "Therefore the best answer to the testing problem can mostly be given by an experienced sales engineer directly at the test location."

The part's size, shape, flatness, surface condition, microstructure (homogenous or non-homogenous), effect of indentation on the part's function and number of components to be tested all need to be considered when choosing a hardness test system.

Webster adds that users need to consider application and whether or not a lot of production is involved. "Will the tester be used for a fast check?" he asks.

Harmonizing testing

The most drastic change to hardness testing was the introduction of NIST hardness reference test blocks as opposed to a new type of hardness tester, Fultz says. "Until this was done there existed differences between manufacturers of test blocks."

Several efforts to harmonize worldwide hardness standards are underway. "Years ago, a company needed traceability to national standards, but there was no agreement between standards," Low says. "Now when calibrating machines, companies must be traceable to NIST. This is bringing some agreement."

One entity, Le Comite International des Poids et Mesures (CIPM)--the International Committee for Weights and Measures--is looking to unify international units of measure, Low says.

The Consultative Committee for Mass and its corresponding Working Group on Hardness (WGH), fall under CIPM. "They are working to define test methods and come up with a single regulation for diamond indenters," Low says.

Another group involved with hardness standards is the International Organization of Legal Metrology (OIML). The organization produces international recommendations--documents that specify how to insure that an instrument meets certain capabilities, according to Low. "These recommendations for hardness are rarely used, but are available in case someone wants to refer to them."

Fultz predicts that hardness testing will continue to be used because of its effectiveness to quickly and easily determine the suitability of a material and to make sure it was processed properly.

"Hardness testers will definitely be around for awhile," Webster adds. "With increased sensitivity to quality control and ISO 9000 requirements, people are more conscious of quality."

Frank says portable hardness testers will continue to flourish. "They are not only a quick and economical supplement to stationary hardness testing in the modern production process, but also open complete new possibilities like testing on-site without having to destroy the part in order to get a small and handy test piece," he says.

"The next generation of portable hardness testers will consider the current technical possibilities with focus on simple and convenient operation, easy data reporting and flexible further processing."

Stephanie J. Fellenstein is editor of T&P's sister publication Quality in Manufacturing.

SOURCES:

Krautkramer Branson, Lewiston, PA, http://www.OneRS.net/106tp-354 or circle 354

King Tester Corp., King of Prussia, PA, http://www.OneRS.net/106tp-355 or circle 355

Webster Instrument Inc., Los Angeles, CA, http://www.OneRS.net/106tp-356 or circle 356

National Institute for Standards and Technology (NIST), Gaithersburg, MD, http://www.OneRS.net/106tp-357 or circle 357

Other sources for mechanical testing or hardness testers include:

MTS Systems, Inc., QTest Division, Cary, NC, http://www.OneRS.net/106tp-358 or circle 358

SATEC Materials Testing Equipment, Grove City, PA, http://www.OneRS.net/106tp-359 or circle 359

Dillon Force Measurement Products, Weigh-Tronix Inc., Fairmount, MN, http://www.OneRS.net/106tp-360 or circle 360

Tinius Olsen, Willow Grove, PA, http://www.OneRS.net/106tp-361 or circle 361

Instron Corp., Canton, MA, http://www.OneRS.net/106tp-362 or circle 362
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Author:Fellenstein, Stephaine J.
Publication:Tooling & Production
Date:Jun 1, 2001
Words:1678
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