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JUNE 2016 ISSUE #90 TECHNICALTIDBITS MATERION PERFORMANCE ALLOYS

HARDNESS VS. STRENGTH Intuitively, most people have an understanding of hardness, even if they can’t provide a metallurgically correct definition. It’s been a Hard Most people have an idea of strength as well, On the positive side: Day’s Night – although most non-technical people may not be + Hardness correlates decently to tensile aware that any given has a number of strength, with some scatter (Figure 1). A brief discussion of different properties that fall under the category the correlation, or lack + It is the only test one can use to test small, of strength ( strength, tensile strength, fatigue finished parts. thereof, between hardness strength, impact strength, etc.) The terms "hardness" + If hardness is tested properly and falls within and "strength" may even be conflated for many and strength. specification, then the other mechanical people. properties are likely, although not guaranteed, It is true that hardness and tensile strength correlate to be within specification as well. fairly well with one another. So, the first question that + Hardness testing may be useful for monitoring Hardness needs to be addressed is, “Can I use hardness test processes. As long as the hardness stays results to make any judgements on the strength of consistent, the process is probably within Indenter my material?” The short answer is “Yes” but take it specification as well. with a grain — or ten thousand — of salt. On the negative side: Load Perhaps it is best to start with the promised - − Hardness correlates only very loosely to yield Scale lurgically correct definition of hardness. “Hardness strength and , with high scatter (Figure 1). is a resistance to permanent indentation by − Hardness testing has many pitfalls that can lead a given indenter at a particular loading condition.” to inaccurate measurements. (Table 1). The bigger and deeper the indentation, the lower the − Even correctly performed hardness tests show hardness and vice versa. The 3 critical components wide variation – so it is difficult to determine of the test are the indenter (sharp object pressed whether or not the part is within specification into the surface), the load used during the test, and the scale (measurement system used to quantify the results.)

Hardness is really a quasi-mechanical property, as it is actually a function of several other mechanical properties such as , yield strength, and tensile strength, which all influence how far the indenter penetrates under a given load, and how much the material springs back when the indenter is removed. So, there are some other questions that would need to be addressed. Figure 1 Hardness and Tensile Property Correlation (or Lack Thereof). Question: How useful is hardness testing Tensile strength and hardness correlate fairly well. There is more spread in yield strength, and The next issue of Technical Tidbits Answer: It depends. Done properly, hardness a greater amount of spread in ductility (percent will discuss different categories of testing can be fairly reliable and repeatable. If not elongation.) This chart is for one particular material properties. done properly, you will find yourself chasing variation, material in multiple heat treat conditions, and rejecting good parts, accepting defective parts, etc. should not be used as a conversion chart.

©2016 Materion Brush Inc. MATERION PERFORMANCE ALLOYS HARDNESS VS. STRENGTH (CONTINUED) Question: Can I use hardness testing to accept/ indenter, and dwell time, and your measurement reject incoming material? is close to the certified value, the error is probably Written by Mike Gedeon of test variation, and you can feel confident. If you Answer: If you are testing hardness on a different Materion Performance Alloys screen incoming material by measuring yield or ten- scale than the one used on the material certification, Marketing Department. Mr. sile strength, you will probably measure within 1% then proceed with extreme caution. If you test on Gedeon’s primary focus is on of the certified values. With hardness, who knows? the same hardness scale using the same scale, load, electronic strip for the automotive, telecom, and computer markets Incorrect load/indenter combination for test scale with emphasis on application Incorrect load/indenter/scale combination for size of test piece development. Incorrect load/indenter/scale combination for hardness of test piece References: Pre-Test Procedures Failure to properly calibrate equipment before testing Technical Tidbits Issue Inadequately fixtured test piece No. 25 – “Hardness Testing.”

Improperly prepared metallographic mount of small test piece Technical Tidbits Issue No. 26 Inadequate spacing between indentations – “Hardness Testing Pitfalls.”

Inadequate number of measurements Dropik, Martin J. Johnson, Conducting test on the wrong surface/area of part Test Procedures David H. P.E. Roth, David E. P.E. “Developing an Conducting test through plating or other surface coating ANSYS Creep Model Failure to account for effect of rough or work hardened surface for Polypropylene from Failure to properly correct measurements for surface curvature Experimental Data”

Incorrect use of conversion charts Post-Test Procedures Please contact your local sales Failure to account for variability of test itself representative for further information on material Figure 2 Potential Pitfalls of Hardness Testing. hardness or other questions The first 13 are all procedural errors that can be avoided by following standard operating procedures and pertaining to Materion or using common sense. The last item is the source of most heartaches, as different machines, different labs, our products. and different operators may come up with wildly different hardness measurements, and the raw material itself may have a number of constituent phases that have different hardness value, which can have a large effect on Health and Safety the overall measured value, particularly on microindentation tests. Handling in solid form poses no Question: Can I use hardness to judge if material the test. However, there are circumstances where special health risk. Like many industrial materials, beryllium- has been properly heat treated? hardness testing is the only possible measurement containing materials may pose that you can make to evaluate materials. You will Answer: You may if you follow the specifications a health risk if recommended and guidelines, and test appropriately. need to exercise good engineering judgement in safe handling practices are deciding whether or not your parts meet your spec- not followed. Inhalation of In summary, there are many caveats to using ifications when hardness is the only property you airborne beryllium may cause hardness testing to evaluate material condition. can measure. In this case, it is best used as a means a serious lung disorder in Hardness tests measure hardness only, and it is risky of statistical process control, to see when and if the susceptible individuals. The to infer other material properties from the result of process drifts. Occupational Safety and Health Administration (OSHA) has set mandatory limits on occupational respiratory exposures. Read and follow the guidance in the Material Safety Data Sheet (MSDS) before working with this material. For additional information on safe handling TECHNICALTIDBITS practices or technical data Materion Performance Alloys Sales on copper beryllium, contact 6070 Parkland Blvd. +1.216.383.6800 Materion Performance Alloys Mayfield Heights, OH 44124 800.321.2076 or your local representative. [email protected] Technical Service +1.216.692.3108 800.375.4205 [email protected]

©2016 Materion Brush Inc.