Page 42 - Metalforming Magazine April 2022

Metal Matters
By Daniel J. Schaeffler, Ph.D.
Brinell, Rockwell and Vickers Hardness Testing: Use and Misuse
Hardness testing often is used to characterize materials due to its ease, speed and relatively low cost. These merits have spurred many rules of thumb over the years, but like most such rules, the basis for these missives suggest they apply to only narrow conditions.
Hardness measures a material’s resistance to permanent indentation. Most hardness tests involve pressing an indenter of a defined shape into the surface of the test piece using a defined load. The resulting hardness reading relates to the size or depth of the indentation.
Indenters average a material’s hardness over the analyzed area. Averaging can prove beneficial when testing non-uni- form microstructures such as found in castings; when testing these products, technicians use higher loads and larger indenters. In contrast, differences in microstructural com- ponent hardness are key to the properties of some advanced high strength steels (AHSS), so lighter loading with smaller indenters find use when testing these grades.
The Three Primary Test Types
Dozens of different hardness tests find use today, including Brinell, Rockwell and Vickers. Each has variants based on the load used to create the indentation. Brinell tests use the largest loads, suiting it for testing thicker test pieces with relatively rough surfaces, such as castings and forgings. A Brinell indenter made from tungsten carbide has a maximum diameter of 10 mm and applies a maximum force of 3000 kgf. The Brinell reading represents the applied load over the concave indentation area created by the spherical indenter. The relatively large size of the indenter and applied load limits its use, as deep penetration over a large area does not suit testing of thin sheet metals. In addition, the Brinell pro- cedure does not allow use of a harder indenter (diamond for example), so this test is not used on hardened steel sur- faces. And, surfaces must be sufficiently smooth to allow for accurate indentation-area measurements. ASTM E10 and
Dr. Danny Schaeffler, with 30 years of materials and applications experience, is president of Engineering Quality Solutions (EQS) and Chief Content Officer of 4M Partners. EQS provides product-applications assistance to materials and manufacturing compa- nies; 4M teaches fundamentals and practical details of material properties, forming technologies, process- es and troubleshooting needed to form high-quality components. Schaeffler is the Metallurgy and Form- ing Technical Editor of the AHSS Application Guide- lines available from WorldAutoSteel at AHSSinsights.org.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
ISO 6506 cover the standard test method for Brinell hardness testing.
Thirty different Rockwell tests allow for use of different combinations of indenter types, sizes and loading conditions. This flexibility contributes to the frequent use of Rockwell testing when characterizing sheet metal hardness. Depending on the specific alloy and thickness, the most-used scales are Rockwell B, C, N and T.
While the Brinell scale is based on the area of the inden- tation, indentation depth defines the Rockwell scale. The test apparatus first applies an initial minor preload that defines a reference depth set at zero. Then, the test setup applies the major load to induce additional penetration. The incremental depth difference relative to the zero point determines the Rockwell hardness value. Each Rockwell point equals a penetration depth of 0.002 mm (2 microns, or 0.00008 in.)
ASTM E18 and ISO 6508 describe the standard test method for Rockwell hardness testing. For the B and C scales, appli- cation of the initial minor load requires 10 kgf, with total test force ranging from 60 to 150 kgf. This contrasts with superficial hardness scales such as N and T, where the initial minor preload is 3 kgf and the total test force ranges from 15 to 45 kgf, making these appropriate only for thinner materials.
C and N scales require use of a conical-shaped diamond indenter, leading to their application on indentation-resistant materials such as tool steels or case-hardened surfaces. B and T scales use a carbide ball, making them the preferred scales when testing softer alloys such as low-carbon steels and aluminum.
Vickers testing, as with Brinell testing, requires meas- urement of the dimensions of the indentation. Where the Brinell test requires calculation of the deformed area, the Vickers test measures the diagonal surface length of the impression formed from a diamond square-based pyramid with a 136-deg. angle between the opposing faces. Polishing the test samples enables accurate measurement of the impression dimensions, and etching the samples facilitates measurement of specific metallurgical phases.
Vickers test procedures allow for use of a wide range of applied loads, from 0.001 to 120 kgf. Tests performed with loads below 1 kgf are known as microhardness tests. These relatively light loads help target measurements of specific microstructural components, as heavier loads leave a larger impression and lead to an averaging of all structures under the indenter. Vickers microhardness testing assists with characterizing case-hardened depths, dimensions of weld
40 MetalForming/April 2022
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