GENERAL KNOWLEDGE ABOUT
HARDNESS TESTING METHODS
Heat treating has evolved into a highly complex, precise process that improves characteristics of metal parts. A critical component of quality heat treating is employing the correct hardness testing method to show manufacturers their parts achieve design requirements.
Hardness testing methods vary based on the material and heat treatment chosen. It’s important that engineers specify hardness testing methods correctly to ensure timely heat treatment and avoid costly delays.
Common hardness testing methods are introduced below.
Rockwell Hardness
This method tests the hardness of steel parts by applying loads to parts via either a tungsten carbide ball or a spheroconical diamond indenter. A part’s testing surface and seating surface must be properly prepared prior to testing. Inadequate surface preparation can lead to failed tests or false readings.
Following surface preparation, a light load is applied to the indenter (usually either 3 or 5 kgf) to zero out the testing machine. Then, a heavy load (anywhere from 15 kgf to 150 kgf, depending on the material and its strength) is applied and held for a period of time.
The downward distance the indenter traveled from the application of the light load to the release of the heavy load is recorded and used to calculate hardness.
Variants on the Rockwell hardness test procedure are used depending on the material and strength of a part. The most common Rockwell variants include:
- HRC – Known as “Rockwell C,” a 150 kgf load is applied via a diamond in this method. It’s most commonly used on steels that are through hardened to increase strength. Parts commonly tested by this method include nuts and bolts, hand tools, seat belt buckles, chains, springs, axles, bearings and blades.
- HR15N – This method is known as “Rockwell 15N.” A relatively light, 15 kgf load is applied to case hardened parts treated to achieve a minimum effective case depth 0.007 inches or a total case of 0.012 inches. Other Rockwell variants (HR30N and HR45N) apply slightly higher loads on parts with incrementally higher minimum effective or total cases.
- HRBW – “Rockwell B” uses the tungsten carbide ball. It’s commonly used for “soft” parts like austenitic stainless steels and for annealed parts.
Brinell Hardness
Relatively high loads are applied to parts via a tungsten carbide ball in this method. Unlike with Rockwell testing, Brinell tests measure the diameter of the indentation made by the ball, not the depth. A 10mm ball with an applied load of 3,000 kgf is most common for Brinell testing.
Brinell tests are ideal for castings and forgings that may have rough surfaces or exhibit some chemical variation. Because the indentation ball is so much larger than in other testing methods, hardness values derived from the test provide a more representative average hardness of the part.
Microhardness testing
Microhardness tests apply considerably lighter loads compared to other methods. Precisely cut diamonds are used for these tests, which most often measure hardness in small, localized regions of parts. Microhardness tests are best used to determine case depths of case hardened parts.
Two types of diamonds are used in microhardness tests. Pyramid-shaped Vickers diamonds are usually used in Europe and Asia. Knoop diamonds, which are pyramidal but with two elongated legs, have historically been more common in the U.S.
It’s important to specify which microhardness testing method to conduct as conversions between the values produced by each type of diamond are only approximate.
It’s also critical to specify the correct load for Knoop tests because too-light loads could produce falsely high hardness readings. For Vickers tests, too-light loads create too-small indentations on a part, which leads to inaccurate hardness readings. On the flip side, too-heavy loads could punch all the way through a case, ruining the chance of getting an accurate hardness reading. Precise specs that include the appropriate testing method and hardness range limit potential error and ensure efficient heat treatment.
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