Rockwell hardness test

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  • 1. Rockwell hardness test Rockwell hardness test History Stanley P. Rockwell invented the Rockwell hardness test. He was a metallurgist for a large ball bearing company and he wanted a fast non-destructive way to determine if the heat treatment process they were doing on the bearing races was successful. Types of Rockwell test There are two types of Rockwell tests: 1. Rockwell: the minor load is 10 kgf, the major load is 60, 100, or 150 kgf. 2. Superficial Rockwell: the minor load is 3 kgf and major loads are 15, 30, or 45 kgf. In both tests, the indenter may be either a diamond cone or tungsten carbide ball, depending upon the characteristics of the material being tested. Theory related to Rockwell hardness Rockwell hardness values are expressed as a combination of a hardness number and a scale symbol representing the indenter and the minor and major loads. The hardness number is expressed by the symbol HR and the scale designation. The Rockwell hardness test is defined in ASTM E 18 and several other standards. Rockwell hardness testing differs from Brinell testing in that the Rockwell hardness number is based on the difference of indenter depth from two load applications. Initially a minor load is applied, and a zero datum is established. A major load is then applied for a specified period of time, causing an additional penetration depth beyond the zero datum point previously established by the minor load. After the specified dwell time for the major load, it is removed while still keeping the minor load applied. The resulting Rockwell number represents the difference in depth from the zero datum position as a result of the application of the major load. The entire procedure requires only 5 to 10 s. In this hardness and differential depth are inversely proportional to each other. Higher the depth and low would be the hardness. Hardness and depth are related as ๐ป๐‘…๐‘ = ๐พโˆ’โ„Ž 0.002 Where Page | 1
  • 2. Rockwell hardness test K = constant, for C scale it is 0.2 and for B scale it is 0.26 h = depth of impression in mm Hence the formulae for HRc = 100 โ€“ e where e = h/0.2 And for HRB = 130 โ€“ e where e h/0.26 Scales and values The most common indenter type is a diamond cone ground at 120 degrees for testing hardened steels and carbides. Softer materials are typically tested using tungsten carbide balls ranging in diameters from 1/16 in up to 1/2 in. The combination of indenter and test force make up the Rockwell scale. These combinations make up 30 different scales and are expressed as the actual hardness number followed by the letters HR and then the respective scale. There are several alternative scales, the most commonly used being the "B" and "C" scales. Both express hardness as an arbitrary dimensionless number Table 1 scale and indenter of Rockwell hardness Scale Abbreviation Load A HRA 50 kgf B HRB 100 kgf C HRC 150 kgf D HRD 100 kgf E HRE 100 kgf F HRF 60 kgf G HRG 150 kgf Indenter 120ยฐ diamond cone 1 โ„16-inch-diameter (1.588 mm) steel sphere 120ยฐ diamond cone Use Tungsten carbide Aluminium, brass, and soft steels Harder steels >B100 120ยฐ diamond cone โ„8-inch-diameter (3.175 mm) steel sphere 1 โ„16-inch-diameter (1.588 mm) steel sphere 1 โ„16-inch-diameter (1.588 mm) steel sphere 1 Page | 2
  • 3. Rockwell hardness test Figure 1 brale indenter Procedure We were provided steel sample and their heat treatment had already done. Roughly polished the surface and checked the sample either it is suitable for test or not. We used the brale indenter. Placed the sample on anvil and applied the minor load of 10kg. Set the zero datum. Applied the load of 45 kg and allowed the needle to settle. And got the result. Calculation 1st readings HRc = 38 2nd reading HRc = 93 3rd reading HRc = 91 Result Acceptable range for Rockwell hardness test on C scale is 19 โ€“ 55 beyond this range results are suspected. In our case the material was soft so it should not be tested on C scale. Advantages of Rockwell hardness There are several reasons for the popularity of the Rockwell test. ๏ƒ˜ The test itself is very rapid. On a manually operated unit, a Rockwell test takes only five to ten seconds, depending upon the size and hardness of the specimen, as well as pre-load and dwell time. ๏ƒ˜ The indentation is extremely small and usually does not need to be removed by machining, making this a non-destructive test. A Page | 3
  • 4. Rockwell hardness test ๏ƒ˜ The Rockwell test is applicable to a wide range of part sizes. Sheet metal as thin as 0.006 inch can be tested on the Rockwell superficial tester, and as long as the surface area is large enough, there is no actual limitation to the size of your specimen. ๏ƒ˜ The Rockwell test is based on measurement of the depth of penetration with the hardness number read directly from the dial gauge or digital display that is part of every tester. ๏ƒ˜ The Rockwell hardness test is an empirical indentation hardness test that can provide useful information about metallic materials. This information may correlate to tensile strength, wear resistance, ductility, and other physical characteristics of metallic materials, and may be useful in quality control and selection of materials. ๏ƒ˜ Rockwell hardness tests are considered satisfactory for acceptance testing of commercial shipments, and have been used extensively in industry for this purpose. ๏ƒ˜ Rockwell hardness testing at a specific location on a part may not represent the physical characteristics of the whole part or end product. Page | 4
  • 5. Rockwell hardness test References ๏‚ท ASM metal handbook volume 8 mechanical testing and evaluation ๏‚ท Mechanical metallurgy by dieter ๏‚ท Fundamental of Rockwell www.wilsoninstrument.com ๏‚ท http://www.wilsonhardness.com/resources/testmethods/rockwelltesting.asx Page | 5