The hardness of a material can be defined as the resistance of that material to indentation and scratching. It is characteristic of materials and very important material property. Different applications required hard materials and the hardness of that material quantifies that property. The more the value of hardness, the more hard the material is, and the more difficult will be to create an indentation under a given load. There are various industry-approved methods for hardness testing like Brinell Hardness, Rockwell Hardness, Vickers Hardness, etc. However, Rockwell hardness testing is one of the widely used, efficient, and industry-recognized methods of hardness test that is represented using Rockwell Hardness Scales. \n\n\n\nFor material selection, process and quality control, and acceptance testing of commercial products, the Rockwell hardness test is useful. In this article, we will learn the procedure of the Rockwell hardness test, related formulas, standards, Rockwell hardness scale, and advantages.\n\n\n\nWhat is the Rockwell Hardness Test?\n\n\n\nRockwell hardness test using Rockwell hardness scale is one of the extensively used and accurate hardness test methods prevalent in industries. This testing is easier to perform as compared to Brinell or Vickers hardness test. As per the name of the inventors, Mr. Hugh M. Rockwell and Mr. Stanley P. Rockwell, this hardness test is well-known as Rockwell hardness testing.\n\n\n\nRockwell hardness test is widely used for thin steel, lead, brass, zinc, aluminum, cemented carbides, iron, titanium, copper alloys, and certain plastics. This test is considered one of the simple and quick hardness testing methods.\n\n\n\nRockwell hardness testing is performed by comparing the penetration depth measurement of an indenter under a large load known as a major load and a smaller preload known as a minor load.\n\n\n\nTypes of Rockwell Hardness Tests\n\n\n\nDepending on the applications of minor and major loads during the test, the Rockwell hardness test is categorized into two groups. They are:\n\n\n\nRegular Rockwell Hardness Test where the minor load is 10 kgf and major load is 60, 100, or 150 kgf.Superficial Rockwell Hardness Test where the applied minor load is 3 kgf and the major loads are 15, 30, or 45 kgf.\n\n\n\nRockwell Hardness Test Procedure\n\n\n\nThe procedure for the Rockwell Hardness test consists of the application of a minor load followed by a major load. The minor load is applied to establish the zero position from where the measurement will be taken. Next, the major load is applied and then removed while still maintaining the minor load. The penetration depth (d) from the zero datum position is measured by a dial.\n\n\n\nA pre-decided minor and major load is applied on the test specimen by a diamond or ball indenter. The minor load (3 kgf to 10 kgf) reduces the effect of surface finish. After that the additional load or major load (15 kgf to 150 kgf) is applied for a specified dwell time. Now the major load is released and the final depth of indentation from the preload position to the combined load position is measured which is converted to the Rockwell hardness number. Then the preload is released and indenter is removed from the sample. Refer to the Fig. 1 below that explains the Rockwell hardness test procedure.\n\n\n\nFig. 1: Rockwell Hardness Test Procedure\n\n\n\nThe procedure of Rockwell hardness test consists of the following steps:\n\n\n\nSetting the test specimen sample on a flat, solid surface.Applying the minor load to create a slight impression (zero point position).Setting the measuring gauge and measuring the initial indentation depth under minor load.Applying the major load and holding for the dwell time.Removing the major load, keeping the minor load.Measuring the depth of penetration from zero position.Finding the Rockwell hardness number from the penetration depth.\n\n\n\nRockwell Hardness Test Formula\n\n\n\nRockwell hardness test is directly read from the equipment. However, there is a formula to convert the measured depth (d) into the Rockwell hardness number. A Rockwell hardness number for a material is a unitless number indicating the hardness value on a specified Rockwell scale.\n\n\n\nThe equation for Rockwell Hardness Number is given by\n\n\n\nHR=N-(d\/s)\n\n\n\nWhere,\n\n\n\nN and s are scale factors depending on the test being used. The value of N is either 100 or 130 depending on the Rockwell hardness scale used. Similarly, the value of s is either 0.001 mm or 0.002 mm.d is the penetration depth measured from zero points (in mm).\n\n\n\nRockwell Hardness Scale\n\n\n\nRockwell hardness scale is a type of scale that indicates the hardness of a material measured using the Rockwell hardness test. It is a unitless number. The Rockwell hardness scale is symbolized by HR followed by a letter indicating any of the possible scales. For example, "HRC 96" for a metal means the hardness of that metal is 96 when measured using Rockwell hardness scale C.\n\n\n\nManufactured products are made of different types of materials. To accommodate the hardness testing of these diverse range, several different indenter types are used in the Rockwell hardness testing along with a range of standard force levels. Each combination of indenter type and applied force levels has been defined using a distinct Rockwell hardness scale. As per ASTM E 18, there are thirty (30) different Rockwell scales divided into two categories: regular Rockwell scales and superficial Rockwell scales.\n\n\n\nThere are several different Rockwell Scales denoted by various letters requiring different loads or indenters during hardness testing. The Rockwell hardness scale chart for regular Rockwell scales and superficial Rockwell scales are provided in table 1 and 2 below:\n\n\n\nRockwell Scale SymbolType of Indenter\/Diameter in case of BallMinor LoadMajor LoadTypical useHRASpheroconical diamond98.07 N (10 kgf)588.4 N (60 kgf)Cemented carbides, thin steel, and shallow case hardened steel.HRBBall, 1.588 mm (\u00b9\u2044\u2081\u2086 inches)98.07 N (10 kgf)980.7 N (100 kgf)Copper alloys, soft steels, aluminum alloys, malleable iron, etc.HRCSpheroconical diamond98.07 N (10 kgf)1471 N (150 kgf)Steel, hard cast irons, pearlitic malleable iron, titanium, deep case hardened steel, and other materials harder than 100 on the Rockwell B scale.HRDSpheroconical diamond98.07 N (10 kgf)980.7 N (100 kgf)Thin steel and medium case hardened steel, and pearlitic malleable iron.HREBall, 3.175 mm (\u215b inches)98.07 N (10 kgf)980.7 N (100 kgf)Cast iron, aluminum and magnesium alloys, and bearing metals.HRFBall, 1.588 mm (\u00b9\u2044\u2081\u2086 inches)98.07 N (10 kgf)588.4 N (60 kgf)Annealed copper alloys, and thin soft sheet metals.HRGBall, 1.588 mm (\u00b9\u2044\u2081\u2086 inches)98.07 N (10 kgf)1471 N (150 kgf)Malleable irons, copper-nickel-zinc and cupronickel alloys.HRHBall, 3.175 mm (\u215b inches)98.07 N (10 kgf)588.4 N (60 kgf)Aluminum, zinc, and lead.HRKBall, 3.175 mm (\u215b\u2033)98.07 N (10 kgf)1471 N (150 kgf)Bearing metals and other very soft or thin materials. HRLBall, 6.350\u00a0mm (\u00bc inches)98.07 N (10 kgf)588.4 N (60 kgf)Bearing metals and other very soft or thin materials. HRMBall, 6.350\u00a0mm (\u00bc inches)98.07 N (10 kgf)980.7 N (100 kgf)Bearing metals and other very soft or thin materials. HRPBall, 6.350\u00a0mm (\u00bc inches)98.07 N (10 kgf)1471 N (150 kgf)Bearing metals and other very soft or thin materials. HRRBall, 12.70\u00a0mm (\u00bd inches)98.07 N (10 kgf)588.4 N (60 kgf)Bearing metals and other very soft or thin materials. HRSBall, 12.70\u00a0mm (\u00bd inches)98.07 N (10 kgf)980.7 N (100 kgf)Bearing metals and other very soft or thin materials. HRVBall, 12.70\u00a0mm (\u00bd inches)98.07 N (10 kgf)1471 N (150 kgf)Bearing metals and other very soft or thin materials. Table 1: Regular Rockwell hardness scale chart\n\n\n\nRockwell Scale SymbolType of Indenter\/Diameter in case of BallMinor Load, N (kgf)Major Load, N (kgf)Typical use15NSpheroconical diamond29.42(3)147.1 (15)Similar to A, C, D scales in table-1 but for thinner gage material.30NSpheroconical diamond29.42(3)294.2 (30)Similar to A, C, D scales in table-1 but for thinner gage material.45NSpheroconical diamond29.42(3)441.3 (45)Similar to A, C, D scales in table-1 but for thinner gage material.15TBall; 1.588 mm (1\/16 inches)29.42(3)147.1 (15)Similar to B, F, G scales in table-1 but for thinner gage material.30TBall; 1.588 mm (1\/16 inches)29.42(3)294.2 (30)Similar to B, F, G scales in table-1 but for thinner gage material.45TBall; 1.588 mm (1\/16 inches)29.42(3)441.3 (45)Similar to B, F, G scales in table-1 but for thinner gage material.15WBall; 3.175 mm (1\/8 inches)29.42(3)147.1 (15)Very Soft Material30WBall; 3.175 mm (1\/8 inches)29.42(3)294.2 (30)Very Soft Material45WBall; 3.175 mm (1\/8 inches)29.42(3)441.3 (45)Very Soft Material15XBall; 6.35 mm (1\/4 inches)29.42(3)147.1 (15)Very Soft Material30XBall; 6.35 mm (1\/4 inches)29.42(3)294.2 (30)Very Soft Material45XBall; 6.35 mm (1\/4 inches)29.42(3)441.3 (45)Very Soft Material15YBall; 12.70 mm (1\/2 inches)29.42(3)147.1 (15)Very Soft Material30YBall; 12.70 mm (1\/2 inches)29.42(3)294.2 (30)Very Soft Material45YBall; 12.70 mm (1\/2 inches)29.42(3)441.3 (45)Very Soft MaterialTable 2: Superficial Rockwell hardness scale charts\n\n\n\nFactors for Selecting Appropriate Rockwell Scale\n\n\n\nChoosing the right Rockwell scale is difficult as various parameters affect the selection process. The major factors to be considered for Rockwell scale selection are:\n\n\n\nthe type of hardness test materialthe thickness of test materialthe area or width of the test materialthe limitations of each Rockwell hardness scalethe test material homogeneity.\n\n\n\nStandards for Rockwell Hardness Testing\n\n\n\nThe widely used standards that govern the Rockwell hardness testing methodology are:\n\n\n\nASTM E18 for MetalsISO 6508 for Metals, ASTM D785 for PlasticsISO 2039 for Plastics\n\n\n\nRockwell Hardness Test Machine\n\n\n\nA wide range of Rockwell hardness tester machines is available to find out the hardness values of materials. It can vary from manual and semi-automatic to fully automatic advanced ones. Refer to Fig. 2 below that provides the major components of a typical Rockwell hardness testing machine.\n\n\n\nFig. 2: Typical Rockwell hardness testing machine\n\n\n\nFactors Affecting the Accuracy and Reliability of Rockwell Hardness Testing\n\n\n\nThere are various parameters that affect the accuracy, precision, and reliability of the Rockwell hardness testing method. Those are:\n\n\n\nRockwell test machine: Test results can vary due to variability from the equipment, operator, and environmental conditions. So, it must be ensured that the machine condition, specimen set-up by the operator are correct.Presence of dirt or grease in the contact area.Specimen Surface: Surface conditions must be of good quality to get more accurate and reproducible test results.Operator Skill: the operator must be highly skilled in proper fixing and testing techniques. Otherwise, test results can vary significantly.\n\n\n\nTypical Rockwell Hardness Values\n\n\n\nThe following table (Table 3) provides some typical values of Rockwell hardness\n\n\n\nMaterialRockwell Hardness ValuesVery Hard SteelHRC 55 to HRC 66AxesHRC 45 to HRC 55BrassHRB 55 to HRB 93Hard Impact Steel BladesHRC 52 to HRC 55Table 3: Typical Rockwell Hardness Values\n\n\n\nAdvantages of Rockwell Hardness Testing\n\n\n\nRockwell hardness testing provides significant advantages with respect to other hard test methods as listed below:\n\n\n\nRockwell hardness test displays hardness values directly. So no calculation is required to find the hardness value.It is a fast, reliable, and robust method with a small area of indentation.\n\n\n\nRockwell Hardness Test vs Brinell hardness Test\n\n\n\nThe details of the Brinell hardness test and the main differences between the Rockwell and Brinell hardness test are published in a separate article. Kindly click here to visit that article.