How Rockwell Hardness Test Is Carried out

   
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Hardness is a characteristic of a material, not a fundamental physical property. It is defined as the resistance to indentation, and it is determined by measuring the permanent depth of the indentation. More simply put, when using a fixed force (load) and a given indenter, the smaller the indentation, the harder the material.

Indentation hardness value is obtained by measuring the depth or the area of the indentation using one of over 12 different test methods. Learn more about hardness testing basics here.

The Rockwell hardness test method, as defined in ASTM E-18, is the most commonly used hardness test method. You should obtain a copy of this standard, read and understand the standard completely before attempting a Rockwell test.

The Rockwell test is generally easier to perform, and more accurate than other types of hardness testing methods. The Rockwell test method is used on all metals, except in condition where the test metal structure or surface conditions would introduce too much variations; where the indentations would be too large for the application; or where the sample size or sample shape prohibits its use. 

The Rockwell method measures the permanent depth of indentation produced by a force/load on an indenter. First, a preliminary test force (commonly referred to as preload or minor load) is applied to a sample using a diamond or ball indenter. This preload breaks through the surface to reduce the effects of surface finish. After holding the preliminary test force for a specified dwell time, the baseline depth of indentation is measured. 

After the preload, an additional load, call the major load, is added to reach the total required test load. This force is held for a predetermined amount of time (dwell time) to allow for elastic recovery. This major load is then released, returning to the preliminary load.  After holding the preliminary test force for a specified dwell time, the final depth of indentation is measured. The Rockwell hardness value is derived from the difference in the baseline and final depth measurements. This distance is converted to a hardness number. The preliminary test force is removed and the indenter is removed from the test specimen.

Preliminary test loads (preloads) range from 3 kgf (used in the "Superficial" Rockwell scale) to 10 kgf (used in the "Regular" Rockwell scale). Total test forces range from 15kgf to 150 kgf (superficial and regular) to 500 to 3000 kgf (macrohardness). 

Test Method Illustration
A = Depth reached by indenter after application of preload (minor load)  
B = Position of indenter during Total load, Minor plus Major loads  
C = Final position reached by indenter after elastic recovery of sample material  
D = Distance measurement taken representing difference between preload and major load position. This distance is used to calculate the Rockwell Hardness Number.
 
Rockwell Hardness Testing Loads


A variety of indenters may be used: conical diamond with a round tip for harder metals to ball indenters ranges with a diameter ranging from 1/16" to ½" for softer materials. 

When selecting a Rockwell scale, a general guide is to select the scale that specifies the largest load and the largest indenter possible without exceeding defined operation conditions and accounting for conditions that may influence the test result. These conditions include test specimens that are below the minimum thickness for the depth of indentation; a test impression that falls too close to the edge of the specimen or another impression; or testing on cylindrical specimens.

Additionally, the test axis should be within 2-degress of perpendicular to ensure precise loading; there should be no deflection of the test sample or tester during the loading application from conditions such as dirt under the test specimen or on the elevating screw. It is important to keep the surface finish clean and decarburization from heat treatment should be removed. 

Sheet metal can be too thin and too soft for testing on a particular Rockwell scale without exceeding minimum thickness requirements and potentially indenting the test anvil. In this case a diamond anvil can be used to provide a consistent influence of the result.

Another special case in testing cold rolled sheet metal is that work hardening can create a gradient of hardness through the sample so any test is measuring the average of the hardness over the depth of indentation effect. In this case any Rockwell test result is going to be subject to doubt, there is often a history of testing using a particular scale on a particular material that operators are used to and able to functionally interpret.

For more information about Rockwell hardness testing  , please contact withus. 

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Hardness is a characteristic of a material, not a fundamental physical property. It is defined as the resistance to indentation, and it is determined by measuring the permanent depth of the indentation.

More simply put, when using a fixed force (load)* and a given indenter, the smaller the indentation, the harder the material. Indentation hardness value is obtained by measuring the depth or the area of the indentation using one of over 12 different test methods.


Hardness testing is used for two general characterizations

1.Material Characteristics
• Test to check material
• Test hardenability
• Test to confirm process 
• Can be used to predict Tensile strength

2. Functionality
• Test to confirm ability to function as designed.
• Wear Resistance
• Toughness
• Resistance to impact

Hardness Testing Considerations
The following sample characteristics should be consider prior to selecting the hardness testing method to use:

• Material
• Sample Size
• Thickness
• Scale
• Shape of sample, round, cylindrical, flat, irregular
• Gage R & R

Material
The type of material and expected hardness will determine test method. Materials such as hardened bearing steels have small grain size and can be measured using the Rockwell scale due to the use of diamond indenters and high PSI loading. Materials such as cast irons and powder metals will need a much larger indenter such as used with Brinell scales. Very small parts or small sections may need to be measured on a microhardness tester using the Vickers or Knoop Scale.

When selecting a hardness  scale, a general guide is to select the scale that specifies the largest load and the largest indenter possible without exceeding defined operation conditions and accounting for conditions that may influence the test result.  


Sample Size 
The smaller the part, the lighter the load required to produce the required indentation. On small parts, it is particularly important to be sure to meet minimum thickness requirements and properly space indentations away from inside and outside edges. Larger parts need to be fixtured properly to ensure secure placement during the test process without the chance for movement or slippage. Parts that either overhang the anvil or are not easily supported on the anvil should be clamped into place or properly supported. 

Roundness Correction

Cylindrical Samples
A correction to a test result is needed when testing on cylinder shapes with small diameters due to a difference between axial and radial material flow. Roundness correction factors are added to your testing result based on the diameter of convex cylinder surfaces. Additionally, it is important to maintain a minimum spacing equal to 2~1/2 times the indentation's diameter from an edge or another indentation. 


Thickness

Sample Thickness 
Your sample should have a minimal thickness that is at least 10x (ten times) the indentation depth that is expected to be attained. There are minimum, allowable thickness recommendations for regular and superficial Rockwell methods 


Scales 
Sometimes it is necessary to test in one scale and report in another scale. Conversions have been established that have some validity, but it is important to note that unless an actual correlation has been completed by testing in different scales, established conversions may or may not provide reliable information. Refer to ASTM scale conversion charts for non-austenitic metals in the high hardness range and low hardness range. Also refer to ASTM standard E140 for more scale conversion information. 


Gage R&R 
Gage Repeatability and Reproducibility Studies were developed to calculate the ability of operators and their instruments to test accordingly within the tolerances of a given test piece. In hardness testing, there are inherent variables that preclude using standard Gage R&R procedures and formulas with actual test pieces. Material variation and the inability to retest the same area on depth measuring testers are two significant factors that affect GR&R results. In order to minimize these effects, it is best to do the study on highly consistent test blocks in order to minimize these built-in variations.

Our Testing Instruments hardness testers operate are ideally suited for these studies. Unfortunately, since these studies can only be effectively done on test blocks, their value does not necessarily translate into actual testing operations. There are a host of factors that can be introduced when testing under real conditions. Newage testers excel at testing in real-world conditions by reducing the effects of vibration, operator influence, part deflection due to dirt, scale, a specimen flexing under load.