Rockwell Hardness ASTM D785
Rockwell Hardness ASTM D785, ISO 2039
The Rockwell Hardness test is a widely used method for measuring the hardness of various materials. It measures the depth of indentation made by an indenter when a load is applied, and the hardness is determined based on the increase in indentation depth. Hardness is reported as unit and the most common scales used are R, L, M, E, and K.
To perform the test, a standard specimen of 6.4mm (0.25 in) thickness is placed on the surface of the Rockwell Hardness tester. A minor load is first applied to set the gauge to zero. Then, a major load is applied by tripping a lever, and after 15 seconds, the major load is removed. The specimen is allowed to recover for 15 seconds, and then the hardness is read off the dial with the minor load still applied.
The Rockwell Hardness test is commonly used in the plastics industry, and the R and M scales are frequently employed for measuring the hardness of plastics. Hardness numbers on these scales have no units, and higher numbers indicate harder materials.
The advantages of the Rockwell Hardness test include its speed, ease of use, and ability to measure the hardness of a wide range of materials. However, it is important to note that the test can only provide an approximate measure of hardness, and it may not be suitable for some materials with unusual shapes or geometries. In such cases, other hardness testing methods, such as Vickers or Brinell hardness testing, may be more appropriate.
Rockwell Hardness Testing Procedure
Procedure A
Scale Selection
Choose the appropriate Rockwell hardness scale for the specimen under test. The hardness value is denoted by a letter representing the scale and a number indicating the reading. Selection of the scale should consider factors such as indentation readings and the final scale reading for the specific material and scale.
Indenter Change and Initial Adjustment
2. After changing a ball indenter, discard the first reading. Hand adjustments may not ensure proper seating of the indenter in the housing chuck. To seat the indenter shoulder correctly, apply the full pressure of the major load. Note that automated electronic instruments might differ in operation; refer to the manufacturer's instructions for specific instrument details.
Minor and Major Load Application
3. Place the specimen on the anvil. Turn the capstan screw until the small pointer is at zero, and the large pointer is within 65 divisions of the B 30 mark or the "set" position on the red scale. This adjustment aligns with the minor load of 10 kg, which is preloaded in the machine. Final adjustment to the "set" position can be made using the knurled ring on some machines, just below the capstan handwheel.
4. Apply the major load by releasing the trip lever within 10 seconds after the minor load is applied. Remove the major load 15 (+1, -0) seconds after application. Read the Rockwell hardness on the red scale to the nearest full-scale division 15 seconds after removing the major load.
Reading Interpretation
5. Record the readings as follows: Count the number of times the needle passes through zero on the red scale when applying and removing the major load. Calculate the difference between these counts. If the difference is zero, record the value as the reading plus 100. If the difference is 1, record the reading without modification. For a difference of 2, record the reading as the scale reading minus 100. Softer plastic materials requiring a less severe scale than the R scale should be tested using Test Method D 2240.
Total Indentation Calculation
6. Determine the total indentation by counting the number of divisions the pointer moves from the zero set position during the 15 seconds after the major load application.
Scale Adjustment
7. If the total indentation reading on a specific scale exceeds the limits of the test machine (150 divisions for regular machines and 250 divisions for PL machines), choose the next less severe scale. For instance, if the M scale indentation exceeds 250 divisions, opt for the L Rockwell scale.
Test Position and Specimen Edges
8. Avoid conducting tests near the edge of the specimen to prevent the indenter from crushing the edge during the application of the major load. Maintain a minimum clearance of 6 mm [1⁄4 in.] from the edge. Tests should not be performed too closely together, as previous indentations may damage the plastic surface. Duplicate tests on the opposite face of a specimen should be avoided to prevent reading discrepancies due to previous indentations.
Procedure B
Appropriate Scale and Load
Utilize the R scale with a 12.7 mm [1⁄2 –in.] indenter and a 60-kg major load for Procedure B.
Determining Spring Constant
2. Calculate the "spring constant" or correlation factor of the machine as follows: Place a soft copper block with plane parallel surfaces on the anvil in the testing position. Raise the sample and anvil by the capstan screw to the 12.7 mm [1⁄2-in.] indenter until the small pointer is at the starting dot and the large pointer reads zero on the black scale. Apply the major load by tripping the load release lever. Repeat this operation multiple times without moving the block, resetting the dial to zero after each test while under minor load, until the dial gage deflection becomes consistent. This constant deflection, in dial divisions, is the correction factor for subsequent calculations.
Test Execution
3. After machine adjustment, position the specimen on the anvil. Apply the minor load of 10 kg and set the zero position within 10 seconds. Immediately apply the major load after zero setting. Observe and record the total divisions the pointer passes through during 15 seconds of major load application. Count the actual scale divisions representing the indentation on the black scale.
Total Indentation Calculation
4. Deduct the spring constant correction from the observed reading. Record this difference as the total indentation under load.
Calculation of Alpha Rockwell Hardness Number
5. Calculate the alpha, a, Rockwell hardness number using the formula: a Rockwell hardness number = 150 - (total indentation under load).
What Influences on Reproducibility and Precision in Rockwell Hardness Testing?
The determination of hardness through Rockwell hardness testing is a crucial endeavor in various industries, providing insights into the structural integrity and durability of materials. However, this assessment is not without its complexities, as multiple factors can influence the reproducibility and accuracy of results.
Impact of Material Characteristics on Reproducibility
The reproducibility of Rockwell hardness readings depends significantly on the material's intrinsic properties. For certain homogenous materials with a Young's modulus in compression exceeding 3400 MPa [53,105 psi], Rockwell hardness readings have demonstrated reproducibility to 62 divisions. Softer plastics and materials with coarse fillers exhibit a broader range of variability. Using a larger ball indenter can distribute the load more uniformly, leading to a narrower range of test results (Note 2). However, it's important to note that the instrument's sensitivity diminishes as the dial reading increases, particularly for readings surpassing 100 due to the shallow indentation of the steel ball. To mitigate this sensitivity loss, it's advisable to utilize the smallest ball and the highest practical load. Readings exceeding 115 are not considered satisfactory and should not be reported. While readings between zero and 100 are recommended, readings up to 115 are permissible. In cases requiring comparison, readings above 115 or below zero on a single scale may be taken, but the accurate readings should be enclosed in parentheses if feasible. It's important to recognize that such alternate readings might not always be attainable under conditions of changing dynamics or irreversible reactions.
Mitigating Vibration Influence
External factors can significantly affect the accuracy of Rockwell hardness testing. If the testing setup is subjected to vibrations, such as those stemming from nearby machinery, it's recommended to mount the Rockwell hardness tester on a metal plate with a 25 mm [1 in.] thick layer of sponge rubber or another suitable dampening material. This prevents vibrations from causing inaccurate indents due to variations in the material's response.
Maintaining Indenter Integrity
The integrity of the indenter is crucial for accurate results. Accumulation of dust, dirt, grease, or rust on the indenter can adversely impact the readings. Additionally, steel ball indenters with nicks, burrs, or deformities should be avoided.
Equipment Condition and Accuracy
The condition of the testing equipment plays a pivotal role in the accuracy of results. Dust, dirt, or heavy oil can act as cushions, yielding erroneous instrument dial readings. It's imperative to keep various components like the instrument housing, indenter chuck, ball seat, capstan, and anvil shoulder seat clean and true. Lightly oiling the capstan and screw is advised. If pitted anvil surfaces are present, refinishing with 600 grit paper can restore accuracy.
Surface Conditions and Anisotropy
Surface conditions significantly influence Rockwell hardness readings. Molded finishes tend to yield higher readings compared to machined faces due to factors such as resin content, material fillers, and plasticizer content. Tubular or curved specimens with no support should be avoided as they can lead to unsymmetrical indentation patterns.
Anisotropic Characteristics
Many plastics exhibit anisotropic characteristics, causing hardness to vary with the testing direction. In such cases, the hardest face is typically perpendicular to the molding pressure. Specimens with flashing on the side supported by the anvil can yield incorrect results.
Impact of Temperature Variation
Ambient temperature fluctuations can significantly influence hardness results for various materials.
Importance of Specimen Thickness
To ensure the highest accuracy, Rockwell hardness tests should be conducted on specimens of sufficient thickness. If there are bulges, color changes, or markings on the underside of the test specimen closest to the anvil, precision testing may be compromised. Stacking thin specimens is allowed if they are flat, parallel, and free from defects, although the precision of stacked specimens is reduced.
In conclusion, Rockwell hardness testing offers invaluable insights into material hardness, but its accuracy hinges on a multitude of factors. Recognizing and addressing these influences are imperative for obtaining reliable and meaningful results in the realm of materials analysis.