Factors affecting the accuracy of hardness measurement and their solutions

There are many instruments and equipment used in measurement work. Each instrument and equipment has many unfavorable factors that affect the accuracy of its measurement value when in use. This article only analyzes some common influencing factors that are easily overlooked by testers when using two conventional instruments (Rockwell hardness tester and Brinell hardness tester), and proposes solutions.

   1. Impact of roughness and solutions

   We know that when using a benchtop hardness tester to measure the Brinell hardness, the indenter of the hardness tester is a steel ball indenter, which is pressed into the surface to be tested under a certain pressure to obtain a circular indentation, and then the diameter of the circular indentation is measured with a reading microscope, and then the corresponding hardness value is found in the Brinell hardness table, that is, the hardness value of the tested sample. The roughness of the tested surface directly affects the accuracy of the hardness measurement value. When the roughness value of the tested surface is greater than Ra=0.8μm, as the roughness value increases, the resistance of the tested surface to the indenter becomes smaller, the plastic deformation becomes greater, the circular indentation becomes larger, and the corresponding hardness value becomes smaller, causing the measured value to be lower than its true value. Experiments have shown that the measurement deviation is above 10HB (Note: When using a benchtop hardness tester to measure Rockwell hardness, the influence of roughness is small, so this article will not analyze it).

   When we use a portable microcomputer ultrasonic hardness tester to measure hardness, the influence of roughness is greater than that of a desktop hardness tester. When the roughness value of the measured surface is greater than Ra=0.8μm, as the roughness value increases, the contact area between the diamond pyramid indenter of the hardness tester and the measured surface will increase. This contact includes indentation contact and non-indentation contact.

   Indentation contact refers to the contact between the indenter and the indentation after the indenter itself is pressed into the measured surface, and the contact area is also extremely small; while non-indentation contact refers to the contact between the conical surface of the indenter of the hardness tester and the inclined surface of the peak of the measured surface profile. Non-indentation contact is not conducive to hardness measurement. Because the working principle of the microcomputer ultrasonic hardness tester is to measure the hardness with the help of the ultrasonic vibration of the sensor rod. Under uniform contact pressure, the resonant frequency of the sensor rod changes with the hardness of the sample. If the hardness of the sample is lower, the indentation contact area is larger, the damping of the measured surface to the sensor rod indenter is greater, the vibration amplitude of the sensor rod indenter is smaller, and the resonant frequency is higher. In other words, the larger the indentation contact area, the lower the indication of the ultrasonic hardness tester. Non-indentation contact greatly increases the contact area between the indenter and the measured surface, causing the hardness tester indication to be lower than the true value. Experiments have shown that the Rockwell hardness measurement deviation is about 10HRC; the Brinell hardness measurement deviation is about 20HB.

   Solution: When measuring the hardness of the sample, we must pay attention to whether the surface roughness of the sample meets the test conditions of the hardness tester. Under normal use of the hardness tester, it must be ensured that the surface roughness value of the sample is less than or equal to Ra = 0.8μm. If the surface roughness value of the sample is greater than Ra = 0.8μm, the surface to be tested can be ground and trimmed by mechanical methods (grinding machine) or manual methods to make the surface roughness of the sample meet the test conditions. Minimize the impact of roughness so that we can obtain accurate measurement values.

   2. Impact of extrusion layer and its solution

   The extrusion layer is a thin hard layer on the surface of the specimen after lathe finishing. When the specimen is being finished, the turning tool also has an extrusion (rolling) effect on the surface of the specimen, which deforms and refines the metal grains on the surface of the finished surface, making them finer and denser than the metal grains in the deep layer of the specimen, thus producing a thin hard layer. The thickness of the hard layer is generally around 0.3 mm. This hard layer causes the hardness measurement value to be higher than the vacuum value, which has different degrees of influence on the accuracy of hardness measurement using a desktop hardness tester and a microcomputer ultrasonic hardness tester.

   When we use a benchtop hardness tester to measure Rockwell hardness, the indenter of the hardness tester is a diamond cone, and the contact area between the indenter (the diameter of the cone top is 0.4 mm) and the measured surface is small. When loaded, the indenter can easily penetrate the extrusion layer, so the measurement deviation of the hardness is small. Tests have shown that the measurement deviation is generally within 5HRC. When measuring Brinell hardness with a benchtop hardness tester, the indenter of the hardness tester is a steel ball indenter, and the contact area between the indenter and the measured surface is large. When loaded, the indenter must overcome the large resistance of the extrusion layer to press into the measured surface, which makes the indenter of the hardness tester insufficient, and the circular indentation obtained by pressing becomes smaller, causing the corresponding hardness value to be higher than its true value. Moreover, the measurement deviation of hardness is large. Tests have shown that the measurement deviation of hardness is about 20HB. In addition, whether measuring Rockwell hardness or Brinell hardness, as the hardness of the specimen itself increases, the measurement deviation of hardness will decrease.

   The extrusion layer has the greatest impact on the accuracy of hardness measurement using a microcomputer ultrasonic hardness tester. Measuring hardness using an ultrasonic hardness tester causes minimal damage to the specimen and is basically a non-destructive test. Under a test load of 10N, the indentation depth is generally around 4μm to 50μm. The thickness of the extrusion layer is generally around 0.3 mm. Therefore, the pyramid indenter of the ultrasonic hardness tester cannot penetrate the extrusion layer at all, and the measured hardness value is only the hardness of the extrusion layer, not the true hardness of the specimen itself. The hardness of the extrusion layer is much higher than the true hardness of the specimen. If we ignore this unfavorable factor, it will cause a large measurement deviation. Experiments have shown that the measurement deviation of Rockwell hardness is generally 5-10HRC; the measurement deviation of Brinell hardness is generally dozens of HB.

   Solution: If we find a fine-turned test piece during the test, before measuring its hardness, we must remove the extrusion layer at the measuring position of the measured surface; we can also remove the extrusion layer of the entire measured surface. The extrusion layer can be removed mechanically (on a grinder) or manually. It should also be emphasized that if we use manual methods to process the extrusion layer, the roughness of the measured surface may be destroyed. If so, the measured surface must also be ground and trimmed to make the measured surface roughness of the test piece meet the test conditions. In this way, we can obtain true and reliable measurement values.

   The two factors that affect the accuracy of hardness measurement analyzed in this article are summarized in actual testing work, and are easily overlooked by testers and are relatively common. Of course, there are still many unfavorable factors in actual testing, which requires us to constantly study, summarize, and take each testing task seriously, so that the test data we get will be true, accurate, and reliable.