The tools, measuring tools, molds, etc. used in mechanical manufacturing should have sufficient hardness to ensure performance and life. Today, the editor will talk to you about topics related to "hardness". Hardness is a measure of the material's ability to resist local deformation, especially plastic deformation, indentation or scratches. Generally, the harder the material, the better its wear resistance. For example, mechanical parts such as gears require a certain hardness to ensure sufficient wear resistance and service life. Types of hardness Types of hardness/Click to view a larger image As shown in the figure above, there are so many types of hardness... Today I will introduce you to the common and practical indentation hardness test in metal hardness. Definition of hardness 1. Brinell hardness/Brinell Hardness The Brinell hardness (symbol HB) test method is the first method developed and summarized among the recognized specifications of hardness, which has contributed to the emergence of other hardness test methods. The principle of the Brinell hardness test is: the indenter (steel ball or carbide ball, diameter Dmm) applies a test force F, and after the sample is pressed, the contact area S (mm2) between the ball indenter and the sample is calculated from the diameter d (mm) of the concave portion left by the indenter, and the value is obtained by dividing the test force. The symbol is HBS when the indenter is a steel ball, and HBW when the indenter is a carbide ball. k is a constant (1/g= 1/9.80665 = 0.102). 2. Vickers Hardness Vickers hardness (symbol HV) is the test method with the widest application range that can be tested with any test force, especially in the field of micro hardness below 9.807N. Vickers hardness is the value obtained by dividing the test force F (N) by the contact area S (mm2) between the standard sheet and the indenter, which is calculated based on the diagonal length d (mm, the average of the lengths in two directions) of the indentation formed on the standard sheet by the indenter (square pyramid diamond, relative face angle = 136˚) under the test force F (N). k is a constant (1/g = 1/9.80665). 3. Knoop Hardness Knoop hardness (symbol HK) is the value calculated by dividing the test force by the indentation projection area A (mm2) as shown in the following formula, which is calculated based on the longer diagonal length d (mm) of the indentation formed on the standard sheet by pressing a rhombus diamond indenter (relative side angles of 172˚30' and 130˚) under the test force F. Knoop hardness can also be measured by replacing the Vickers indenter of the microhardness tester with the Knoop indenter.
4. Rockwell and Rockwell surface hardness Rockwell Hardness Before measuring Rockwell hardness (symbol HR) or Rockwell surface hardness, it is necessary to use a diamond indenter (tip cone angle: 120˚, tip radius: 0.2mm) or a spherical indenter (steel ball or carbide ball) to apply a preload force to the standard sheet, then apply the test force, and restore the preload force. The hardness value is derived from the hardness formula, which is expressed as the difference in indentation depth h (μm) between the preload force and the test force. The Rockwell hardness test uses a preload force of 98.07N, while the Rockwell surface hardness test uses a preload force of 29.42N. The specific symbol provided together with the indenter type, test force and hardness formula is called a scale. The Japanese Industrial Standard (JIS) defines various related hardness scales. The hardness tester is relatively simple and quick to operate, and can be tested directly on the raw material or the surface of the part, so it is widely used. A guide to selecting hardness test methods for your reference: Hardness Selection Guide Hardness Selection Conversion 1. Conversion between Knoop hardness and Vickers hardness (1) Based on the assumption that objects of the same hardness have equal resistance to the Knoop and Vickers indenters, the stresses of the Vickers and Knoop indenters under load are deduced, and then based on σHK=σHV, it is concluded that: HV=0.968HK. This formula is measured under low load and has a relatively large error. In addition, when the hardness value is greater than HV900, the error of this formula is very large and loses its reference value. (2) After derivation and correction, the conversion formula between Knoop hardness and Vickers hardness is proposed. After verification by actual data, the maximum relative conversion error of this formula is 0.75%, which has a high reference value.
2. Conversion between Rockwell hardness and Vickers hardness
(1) After modifying the Qvarnstorm conversion formula proposed by Hans Qvarnstorm, the conversion formula between Rockwell hardness and Vickers hardness is obtained: This formula uses the standard data of ferrous metal hardness published in my country for conversion. Its HRC error is basically within the range of ±0.4HRC, and its maximum error is only ±0.9HRC. The maximum HV error is ±15HV. (2) According to the stress σHRC=σHV of different indenters, the formula is obtained by analyzing the relationship curve between Rockwell hardness and Vickers hardness indentation depth. This formula is compared with the national standard experimental conversion value. The error between the conversion result and the standard experimental value is ±0.1HRC. (3) Based on the actual experimental data, the conversion between Rockwell hardness and Vickers hardness is discussed using the linear regression method, and the formula is obtained: This formula has a small application range and a large error, but it is simple to calculate and can be used when the accuracy requirement is not high. 3. Conversion between Rockwell hardness and Brinell hardness (1) Analyze the relationship between the depth of Brinell indentation and Rockwell indentation, and obtain the conversion formula based on the stress of the indenter σHRC=σHB. The calculation results are compared with the national standard experimental value. The error between the conversion formula calculation result and the standard experimental value is ±0.1HRC. (2) The formula is obtained by linear regression method based on actual experimental data. The formula error is large and the application range is small, but the calculation is simple and can be used when the accuracy requirement is not high. 4. Conversion between Brinell hardness and Vickers hardness The relationship between Brinell hardness and Vickers hardness is also obtained according to σHB=σHV. The conversion result of this formula is compared with the national standard conversion value, and the conversion error is ±2HV. 5. Conversion between Knoop hardness and Rockwell hardness Because the corresponding curves of Knoop hardness and Rockwell hardness are similar to parabolas, the approximate conversion formula obtained from the curve is relatively accurate and can be used as a reference.
