Tools, measuring tools, molds, etc. used in machinery manufacturing should have sufficient hardness to ensure performance and life. Today, I will talk to you about hardness-related topics.
Hardness is a measure of ability of a material to resist localized deformation, especially plastic deformation, indentation or scratching. Generally, the harder material, the better its wear resistance. For example, mechanical parts such as gears will require a certain hardness to ensure sufficient wear resistance and service life.

 

 

As shown above, there are so many types of hardness.
Today, I will introduce to you common and practical indentation hardness test in metal hardness.

Brinell hardness (symbol HB) test method is one of the earliest methods developed and summarized in hardness that has become a recognized specification, and it has contributed to emergence of other hardness test methods.

 

Principle of Brinell hardness test is as follows: indenter (steel ball or carbide ball, diameter D/mm) applies test force F, after sample is pressed, diameter of concave part left by lifting indenter d (mm) is calculated to calculate ball. Contact area S (mm2) of indenter and sample, value obtained by dividing test force. When indenter is a steel ball, the symbol is HBS, and when indenter is a carbide ball, it is HBW. k is a constant (1/g = 1/9.80665 = 0.102).

 

Vickers hardness (symbol HV) is the most widely used test method that can be tested with any test force, especially in the field of micro hardness below 9.807N.

 

Vickers hardness is value obtained by dividing test force F (N) by contact area S (mm2) between standard piece and indenter, which is calculated from diagonal length d (mm, average length of the two directions) of indentation formed on standard sheet by indenter (square pyramid diamond, relative face angle = 136˚) under test force F (N). k is a constant (1/g=1/9.80665).

 

Knoop hardness (symbol HK) is calculated by dividing test force by projected area A (mm2) of indentation, which is calculated from longer diagonal length d (mm) of indentation formed by pressing rhomboid diamond indenter (opposite corners are 172˚30' and 130˚) on standard at test force F, as shown in following formula. Knoop hardness can also be measured by replacing Vickers indenter of a microhardness tester with a Knoop indenter.

 

Before measurement of 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 standard piece, then apply a test force, and restore preload force.

 

This hardness value is derived from hardness formula, which is expressed as difference in indentation depth h (μm) between preload force and test force. Rockwell hardness test uses a preload force of 98.07N, and Rockwell surface hardness test uses a preload force of 29.42N. Specific symbols provided in conjunction with indenter type, test force and hardness formula are called scales. Japanese Industrial Standards (JIS) define various relevant hardness scales.
Hardness testing machine is relatively simple and quick to operate, and can be tested directly on the surface of raw materials or parts, so it is widely used.

 

Micro Vickers Hardness Tester HM Series

 

Vickers hardness testing machine HV series

 

Rockwell hardness testing machine HR series

 

Portable Leeb Hardness Tester HH Series
Guidelines for selection of hardness test methods:

 

 

 

(1) On basis that objects of same hardness have equal resistance to two Knoop-Vickers indenters, stress of two Vickers-Knoop indenters under applied load is deduced respectively, then obtained according to σHK=σHV : HV=0.968HK. This formula is measured under low load, and error is relatively large. In addition, when hardness value is greater than HV900, error of this formula is very large, and reference value is lost.
(2) After derivation and correction, conversion formula of Knoop hardness and Vickers hardness is proposed as

 

Verified by actual data, the maximum relative conversion error of this formula is 0.75%, which has a high reference value.

(1) Conversion formula for Qvarnstorm proposed by Hans Qvarnstorm

 

After correction, conversion formula of Rockwell hardness and Vickers hardness is obtained as

 

This formula is converted with standard data of ferrous metal hardness published in my country, HRC error is basically within range of ±0.4HRC, maximum error is only ±0.9HRC, and maximum calculated HV error is ±15HV.

(2) According to stress σHRC=σHV of different indenters, formula is obtained by analyzing relationship between Rockwell hardness and Vickers hardness indentation depth

 

This formula is compared with conversion value of national standard experiment, and error between calculation result of conversion formula and standard experimental value is ±0.1HRC.

(3) According to actual experimental data, conversion of Rockwell hardness and Vickers hardness is discussed by method of linear regression, and formula is obtained:

 

This formula has a small range of use and a large error, but calculation is simple and can be used when accuracy requirements are not high.

(1) Analyze relationship between Brinell indentation and Rockwell indentation depth, and obtain conversion formula according to stress of indenter σHRC=σHB

 

Calculation result is compared with national standard experimental value, error between conversion formula calculation result and standard experimental value is ±0.1HRC.

(2) According to actual experimental data, linear regression method is used to obtain formula

 

Formula error is too large and range of use is too small, but calculation is simple and can be used when accuracy is not high.

Relationship between Brinell hardness and Vickers hardness is also derived from formula according to σHB=σHV

 

Conversion result of this formula is compared with national standard conversion value, and conversion error is ±2HV.

Because corresponding curve of Knoop hardness and Rockwell hardness is similar to a parabola, approximate conversion formula obtained from curve is:

 

This formula is more accurate and can be used as a reference.