How much do you know about the hardness of the material?

In order to ensure the performance and life,the material of the cutter,measuring tools and molds which used in mechanical manufacturing,should have enough hardness should have enough hardness.

Today,I will discuss about the hardness of material with you

Hardness is a measure of material’s ability to resist local deformation,especially plastic deformation,indentation or scratch.Generally,the more hardness of the material,the better its wear resistance,such as gear and other mechanical parts will require a certain hardness to ensure sufficient wear resistance and service life.

Types of hardness

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

Definition of hardness

1. The Brinell Hardness

The Brinell hardness(symbol HB) test method,which has become an accepted hardness specification,is one of the first methods to be developed and summarized,and it has contributed to the emergence of other hardness test methods.

The principle of Brinell hardness test is: the indenter (steel ball or carbide ball, diameter Dmm) applies the test force F, after the specimen is pressed, the contact area S(mm2) between the ball indenter and the specimen is calculated in the concave diameter d(mm) left by the indenter, and the value obtained by the test force is excluded. When the indenter is a steel ball, the symbol is HBS, and when the cemented carbide ball is HBW. k is a constant (1/g= 1/9.80665 = 0.102).

2. The Vickers hardness

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

The Vickers hardness is the value obtained by dividing the test force F(N) by the contact area S(mm2) between the standard plate and the indenter, calculated based on the diagonal length d(mm, the average length in both directions) of the indentation formed on the standard plate by the indenter (tetragonal coned diamond, relative surface Angle =136˚) at the test force F(N). k is a constant (1/g=1/9.80665)

3. The knoop hardness

The Knoop hardness (symbol HK), as shown in the following formula, is calculated by dividing the test force by the indentation projection area A (mm2) based on the longer diagonal length d (mm) of the indentation formed on the standard sheet at the test force F by pressing the long diamond indenter with relative side angles of 172˚30' and 130˚.

Knoop hardness can also be measured by replacing the Vickers indenter of a microhardness tester with Knoop indenter.

4. The Rockwell hardness

The Rockwell hardness (symbol HR) or Rockwell surface hardness is measured by applying a preload force to the standard sheet using a diamond indenter (tip cone Angle: 120˚, tip radius: 0.2mm) or a spherical indenter (steel ball or carbide ball), then applying a test force and restoring the preload force.

This hardness value is derived from the hardness formula, which is expressed as the difference between the indentation depth h(μm) between the preloaded force and the test force. The Rockwell hardness test uses a preload force of 98.07N, and the Rockwell surface hardness test uses a preload force of 29.42N. The specific symbol provided in combination with the indenter type, test force, and hardness formula is called a scale. The Japanese Industrial Standards (JIS) define various related hardness scales.

HR(Diamond indenter, Rockwell hardness)=100-h/0.002 h:mm

HR(Ball indenter, Rockwell hardness)=130-h/0.002 h:mm

HR(Diamond/ball indenter, surface Rockwell hardness)=100-h/0.001 h:mm

Hardness testing machines are widely used because they are simple and quick to operate and can be tested directly on the surface of raw materials or parts.

Hardness Selection Guide

Selection guide for hardness testing methods for your reference:

Hardness selection conversion

• Knoop to Vickers conversion

• Based on the fact that objects with the same hardness have equal resistance to the two types of Knoop Vickers indenters, the stress of the two types of Vickers Knoop indenters under load is deduced respectively, and then according to σHK=σHV, HV=0.968HK is obtained. This formula is measured under low load, and the error is relatively large. In addition, when the hardness value is greater than HV900, the error of this formula is very large, and the reference value is lost.
• After derivation and correction, the conversion formula of Knoop hardness and Vickers hardness is proposed.

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

• Conversion of Rockwell to Vickers

• To Hans· The Qvarnstorm conversion formula proposed by Qvarnstorm is modified to obtain the conversion formula of Rockwell hardness to Vickers hardness:

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

• 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, and the error between the calculation result of the conversion formula and the standard experimental value is ±0.1HRC.

• According to the actual experimental data, the conversion of Rockwell hardness to Vickers hardness is discussed by linear regression, and the formula is obtained:

This formula has a small range of use and a large error, but it is easy to calculate and can be used when the accuracy is not high.

• Conversion of Rockwell to Brinell hardness

• The relationship between Brinell indentation and Rockwell indentation depth was analyzed, and the conversion formula was obtained according to the stress σHRC=σHB of the indenter.

The error between the calculated results and the standard experimental values is ±0.1HRC.

• According to the actual experimental data, the formula was obtained by linear regression method.

The formula error is large, and the range of use is small, but the calculation is simple, and it can be used when the accuracy is not high.

• Conversion of Brinell to Vickers

The relationship between Brinell hardness and Vickers hardness is also based on σHB=σHV.

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

• Knoop to Rockwell conversion

Because the corresponding curves of Knoop and Rockwell are similar to parabolas, the approximate conversion formula is derived from the curves.

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