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What is the hardness of metal materials？

Hardness is one of the most critical parameters in any machining process, as it affects material removal rate, machine efficiency, and tool life. It is the main parameter for understanding raw materials and cutting materials. Today let's talk about this topic. For those who are engaged in processing, it is still necessary to understand the topic of "hardness". You may ask, isn't hardness just about being hard or not? What's there to say? Don't worry, there are many tricks here. If you don't understand them, you might fall into a trap when choosing materials.

01. What is the hardness of metal

First, let's talk about what hardness is. Simply put, hardness is the ability of a metal material to resist external pressure. Imagine using a hammer to smash iron blocks and cotton. The iron block is definitely harder because it can resist the impact of the hammer and will not easily deform. Cotton, on the other hand, sinks into the ground with a hammer, which is a sign of low hardness. Image: Hardness Test

In the field of metal processing, hardness is not just about whether it is hard or not. It is directly related to a series of important indicators such as the processing performance, wear resistance, and service life of metal materials. For example, if a cutting tool is not hard enough, it will quickly become dull when cutting things; And some machine parts, if their hardness is not enough, will wear out soon, affecting the normal operation of the machine.

02. Several common methods for representing hardness

When it comes to hardness, we cannot ignore several common methods of expressing hardness. Why are there so many types? Because different testing methods are suitable for different materials and application scenarios, let's take a look at them one by one.

2.1 Brinell hardness (HB)

Image: Brinell hardness testing method

Brinell hardness [HB] is one of the most commonly used units for listing the hardness of steel. The test was conducted using a 10mm steel ball under a pressure of 3000 Kgf (6614 Lbf). The commonly used values for machined materials range from 100 HB for very soft materials to 650 HB for heat-treated steel.

The advantage of Brinell hardness [HB] over Rockwell hardness [HRC/HRB] is that the Brinell hardness scale covers the entire range, while in the Rockwell C scale, when the hardness is below 180 HB, it must switch to Rockwell B.

Brinell hardness is one of the earliest widely used hardness testing methods. Its testing principle is to use a certain size of test load, press a hardened steel ball or hard alloy ball of a certain diameter into the surface of the tested metal, maintain it for a specified time, and then remove the load. The Brinell hardness value (HB) is the quotient obtained by dividing the load by the surface area of the indentation sphere.

The advantage of Brinell hardness is that the testing process is relatively simple and suitable for testing medium hardness metal materials, such as some common steel, cast iron, etc. However, its drawbacks are also obvious, such as longer testing time and higher requirements for surface quality. If there are scratches or unevenness on the material surface, it will affect the test results.

2.2 Rockwell hardness (HR)

Image: Rockwell hardness testing method

Rockwell hardness [HRC/HRB/HRA] is one of the most commonly used units for listing the hardness of machined materials. The test is completed by comparing the penetration depth of the sphere under high load with the penetration depth under reference preload. The Rockwell scale is divided into 9 sub scales, marked with the letter AK. Each ruler uses different reference loads and different sphere sizes. In machining, the most common is the C-scale (HRc). The hardness of machined metals is usually between 10-65 HRC.

Rockwell hardness is currently one of the most widely used hardness testing methods. Its testing principle is to use a diamond cone with a vertex angle of 120 degrees or a quenched steel ball with a diameter of 1.59mm and 3.18mm to apply a certain load to the surface of the tested metal, hold it for a period of time according to regulations, and then remove the load. Calculate the hardness value based on the load and indentation depth.

The Rockwell hardness testing method is relatively flexible, and different scales can be selected according to the hardness range of the material, such as HRA (for hard materials), HRB (for medium hardness materials), and HRC (for high hardness materials). Its advantages are fast testing speed, simple operation, relatively low requirements for material surface, and suitable for rapid on-site inspection.

2.3 Vickers hardness (HV)

Image: Vickers hardness testing method

Vickers hardness [HV] is a commonly used unit to list the hardness grades of hard alloys and other hard cutting materials such as ceramics, CBN, and PCD. The test is completed by measuring the surface area of the indentation generated by diamond in the shape of a square pyramid. The HV value is equal to the force divided by the area (F/a). The grade of hard alloy is usually in the range of 1300-1900 HV. The hardness of ceramics can reach 2000 HV, CBN can reach 3000 HV, and PCD can reach 6000 HV.

The testing principle of Vickers hardness is somewhat similar to Brinell hardness, which also uses a certain size of test load to press a diamond pyramid with a vertex angle of 136 degrees into the surface of the tested metal, maintain it for a specified time, and then remove the load. Differently, the indentation of Vickers hardness is a square, and the hardness value is calculated by measuring the diagonal length of the indentation.

The advantage of Vickers hardness is high accuracy, which can test various materials from very soft to very hard, and the requirements for material surface are also relatively low. However, its disadvantage is that the testing equipment is relatively expensive, the operation is relatively complex, and it is generally used for precise measurements in the laboratory.

03. The Effect of Hardness on Metal Materials

Hardness is not an isolated indicator, it is closely related to the properties of other materials. Let's take a look at several important effects of hardness on metal materials.

3.1 Processing performance

High hardness metal materials are relatively difficult to process. For example, high hardness steel will experience faster tool wear and reduced machining efficiency during the cutting process. Although materials with low hardness are easy to process, their service life may be affected due to insufficient strength. So, when choosing materials, hardness should be comprehensively considered based on actual processing needs and usage scenarios.

3.2 Wear resistance picture:

Generally speaking, materials with high hardness have better wear resistance. For example, some high hardness alloy steels have significantly increased surface hardness and wear resistance after heat treatment. This is very important in some mechanical parts that require long-term friction, such as bearings, gears, etc. However, wear resistance not only depends on hardness, but also on factors such as material composition and microstructure.

3.3 Fatigue resistance performance

Materials with high hardness also have certain advantages in terms of fatigue resistance. Because materials with high hardness are less prone to surface cracking when subjected to repeated stress, thus improving the service life of the parts. However, the higher the hardness, the better. If the material is too hard, it may become brittle and prone to fracture.

04. How to choose the appropriate hardness material

Choosing the appropriate hardness material is not a simple task, and it requires comprehensive consideration of factors such as the material's usage scenario, processing technology, and cost. Let's take a look at several common application scenarios.

4.1 Tool Materials

The hardness of the tool material must be high in order to ensure cutting efficiency and service life. Generally speaking, the hardness of cutting tool materials should be above HRC60, and for some high-speed steel cutting tools, the hardness can reach HRC62-68. However, the tool material should not be too hard, otherwise it will become brittle and prone to chipping.

4.2 Mold Materials

The hardness of mold materials is also important because they have to withstand high temperatures, high pressures, and repeated impacts. Generally speaking, the hardness of mold materials should be above HRC45, and for some hot work mold steels, the hardness can reach HRC48-52. At the same time, the mold material also needs to have a certain degree of toughness to prevent cracks from occurring during use.

4.3 Mechanical Parts

The hardness selection of mechanical parts should be determined according to specific usage scenarios. For example, some parts that bear high loads, such as gears and shafts, generally require a hardness of HRC35 or higher to ensure the strength and wear resistance of the parts. For some parts that do not need to withstand high loads, the hardness can be appropriately reduced to improve processing efficiency and reduce costs.

05. Precautions for Hardness Testing

Although hardness testing may seem simple, there are actually many things to pay attention to. Let's take a look at a few common questions.

5.1 Selection of Testing Equipment

Different hardness testing methods require different equipment, and choosing the appropriate equipment is very important. For example, if you want to test materials with high hardness, do not choose a Brinell hardness tester as it may damage the equipment. If you need to quickly test the hardness of materials, a Rockwell hardness tester may be a better choice.

5.2 Control of Testing Conditions

The conditions of hardness testing have a significant impact on the test results, such as load size, testing time, ambient temperature, etc. If the testing conditions are unstable, it can lead to significant deviations in the test results. So, during the testing process, it is necessary to strictly follow the standard operation to ensure the stability of the testing conditions.

5.3 Analysis of Test Results

After the test results are released, it is not enough to simply look at the hardness value, but also to conduct a comprehensive analysis based on factors such as the composition and organizational structure of the material. For example, the same HRC50 material may have different usage performance due to different compositions and heat treatment processes.

06. Future Development Direction of Hardness

Image: Non destructive testing

With the continuous advancement of technology, hardness testing technology is also constantly developing. For example, there are now some non-destructive hardness testing techniques that can measure hardness without damaging the material, which is very important for the detection of some high-value materials. At the same time, some new types of hardness testing equipment are constantly emerging, which are more intelligent and automated, and can greatly improve testing efficiency and accuracy.