Welcome to our comprehensive guide on mastering the material hardness test for CNC machined parts! As a CNC machining expert and blogger, I’m excited to share with you the essential knowledge and techniques needed to ensure the quality and performance of your machined components. Understanding material hardness is crucial in CNC machining, as it directly impacts factors like durability, wear resistance, and overall functionality.
In this blog post, we’ll explore the various hardness testing methods, step-by-step procedures for conducting tests, common challenges faced, and practical tips for accuracy and precision.
Whether you’re a seasoned CNC machinist or just starting out, this guide will equip you with the expertise to confidently assess the hardness of your machined parts. Let’s dive in!
5 Common Methods for Material Hardness Test of CNC Machined Parts
When it comes to the material hardness test in CNC machining, there are several methods available, each with its own unique approach and advantages. Let’s explore five of the most common material hardness testing equipment and methods:
Rockwell Hardness Test
The Rockwell hardness test is widely used due to its simplicity and speed. It involves applying a minor load followed by a major load to the material surface using a diamond or hardened steel ball. The depth of penetration of the indenter into the material is measured, providing a numerical hardness value. This method is suitable for a wide range of manufacturing materials and can quickly determine hardness variations within a material. It’s a non-destructive methodology.
Rockwell hardness, HRC = [0.2-permanent indentation depth (mm)] x 500
Brinell Hardness Test
In the Brinell hardness test, a hard spherical indenter (usually made of tungsten carbide) is pressed into the material surface under a predetermined load. The diameter of the indentation left on the material surface is measured, and the hardness value is calculated based on the applied load and the diameter of the indentation. This method is particularly useful for testing materials with coarse grain structures, rough surfaces, or softer materials.
Where:
HB is the Brinell hardness number.
P is the applied force in kilograms-force (kgf).
D is the diameter of the indenter ball in millimeters (mm).
d is the diameter of the indentation left on the material surface after applying the force, also in millimeters (mm).
This formula calculates the Brinell hardness number based on the applied force and the diameter of both the indenter ball and the resulting indentation on the material surface. The value of 𝜋 represents the mathematical constant pi (approximately 3.14159).
Vickers Hardness Test
The Vickers hardness test involves pressing a square-based pyramid-shaped diamond indenter into the material surface under a specific load. The size of the resulting indentation is measured, and the hardness value is calculated based on the applied load and the surface area of the indentation. If you need to make accurate measurements of materials with high hardness or thin coatings, Vickers hardness may be the best test method for you.
Where:
HV is the Vickers hardness number.
F is the applied force in kilograms-force (kgf).
d is the average length of the two diagonals of the indentation left on the material surface after applying the force, measured in millimeters (mm).
This formula calculates the Vickers hardness number based on the applied force and the dimensions of the indentation produced on the material surface. The value of 1.854 is a constant that converts the applied force and indentation dimensions into a hardness value.
In conclusion, the Vickers hardness number provides a measure of the material’s resistance to indentation by a pyramidal diamond indenter under a specified load. Higher Vickers hardness numbers indicate greater hardness and resistance to deformation.
Mohs Hardness Test
The Mohs hardness test is a qualitative method used to determine the scratch resistance of materials. It involves scratching a material surface with reference materials of known hardness (e.g., minerals) and comparing the scratch patterns to determine the relative hardness of the material. While less precise than other hardness testing methods, the Mohs scale is useful for quickly assessing the hardness of minerals and geological specimens.
The hardness increment on the Mohs hardness scale is very arbitrary and is not linear or proportional. It is for this reason that most of these tests are only used to determine the relative hardness of minerals and not of other materials, such as metals.
Knoop Hardness Test
The Knoop hardness test is similar to the Vickers test but uses a rhombic-based pyramidal diamond indenter. It is particularly well-suited for testing thin or brittle materials, as it produces smaller, elongated indentations. The Knoop hardness value is calculated based on the applied load and the length of the indentation. This method is commonly used in microhardness testing applications.
In CNC machining, selecting the appropriate hardness testing method depends on factors such as the material type, surface condition, and desired level of accuracy. By understanding the principles and applications of these common hardness testing methods, you can ensure the quality and performance of their machined parts.
Factors Affecting Material Hardness Testing
Surface Finish: Irregularities or roughness on the material surface can affect the accuracy of hardness test results by influencing the contact between the indenter and the material.
Temperature and Humidity: Variations in temperature and humidity levels in the testing environment can impact the material’s mechanical properties, leading to fluctuations in hardness measurements.
Sample Size and Geometry: The size and shape of the material sample can affect the distribution of stress during testing, potentially influencing hardness values.
Material Composition: Variations in material composition, such as alloying elements or impurities, can affect the material’s hardness and alter hardness test results.
Testing Methodology: Differences in testing procedures and equipment calibration can introduce variability in hardness measurements.
How To Make Low-Hardness Material Harder?
To enhance the hardness of low-hardness materials, several methods can be employed, each offering unique advantages and considerations. Let’s compare these options:
Case Hardening
Process: The surface of the material is infused with a hardening agent, typically carbon, to form a hardened outer layer.
Suitable for: Low-carbon steels and alloys.
Carburizing
Process: Introducing carbon into the surface of the material at high temperatures to create a carbon-rich layer, increasing hardness.
Suitable for: Low-carbon steels.
Nitriding
Process: Introducing nitrogen into the material surface to form hard nitride compounds, improving surface hardness and wear resistance.
Suitable for: Low-alloy steels, tool steels, and stainless steels.
Carbonitriding
Process: Combination of carburizing and nitriding, introducing both carbon and nitrogen to enhance surface hardness.
Suitable for: Low-alloy steels.
Induction Hardening
Process: Heating the material surface with high-frequency induction coils, followed by rapid quenching to increase surface hardness.
Suitable for: Various steel alloys.
Flame Hardening
Process: Heating the material surface with a high-temperature flame, followed by quenching, to achieve surface hardening.
Suitable for: Carbon steels and low-alloy steels.
Quenching
Process: Rapidly cooling the material from high temperatures, typically in oil or water, to achieve high hardness.
Suitable for: Various steel alloys.
Precipitation Hardening
Process: Heat treatment to form precipitates within the material, increasing strength and hardness.
Suitable for: Aluminum, titanium, and some stainless steels.
Anodizing
Process: Electrochemical treatment to form an oxide layer on the material surface, enhancing hardness, corrosion resistance, and durability.
Suitable for: Aluminum and its alloys.
Each method has its advantages and limitations, and the choice depends on factors such as material composition, desired hardness, surface area, and specific application requirements. Consulting with materials and manufacturing experts can help determine the most suitable method for your low-hardness material.
Conclusion
In conclusion, mastering the material hardness test for CNC machined parts is essential for ensuring the quality, durability, and performance of components. By understanding the various hardness testing methods, from Rockwell to Vickers, and considering factors like surface finish and material composition, machinists can accurately assess the hardness of their parts. Whether employing case hardening, nitriding, or other hardening techniques, choosing the right method is crucial.
Through meticulous testing, calibration, and adherence to best practices, CNC machinists can confidently produce parts that meet stringent hardness requirements, contributing to the reliability and longevity of the final products.