The Critical Role of Hardness: Insights for Machining Professionals

March 28, 2024 8:57 am

What is Hardness?

Hardness is a physical property that describes a material’s resistance level to localized plastic deformation caused by external forces or abrasion. Essentially, it measures how difficult it is to make a dent or scratch on the surface of a material. The hardness of a material is typically proportional to its strength and wear resistance, meaning that materials with high hardness tend to be stronger and more resistant to wear and tear. On the other hand, materials with high hardness are brittle and more prone to fracture under stress, especially when compared to materials with lower hardness. It is a critical factor when selecting materials for various components, as it impacts their durability, mechanical properties, and production costs. When machining, it is essential to know the hardness of the material we are about to cut since it will influence our choice of cutting materials, carbide grades, and cutting conditions. We will get into this in more detail later in this article.

 

Hardness Units

There are various methods and units to measure hardness. The most commonly used units in the machining industry include Rockwell, Brinell, and Vickers. In this text, we will explore these units, identify their differences in usage and measurement, and learn how to convert between them.

 

Rockwell (HRC/HRB/HRA)

Rockwell is the most common unit used on drawing callouts. The test is done by measuring a sphere’s penetration depth under a large load compared to the penetration made by a reference preload. The Rockwell scale is divided into 9 sub-scales marked by letters A-K. Each scale uses a different reference load and different sphere sizes. In machining, the most common is the C scale (HRC). Machined metal is usually between 10-65 HRC.

Brinell [HB]

Brinell is the most common unit used for listing the hardness of machined materials (For example, in cutting condition charts that the cutting tool suppliers provide). The test is done with a 10 mm steel ball pressed with 3000 Kgf (6,614 Lbf). Typical values for machined materials range from 100 HB for very soft materials to 650 HB for heat-treated steels.

The Rockwell C scale only covers hardness above 180 HB, requiring a switch to Rockwell B for lower values. In contrast, the Brinell scale covers the entire range.

Vickers [HV]

Vickers is a go-to unit for listing cutting materials such as Carbide grades, Ceramics, CBN, and PCD. The HV number is determined by dividing the force by the surface area of the indentation left by a diamond in the shape of a square-based pyramid. Carbide grades are usually in the range of 1,300-1,900 HV. Ceramics can reach a hardness of 2,000 HV, CBN up to 3,000 HV, and PCD up to 6,000 HV.

 

Converting between hardness units

As different units are measured using various scales and metrics, it is impossible to establish a general mathematical formula to convert between them. The most commonly used method in the industry to convert units is to refer to conversion tables. However, in today’s digital age, an easier alternative is to utilize online calculators, which offer a quick and convenient way to convert between the different hardness units. Simply enter the value you wish to convert and select the input and output units, and the calculator will provide you with the converted value.

 

Hardness and Machining

The parameter of ‘hardness’ is crucial in any machining process as it impacts the Material Removal Rate, the machine’s power consumption, and the tool’s lifespan. It is essential to determine the hardness of both raw and cutting materials.

Raw Materials

It may seem counter-intuitive, but very soft materials are not ideal for machining. This is because controlling the swarf below a certain hardness can be challenging, and the raw material tends to stick to the cutting edge, causing BUE (Built-Up Edge). When the hardness of the material surpasses a certain point, increasing the hardness any further will result in increased wear, requiring you to reduce the cutting speed or accept a lower tool life (The sweet spot is around 200 HB/15 HRC). Once you exceed a certain threshold, using a conventional carbide insert to machine the material will become impossible. Advanced materials such as Ceramics and CBN (Cubic Boron Nitride) will be necessary.

 

All images provided courtesy of MachiningDoctor.com

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