Hardness is a metric of a material’s or steel’s resistance to deformation when subjected to weight or pressure. The higher the hardness, the more challenging it is to distort. When it comes to determining the criteria and requirements of fasteners, screws, and bolts made of carbon steel or alloy, core hardness and surface hardness are two essential considerations. 

This article will discuss the importance of core hardness and surface hardness for fasteners. 

Why is hardness important in fasteners?

Fasteners are subjected to air, water, and other environmental factors. 

  • The ability of a metal to survive persistent incision, distortion, scratching, or grinding is decided mainly by its hardness.
  • Hardness is also crucial for metal tensile strength or the ability to resist external tension because as hardness increases, so does tensile strength.

As a result, several fastener norms demand hardness certification before components are authenticated as pliable. 

What is core hardness?

The hardness of metal till the innermost area is referred to as core hardness. A core hardness is a measure of resistance to incision or penetration. 

How is core hardness tested?

A penetrometer can be used to determine this. Metal is sliced in a cross-section to expose the core for testing. As a result, the core hardness test is sometimes referred to as a distortion test

What is surface hardness?

Surface hardness is a metric of the ability to withstand rupture on the surface. It is used to assess if steel can be retrieved despite external damage such as scraping, cutting, or abrasion. 

How is surface hardness determined?

It can be determined with a Vickers hardness or diamond point test. The surface test method does not involve any slicing of the material, thereby making it non-destructive and swift.

What is the distinction between core hardness and surface hardness?

  • Depth Perception: Core hardness is the actual depth of the indentation, while surface hardness is the apparent depth on the surface. 
  • Latency: Core hardness is the hardness of the metal at its center, which takes time to change. The surface hardness is the hardness of the metal on its surface, which changes faster.
  • Density: Core hardness is lesser because of slower cooling as it is not directly exposed to air. Surface hardness is more because of the fast cooling of the surface due to immediate exposure to air.
  • Complexity: Core hardness is not as easy to determine as surface hardness because it requires more time and effort to get to the inner layers of steel. Surface hardness can be determined easily by scratching with your other equipment. 
  • Sensitivity: The difference between these two measurements is that the surface hardness is not affected by the direction of force applied, and it doesn’t matter if you are applying a force perpendicular or parallel to the surface. However, the core hardness is much more sensitive to the direction and pressure applied on the material’s surface. For example, if you apply a force parallel to an object’s surface, it will cause it to bow outwards and increase in radius, but this will not change its core hardness.

Why is surface hardness more than core hardness?

The surface of steel is exposed to air directly, unlike the core of the material. It creates a time difference in the process of cooling and hardening of both. While the surface becomes hard faster and without much time, the core takes time to cool down and remains softer compared to surface hardness for a long time or permanently. 

What to do to improve core hardness?

Sometimes, core hardness needs an improvement to match up with surface hardness as it could be lesser or not equal. To improve core hardness and reduce the gap between the core and surface, lab technicians are required to change in chemistry, use better charge mix options, improve solidification trend or control the cooling cycle. 

Finally, while core hardness and surface hardness are distinct, they are both necessary to assess how fasteners will resist deformation when subjected to external force or tension. 

In conclusion, understanding the distinction between core hardness and surface hardness is crucial for ensuring the quality and performance of the finished product. At Syscon Wires, we have the expertise and experience to provide high-quality CHQ wire that meets the specific needs of our clients. If you have any wire needs, don’t hesitate to contact us. Our team would be happy to assist you in finding the best solution for your project. Thank you for reading and we look forward to the opportunity to work with you.