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Traditional and new methods of hardness measurement are available. Deciding which one is right for an application depends on several factors.

Case hardening is done for many applications and many industries. It is a process used to achieve greater wear resistance and other important attributes. The depth of the case hardness produced by case hardening is important to the part’s performance, and measurement of the depth typically involves a labor-intensive process. This article reviews the typical measurement methods used and describes methods for case-depth measurement.

What is case hardness?

Case hardness refers to the surface hardness of a metal subjected to case hardening. It is a measure of the resistance of the surface of a metal to penetration and, by corollary, wear. Case hardness is usually measured using the Rockwell scale, which is a scale that uses a small diamond indenter to measure the hardness of a material by measuring the depth of the indentation it leaves. The Rockwell scale is expressed in terms of a number followed by a letter, with higher numbers indicating a harder material. For example, a material with a Rockwell hardness of 60HRC is harder than a material with a Rockwell hardness of 50HRC.

Case hardness is important in many applications, such as in the production of cutting tools, dies and parts (like gears and other drive components) that are subjected to wear and abrasion. Case hardness is a measure of the resistance of a material to surface indentation or wear.

To protect the metal from wear or other forms of surface damage, the metal object is subjected to a protection process that may include a coating step to improve corrosion performance to increase the hardness of the outer layer. There are several methods of case hardening, including carburizing, cyaniding, nitriding and induction hardening.

There are also several methods used to measure case hardness, including Rockwell, Brinell, Vickers and Knoop hardness testing (Table 1). The appropriate method to use will depend on the type and depth of case hardening, the type of material being tested and the desired level of accuracy.

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The depth of case hardening in a metal object depends on the specific case-hardening process used and the conditions under which it was performed. In general, the depth of case hardening is typically measured in millimeters or micrometers and can range from a few micrometers to several millimeters. Hardness and microstructure of the case is very strongly affected by time, temperature and carbon or nitrogen content of the case-hardening environment.

In carburizing, for example, the depth of the case-hardening layer can be controlled by adjusting the temperature, time and the cooling rate of the material after heating. In nitriding, the depth of the case-hardening layer can be controlled by adjusting the temperature and time.

In general, the depth of case hardening is typically chosen based on the specific requirements for the part and its application. For example, if the part is subjected to high levels of wear or high point loading, a deeper case-hardening layer may be needed to provide sufficient protection. On the other hand, if the component is only subjected to moderate levels of wear or simple sliding wear, a shallower case-hardening layer may be sufficient.

A component with a deep and effective case hardness will have improved wear resistance, fatigue strength and corrosion resistance compared to a component with a shallow or incomplete case hardness.

Case hardness is especially important in applications where the component is subjected to high levels of wear and fatigue, such as in gears, bearings, rotating shafts and cutting tools. In these applications, a component with an appropriately deep and effective case hardness will be able to withstand the stresses of use for a longer period compared to a component with a shallow or incomplete case hardness.

Case hardness is also important in applications where erosion resistance is a concern. A component with a deep and effective case hardness will be less prone to erosion compared to a component with a shallow or incomplete case hardness.

Overall, case hardness is an important factor to consider when selecting materials and designing components for a given application because it can have a significant impact on the performance and lifespan of the component.

Measuring Case Depth

After heat treatment, case hardness can be measured by performing a hardness test on a cross section of the material. This can be done by preparing a sample of the material by sectioning, grinding or polishing the sample to a suitable surface finish. The sample should be cleaned and properly mounted for testing. The hardness tester is then applied to the surface of the sample using a specified load and indenter. The resulting indentation depth is then measured and converted to a hardness value using a standard conversion chart or formula.

Fig. 1. A Brinell tester is designed to make impressions that are used to measure the hardness of a metal.

It is important to note that case hardness is not always a reliable indicator of the overall strength or performance of a material. It only measures the resistance to surface indentation and does not consider factors such as microstructure, toughness or fatigue resistance.

Here is a summary of the typical process to measure case hardness.

1. Prepare a sample of the material by sectioning, grinding or polishing it to a suitable surface finish.
2. Clean the sample and properly mount it for testing.
3. Select the appropriate hardness or microhardness testing method based on the type of material and desired level of accuracy. Options include Rockwell, Brinell, Vickers or Knoop hardness or microhardness testing.
4. Apply the hardness tester to the surface of the sample using a specified load and indenter.
5. Measure the resulting indentation depth and convert the result to a hardness value using a standard conversion chart or formula.
6. Record the hardness value and compare the result to the desired specifications or to the hardness values of other materials.

The actual time it takes to complete a hardness test can vary significantly depending on the specific equipment being used and the complexity of the part being tested. Time involved has been the major drawback in case-depth measurement. The typical case-depth measurement takes over an hour, which can be costly.

Faster Case-Depth Measurement

Recently, a new method for measuring case depth has emerged (Fig. 2). Users who have adopted this approach are seeing significant time and cost savings. This new measurement method, dynamic case-depth measurement (DCDM), involves the continual application of force to the part and does not require the part to be cut into a sample, polished or mounted. In addition, the testing can be completed in under two minutes (Fig. 3). Contrasted with the typical method of measuring case depth, which can take over one hour, this new approach is a meaningful advantage and yields results that are consistent in accuracy with the more typical time-intensive testing method.

Fig. 2. A newer test method for case-depth testing performs continuous acquisition testing to determine the hardness of the metal throughout the case, significantly reducing the time and cost of testing case depth.

Regardless of the specific setting, technicians who measure case-depth hardness are typically trained to use specialized equipment, such as Rockwell hardness testers, Brinell hardness testers (Fig. 1) or Vickers or Knoop hardness or microhardness testers. They must also be familiar with appropriate test methods and procedures and the safety precautions that must be followed when performing hardness tests. Case-depth hardness is typically measured by trained and certified technicians who have the knowledge and skills to perform the necessary tests.

Fig. 3. This new measurement method, dynamic case-depth measurement (DCDM), involves the continual application of force to the part and does not require the part be cut into a sample, polished or mounted. The testing can be completed in under two minutes.

Keep in mind that regardless of the method used, measurement of case depth should be approached with caution. It is always important to follow the equipment manufacturer’s instructions and use proper handling and safety precautions when performing a hardness test.

The choice of the measuring method depends on specific circumstances. Factors to consider include:

• Material to be tested
• Specifications and part use
• Desired measurement accuracy
• Desired case depth
• Equipment to be used
• Technician skill level

Conclusion

Case hardness is used in many applications, including but not limited to: automotive for engine components, transmission parts and other mechanical components; aerospace for structural components such as landing gear; defense for weapons and armor; mining for equipment such as crushers, conveyors and other critical components; and medical for parts. Getting the depth correct is critical to the performance of the parts, and proper measurement plays a key role in the right depth and consistency of the depth across parts.

For more information: Jim Knight is president of King Tester Corp. in Phoenixville, Pa. He can be reached at 610-279-6010, king@kingtester.com or www.kingtester.com.

All graphics provided by the author.