Accuracy and repeatability are key when measuring the microhardnesses of surface-hardened steels. What are the problems, and what can be done to assure accuracy?

Surface hardening of steel is performed to increase the strength and wear properties of the material. One of the prerequisites for hardening is sufficient carbon and alloy content. If there is sufficient carbon content, the steel can be directly hardened. Otherwise, the surface of the part has to be carbon or nitrogen enriched using diffusion-treatment hardening techniques. Hardened steel parts are typically used in moving or rotating applications where high wear resistance and/or strength is required, such as gear and engine parts, injection pumps and nozzles, etc. Since hardening makes the steel brittle, and it is desired to retain the ductility in the material core, it is required to control the hardness depth - thus the term surface hardening.

Metallographic Preparation of the Sample Prior to Hardness Testing

The surface – a transverse section on which the measurement is to be made – must be polished to smoothness sufficient to permit correct measurements of the microhardness indentation (Vickers or Knoop, loads £1 kgf). In this case, precautions shall be taken to avoid damage to the edges of the specimen such as edge rounding, overheating and changes to the surface.

In order to ensure a reproducible test result, it is important that the metallographic preparation is performed in a controlled manner, with preparation parameters – surface grinding and polishing, rotational speed, abrasive dosing, force, preparation time – strictly under control. For each application, it is recommended that a standard preparation method is developed, defined and used during the metallographic preparation to avoid variation in the sample surface.

Table 1 gives a recommended method for metallographic preparation of carburized parts suitable for hardness testing and metallographic examination.

Procedure

The surface-hardening depth can be evaluated by means of a series of hardness indentations, and this is described in various standards such as DIN 50190, ISO 4507 and ISO 2639. The surface-hardening depth (case depth) is the distance from the sample surface to a position within the specimen where the hardness has dropped to a certain value – the “hardness limit.” The applicable procedure depends on the surface treatment used, such as carburization, induction or flame hardening, or nitriding.

Hardness depth of surface-hardened materials is commonly calculated as follows:

Carburized Parts
  • Methods used: HV0.1-HV1/HV5
  • Hardness limit = 550HV (50HRC)
  • Case hardness depth, CHD = Distance from surface to point where hardness is 550HV
Induction / Flame-Hardened Parts
  • Methods used: HV1, (HV0.5-HV5)
  • Hardness limit = 80% x (Minimum) surface hardness
  • Case hardness depth, Rht = Distance from surface to point where hardness is 80% of (minimum) surface hardness
Nitrided Parts
  • Methods used: HV0.3-HV2
  • Hardness limit = Core hardness + 50HV.
  • Case hardness depth, Nht = (Max.) Distance from the surface to the point where hardness is 50HV1 above core hardness


Figure 1. Placement of test series using the overview camera

Challenges in Case-Depth-Hardness Testing and Solutions

Some applications require that the test points are placed over a relatively large area. This is true for Nht measurements (Nitrided Case Depth), where it is necessary to measure the core hardness in order to determine the hardness limit. It is also true for weldings, where a series of test points typically is placed over a distance of several centimeters.

Solutions where you can see the entire sample at once – in one field of view – will therefore make the task easier and highly improve the test procedure.

Templates
If you often perform hardness tests on similar samples, you can define templates that will simplify and automate your entire process. Once created, a template allows for easy and fast setup of test runs. Figure 1 illustrates how a template can be used with the tester’s overview camera to easily set up and run a test series. In this example, 10 test series (the red T-lines numbered 1-10) are being applied on the test piece. The blue and green lines are help-lines that easily allow you to position your test series. The advantage of this is repeatability. The operator does not need to set up anything. Procedure standardization is ensured independent of the operator. Operators without extensive training are thus able to perform sophisticated hardness testing with ease.

Automatic Indent Spacing
When running case-depth measurements, you traditionally need to set a fixed spacing (in µm) between your indents, regardless of the fact that the size of your indentations changes through the case. Duramin-A300 offers the unique automatic indent spacing feature, allowing the spacing to be set according to actual indentation size (e.g., 3x indentation diagonal).

The spacing between each indentation is calculated individually, ensuring that spacing increases as the indentation increases in size. The result is properly spaced indentations (not risking the indentations influencing each other), resulting in a smooth and accurate projection of the case-depth curve.

Figure 2. Hardness-value variations due to different illumination

Illumination
In quality control it is crucial to have your inspection parameters under control. Without a repeatable and reproducible testing procedure your quality control loses value and credibility.

In surface-hardness testing, illumination is just one of several factors that affect the measured hardness value. Illumination intensity influences how large/small Vickers, Brinell and Knoop indents appear on screen. It is essential to control this parameter to ensure repeatable and reproducible tests.

Figure 2 is a random example showing the measured hardness value of the same Vickers indent at different illumination and contrast settings. The measured hardness value varies from 219 to 202HV1 - a difference of almost 10%. This clearly shows the importance of having the illumination parameters under control.

Additionally, many hardness-testing systems are based on a halogen illumination system. The typical halogen light bulb has a relatively short operational life. The issue here is that illumination intensity constantly degrades as the bulb ages. To combat this, modern testers utilize LED illumination, which produces constant illumination intensity over its lifetime. This is yet another factor influencing reliability and reproducibility of your test.

Figure 3. Case-hardness depth statistics

Evaluation of Results

When evaluating the test results, it may be of interest to review them statistically. One possibility is to evaluate the process capability index, the so-called Cp. Cp measures the capability of a process to meet its specification limits. It is the ratio between the required and actual variability of the process.

Cp = (USL - LSL) / (6 x s)

s = Standard Deviation; USL = Upper Specification Limit; LSL = Lower Specification Limit; µ = Mean Value

Process capability is a measure of the ability of a process to produce consistent results – in this case the surface-hardening process. Cp<1 is indicative of a process that is producing considerable nonconforming products. Cp=1 is indicative of a process that is meeting requirements but includes some nonconforming products. Cp>1 is indicative of a process that is under control and thus producing very little if any nonconforming product.

Cp is best when results are evenly dispersed around the control target, centerline. If the data is evenly spread either above or below the control line, the Cpk index should be considered. Cpk takes into account the so-called off-centeredness of the process. Cpk is the smaller of the following:

(USL - µ) / (3 x s) or (µ - LSL) / (3 x s)

Figure 3 shows the Cpk statistics for a series of CHD measurements.

Summary

Many factors influence the hardness-testing procedure and result. These are among others related to the material surface quality, the instrument and the procedure used. Therefore, it is crucial that as many of these factors as possible are kept under control. Various solutions exist today that can control these factors, which are suitable for different applications and needs.

All examples and images are taken from the Struers Duramin-A300 Hardness Tester.

For more information: Contact Steve Glancy, Struers Inc., 24766 Detroit Road, Westlake, OH 44145; tel: 1-800-321-5834; fax: 1-440-871-8188; e-mail: info@struers.com; web: www.struers.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: metallographic preparation, nitrided case, surface hardening, process capability