CHD Heat Treatment: How to Make Efficiency Savings

The efficiency of sample processing in Core Hardness Depth (CHD) measurements can be enhanced using increased automation. This article aims to highlight the superiority of semi-automatic sample preparation when compared to classic of sample preparation for hardness testing.

A typical heat treated part from Automotive industry was analysed to demonstrate an efficient preparation method and subsequent time savings when automation is used in hardness testing.

PlanarMet 300 and EcoMet AutoMet 300 Bundle

Grinding Polishing Procedure

The PlanarMet 300 planar grinder and EcoMet AutoMet 300 polishing machine were used to carry out a 3-step process (clampling, grinding and polishing) in which 10 mounted samples are prepared.

Figure 1. Polishing process steps: clamping, grinding and polishing

Traditional manual methods take 26 minutes of preparation time with 26 minutes of operator time required. The semi-automatic methods take 10 minutes of preparation and 3.30 minutes operator time required. However, the updated method only takes 8 minutes with 3.30 minutes operator time required. This highlights how optimised methodology can save both money and time and increased yield and quality of the laboratory, whilst the safety standards are maintained.

Table 1. Parameters for PlanarMet 300 and Ecomet / Automet 300 polishing

Note: Polishing cloth selection depends on the type of analysis desired:

• For a 9 µm cloth selection. Hercules S is preferred for a fast and powerful action on standard heat treated parts and to use UltraPad cloth for heat treated part with brittle white later as found on Nitrided components or for unmounted sample polishing applications.
• For a 3 µm cloth selection, VerduTex is preferred for excellent flatness and fast damage removal but residual scratches from 3 µm might be evident and aid to get the right focus during hardness measurements procedure. For scratch-free surfaces, MicroFloc cloth is the preferred surface for microstructural analysis.

Table 2. Polishing Methods Comparison

Operator interaction is required during all the steps

Image Credit: Buehler

Due to the high stock/material removal rates achievable on a PlanarMet 300 and the resultant surface finish, the method highlighted allows high sample throughput whilst minimising operator time. The procedure is also suitable for the preparation of unmounted medium to larger samples with high quality finish and flatness.

Table 3. Quality Consistency

	Traditional Manual Way	Semi-Automatic Way	Up to Date Way
Typical surface quality from each method
Comments	Over polished edges with edge rounding affecting focus issues during CHD or decarburization inspection	Remaining deep scratches on the edges of larger unmounted samples can affect accuracy of CHD measurements	Perfect flatness with a scratch free surface ideal for both CHD and microstructural analysis
Grinding steps thickness removal rate for 6 samples	0,05 mm/min	0,15 mm/min	0,3 – 0,5 mm/min – depend on desired stock removal level
Edge retention, flatness, quality	Poor	Average	Very Good
Operator dependent	100%	40%	20%
Safety	Poor	Very Good	Very Good

With the traditional manual method, the edges of the samples are excessively polished causing edge rounding, which leads to focussing issues during cursory metallographic examination or throughout case hardness depth (CHD) or decarburization inspections. This method is also 100% operator dependent and has poor safety levels.

The semi-automatic method produces deep scratched around the edges of bigger unmounted samples. These scratches can alter the CHD measurement accuracy; however, this method is only 40% operator dependent and has a very good safety level.

With the optimised 3 step procedure, the samples produced are faultless, with excellent edge flatness and provides a perfect surface for microstructural and CHD analysis. In comparison to the other methods, the edge retention, flatness and quality is exceptional. It also only requires 20% operator dependency and has very high safety levels.

Hardness Control, Resource Time Savings with DiaMet Automation

Lab Case

This example aims to highlight the demanding CHD measurement control which is required for a heat-treated automotive shaft. Testing of the shaft involves sectioning out samples at 0, 120 and 240 degrees around the shaft. These are then mounted and semi-automatically prepared as previously described. CHD control measurements are then selected at 8 different sites on each sample containing 12 indentation points (a total of 96 indents per sample). To test six samples, a total of 576 indentations are required, which for a manual or a semi-automatic hardness testing machine would be painstakingly slow

Automation Process

The polished samples are fixed on a magnetic sample holder followed by a contour scan of one of the sample and stored within DiaMet as a template. Since the remaining 6 samples have similar geometry, the saved template is then automatically superimposed on the contour scans and the software automatically alters the profile to match the sample irrespective of how it was placed in the magnetic holder. Both the 8 locations/site and their corresponding 12 indent positions are instantaneously fixed.

Figure 2. Shows a 6-sample holder clamping fixture for multiple sample testing

Image Credit: Buehler

This assignment demands a large amount of time from an operator within a standard system, and has a big potential for error. Within the Automotive and Aerospace fields, the improved DiaMet template functionality is fundamental various heat treatment control processes. Significant time savings can be achieved using Buehler DiaMet hardness automation software, which functions to assimilate the Template and Auto-Snap functions to instantaneously place the sites to be tested, with several indent points at each individual site.

The Auto Snap functionality enables automatic adjustments to be made to each location/site, positioning them to the edge of the sample at high magnifications, to maximize accuracy. All indents, autofocuses and measurements are automatically carried out by DiaMet. The measured indents are also saved to allow for revalidation later. This can be useful in later adjustments and ensuring accuracy.

Figure 3. (a) Illustrates auto-snap and validation of location placement on the samples at higher magnification and (b) shows a saved high magnification indent including a magnifying window for fine adjustments if desired

Image Credit: Buehler

The software also contains a graphical display which plots the various locations/sites instantaneously, to check for conformance and to focus on the crossover points. These crossover points are the positions at which the surface hardness level falls below the pre-adjusted minimum. The case depth distances at these points are also displayed on the carts. This allows for validations if they fit with the tolerance for the specific component.

Figure 4. Shows a chart within Diamet visually displaying the CHD plots from 8 locations highlighting the crossover points (red line) where hardness drops below the preset minimum

Your Savings vs Manual & Semi-Automatic Process

Return on Investment

Conclusions

A planar grinder is utilized for semi-automatic sample preparation for rapid initial stock removal and providing an optimal surface for the integrity process, before the final polishing stage is carried out. Overall this process develops a faster 3-step process to enable savings of time for both the overall process and the operator.

Hardness testing is carried out by the DiaMet controlled automation technology, which notably lowers the times taken to carry out hardness checks. This is significantly reduced to just a few hours down from a whole day. This suggests that there is significant return on investment from using this automation process in order to analyze case hardening depth. Specifically, in laboratories which require a high throughput of samples for quality control, this process could prove vital. This process is extremely simple and accurate, acquiring both reports and graphics in just one click.

This information has been sourced, reviewed and adapted from materials provided by Buehler.

For more information on this source, please visit Buehler.