In some specific cases, conventional Rockwell hardness testers are not the correct instrument to resolve a certain measurement requirement. Getting the correct measurement, however, is very significant. The following case provokes interest and raises a lot of questions regarding the care of obtaining reliable results.  

Case Study

The apparatus used was a hardness tester equipped with an AFFRI system – the AFFRI 3332 MRSA model. The system was also equipped with a Rockwell measurement and optical system to determine

This study was conducted at Caterpillar Company. The sample was a sectioned heavy truck wheel. The scope was to determine the hardness homogeneity from the outer surface to the center of the wheel.

Sample Details
The sample was heat treated at 850°C (1562°F). A transverse section of 170 mm long, 50 mm wide and 7 mm thick was sectioned. The surface area to be examined was very extensive, commencing from the exterior profile and working toward the core. The transverse indentation profile was tabulated until a core hardness of 45 HRC was reached. The total case depth was 20 mm. The same hardness depth profile was repeated over various areas of the transverse sample.

The largest problem encountered was the non-flatness planarity of the sample due to variable thickness from area to area of the cut sample, which created instability.

Testing Hardness
The hardness scale chosen was Rockwell C equivalent to 2 µm depth of penetration with a test load of 150 kg and a diamond indenter with a 120-degree angle. Under the effect of the 150-kg test-load pressure, the stability of the test is very important during the testing cycle. Any type of specimen movement, as little as 1 µm during the testing cycle, would cause loss in precision and reliability of results. In this specific case, any sudden change of the fulcrum creates loss of stability of the reference point, thus skewing the depth of penetration.

Testing Equipment
The non-flatness planarity of the sample due to variable thickness produced an unstable and flexible surface with considerable tendency to exhibit soft readings during hardness testing. It also created difficulty in measuring the depth of penetration of the Rockwell hardness, rendering it very difficult to obtain repeatable results without the use of very expensive and complex instrumentation.

The AFFRI 3332 MRSA was modified with auto-sample surface-clamping contact, which auto-compensated for any sample deflection from the load forces. It was also equipped with a motorized specimen table and optical display system. The 330- x 390-mm base of the tester supports the load force of the instrument and that of the X-Y stage, including the weight of the specimen up to 200 kg.

The head of this specific hardness tester automatically descends and makes contact with the specimen surface and clamps the specimen. During this movement, the indenter remains retracted. After the specimen is clamped, the indenter travels down to penetrate the specimen surface. The head of the tester also compensates for any bending or relief of the specimen to reveal its true hardness value. The final result is then transferred to the computer memory for storage. The auto-stage then incrementally moves to the next position, and the test cycle is repeated again until a total of 20-mm cross-sectional depth and the 170-mm length is reached.

The software operates under a Windows environment. The results are saved in an Excel spreadsheet, and the material hardness is plotted over the X-Y stage positions to a resolution of 10 µm, providing a hardness depth map of the entire specimen. Every result assists in the determination of the material heat-treat homogeneity.

Conclusion

In our case study, every measurement is accurate and reliable without the influence of specimen distortion over all the various heat-treated zones. As a result, the true overall hardness homogeneity of the tested sample was able to be determined in spite of sample geometry challenges. IH  

For more information:  Please contact Mr. Roberto Affri; AFFRI Inc., 170 N. Brandon Dr., Glendale Heights. IL 60139; tel: 630-303-1588 or 224-374-0931; e-mail: roberto.affri@affri.com or usa@affri.com; web: www.affri.com