Rockwell and Vickers hardness tests have their place in the metrologist’s armoury but, for large cast components, the Brinell hardness test is still very much the hardness test. In the Brinell test a tungsten carbide ball-indenter is forced into the test material and the diameter of the resultant indentation is used to calculate the material’s hardness. Measurement of that diameter is not, however, straightforward. In the blog of 24th August 2020, Foundrax MD Alex Austin covered some of the difficulties. Here we consider another one: the effect that the size of the indenter has on the material being tested.
It may be thought that one 3mm diameter indentation is much like any other but this is not the case. For example, the profile of a 3mm indentation made with a 5mm ball is quite different to one made with a 10mm ball. Here’s a diagram illustrating this:
It’s pretty obvious that, once an indentation has reached a diameter of 3mm, more of the test material will have been disturbed by a 5mm indenter than by a 10mm one. The material that has been pushed downwards and sideways has to go somewhere and there is, of course, much less resistance to ‘sideways’ deformation than to ‘downwards’. The consequence of this is that small indenters produce a bigger ‘pile up’ of displaced metal at the edge of an indent than large ones (as an aside, a well-used Brinell test calibration block with multiple indentations in it is slightly convex when viewed from above because the surface area has been increased by all the indentations stretching the material).
Now, for those whose task it is to measure indentation diameters, the question of pile-up is significant. Where does the indentation end and the pile-up begin? That’s tricky enough as it is. Add to that the fact that the edge of an indentation can appear to move as you adjust the angle of the light shining on it and the fact that different types of steel have different deformation characteristics and the fact that metal will deform more easily in one direction than another because of the molecular grain structure (and thus produce an elliptical indentation not a truly round one) and it quickly becomes apparent what a challenge this is.
Foundrax produced the first automatic measurement system in Brinell hardness testing equipment in the early 1980s, wringing every last drop of computing power out of a 4K(!) machine. Since then the algorithms have been refined and refined and we have a system that can detect hundreds of diameters, accurately, in well under a second. The system can distinguish ‘pile-up’ from the true edge, no matter what the indenter size. It took over 10 years to get from the pioneering kit of 1982 to a reliable production model and our range of equipment is still being refined for user-friendliness and portability. It’s also even more accurate than it was ten years ago – and even then it comfortably exceeded the accuracy required by international standards. The equipment is integral to some of our larger machines and available as a separate portable unit; the ‘BRINtronic’.