One of the challenges we face when analyzing steel parts such as springs is their complex geometrical configurations. It is therefore difficult, dare I say impossible in the case of springs, to directly assess the latter and measure their hardness. In the best of worlds, a section of the spring is cut and shaped into a cube which is then easier to manipulate, with no risk of moving during the hardness test. However, cutting quenched steel means generating heat which may affect the data due to migrating carbon atoms inside the steel matrix. Taking into account the cooling time, we obtain what is closer to air quenching, which assuredly affects the hardness measurements.
In order to maintain the same properties as those of the original heat-treated part, the quenched steel part must be cut without overheating it. The ideal maximum temp to reach is 200°C (400°F), which doesn’t give much wiggle room to find an easy and effective cutting technique. In practice, immediately after cutting, the piece should be easy to manage with bare hands, at a maximum temp of around 50°C (120°F). Tolerance to heat may of course vary from one person to another.
Paying attention to the heat of a cut part is therefore a good place to start to ultimately obtain accurate hardness data. Most metallurgy laboratories have a diamond or carbon blade saw with permanent water cooling to ensure a low, uniform cutting temperature. When using a grindstone (a common practice), cutting must be performed slowly, with the part ideally plunged into fluid (such as water) during the process. Remember: the more you heat the part during cutting, the more it will lose its original hardness properties. This cannot be emphasized enough if we want to make sure that we have the right heat treatment recipe to ensure maximum control of the quality of our parts.