The first is the requirements for Hardness. For some occasions, the higher the hardness of the material, the better. It is only limited to the hardness requirements of the machined parts. The machined materials cannot be too hard. If they are harder than the machined parts, they cannot be machined.
Secondly, the material should be moderately soft and hard, at least one grade lower than the hardness of the machine parts, and it also depends on the purpose of the processed components and the reasonable selection of materials for the machine parts. In short, precision machining still has certain requirements for materials, and not all materials are suitable for processing, such as materials that are too soft or too hard. The former is not necessary for processing, while the latter cannot.
So, the basic rule is to pay attention to the density of the material before processing. If the density is too high, it is equivalent to a high hardness. If the hardness exceeds the hardness of the machine (lathe tool), it cannot be processed. Not only will it damage the parts, but it will also cause danger, such as the tool flying out and injuring people. So, generally speaking, for mechanical processing, the material material must be lower than the hardness of the machine tool in order to be processed.