This product is a so-called file hardness, a dedicated file that hits the file. The hardness of a file made with a single iron file depends on the experience and intuition of an expert, but the hardness check is a convenient tool that anyone can use. It is very convenient to check the hardness of foreign objects such as unknown knives, hoes, shovels, etc. Since the hardness of quenched steel is about HRC35 to 70, almost all steel products can be examined with this one set. I try to use various types of nails for things with low hardness such as non-hardened steel. If you are using it for the first time, I think you should practice with an OPINEL knife (HRC57-58) whose hardness you know. 60 will scratch it. If it is 55, there will be a thin line that can not be distinguished even if you trace it with your finger. At 50, there will be even thinner lines or dirt marks. Rather than judging by appearance, it is better to judge by the feeling of scraping and the unevenness of scratches. In addition to this product, you can obtain higher accuracy by collecting cutlery, files, knives, nails, etc. whose hardness can be specified. For example, using OPINEL, if you scratch other knives or kitchen knives, you will know that if there is a scratch, it is 57 or less, and if it is not scratched, it is 57 or more. If you apply force when using it, the eyes of the file will be crushed and its life will be shortened, and if you are gentle, it will last longer. In particular, it is better to be careful with the three low hardness 40, 45, and 50.
(Translated by machine.)

I bought it because I wanted to do blacksmithing work in the future and to determine the hardness of cheap stainless steel blades at home, which is not enough for durability. Since the width of the velcro that covers the case is narrow, it may open and drop when you carry it. I was a little worried. Therefore, I store it in the paper box that contains the case at the time of purchase, but since the thickness of the velcro is so thick that it is difficult to put it in, I thought that this is the neck if I dare to use it every day, the hardness check body I have no complaints about it.
(Translated by machine.)

I think that it is a must-have checker when judging whether or not the privater can cut the material that I received and whose identity is unknown. You can reduce the risk of the blade being chipped or worn, and it is also useful for determining quenching after making your own blade. Conversely, it can also be used to determine whether a hardened product can be annealed and scraped. I thought it was expensive at first, but it would save time and money.
(Translated by machine.)