For a long time, the “weapon of last chance” for an infantryman was a bayonet, which was attached to a rifle. However, as history approached the Second World War, the armies of the leading countries began to fill with the first machine guns. Submachine guns, due to the design features, did not have and could not have any bayonets. In order not to leave the fighters without edged weapons, it was decided to return to the idea of an infantry knife. In the Soviet Union, the HP-40 became such.
As you might guess from the factory index, the Soviet infantry knife HP-40 was born in 1940 and immediately entered service with the Red Army. The combat "Knife of the Scout" was produced at the Zlatoust Tool Plant No. 259. The basis of the soldier's HP-40 was taken terribly popular in the early Soviet Union, and before that in the Russian Empire - a Finnish knife, which in turn was the result of the development of a traditional northern European household knife puukko.
The reconnaissance knife had to be as simple as possible, reliable enough and at the same time as cheap as possible. For its production in the USSR, U7 steel was used. This is a tool steel belonging to the class of high-strength and unalloyed. The percentage of carbon in it was 0.7%. In addition to the latter, chromium, silicon, phosphorus, manganese, and sulfur were added to U7. Briefly and clearly, the qualities of U7 steel are characterized by an old folk joke: if you peel a potato - wipe it, cut the fat - do not touch it. This, by the way, is about the fact that, like any other non-alloy steel, U7 rusts quite easily without proper and timely care.
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Why did Soviet designers choose this particular steel? In fact, everything is quite banal, because the U7 is cheap and cheerful. On the one hand, the material has an optimal strength range of 50-55 HRC for a mass combat knife. This material is easy to process, has sufficient viscosity, which allows it to be used for the rapid production of a huge number of products. U7 steel can be safely used under conditions of dynamic and increased loads, it heats up slightly, and it also breaks very badly under physical impact. Finally, this material is very cheap. What plays an important role in the weapons industry.
If you want to know even more interesting things, then you should read about why the PK MOOiR knife that did not take root in the USSR is in great demand in the West.
Source: https://novate.ru/blogs/191221/61598/