ATS-55 Stainless Steel

ATS-55 is a high-carbon stainless steel developed by Hitachi Metals. At the time of its development, ATS-55 was considered a premium steel with a performance level of 440C. Its optimal hardness is 59-62 HRC. It is slightly more susceptible to corrosion than ATS-34. ATS-55 is a very good cutlery steel from Japan, very similar to ATS-34, but with much less molybdenum and the addition of cobalt and copper.

This steel was custom made by Hitachi Metals for Spyderco, and since Spyderco no longer uses this product, it is no longer seen for sale.

Its performance is comparable to 440C. ATS-55 metal has a very thin edge and cuts very well. It is easier to sharpen than its big brother, ATS-34, but it also holds an edge longer.

Since only Spyderco used this steel in their knives, it is not a well-known product and after its use was discontinued in the early 2000s, the steel is no longer often discussed. However, Spyderco collectors are very familiar with this use due to its widespread use in mid-grade Spyderco knives of the era.

Chemical composition

So, ATS-55 is an offspring of its parent steel, ATS-34, which was a popular high carbide stainless steel. To create ATS-55, the molybdenum was removed from ATS-34, making it a non-high speed steel with the added benefit of being economical to manufacture. ATS-55 is considered to have better strength than ATS-34, but is more difficult to heat treat.