The hardest steel is maraging steel. Maraging steel is a low-carbon steel that is composed of iron alloyed with 15 to 25 percent nickel and other strong metals such as titanium, cobalt and molybdenum. When chromium is added to the alloy, it creates a stainless steel.
Maraging steel gets many of its properties from being heat treated. The metal has a high tensile strength and continues to be malleable while remaining uncommonly tough and strong. It can be successfully welded and machined, as it can be easily cut, does not unduly wear down cutting tools and has a good finish.