Alloy steel refers to a type of steel that is alloyed with various elements. In theory, every steel can be referred to as alloy steel since the simplest steel is iron alloyed with up to 2.06% of carbon. However, the term “alloy steel” commonly refers to steels that are alloyed with elements other than carbon. The total weight of alloying elements can amount up to 50% to give the material improved properties such as better wear protection or ductility. A distinction is made between low-alloyed and high alloyed steels. Low-alloyed steels are characterized by their low amount of alloys, which in summation make up to less than 5%. The amount of elements in high-alloyed steels can be greater or equal 5%, making the material more expensive by tendency. Aside from those two groups, there are also unalloyed steels which carry an extremely small amount of alloys.
As an alloying element, even small amounts of boron (0.001–0.003 %) can greatly increase hardenability. However, boron-steel is not easy to find as it only makes up to 0.003% of the alloy steels. Boriding is a more popular approach to introduce boron to steels. With BorTec’s Borocoat diffusion layers, a hardness of up to 2.800HV can be reached. Furthermore, BorTec’s technology ensure high layer hardness even on unalloyed steels, improved adhesion and high resistance against wear. Read more here.
Chromium is one of the most popular alloying metals for steel due to its high hardness and corrosion resistance. On its own, chromium is a grey, hard and brittle metal with a high temperature resistance and a melting point of 1907°C (3465°F). In steel, chromium alloyed to increase hardenability. Higher chromium contents of 4 to up to 18% result in better corrosion resistance. For example: One of the most widespread steel alloys, namely stainless steel, has a chromium content of at least 10.5%, making it more resistant against water, heat or corrosion damage. In contrast to iron oxide in unprotected carbon steel, chromium oxide does not spread and fall away from the material. It creates a film of dense chromium oxide on the surface that blocks out any further corrosion attacks.
With more than 400 alloys, copper is one of the most common alloying elements. For steel, copper is solved into the material by 0.1 to 0.4 % to increase corrosion resistance. Copper is also known for its high thermal and electrical conductivity. Its copper oxides work in a similar way as the chromium oxides.
In alloy steel, manganese is typically used in combination with sulfur and phosphorus. Managanese helps to reduce brittleness and improves forgeability, tensile strength and resistance to wear. Manganese reacts with sulphur, resulting in manganese sulfides which prevents the formation of iron sulfides. Manganese is also added for better hardenability as it leads to slower quenching rates in hardening techniques. Excess oxygen can be removed in molten steel by using manganese.
Austenitic stainless steels are most known for their high content in nickel and chromium. It is used to increase strength, hardness, impact toughness, and corrosion resistance. Nickel-alloyed steels are often found in combination with chromium, resulting in an even higher hardness.
Titanium is used as a stabilizing element in stainless steels. It fixes carbon in inert particles, improving corrosion resistance and weldability. Since Titanium is a rare element on Earth, this alloying element can be up to 200 times more expensive than regular carbon steel.