Key Alloying Elements in Steel: Properties and Uses
Nickel
Nickel was one of the first metals used as an alloying element in the manufacture of special steels. This element was first used in the late nineteenth century. It was noted that adding 2-5% nickel to carbon steels increased their strength and yield strength without reducing toughness.
Nickel is an extremely important element in the manufacture of stainless steel and provides resistance to high temperatures. These steels are austenitic at room temperature and do not support tempering. The iron-nickel alloy with less than 10% carbon and 36% nickel has a very low thermal expansion (null between 0° and 100°) and is called Invar.
Chromium
Chromium is one of the most important elements used in alloy manufacturing. It is used interchangeably in construction steel, tools, stainless steel, and heat-resistant steel. It is used in varying amounts from 0.30 to 30%, as appropriate. Chromium serves to:
- Increase hardness and tensile strength of steel
- Improve hardenability
- Prevent deformation during hardening
- Increase wear resistance and corrosion resistance
Chromium-Nickel
Series 3000 improves ductility and toughness. It has very good temperability and wear resistance, so it is used extensively in tool manufacturing.
Molybdenum
Series 4000 is quite expensive. It dissociates ferrite and carbon and is very strong, improving temperability. When alloyed with chromium, it increases hardness at high temperatures. It is very good for fatigue resistance and is always accompanied by chromium and nickel. When it is alone (series 40xx and 44xx), it has a low carbon percentage and is used for cementation, such as in gears and transmission shafts.
Vanadium
Vanadium is very expensive because it is toxic. It is a potent deoxidizer and provides very good temperability. Its principal property is that it increases tenacity.
Chromium-Vanadium
Chromium-Vanadium is used in the manufacture of fasteners, crankshafts, axles, and springs due to its great tenacity.
Types of Stainless Steel
Martensitic Steels
Martensitic steels (series 400 and 500) contain 11-18% chromium. They can be tuned and have high mechanical strength. They can be used to cast parts and are used in turbine blades and surgical instruments due to their high resistance to corrosion at high temperatures.
Austenitic Steels
Austenitic steels (series 300) are not magnetic and are not hardened by heat treatment. They contain nickel and chromium, with no less than 23% of the alloy between the two. They have much more nickel than chromium. Due to their low carbon content, they can be welded and forged and are used in the manufacture of food containers. They do not undergo allotropic changes and cannot be tuned.
Ferritic Steels
Ferritic steels (series 400) have a high percentage of chromium (14-27%) and no nickel. They can be worked both hot and cold. They are used in chemical applications, architectural ornaments, and automobiles.
Precipitation Hardening Stainless Steels
Precipitation hardening stainless steels are chromium-nickel steels containing additional alloying elements such as copper and aluminum. These elements favor the precipitation of secondary phases and significantly increase the hardness and mechanical strength of the material when subjected to aging heat treatment. These materials offer an alternative to obtain good mechanical strength by a heat treatment at a lower temperature, which can be applied even after the manufacture of the part or mechanical element. The mechanical properties (strength and hardness) that can be achieved with these alloys are superior even to those obtained by martensitic stainless steels (approximately 1480 MPa). Also, because the chromium content is greater than in the latter, the corrosion resistance is also superior. Precipitation hardening stainless steels may be martensitic, semi-austenitic, or austenitic.