Steel is pervasive in almost every aspect of our lives, found in nearly every building, bridge, vehicle, appliance, and machine. According to the World Steel Association, more than 1,500 megatonnes of steel are produced annually, worldwide.
Steel is made primarily from iron and carbon in a two-step process. In the first step, primary steelmaking, liquid iron is converted into steel by the basic oxygen furnace (BOF) process, or by melting scrap steel or direct reduced iron (DRI) in an electric arc furnace. Secondary steelmaking, or metallurgy, is a refining process in which alloying metals are added and impurities are removed to ensure the quality of the finished steel product.
The secondary step is critical to produce steel that is “clean,” or free from contaminants such as carbon (C), nitrogen (N), oxygen (O), sulfur (S), hydrogen (H), and phosphorous (P). This is especially important when dealing with scrap iron, which will bring unwanted or unknown elements into the mix. While the primary and secondary steelmaking stages will reduce these impurities to low levels, enough remain to affect steel mechanical properties, such as strength, formability, toughness, weldability, and fatigue resistance, and must be carefully controlled during the steel manufacturing process. Clean steel also depends on reducing impurities known as nonmetallic inclusions, such as oxide and sulphide inclusions which can result in corroded, brittle or cracked steel.
While impurities are removed, a variety of alloying elements are added to produce different grades of steel with specific properties, depending on the application for which the steel will be used. Chromium and nickel, for example, are added to create stainless steel, which is very resistant to rust. Some other alloying elements that can be added to steel to increase strength and provide other properties include:
- Boron: Improves deformability and machinability.
- Cobalt: Improves strength at high temperatures and magnetic permeability.
- Copper: Produces precipitation hardening properties and increase corrosion resistance.
- Manganese: Increases strength at high temperatures and improves hardenability, ductility and wear resistance.
- Molybdenum: Increases hardenability and strength at high temperatures and protects against pitting corrosion caused by chlorides and sulphur chemicals.
- Selenium: Increases machinability.
- Silicon: Improves strength, elasticity, acid resistance and leading to greater magnetic permeability.
- Titanium: Improves strength and corrosion resistance.
- Tungsten: Increases hardness at high temperatures.
Clean steel that meets exact mechanical specifications is a competitive necessity. Rapid, accurate detection and analysis of the levels of C, N and O at low concentrations, in addition to the other elements, is especially important for steel manufacturers to be able to guarantee the quality of their products to their customers. Improved technologies have emerged for elemental analysis during the steelmaking process to ensure that the finished product contains the appropriate elements in the correct percentages for their application. One such technology is Optical Emission Spectroscopy (OES).
OES is a robust, reliable, and widely-used technology for the analysis of metals and alloys. Compared with traditional analyzers, (Note: combustion is specific to some elements, like C, N and O. Other analysis techniques can also be used for other elements) OES provides faster and cheaper elemental analysis of most needed elements with high precision and accuracy in iron and steel, aluminum, copper, magnesium, precious metals and other specialty metals/alloys.
OES analysis is based on the ablation of sample material by electrical sparks plasma. The ablated material is excited in the plasma and a corresponding light in the VUV-visible range is emitted. The emitted wavelengths are characteristic of each element and their intensity is proportional to the element’s concentration in the sample. The light emitted is directed towards the optical system. The main component of the system, the diffraction grating, separates the polychromatic light into its monochromatic constituents by dispersion over a certain wavelength range. The photons of the wavelengths of interest are usually detected with PMTs (Photo-Multiplier Tubes) and transformed into electrical signals. Alternatively the full spectrum or part of it may be collected with CCD detectors.
OES has demonstrated its capability to provide more efficient control of steel production by providing accurate determination of all the needed elements during the manufacturing process. See detailed analysis, including data charts illustrating how one OES instrument fulfills the latest analytical requirements of steel producers regarding C, N and O.