We continue to review some of the most important materials in heat treatment and metallurgy.
Oxygen (chemical symbol: O)
Oxygen is a colorless, odorless, tasteless gas that is highly reactive due to its high electronegativity, second only to fluorine. The characteristic of electronegativity is due to its position on the periodic table, reflecting the fact that it needs two electrons to fill its outer electron shell. This means that O2 tries hard to acquire electrons, and this is accomplished by bonding with any less electronegative element. As a result, oxygen forms compounds (oxides) with almost all other elements.
This quality makes free oxygen (dioxygen or O2), rare in earth’s atmosphere. It bonds readily with hydrogen (Fig. 1) by sharing two electron pairs. Because oxygen comprises most of the mass of water (H2O), it also comprises most of the mass of living organisms, including human beings. It is the third-most abundant element in the universe, after hydrogen and helium. Due to its strong tendency to bond with other elements, oxygen gas would not exist in the atmosphere without its production via photosynthesis in plants. Oxygen started to accumulate in the atmosphere after the evolutionary arrival of photosynthetic organisms – roughly 2.5 billion years ago.
Perhaps the most important contribution of oxygen to metallurgy is in steelmaking. In a process known as basic oxygen steelmaking (BOS aka Linz–Donawitz-steelmaking), or the oxygen converter process, carbon-rich pig iron is made into steel with the use of oxygen.
In BOS, the pig iron is first liquefied and then highly pure oxygen is injected into the molten metal at a pressure of 700-1,000 kPa (100-150 psi) while at the same time being sprayed at supersonic speed onto the surface of the molten iron through a water-cooled lance, which is suspended a few feet above it (Fig. 2). This rapidly ignites (oxidizes) much of the carbon in the molten pig iron, and the oxygen combines with it to form carbon monoxide (CO) and carbon dioxide (CO2).
The process is highly exothermic, and scrap iron is added at the same time to absorb the excess generated heat and to prevent overheating. The end product is low-carbon steel. BOS is a successor to the original Bessemer steel process, which used air to oxidize the carbon. The Bessemer process was less efficient, and it was only after commercially available oxygen became more affordable that Swiss engineer Robert Durre developed the early BOS process in 1943.
Here are a few important facts about oxygen.
- Atomic number: 8
- Atomic weight: 15.9994
- Melting point: 54.36 K (-218.79°C or -361.82°F)
- Boiling point: 90.20 K (-182.95°C or -297.31°F)
- Density: 0.001429 grams per cubic centimeter
- Phase at room temperature: Gas
- Element classification: Nonmetal
- Period number: 2
- Group number: 16
- Group name: Chalcogen