Molten iron produced in a blast furnace may contain up to 5% carbon. To reduce the carbon content, the molten iron is transferred to a second furnace into which hot O2 and additional CaO are injected. In this basic oxygen process (Figure 11.14 b), some of the carbon is oxidized to CO2 and any remaining silicon impurities form a slag of CaSiO3. Sulfur and phosphorus impurities in the fuels used in the blast furnace also end up in the slag. Worldwide about 60% of the iron used to make steel is refined using the basic oxygen process.
When molten iron cools to its melting point point of 1538 degree, it crystallizes in a face-centered cubic structure called austentite. With further cooling the structure called the structure changes into a body-centered cubic structure called ferrite. The spaces, or holes, forming an interstitial alloy because the carbon atoms occupy spaces, or interstices, between the iron atoms (Figure 11.16).
There are two kinds of holes between the atoms in closest-packed crystal structures (Figure 11.17). The smaller of the two are located between clusters of 4 host atoms and are called tetrahedral holes. The larger holes are surrounded by clusters of 6 host atoms in the shpe of an octahedron and are called octahedron holes. The data in Table 11.2 provide guidelines on which holes are more likely to be occupied based on the relative sizes of the smaller atoms in the holes and larger host atoms. According to the data in Appendix 3, the atomic radii of C and Fe are 77 and 124 pm, respectively. The ratio of the two is 77/124 = 0.62. According to the guidelines in Table 11.2, this ratio means that C atoms should fit in the octahedral holes of austentite, as shown in Figure 11.16, but not in the tetrahedral holes.
As austentite iron cools to room temperature, it turns into body-centered cubic ferrite iron. The octahedral holes in ferrite are smaller than those in austentite: too small to accommodate carbon atoms. As a result, carbon precipitates as clusters of carbon atoms, or it may react with iron forming iron carbide, Fe3C. The clusters of carbon and Fe3C disrupt ferrite's crystal structure and inhibit the host iron atoms from slipping past each other when a stress is applied. This resistance to slippage, which is much like that experienced by the copper atoms in bronze (Figure 11.12), makes iron-carbon alloys, known as carbon steel, much harder and stronger than pure iron. In general, the higher the carbon concentration, the stronger the steel, as indicated by the information in Table 11.3. Note from the values in the table that increased strength and hardness comes at the cost of increased brittleness.