In our last post, we looked at a modern method of manufacturing steel, Basic Oxygen Steelmaking (a.k.a) the BOS process. As we saw, this is based on the Bessemer process, except that we use oxygen instead of air to burn off impurities. When we studied the Bessemer process, shortly after that, we studied how fluid compressed steel was made from steel made by the Bessemer process. The purpose of compressing the steel was to eliminate gas bubbles and hairline cracks in the ingot. Well, the BOS process also could have these problems for the same reasons as well, so we will study how these problems are tackled in today's post.
The problem is that when steel is manufactured using the BOS process, oxygen is injected over the molten metal to burn off impurities. As it turns out, not all of this oxygen gets used up to burn impurities, some of the excess oxygen gets dissolved in the molten steel as well. When the metal solidifies, this oxygen is released out and can do bad things to the steel. For one, it can combine with the iron in the steel, to form iron oxide (i.e. rust). The second is that the oxygen gas can form gas bubbles (blowholes) in the ingot. Thirdly, it can combine with the carbon in the steel, forming carbon monoxide and carbon dioxide, which reduces the carbon content of the steel and weakens it. Also, the carbon monoxide and carbon dioxide gas can form blowholes in the steel as well. Gas bubbles and blowholes cause the steel to have pores in it. One more problem is that the carbon monoxide tends to form more on the outside of the ingot and escapes out. This causes non-uniform distribution of the carbon in the steel, because the outside of the ingot now becomes relatively pure iron, while the inside of the ingot is carbon steel. Also, steel shrinks considerably as it cools and trapped gas in the metal can cause gaps and hairline cracks in the ingot as well. For firearm applications, the presence of rust, bubbles, cracks and pores is undesirable, as is the non-uniform distribution of carbon in the steel.
So clearly, we must minimize the oxygen in the molten steel before it solidifies and preferably remove it without forming a gas like carbon monoxide, because the gas could cause bubbles and cracks to form. In modern times, this is done right after the molten steel is tapped out of the BOS furnace and poured into molds, by adding deoxydizing agents to the molten steel. Basically, a deoxydizing agent is a chemical that strongly combines with oxygen better than carbon and iron do. Therefore, as the molten steel cools, the dissolved oxygen combines with the deoxydizing agents first, before it has a chance to react with the iron or carbon in the steel. A good deoxydizing agent also forms solid slag rather than a gas, so that there are no gas bubbles or cracks formed as the steel cools. Such a steel is called "Killed Steel".
Typical deoxydizing agents are aluminum, ferrosilicon (an alloy of iron and silicon) or ferromanganese (an alloy of iron and manganese). These combine with the oxygen dissolved in the molten steel to form aluminum oxide (alumina) or silicon dioxide (silica). Deoxydizing agents are added as soon as the steel is poured out from the furnace into molds and may be added individually or together, depending on the type of steel desired.
As the molten killed steel hardens in the mold, there are practically no gas bubbles seen, because most of the dissolved oxygen has been removed by the deoxydizing agents. Since there are no bubbles formed, the steel quietly solidifies in the mold and this is why it is called "killed steel". The ingot is generally free from blowholes and the distribution of carbon and other alloying elements in the steel is more uniform. This ensures that the killed steel ingot has excellent chemical and mechanical properties that are uniform throughout the entire length of the ingot. Killed steel ingots are sometimes marked with the letter "K", to indicate how they were manufactured.
Not all steel manufactured is killed, but any steel with carbon content greater than 0.25%, or in general, any steel that is meant to be forged later, is killed, Stainless steel and alloy steels are also killed as part of their manufacturing process. As we saw earlier in the series, 4140 and 4150 steels that are used in firearms have 0.40% or 0.50% carbon content. Stainless steel is also used in the firearms industry.