Metals Used in Firearms - XVIII

In our last post, we studied one of the modern methods of steel making, the electric arc furnace. In today's post, we will study another method that is commonly used today, the Basic Oxygen Furnace (BOF) otherwise known as the Basic Oxygen Steelmaking (BOS) process.

The interesting thing about the BOS process is that the original concept is actually from the 19th century. Recall that the Bessemer process that we studied earlier, works by blowing air through hot molten metal and the oxygen in the air burns off the impurities in the molten iron. Well, the reader is probably thinking that since air consists of a mixture of nitrogen, oxygen, carbon dioxide and other gases, and since only oxygen is needed in this process, wouldn't the process become more efficient if we directly blew pure oxygen over the molten metal? The same idea occurred to Henry Bessemer (allegedly suggested to him by his father as a joke) and he received a patent on October 5th, 1858 for this concept. Unfortunately for him, this idea was not practical in the 19th century, because bottled oxygen was not available at reasonable cost or in large quantities at that time.

Also recall when we studied the Bessemer process, there are two types: the acid bessemer process and the basic bessemer process. The "basic bessmer process" is called that, because it uses an alkaline (i.e. basic) lining in the vessel (as opposed to an acidic lining). The Basic Oxygen Steelmaking process is also called "basic" because it uses an alkaline lining (usually, Magnesium Oxide (MgO)) in the vessel. The purpose of this alkaline lining is to remove elements such as phosphorus and sulfur from the molten metal, as these elements are harmful to steel's properties.

The idea of using oxygen in the furnace was revisited in the 20th century and made practical during the late 1940s. Interestingly, the modern BOS process was developed, not by any large steel companies, but mainly due to the efforts of one man and the support of a few managers in a small company that he worked for. Our story starts with a Swiss metallurgist, Robert Durrer, who graduated from Aachen university in Germany in 1915 and remained there until 1943. He served as a professor of steelmaking in Berlin's Technishe Hochschule (Berlin Institute of Technology) between 1928 and 1943, where he performed many years of experiments using oxygen for steel refining. In 1943, he returned to Switzerland and joined a small Swiss company called Von Roll AG. Here, he continued his experiments in the town of Gerlafingen, with a German colleague, Dr. Heinrich Hellbrugge. In 1947, Durrer bought a small 2.5 ton converter from the US and with it, he reported his first success in the internal plant newspaper in May 1948:

"On the first day of spring, our "oxygen man", Dr. Heinrich Hellbrugge carried out the initial tests and thereby, for the first time in Switzerland, hot metal was converted into steel by blowing with pure oxygen... On Sunday, the 3rd of April 1948 ... results showed that more than half the hot-metal weight could be added in the form of cold scrap ... which is melted through the blast produced heat"

Soon after this, two Austrian steelmakers, VOEST and Alpine Montan AG (OAMG), got interested in these developments and worked with Von Roll to commercialize this process. Theodor Suess of VOEST's plant in Linz and the managers of the Alpine Montan plant in Donawitz organized the actual experiments and worked out all the technical issues and decided to construct two 30-ton furnaces in 1949. On November 27th 1952, the first steel was produced by this new type furnace. Since the VOEST plant in Linz and the Alpine Montan plant in Donawitz were instrumental in commercializing this technology, their version is called the Linz-Donawitz process.

Since oxygen containers became available in large quantities and low cost after the 1940s, this process was very efficient and cheap. Readers interested in history might be amused to learn that the reason that methods to produce low-cost oxygen at large volumes were developed was mainly because of the German V2 rocket program! After World War II, the Germans were not allowed to manufacture oxygen in large quantities, but the factories and equipment that they had pioneered were shipped off to other countries.

In the beginning, big steel manufacturers in the US paid no attention to this innovation by a small Central European company, whose total steel making output was less than one third that of a single US Steel factory! A smaller American company, McLouth Steel in Michigan, was the first to install BOS furnaces in the US in 1954. The larger American companies, such as US Steel and Bethlehem Steel only built their first BOS furnaces in 1964. However, the rest of the world quickly adopted this new technology and by 1970, 50% of the world's steel (and 80% of Japan's steel) came from BOS furrnaces. As recently as 2011, about 70% of the world's steel output was still made using this method.

A large container, called a ladle, is lined with refractory materials, such as magnesium oxide (MgO). The ladle is tilted about 45 degrees and is charged with scrap steel and then molten pig iron from a blast furnace is also added. The ratio is about 20-30% of scrap steel to about 70-80% of molten pig iron, based on the requirements of the final steel to be produced. This takes a couple of minutes. After this, fluxes such as magnesium or lime are added to remove sulfur and phosphorus. Then the vessel is turned back to the vertical position and a water-cooled lance with a copper tip is lowered down within a few feet of the bottom of the vessel. Through this lance, pure oxygen (greater than 99% pure) is blown over the hot metal at supersonic speeds (about 2x the speed of sound). The oxygen ignites the carbon in the molten iron, forming carbon monoxide and carbon dioxide. These reactions are exothermic (i.e. they produce heat), so the temperature of the molten iron increases even more. The magnesium burns with the sulfur, forming magnesium sulfide, which is also an exothermic reaction, contributing to the rise in temperature. Silicon combines with the oxygen forming silicon dioxide slag. The blowing of the oxygen also churns the molten metal and fluxes, which helps the refining process. The slag, being lighter than the molten steel, floats on top of it.

Click on the image to enlarge

The temperature of the furnace is closely monitored and after about 15-20 minutes, a small sample of the steel is taken and analyzed to make sure that its chemistry is correct. After that, the furnace is tilted horizontally and the molten steel is tapped out into another ladle. At this point, other alloying elements such as nickel, chromium etc. may be added. Sometimes, an inert gas, such as argon may be bubbled through the ladle, to mix the alloying elements properly into the steel. To prevent slag from being poured out with the steel at the end of the tapping process, various "slag stoppers" are used, but a human eye remains the best device to determine when to stop tapping the steel. After tapping the steel out, the vessel is turned upside down and the remaining slag is poured out into a separate slag pot. The vessel is examined to make sure its refractory lining is intact and more lining material is added if needed and the vessel is prepared for the next batch.

The entire process takes about 40 minutes, which is substantially faster than the 10-12 hours that the Open Hearth Process takes. This is why it quickly replaced the open hearth process in many places around the world. Using pure oxygen instead of air makes the process more efficient and it also avoids piping nitrogen and other undesirable gases in the air through the molten steel. The process can take about 250-350 tons of metal in one charge. Unlike the electric arc furnace, this is a primary steelmaking process (i.e.) it works mostly with pig iron rather than scrap steel. This process increases the productivity of steel making -- in fact, as this process became popular, the labor requirements of steel making went down by a factor of 1000. Instead of taking 3 man-hours per ton of steel produced, it now takes 0.003 man-hours per ton of steel. The only disadvantage of this over the open-hearth process is the reduced flexibility of the charge -- the open hearth process can use up to 80% scrap steel, whereas the BOS process can only use a maximum of about 30% of scrap steel. About 70% of the world's steel today is made by the BOS process.

In our next post, we will look at some finishing up processes for steel and after that, we will look at a factory producing rifle barrels at the beginning of the 20th century.

Metals Used in Firearms - XVII

In our last post, we studied the invention of the Siemens-Martin process to make steel. In today's post, we will study a type of furnace that was invented in the early 1900s, gained popularity around World War II and is still in use today. We are talking about the electric arc furnace.

To understand this type of furnace, we must understand what an electric arc is. An electric arc is a form of electrical discharge between two electrodes, separated by a small gap (typically, normal air). The best known example of this is lightning. Anyone who has performed arc welding is also familiar with electric arcs: you connect the work piece to the negative side of a DC power source and an electrode to the positive side, touch the electrode to the workpiece momentarily and then draw it a small distance apart from the work piece. A stable electric arc forms between the electrode and the work piece and the heat from this arc is sufficient to melt the electrode and weld the workpieces together. The same idea is used in a larger scale in an electric arc furnace.

The idea of electric arcs was first demonstrated by Sir Humphry Davy in 1810 in England and several people after him tried experiments and patented processes in the 19th century, including Carl Wilhelm Siemens, who we read about in our previous article. However, the first successful electric arc furnace was due to the Frenchman, Paul Heroult, in 1900. He was later invited to the United States in 1905, to set up furnaces for American companies, such as US Steel and Halcomb Steel. The process really gained popularity during World War II and afterwards, because of the low costs associated with setting up an electric arc furnace, compared to a complete integrated steel mill.

The furnace is a kettle made with a dished bottom, mounted so that it can be tilted forward and drained. The kettle is lined with fire brick which can withstand very high temperatures. There are doors on either side to put in raw material and the front has a spout to pour out the molten steel. The roof of the furnace is a dome lined with firebrick and has two or three carbon electrodes in it.

Electric furnaces are typically charged with scrap steel, though they may also be used with hot pig iron directly from a blast furnace. Usually though, scrap steel is used. The scrap is prepared based on the grade of steel to be made and the scrap pieces are arranged so that large heavy pieces of scrap metal don't lie in front of the burner ports. Some lime and carbon may also be added at this stage, although more may be injected at a later stage. After the charge is put in the furnace, the roof is lowered on the furnace and an intermediate amount of electricity is sent through, to start the electric arc, until the electrodes bore into the scrap sufficiently. Usually, light scrap is placed on the top of the pile to accelerate the bore-in process. After a few minutes, the electrodes melt enough of the scrap that they can be pushed deeper in and the high voltage can be fed in without fear of electric arcs hitting the roof of the furnace. As the furnace heats up, the electric arc becomes stable and starts melting the material. At this point, air (or oxygen) may be fed into the furnace to burn up the carbon, silicon, manganese etc. and form steel. More carbon and limestone and other elements may be added at this stage to form the steel.

As we have studied before, phosphorus and sulfur tend to weaken the steel and must be removed. As it turns out, the conditions favorable to remove phosphorus are opposite to those favorable to remove sulfur and vice versa. As a result of this, there is a chance that one of these elements may revert back into the steel from the slag, if proper steps are not taken. Therefore, the phosphorus removal is carried out very early on in the process -- while the temperature is still relatively low, the furnace is tilted to pour out the initial slag formed, which gets rid of much of the phosphorus. If this high phosphorus slag is not removed early on, it will revert back into the steel later on. Then the furnace continues to be heated, and more slag formers are introducted to remove the other elements, such as silicon, sulfur, calcium etc.

The molten metal is analyzed via a spectrometer to make sure that the carbon content and oxygen are correct. Once the correct temperature and chemical contents are achieved, the steel is tapped out as shown in the illustration above. At this point, beneficial alloying elements such as nickel or vanadium may be added to the tapped metal stream.

After all the metal is tapped out, the solid slag is cleaned out of the vessel, the electrodes are checked for damage and the new charge is prepared to be introduced into the vessel. The entire process of preparing a charge, melting it, tapping it, cleaning out the vessel and recharging it, takes about 60 minutes on a medium-sized furnace (capacity of 90 tonnes or so).

Electric arc furnaces can range from really small sizes suitable for research labs to large ones capable of working with 400 tons of metal at a time. The nice thing about them is that they can work with 100% scrap metal, which means they are very handy for recycling old steel, which can be bought for far cheaper than iron ore. They can easily be started and stopped, unlike other furnaces. They are also very energy efficient, compared to methods that make steel from raw iron ore. They can produce very high grade steel from cheap and impure metals and even better than the Siemens-Martin process. Since they run at higher temperatures, they allow the operator to make slags that are normally difficult to melt, but useful to remove small traces of impurities. They can be used for superior stainless steel alloys as well. Nucor, one of the largest steel manufacturers in the US, uses electric arc furnaces a lot, because it allows them to put up smaller mini-mill plants near where the steel is needed and they can vary production quickly, depending on the demand.

Electric arc furnaces are also used as part of the process in vaccuum arc remelting (VAR), which is used to produce specialty steels. In this process, the steel is first melted in an electric-arc furnace and then alloyed in an argon oxygen decarburizing vessel and poured into ingots. Then, the ingots are put into another container and the air is removed from it to form a vacuum. An electric arc is used to remelt the steel, since the arc can form without the need for oxygen. Any dissolved gases (such as nitrogen and oxygen) escape out under the vacuum conditions, as do elements such as sulfur and magnesium, which have high vapor pressure. The molten steel is solidified at a controlled rate, using a water jacket around the vessel to control the cooling rate and ensure uniformity.  It is known that the VAR process is used to produce 9310, 4340, Aermet 100 and maraging steels, which we studied earlier when studying steels used for rifle barrels, bolts and firing pins at the start of this series. The process can also be used to produce titanium, which is also sometimes used in the firearms industry, as we studied before.