Tuesday, December 16, 2014

What's the deal with Barrel Shrouds?

So what is a barrel shroud? It is simply a hollow covering tube that surrounds a barrel (either partially or fully). What does it do? Well, it protects the user of the firearm from accidentally burning himself or herself with the hot barrel.

A typical barrel shroud

A barrel shroud typically has many holes throughout its length. The holes serve to reduce its weight and also dissipate heat by venting out hot air. The next picture shows a barrel shroud attached to a rifle.

As you can see in the above image, the barrel shroud is simply that tube with holes that surrounds the barrel. In the above example, the user has also attached an extra hand grip to the barrel shroud. Since much of the barrel shroud is not in contact with the hot barrel, if the user was to accidentally touch the front of the firearm, the user will not get burned by the barrel.

The curious reader is probably thinking now, "isn't that what the stock of a firearm is designed to do?", Yes, the stock and the receiver do protect the user's hands as well, but they are not considered as barrel shrouds, because they serve other purposes as well, whereas the barrel shroud is a separate component that is screwed on around the barrel and explicitly designed to protect the user's hands (or other body parts) from heat.

Barrel shrouds are generally commonly seen with air-cooled machine guns, but they are also optional components for many semi-automatic models. Some shotguns also feature barrel shrouds. There are many third party component makers that make barrel shrouds for various rifle and shotgun models. In general, they are useful to have with weapons that fire rapidly, because the barrel can heat up quickly after a few shots.

If a barrel shroud is simply a covering tube to protect a user from touching a hotter part of the gun, then what's the big deal about them? Well, for a while, barrel shrouds were targets of legislative restrictions in the United States. The now expired Federal Assault Weapons Ban explicitly included barrel shrouds in its list of features for which a semi-automatic pistol could be banned (if a firearm had two features in the list, it could be banned under this law). After the law expired, proposals were made to renew the ban, including this provision, but have not been successful so far.

Amusingly, during an interview on MSNBC in 2007, Representative Carolyn McCarthy was asked about her gun control legislation and why it prohibited people from purchasing firearms that have barrel shrouds and if she even knew what a barrel shroud was. After attempting to avoid the questions twice, she finally admitted, "I don't know what it is, I think it is a shoulder thing that goes up!"

It is amazing that she was trying to introduce a law to ban something without even knowing what it was!

Sunday, December 14, 2014

Forging Rifle Barrel Blanks in the 1920s - II

In our last post, we studied some parts of a factory designed to produce rifle barrel blanks. In today's post, we will continue studying the process. As noted before, some of the details come from a book, "The Working of Steel" by Fred Colvin and K.A. Juthe, and in addition, K.A. Juthe was the designer of the factory as well.

Where we last left off, the barrel blanks were straightened out and tested for straightness. The next process was to heat treat the barrel blanks and increase their hardness. We will discuss this heat treating process shortly.

The last process was to grind the ends of each blank and then grind a spot on the enlarged end of each blank and test the hardness of the blank on a Brinell machine, to ensure that the blanks met the required hardness nunbers

The Brinell hardness test was invented by Swedish engineer Johan Brinell in 1900. It was one of the first standardized hardness tests used in engineering and is still used today. The test is very simple. It uses a steel or tungsten carbide ball of diameter 10 mm. (0.39 inches), which is used as an indenter. The ball is placed on the surface of the object to be tested and a 3000 kg. (or 6600 lbf.) test force is applied to the ball for a specific time (normally 10 to 15 seconds). After this, the ball is removed and it leaves a round indentation on the surface of the object. The diameter of this indentation is measured and the Brinell Hardness Number (BHN) is calculated, based upon the diameter of the ball, diameter of the indentation and the force applied to the ball. For softer materials, such as aluminum, a smaller test force (e.g. 500 kg. (or 1100 lbf.) is used instead.

The above image shows a line drawing of the concept and the formula actually used to calculate the Brinell Hardness Number (HB in the image above).

Returning back to our study of the factory process, the barrel blanks were tested for hardness to make sure that they had a Brinell Hardness Number (BHN) of at least 240.

At this point, the barrel blanks were shipped off to a barrel manufacturer, who would then drill, ream, finish-turn and rifle the blanks into complete barrels.

Now. all through the description of the process so far, we've been talking about heating the blanks for various purposes. We will cover the heat treatments in detail here. There were actually four separate heat treatments done to the blanks.

  1. Heating and soaking the steel above the critical temperature and quenching it in oil, to harden the steel through to the center of the blanks.
  2. Reheating the steel for drawing of temper  for the purpose of meeting the physical specifications of the blank
  3. Reheating the blanks to meet the machineability test for production purposes
  4. Reheating to straighten out the blanks when hot.
We will study each of the four heating processes in detail. 

For the first heating process, the blanks were slowly brought up to the required heat, which is about 150 degrees Fahrenheit (65.5 degrees centigrade) above the critical temperature of the steel. The blanks were then soaked at a high heat for about one hour before quenching in oil. The purpose of this treatment was to eliminate any strains already existing in the bars that may have been put there from milling operations done to the bars. Remember that steel is an elastic substance and working it puts stress on the bars. For instance, during the production of steel, the manufacturer rolls the bars through various rollers to make them the required diameter, which causes the bars to come out stressed. The heat treatment process removed the stress caused by rolling, hammering, cutting etc. It also ensured that the heat treatment applied to the entire cross-section of the bar and not just the surface. In addition, if a blank had seams or slight flaws, these opened up drastically during the quenching process and made it easy to determine if a blank was defective or not.

The oil used for quenching was kept at a temperature of  around 100 degrees fahrenheit (38 degrees centigrade). This is an ideal temperature is to prevent shock to the steel when it is dropped into the quenching oil, otherwise it could develop surface cracks on the piece.

The second heating process (the one for drawomg the temper of the steel) was a very critical operation and had to be done carefully. The steel had to be kept heated within 10 degrees of temperature fluctuation in the process. The degree of heat necessary for this operation depended entirely on analyzing the steel. Even if the steel was purchased from the same manufacturer, there was always some variation in different batches received from the manufacturer.

The third heating process (reheating for machineability) was done at a temperature of around 100 degrees Fahrenheit (38 degrees centigrade) less than the drawing temperature used for the second heating process. However, the time of soaking was almost double that of the second process.

For both the second and third heating process, after the heating was done. the blanks were buried in lime so that they would be out of contact with air, until their temperature had dropped down to room temperature.

The fourth heating process was used when straightening the blanks. In this process, the blanks were first heated to about 900-1000 degrees Fahrenheit (482-538 degrees centigrade) in an automatic furnace for 2 hours before straightening them. The purpose of heating before the straightening was to prevent any stresses being put into the blanks during the straightening operation. This is necessary because when later processes such as drilling, turning and rifling are done to the blanks, they have a tendency to spring back into the shape they were in when they left the quenching bath. By heating before straightening, the blanks are prevented from doing this.

Another method was later found to produce an even better barrel blank. The blanks were first rough-turned to the final barrel diameter and then heated to about 1000 degrees Fahrenheit (538 degrees centigrade) for about 4 hours before sending them to the barrel manufacturer. Blanks produced with this method remained practically straight during the different barrel making operations (drilling, reaming, finish-turning and rifling). This meant that the barrel manufacturers didn't need to straighten barrels after they were finished (which was a much more expensive operation). This method was tested out with one of the largest barrel manufacturers in the US and it proved to be very effective.

As the reader might be wondering, all this heat-treating needed a large amount of oil for cooling and one of the problems was how to keep all this oil at the proper temperature. After much study, a cooling system was developed for the factory. The next two images show the cooling system as seen on the roof from the outside of the factory.

Click on the images to enlarge. Public domain images.

The next image shows the details of the cooling system:

The hot oil is pumped up from the quenching tanks through the pipe A into the tank B, From here, the oil runs down onto the separators C, which break the oil up into fine particles, that are blown upwards by the fans D. The spray of oil particles is blown up into the cooling tower E, which contains banks of cooling pipes and baffles F. Cold water is pumped through the inside of the pipes. The spray of oil particles collects on the outside of the cold pipes and forms larger drops, which fall downwards onto the curved plates G and then run back to the oil-storage tank below ground. The water pumped through the cooling pipes comes from 10 natural artesian wells at a rate of 60 gallons per minute and this serves to cool about 90 gallons of oil per minute, lowering it from a temperature of about 130-140 degrees Fahrenheit to about 100 degrees Fahrenheit. The water comes out of the wells at an average temperature of 52 degrees Fahrenheit. The pump is driven by a 7.5 HP motor and the speed can be varied to suit the amount of oil to be cooled. The plant was designed to handle up to 300 gallons of oil per minute.

The finished blanks from this factory were sent to different barrel manufacturers to drill, ream, rifle etc. to their requirements.

Tuesday, December 9, 2014

Forging Rifle Barrel Blanks in the 1920s - I

After all the stuff we studied about metallurgy in the last several posts, we will look at an ancillary subject today, forging of rifle barrel blanks. We have already covered barrel manufacture from barrel blanks in some detail in previous posts many months ago. In today's post, we will study the process of manufacturing the barrel blanks as it was done in a factory in America in the 1920s. In particular, this was a factory belonging to Wheelock, Lovejoy & Company, which was designed to mass-produce rifle barrels designed to meet specifications demanded by some foreign governments. Some of the pictures and information in this post was taken from the book "The Working of Steel" by Fred Colvin and K.A. Juthe, and in addition, K.A. Juthe was the designer of the factory as well.

This factory did not manufacture its own steel: instead, they bought what they needed from a large steel manufacturer. The steel manufacturer made the steel to the required specifications and supplied them in the form of bar stock, but the length of the supplied bars was longer, typically each bar measured something like 30 or 35 feet long (about 9.1 to 10.7 meters long).

Cutting the bar stock to size.

Therefore, the first step in the process was to cut each steel bar into smaller lengths to make barrels. The bars came on trucks and were fed through the cutting-off shear, where they were cut into pieces of the proper length. The pieces were actually a little longer than the final barrel lengths, to allow for trimming during the machining process.

A close up of the details of the cutting off is shown in the next image.

A is the stock stop bolted to the side of the frame and the ledge formed by the strip bolted to the stop, keeps the bar stock level during the cutting process. The hold-down B prevents the back end of the steel bar from flying up when the bar is cut. The knife C has several notched edges with which the barrels can be cut, so that it need not be taken out for resharpening, until all the notches are dull.

The cut barrel pieces then passed into the next room, where there was a forging or upsetting press.

Upsetting (or more properly, upset forging) is a process of increasing the diameter of the end of a work piece, by compressing it along its length inside a die. The images below show the process.

The barrel pieces were heated in a furnace to soften them, before being sent through the upsetting press. The press could handle the barrels from all the heating furnaces shown in the room. The men changed work at frequent intervals, to avoid excessive fatigue.

The barrels were then sent through a continuous heating furnace to be reheated and then straightened out as much as possible before being tested for straightness.

A Continuous Heating Furnace

In the above machine, each barrel was tested for straightness by placing it on the rollers as shown in the image above. The screw on the press was used to apply pressure and straighten out the barrel as needed.

We will continue our study of the process in the next post or two.

Thursday, December 4, 2014

Questionable Tactics

After that long series about different metals used in firearms manufacturing, might as well take a break from a dry topic and watch something else instead.

Initially, your humble editor thought that someone was parodying the infamous Rex Kwon Do scene from the movie Napoleon Dynamite. (In case you haven't seen the movie, here's clip 1 and clip 2). Unfortunately, what you're about to see is not a joke, there is actually someone trying to train people to use tactics like this. This group is called the "Sulsa Do Corps" (no joke, that's what they call themselves). See for yourself:

This video was originally posted at a youtube channel called God Rock Ministries / Expert Karate, which appears to be some sort of combination of church and karate school (dojo). This school appears to be run by a Mr. David Bateman. They removed the original video from the channel, when word about its unintentional comedy started to spread. Unfortunately for them, the video was saved by someone else and here it is :).

It is a darn good thing they are using pellet guns with no ammunition, instead of real pistols. Questionable tactics? Where do we start. First, we have quite a few instances of them sweeping each other with the muzzles of their pistols (big safety no-no). Then, we have fingers placed on the trigger at all times (bad idea). Next we have a couple of cases of shooting at the ceiling and open door, while moving and rolling around (another safety no-no). What's with the backwards dive to the ground anyway and since when is running backwards without looking at where you're going ever a good idea. Then, we have firing the pistols close to own face/someone else's face (if those were real pistols, guess whose eardrums are getting blasted to hell, not to mention hot cartridge brass getting ejected on someone else). We also have thumbs placed behind the slide in some instances (if that was a real pistol, someone is going to have a broken thumb when the slide moves backwards at high speed after the cartridge is fired). Then, they run in front of each other, with the ones in the back shooting (good chance of getting shot in the back if they were using real pistols). Then, there's the firing between the legs position (guess whose kneecap is going to receive some hot cartridge brass if those were real pistols). Firearm sights are there for a reason, but they don't seem to know how to use them. We even have a few instances of triggers pulled when one of the others was in the line of fire. The funniest part in your humble editor's opinion is around 0:13 of the video, when the gentleman starts his solo run backwards, holds his pistol practically next to his cheek, then shoots one through the open door, then falls over backwards and bangs his head against the wall! There's probably a few more bad things I missed because I was laughing too hard.

The scary part is that they appear to be dead serious and actually imagine that this is good training. If those were real pistols, someone is definitely going to get hurt or worse. In case you're wondering, this David Bateman has a few more videos about his martial arts training academy, including this gem:

Yep, seems he's the second coming of Rex Kwon Do himself.


Tuesday, December 2, 2014

Metals Used in Firearms - XIX

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.

Sunday, November 30, 2014

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 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 in 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 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.

Monday, November 24, 2014

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.