THERMIT HEAT PROCESS
As a result of his discovery that by starting a terrific battle for oxygen between two metals he could reduce one of them to almost absolute purity, Dr. Hans Goldschmidt has converted to the use of man a process of welding so simple and yet so forceful that it is making world-wide changes in the working of metals. This battle itself is the most interesting feature of the Goldschmidt process because of the terrific heat it generates.
Imagine sticking your finger into boiling water. By so doing you would be exposing your flesh to a temperature of 212 degrees Fahrenheit. Imagine sticking your finger into a pot of molten lead if even for the fraction of a second. You know very well what the effect would be. The temperature is 618 degrees Fahrenheit. Still again, think of a redhot iron. This is about 1,652 degrees Fahrenheit. Steel boils at 3,500 degrees.
They are all hot enough, but compare them with the temperature of 5,400 degrees Fahrenheit or about 3,000 degrees Centigrade, which is attained by the thermit reaction. The range of temperature in which we can live extends from a little over 100 degrees to 70 or 80 degrees below zero, and yet man can so direct the heat of the thermit reaction that it will work for him.
The commonest use of the process is in welding steel or iron, such as broken parts of machinery and welding steel rails, and steel or iron pipes. Besides this, the thermit process will reduce many metals to a high degree of purity. After spending a few minutes in seeing how the inventor of this process came to discover it, we will take a little trip in our mind's eye to some of the places where the thermit process is in use, and see what happens.
As you know, metals rarely come from the mines in a state of purity. They usually are very much mixed up with rock, slag, and other minerals, so that it takes a complicated process called smelting to separate them. Even then they are not pure, and more complicated processes have to be gone through with. Oxides, or metals that have been oxidized, are common because oxidization merely means that the metal has been burned so that each atom of metal has taken up an atom of oxygen to make what is called a molecule of oxide. Iron ore is usually found in the form of iron oxide, because when this great earth was nothing but a swirling ball of burning gases, probably as hot as the sun, gradually cooling and forming a great cauldron of molten matter, boiling and bubbling more fiercely than the hottest cauldron of molten metal in any steel mill, much of the matter that later became iron ore was burned or oxidized. Other chemical actions too technical for our attention just now were responsible for other forms of ore, such as sulphides, etc. When the earth cooled sufficiently to become solid, these things were completed, and they only had to remain hidden away under the surface for ages and ages until a little man who could live but a hundred years at the utmost solved the deepest secrets of the earth's formation.
Thus, to obtain pure metals the oxygen must be removed from the oxide. In other words, it must be reduced. Plainly such reduction was a problem of smelting, but Doctor Goldschmidt in his efforts to obtain purity was working along lines of smelting, in his little German laboratory, very different from the ones in general use.
His first object was to reduce iron oxides. First, he knew that aluminum has a great affinity for oxygen, or, in other words, when the two are heated will absorb oxygen like a sponge will absorb water, only more forcibly and more violently than any such comparison even faintly suggests. In yet other words, aluminum wants oxygen more than any other metal does. Of course no chemical changes would occur if a piece of iron oxide and a piece of aluminum were set side by side, any more than we would have gunpowder if we set a chunk of saltpetre, a chunk of sulphur, and a chunk of charcoal all in a row. The iron oxide and the aluminum would have to be mixed by cutting or filing them into small pieces and making a coarse powder. Still nothing would happen without heat to start it.
If you collected some flakes of iron oxide in the palm of your hand they wouldn't look to you like very promising material for a bonfire, and you wouldn't be in any danger of an explosion, but you would have something in your hand that would burn, nevertheless. If you sprinkled your iron filings over a gas flame, Welsbach burner, or over a common lamp chimney the heat would cause them to splutter and fly out with all the brilliancy you know so well when the blacksmith gives the redhot horseshoe the first pound.
Of course Doctor Goldschmidt knew all this, just as he knew that the way the aluminum would take the oxygen away from the iron oxide was through heating the coarse powder of filings to a very high temperature. But this was attended with serious troubles and many times the German scientist came near losing his life in explosions in his laboratory.
At first he failed to get the mixture hot enough and nothing happened. Bit by bit he increased the heat under the crucible containing the filings until it reached about 3,000 degrees Fahrenheit. At this point the metals were hot enough to fuse or run together and the whole thing reacted with such violence that it amounted to an explosion. What really happened was that the mass reached the temperature where the aluminum could take the oxygen from the iron oxide, and it did so with such force that an explosion resulted.
Doctor Goldschmidt then saw his problem. It was that of devising some way of heating the mixture to a temperature sufficient to gain the reaction, but without an explosion.
After trying everything that he could think of, he conceived the plan of leaving the crucible in the open air and starting the heat at just one point first, instead of heating the whole thing in a furnace. He did this with a pinch of ignition powder placed on the top of his pile of iron oxide and aluminum. The ignition powder was simply lighted with a match.
What happened?
Thermit was discovered.
The heat, or reaction started at one point, gradually spread through the whole mass, and reduced it to white-hot molten material.
In other words the application of intense heat at one point in the mixture was sufficient to fuse the metals and start the battle between the iron oxide on one side and the aluminum on the other, in the immediate vicinity of the point where the heat was applied. As the few particles set off by the ignition powder struggled for the oxygen they themselves generated heat—terriffic heat—which gave a high enough temperature to start the particles that were their next-door neighbours to struggling for the oxygen. These in turn generated heat to set off their own neighbours, and so it went.
In far less time than it takes to read this, Doctor Goldschmidt saw the whole crucible of dead mineral particles take on life and become white-hot liquid metal. Scientifically speaking, the reaction had spread through the whole mass in less than a minute, but what Doctor Goldschmidt saw was a blinding white light, more intense than any arc lamp, throwing off a little cloud of white smoke or vapour. Apparently the whole thing was burning up. He only heard a little hissing as the metals battled for the precious oxygen.
There was no explosion, there was no violent scattering of molten particles, and there were no noxious life-destroying gases such as come from the explosion of gunpowder, dynamite, or even the burning of coal. And yet the seething, molten metals in the crucible reached a temperature second or third to the highest ever registered by man. Five thousand four hundred degrees—think of it!—more than half as hot as science tells us is the sun which makes this world of ours habitable.
But what was the result of this temperature which staggers the imagination?
Just this. Doctor Goldschmidt knew that the aluminum had won the prize of battle and had paid the price of victory.
The conquered iron was at the bottom of the crucible, a molten mass of pure metal, while the victorious aluminum, seething on the top, was nothing but slag (aluminum oxide).
Perhaps there may be a little lesson in this drama of the metals, because while the iron was vanquished it emerged from the stress of conflict purified and fitted for its high service to mankind, while the more aggressive aluminum came to the top an almost useless product, ruined by the prize for which it had fought.
Another interesting point about this reaction is that the heat produced by a certain quantity of the mixture is no greater in total volume than the heat that would be produced by the burning of an equal amount of anthracite coal. The difference is that the thermit process concentrates all the heat in a few seconds whereas the coal gives off its heat bit by bit for a long period of time.
The mixture of filings used in this process is called thermit. A technical definition of the product is as follows: "Thermit is a mixture of finely divided aluminum and iron oxide. When ignited in one spot, the combustion so started continues throughout the entire mass without supply of heat or power from outside and produces superheated liquid steel and superheated liquid slag (aluminum oxide)."
Thus the makers of thermit call the pure metal that results from the combustion, thermit steel.
For the boy who has studied chemistry the simple equation by which the scientist described the process to his young friend will mean as much as his long explanation. The equation is:
Fe2O3 + 2Al = Al2O3 + 2Fe.
The scientist simply went on to say that Fe2, iron, and O3, oxygen, in the equation means iron oxide, while 2Al means aluminum. Thus we have iron oxide plus aluminum, heated to 5,400 degrees Fahrenheit, equals aluminum oxide, Al2O3, plus pure iron, 2Fe. These signs are simply the abbreviations scientists use for expressing processes in the terms of mathematical equations.
With this general outline of the principle of the thermit process in mind its actual application will seem a simple matter. Suppose that a great steel ship ploughing her way through a storm breaks her sternframe. This is the steel framework upon which the rudder post is mounted, and naturally a fracture puts the rudder out of commission. Repairs must be made before the ship can make another trip. Quick repairs are desired by the owners. Perhaps the ship is a passenger steamer due to leave port in a few days with passengers and mail, so to put the liner in drydock, wait for the steel mills to cast a new sternframe, wait for it to come by freight, and then wait for the steelworkers to fit the piece in the place of the broken one is a matter of weeks, perhaps more.
With the thermit process at hand this is not necessary. The company that manufactures and sells thermit has big plants in several cities in various parts of the world, but if there is steel repairing to be done elsewhere the company will send its materials and expert workmen on a minute's notice. So if the crippled ship limps into the port where there is a thermit plant the repairs can begin at once, but there need be only a little delay otherwise, because the captain of the ship can notify his owners of the damage by wireless while still out at sea, and long before he reaches the port he is making for they can have a complete thermit outfit on the way.
One of the biggest advantages of the thermit process of repairing machinery or structural steel is that the welding in a great many cases can be made without taking the complicated parts to pieces. Consequently after the ship is in drydock the workmen build a wooden scaffolding about the broken sternframe, so that they can work the better.
The next step is the preparation of the broken parts for welding. Most boys know how the doctor has to put splints on a broken arm so that it will knit properly. It is something like that with a thermit weld.
The broken parts are supported in exact alignment by heavy blocks of concrete, and the fractured ends sliced off clean by the oxygen-gas torch. This leaves a space of from one inch to two and a half inches between the fractured ends, just according to the size of the piece to be welded. After the parts are all thoroughly cleaned the workmen are ready to take the next step.
This is the preparation of the mould for the weld. First, a pattern of the weld, as it will appear when completed, is put on the fracture with beeswax. The space between the broken ends is filled in and a thick "collar" of wax is packed around the parts, so that when this is done the pattern looks like a swelling on the frame. The mould is then built around this wax pattern.
The inventor of the thermit process had to make a number of experiments before he found a material refractory enough to stand the terrific heat to which the mould had to be exposed. Finally he decided upon an equal mixture of fire brick, fire clay, and fire sand.
With this material, then, the workmen go about making the mould. It is solid, with the exception of three apertures or tunnels, which are left by inserting in the moulding clay, wooden models of the size and shape desired. These are a gate, or place into which the molten welding material is to be poured, a "riser" or larger hole into which the surplus material can run for the overflow, and a heating aperture. The gate runs from the top of the mould down to the lowest point of the wax pattern, while the "riser" extends from the top of the wax pattern to the top of the mould. Thus we really have a small inlet and large outlet, although it is always arranged so that the surplus metal remains in the riser, and as little as possible runs over. The heating aperture is a small hole in the side of the mould extending to the bottom of the wax pattern.
With the mould complete the wooden models of the gate, riser, and heating aperture are pulled out and the first step in the process of welding is taken. The long pipe of a specially constructed gasoline compressed-air torch is inserted in the heating aperture and the process called preheating started. The gasoline torch, of course, quickly melts the beeswax, and leaves the space occupied by the pattern clear for the molten metal that is to be introduced to make the weld. The blast from the torch is continued through this heating aperture until the parts to be welded have reached a red heat, because if this were not done the cold steel would so chill the molten thermit steel that the weld could not be accomplished. The length of time taken by this preheating is governed, of course, by the size of the parts to be welded. Sometimes it is many hours.
Everything is now ready for the thermit. There has been some elaborate preparation of the thermit too. The coarse powder or grains of iron oxide and aluminum previously have been prepared according to the job to be done. In very large welds, or welds where very hard steel is required, certain additions, to be explained later, are made to the thermit.
The amount of thermit to be used is an important factor, of course, as there must be plenty to fill the mould, and yet not so much that it will overflow the riser. To decide on the amount takes a careful calculation because in large operations there are certain additions to the thermit which have to be considered. In general, however, the engineer must remember that he must have just twice as much molten thermit steel as he needs to fill the space left by the melting of the wax pattern. The surplus flows up into the riser, heating aperture, and gate, effectually closing all of them. The calculation, then, is that it takes four and a half ounces of steel to fill a cubic inch. It takes nine ounces of thermit to produce four and a half ounces of steel, so the engineer directing the weld must figure on eighteen ounces of thermit to each cubic inch in the wax pattern, including the space between the parts to be welded.
After seeing that the proper amount of thermit is measured out the engineer must see that the crucible in which the reaction is to take place is ready to contain the strenuous battle that is to be fought in it.
As before mentioned there are very few products that can withstand the heat of the fire produced by thermit. Ordinary fire brick and mortar would melt or be burned to powder in a few seconds. Metal would go the same way that the metal in the crucible goes. Science, however, has established that magnesia tar is not affected by the thermit fire, so the crucible in which the thermit is reduced is heavily lined with magnesia tar. The crucible itself is shaped like a cone with the point downward. At the bottom is a magnesia stone, which has a conical-shaped hole for the "thimble." This "thimble" also is made of magnesia stone, and has a hole through it for the molten thermit steel to run through after the reaction has taken place. Before filling the crucible with the thermit, however, the pouring hole is very carefully plugged up by a special process, with a little steel pin protected by fire sand and fire clay. This pin extends below the lowest point of the crucible a couple of inches, and by knocking it upward the molten metal is allowed to flow out. The upper end of this little plug that otherwise would be melted instantaneously by contact with the burning thermit, as indicated above, has to be protected by a layer of fire sand. The hole through which the metal flows is never more than half an inch in diameter.
With the crucible, mould, and thermit prepared, the next thing is to put the thermit in the crucible and put the crucible in place. There are many ways of placing the crucible. In some cases, it is hung by a chain and in others it is supported by a tripod or wooden scaffolding. The latter is the better because, though the wood always catches fire from the heat, it can be kept standing by throwing on water, whereas steel or iron would be eaten in two in an instant by the touch of a few sparks of flying thermit. The point is to support the crucible so that the pouring hole is directly over the entering gate, or pouring gate of the mould.
THERMIT WELD ON STERNFRAME OF A STEAMSHIP
Notice metal left above weld, where it flowed up into the riser.
A LARGE SHAFT WELDED BY THE THERMIT PROCESS
Protruding metal is that which flowed up into gate and riser. It is cut away by the gas torch to leave a neat weld.
Courtesy of the American Machinist
CUTTING UP THE OLD BATTLESHIP MAINE WITH AN OXY-ACETYLENE GAS TORCH
Picture shows end of boat crane over exploded magazine, which was cut off in fifteen minutes.
Courtesy of the American Machinist
CUTTING AWAY THE DECKS
Oxygen and acetylene generators can be seen on top of after-turret.
Things move with a rush now, for all these arrangements are made ahead of time, and as soon as the workmen are sure that the parts in the mould are redhot the heating aperture is carefully plugged with fire sand and the thermit is ignited. From a mere pinch to half a teaspoonful of the ignition powder is put into a little hollow in the thermit so that the heat may be communicated at once to as much of the thermit as possible. This is then set off with a storm match. The workman quickly withdraws his hand, slams the lid on to the crucible and gets out of the way of flying sparks.
There is a hiss, a puff of white smoke, a blinding glare from the hole in the top of the crucible, and that is all, beside a few sparks, to indicate that a heat second only to that of the sun is being generated within.
One cannot help but marvel at the wonders of science as this inconceivable heat is being produced, the process is seemingly so simple, so easily handled, and so accessible for all kinds of work where steel welding is necessary.
Half a minute to a minute (according to the amount of thermit used) after the match has been applied a workman holding at arm's length a long tool called a "tapping spade" gives a few upward knocks to the little metal pin extending down from the closed pouring aperture. He jumps back for the heat is enough to set his clothes afire, even at a considerable distance, and a few flying particles of the molten thermit would inflict a serious burn.
Down through the little hole the thermit, that a minute before had been only a coarse dark gray powder like metal filings, seemingly the last thing on earth that would catch fire, flows into the pouring gate of the mould in a steady stream of white-hot liquid steel. The white glow from the metal is brighter than any electric light. It is so intense that although the workmen wear heavy dark goggles, they shade their eyes and turn their heads away.
Now you will be wondering, if you know anything about steel and its wonderful properties, how it is that this can be good steel when it is all mixed up with the aluminum oxide or slag. The reason it is of best quality is that as soon as the reaction reduces the whole mass to a molten liquid the heavier steel, set free, as the scientists say, but as we have chosen to think of it, robbed of the aluminum, sinks to the bottom, while the lighter aluminum oxide rises to the top. Consequently the steel goes into the mould to make the weld while the slag, having risen to the top, will be found at the top of the pouring gate, and only around the outer edges of the weld.
When the pour is completed the workmen go away and leave it to cool. It is usually left over night, sometimes as long as forty hours, when the weld is a very large one.
Finally the mould is broken down and the weld is found complete, with big extensions of the steel extending from the weld, in just the shape of the pouring gate "riser" and heating aperture.
The molten thermit steel rushing in at the bottom of the mould has risen between the heated broken ends, and all around them, in just the shape left by the wax pattern. As the scientists say, the thermit steel has united the broken sternframe and formed a homogeneous mass with it. In other words the terrific heat of the thermit rushing on the heated ends has resulted in the two parts becoming one with the added thermit steel.
After the mould is broken down the oxygen-gas torch comes into use again to cut away the ends of steel sticking up where they had cooled in the pouring gate, "riser" and heating aperture. After this the weld looks like a great swelling upon the sternframe, and if the swelling is where it will not interfere with the working of the rudder or steamer propellers, nothing more need be done. On the other hand, if the swelling is in the way, it can be reduced to the size of the frame, and squared off with machines built for the purpose.
Thus the ship is repaired and is ready to be taken out of drydock for her next trip, as good as new.
About the same plan is followed out on all kinds of welding except pipes and rails. Locomotives can be repaired without taking the complicated machinery apart just by working around until the crucible can be so hung, and the pouring gate so arranged that the metal can be poured into the place designed for it. The chief difference lies in the size of the weld to be made and the consequent amount of thermit to be used. Welds have been made where as much as 2,000 pounds of thermit—enough to make 1,000 pounds of steel—have been run into a mould. In these very big welds a certain percentage of steel "punchings," or small pieces of steel, and a little pure manganese are used to give the additional hardness to the weld.
Without going into details as to the manner in which the principle of the thermit process is applied on rails or pipes, it will be enough to say that in welding rails three different systems are used. The first is done by building the mould around the ends of the two rails to be welded together and letting the thermit steel run in and completely surround the rails and the space between them. This gives one continuous rail just as far as the welding is carried on, and one through which the electric current of an electric road can pass without any trouble at all. It is plain, then, why this system is used so much on third rails of electric roads. The trouble with it is that the swelling on the top and inside of the rails must be machined down to present a smooth running surface to the wheels.
The next system, which is now almost out of date, is one in which two moulds are used so that the thermit does not come up over the running surface of the rails. This relieves the engineers of the necessity of machining the welded joints.
The third system is a mixture of the joining by plates and the thermit process. This is called the "Clark joint," after the name of Chief Engineer Charles H. Clark of the Cleveland, (Ohio) Electric Company, who formulated the plan. The rails are joined with plates and bolted, or riveted together in the old way, but a thermit weld is made at the base of the rail, welding the bases of the two rails together and to the plate.
The method of welding steel pipes is an exact reversal of the principle of welding together solid pieces of steel or iron. After the pipes are cut off clean, the mould, which is made of cast iron, is placed around them with specially constructed clamps to force the two ends closer together after the thermit has been poured in. The thermit is then set off in a flat-bottomed crucible like a long-handled ladle, and poured into the mould by hand as if from a ladle. As the slag rises to the top it goes into the mould first and coats the pipes. The thermit steel does not touch the pipes, but merely supplies the heat to weld them perfectly, so that they are as strong as the piping itself. Just after the pour has been made, the clamps are tightened up and the white-hot pipe ends forced together. They are thus held until cold, when the mould is broken away. The slag coats the outside of the pipes and this is chipped away, leaving a perfect weld.
Another interesting use of thermit is in the great foundries where cauldrons of metal have to be kept at a very high temperature. To help keep the mass in a liquid state thermit can be introduced in it either by throwing it into the cauldrons in bags, with a little ignition powder so fixed that it will be touched off by the heat of the boiling metal, or by putting it in especially designed cans affixed to the ends of long rods. By these rods the thermit can be plunged to the bottom of the cauldron before it "burns." The reaction of the thermit, with the intense heat caused by it, helps to keep the mass at the proper temperature.
Also thermit is used in the same way with a small amount of titanium oxide, to purify iron and steel. The metal becomes much more liquid, and a commotion like boiling is started. This is the result of the titanium driving out the impure gases and driving other impurities such as metallic oxides and sulphur contents to the top. Chemically what happens when the titanium is introduced by the thermit process is that the titanium combines with the nitrogen in the molten iron, giving it a much finer grain, and making it a much lighter colour, more like steel, than previously.
One of the things thermit is not extensively used for is the repairing of gray iron castings. The first reason is that gray iron is cheaper than steel, and a new casting often can be turned out by the mills quickly.
Another and a more interesting reason is that gray iron melts in a much lower temperature than does thermit steel and consequently has a lower shrinkage. Therefore when the molten thermit, with its terrific heat, cools there is a large shrinkage. Thermit steel being much stronger than gray iron, its shrinking sometimes strains and cracks the iron casting.
In spite of this difficulty very successful repairs have been made on cast-iron and it has been found that by mixing 2 per cent. of ferro-silicon and 1 per cent. pure manganese with the thermit for welding, a thermit steel is formed which is very soft and comes close to the properties of gray iron. By using this mixture important welds have been made on cast-iron flywheels, water wheels, and other cast-iron parts with great success.
While industry is making progress with all these uses of thermit, science is experimenting all the time to add to the scope of the process. As was pointed out before, many other metals can be reduced to a high degree of purity with this process and in the laboratories they are always trying new ones and working out new formulas. Of the pure metals that can be reduced by the thermit process there are chromium, which is 98 to 99 per cent. pure; manganese, which is 96 per cent. pure; and molybdenum, which is 98 to 99 per cent. pure. These are used in the manufacture of very hard steel, such as armour plate, and "high speed steel." Among the alloys, or mixtures of metals, there are chromium-manganese, manganese titanium, ferro-titanium, ferro-vanadium, and ferro-boron, all of which have uses in industry and help us to travel faster and more safely by railroad, electric train, and steamship.
It may have occurred to some bright boy that, since this heat is so intense and so handy, it might be a good way to make steam in locomotive boilers, or cook our meals, but it will be remembered that the heat is all over within a few minutes. In other words, where a terrific heat is required for a few seconds, thermit will fill the bill, but where a continuous heat for many hours is needed, electricity, gas, coal, coke, oil, or wood are better. The high cost of aluminum would probably prevent the thermit process coming into use in the manufacture of steel for our armour plate, ship plate, or structural steel, at least for a good many years.
Earlier in this chapter I said that the slag, or aluminum oxide, from the thermit process was an almost useless product. This is not the precise scientific truth, for the slag becomes a black powder such as is used in making emery wheels, but the slag from thermit is never actually used for this. Another use for the slag from the thermit process in which chromium is used has been discovered. Potters use a material called corundum, which this slag resembles, except that it is superior to natural corundum in pottery manufacturing because of its freedom from metallic impurities. The slag can be mixed with clay and baked. It is especially useful in chemical apparatus that must withstand great extremes of temperature, because its experience has so tempered it that nothing less than a heat equal to that of the sun would give it much concern.
Another interesting thing about the slag from chromium thermit is that small rubies have been found in it. The scientific explanation is that they are nothing but crystallized alumina, coloured with chromium. The jewels usually are too small for any commercial purpose but serve as a very striking example of the intensity of the thermit fire. All the real jewels, diamonds, rubies, emeralds, amethysts, and so on, were formed by the terrific heat in the bosom of the earth millions of years ago when it was cooling down from gases hotter than anything we can possibly conceive of, to a molten ball, then to a solid redhot mass and then to a globe sufficiently cool on the outside to be crusted over. That they can be made in this little chemical furnace shows how far science has gone in imitation of the wonders of nature.