WORKING OF STEEL
ANNEALING, HEAT TREATING
HARDENING OF CARBON AND ALLOY STEEL
FRED H. COLVIN
Member American Society of Mechanical Engineers and Franklin Institute; Editor of the American Machinist, Author of "Machine Shop Arithmetic," "Machine Shop Calculations," "American Machinists' Hand Book."
K. A. JUTHE, M.E.
Chief Engineer, American Metallurgical Corp. Member American Society Mechanical Engineers, American Society Testing Materials, Heat Treatment Association, Etc.
McGRAW-HILL BOOK COMPANY, Inc.
NEW YORK: 370 SEVENTH AVENUE
LONDON: 6 & 8 BOUVERIE ST., E. C. 4
PREFACE TO SECOND EDITION
Advantage has been taken of a reprinting to revise, extensively, the portions of the book relating to the modern science of metallography. Considerable of the matter relating to the influence of chemical composition upon the properties of alloy steels has been rewritten. Furthermore, opportunity has been taken to include some brief notes on methods of physical testing—whereby the metallurgist judges of the excellence of his metal in advance of its actual performance in service.
NEW YORK, N. Y.,
PREFACE TO FIRST EDITION
The ever increasing uses of steel in all industries and the necessity of securing the best results with the material used, make a knowledge of the proper working of steel more important than ever before. For it is not alone the quality of the steel itself or the alloys used in its composition, but the proper working or treatment of the steel which determines whether or not the best possible use has been made of it.
With this in mind, the authors have drawn, not only from their own experience but from the best sources available, information as to the most approved methods of working the various kinds of steel now in commercial use. These include low carbon, high carbon and alloy steels of various kinds, and from a variety of industries. The automotive field has done much to develop not only new alloys but efficient methods of working them and has been drawn on liberally so as to show the best practice. The practice in government arsenals on steels used in fire arms is also given.
While not intended as a treatise on steel making or metallurgy in any sense, it has seemed best to include a little information as to the making of different steels and to give considerable general information which it is believed will be helpful to those who desire to become familiar with the most modern methods of working steel.
It is with the hope that this volume, which has endeavored to give due credit to all sources of information, may prove of value to its readers and through them to the industry at large.
CHAPTER I. STEEL MAKING II. COMPOSITION AND PROPERTIES OF STEELS III. ALLOYS AND THEIR EFFECT UPON STEEL IV. APPLICATION OF LIBERTY ENGINE MATERIALS TO THE AUTOMOTIVE INDUSTRY V. THE FORGING OF STEEL VI. ANNEALING VII. CASE-HARDENING OR SURFACE-CARBURIZING VIII. HEAT TREATMENT OF STEEL IX. HARDENING CARBON STEEL FOR TOOLS X. HIGH SPEED STEEL XI. FURNACES XII. PYROMETRY AND PYROMETERS
THE ABC OF IRON AND STEEL
In spite of all that has been written about iron and steel there are many hazy notions in the minds of many mechanics regarding them. It is not always clear as to just what makes the difference between iron and steel. We know that high-carbon steel makes a better cutting tool than low-carbon steel. And yet carbon alone does not make all the difference because we know that cast iron has more carbon than tool steel and yet it does not make a good cutting tool.
Pig iron or cast iron has from 3 to 5 per cent carbon, while good tool steel rarely has more than 1-1/4 per cent of carbon, yet one is soft and has a coarse grain, while the other has a fine grain and can be hardened by heating and dipping in water. Most of the carbon in cast iron is in a form like graphite, which is almost pure carbon, and is therefore called graphitic carbon. The resemblance can be seen by noting how cast-iron borings blacken the hands just as does graphite, while steel turnings do not have the same effect. The difference is due to the fact that the carbon in steel is not in a graphitic form as well as because it is present in smaller quantities.
In making steel in the old way the cast iron was melted and the carbon and other impurities burned out of it, the melted iron being stirred or "puddled," meanwhile. The resulting puddled iron, also known as wrought iron, is very low in carbon; it is tough, and on being broken appears to be made up of a bundle of long fibers. Then the iron was heated to redness for several days in material containing carbon (charcoal) until it absorbed the desired amount, which made it steel, just as case-hardening iron or steel adds carbon to the outer surface of the metal. The carbon absorbed by the iron does not take on a graphitic form, however, as in the case of cast iron, but enters into a chemical compound with the iron, a hard brittle substance called "cementite" by metallurgists. In fact, the difference between the hard, brittle cementite and the soft, greasy graphite, accounts for many of the differences between steel and gray cast iron. Wrought iron, which has very little carbon of any sort in it, is fairly soft and tough. The properties of wrought iron are the properties of pure iron. As more and more carbon is introduced into the iron, it combines with the iron and distributes itself throughout the metal in extremely small crystals of cementite, and this brittle, hard substance lends more and more hardness and strength to the steel, at the expense of the original toughness of the iron. As more and more carbon is contained in the alloy—for steel is a true alloy—it begins to appear as graphite, and its properties counteract the remaining brittle cementite. Eventually, in gray cast iron, we have properties which would be expected of wrought iron, whose tough metallic texture was shot through with flakes of slippery, weak graphite.
But to return to the methods of making steel tools in use 100 years ago.
The iron bars, after heating in charcoal, were broken and the carbon content judged by the fracture. Those which had been in the hottest part of the furnace would have the deepest "case" and highest carbon. So when the steel was graded, and separated into different piles, a few bars of like kind were broken into short lengths, melted in fire-clay crucibles at an intense white heat, cast carefully into iron molds, and the resulting ingot forged into bars under a crude trip hammer. This melting practice is still in use for crucible steel, and will be described further on page 4.
THE WORKING OF STEEL
ANNEALING, HEAT TREATING AND HARDENING
CARBON AND ALLOY STEEL
There are four processes now used for the manufacture of steel. These are: The Bessemer, Open Hearth, Crucible and Electric Furnace Methods.
The bessemer process consists of charging molten pig iron into a huge, brick-lined pot called the bessemer converter, and then in blowing a current of air through holes in the bottom of the vessel into the liquid metal.
The air blast burns the white hot metal, and the temperature increases. The action is exactly similar to what happens in a fire box under forced draft. And in both cases some parts of the material burn easier and more quickly than others. Thus it is that some of the impurities in the pig iron—including the carbon—burn first, and if the blast is shut off when they are gone but little of the iron is destroyed. Unfortunately sulphur, one of the most dangerous impurities, is not expelled in the process.
A bessemer converter is shown in Fig. 1, while Fig. 2 shows the details of its construction. This shows how the air blast is forced in from one side, through the trunnion, and up through the metal. Where the steel is finished the converter is tilted, or swung on its trunnions, the blast turned off, and the steel poured out of the top.
OPEN HEARTH PROCESS
The open hearth furnace consists of a big brick room with a low arched roof. It is charged with pig iron and scrap through doors in the side walls.
Through openings at one end of the furnace come hot air and gas, which burn in the furnace, producing sufficient heat to melt the charge and refine it of its impurities. Lime and other nonmetallic substances are put in the furnace. These melt, forming a "slag" which floats on the metal and aids materially in the refining operations.
In the bessemer process air is forced through the metal. In the open-hearth furnace the metal is protected from the flaming gases by a slag covering. Therefore it is reasonable to suppose that the final product will not contain so much gas.
A diagram of a modern regenerative furnace is shown in Fig. 3. Air and gas enter the hearth through chambers loosely packed with hot fire brick, burn, and exit to the chimney through another pair of chambers, giving to them some of the heat which would otherwise waste. The direction is reversed about every twenty minutes by changing the position of the dampers.
Crucible steel is still made by melting material in a clay or graphite crucible. Each crucible contains about 40 lb. of best puddled iron, 40 lb. of clean "mill scrap"—ends trimmed from tool steel bars—and sufficient rich alloys and charcoal to make the mixture conform to the desired chemical analysis. The crucible is covered, lowered into a melting hole (Fig. 4) and entirely surrounded by burning coke. In about four hours the metal is converted into a quiet white hot liquid. Several crucibles are then pulled out of the hole, and their contents carefully poured into a metal mold, forming an ingot.
If modern high-speed steel is being made, the ingots are taken out of the molds while still red hot and placed in a furnace which keeps them at this temperature for some hours, an operation known as annealing. After slow cooling any surface defects are ground out. Ingots are then reheated to forging temperature, hammered down into "billets" of about one-quarter size, and 10 to 20 per cent of the length cut from the top. After reheating the billets are hammered or rolled into bars of desired size. Finished bars are packed with a little charcoal into large pipes, the ends sealed, and annealed for two or three days. After careful inspection and testing the steel is ready for market.
THE ELECTRIC PROCESS
The fourth method of manufacturing steel is by the electric furnace. These furnaces are of various sizes and designs; their size may be sufficient for only 100 lb. of metal—on the other hand electric furnaces for making armor-plate steel will hold 40 tons of steel. Designs vary widely according to the electrical principles used. A popular furnace is the 6-ton Heroult furnace illustrated in Fig. 5.
It is seen to be a squat kettle, made of heavy sheet steel, with a dished bottom and mounted so it can be tilted forward slightly and completely drained. This kettle is lined with special fire brick which will withstand most intense heat and resist the cutting action of hot metal and slag. For a roof, a low dome of fire brick is provided. The shell and lining is pierced in front for a pouring spout, and on either side by doors, through which the raw material is charged.
Two or three carbon "electrodes"—18-in. cylinders of specially prepared coke or graphite—extend through holes in the roof. Electrical connections are made to the upper ends, and a very high current sent through them. This causes tremendous arcs to form between the lower ends of the electrodes and the metal below, and these electric arcs are the only source of heat in this style of furnace.
Electric furnaces can be used to do the same work as is done in crucible furnaces—that is to say, merely melt a charge of carefully selected pure raw materials. On the other hand it can be used to produce very high-grade steel from cheap and impure metal, when it acts more like an open-hearth furnace. It can push the refining even further than the latter furnace does, for two reasons: first the bath is not swept continuously by a flaming mass of gases; second, the temperature can be run up higher, enabling the operator to make up slags which are difficult to melt but very useful to remove small traces of impurities from the metal.
Electric furnaces are widely used, not only in the iron industry, but in brass, copper and aluminum works. It is a useful melter of cold metal for making castings. It can be used to convert iron into steel or vice versa. Its most useful sphere, however, is as a refiner of metal, wherein it takes either cold steel or molten steel from open hearth or bessemer furnaces, and gives it the finishing touches.
As an illustration of the furnace reactions that take place the following schedule is given, showing the various stages in the making of a heat of electric steel. The steel to be made was a high-carbon chrome steel used for balls for ball bearings:
6-TON HEROULT FURNACE
11:50 A.M.—Material charged: Boiler plate 5,980 lb. Stampings 5,991 lb. —————- 11,971 lb. Limestone 700 lb. 12:29 P.M.—Completed charging (current switched on). 3:20 P.M.—Charge melted down. Preliminary analysis under black slag. Analysis: Carbon Silicon Sulphur Phosphorus Manganese 0.06 0.014 0.032 0.009 0.08 Note the practical elimination of phosphorus. 3:40 P.M.—The oxidizing (black) slag is now poured and skimmed off as clean as possible to prevent rephosphorizing and to permit of adding carburizing materials. For this purpose carbon is added in the form of powdered coke, ground electrodes or other forms of pure carbon.
The deoxidizing slag is now formed by additions of lime, coke and fluorspar (and for some analyses ferrosilicon). The slag changes from black to white as the metallic oxides are reduced by these deoxidizing additions and the reduced metals return to the bath. A good finishing slag is creamy white, porous and viscous. After the slag becomes white, some time is necessary for the absorption of the sulphur in the bath by the slag.
The white slag disintegrates to a powder when exposed to the atmosphere and has a pronounced odor of acetylene when wet.
Further additions of recarburizing material are added as needed to meet the analysis. The further reactions are shown by the following:
3:40 P.M.—Recarburizing material added: 130 lb. ground electrodes. 25 lb. ferromanganese. Analysis: Carbon Silicon Sulphur Phosphorus Manganese 0.76 0.011 0.030 0.008 0.26
To form white slag there was added:
225 lb. lime. 75 lb. powdered coke. 55 lb. fluorspar. 4:50 P.M.— Analysis: Carbon Silicon Sulphur Phosphorus Manganese 0.75 0.014 0.012 0.008 0.28 Note reduction of the sulphur content.
During the white-slag period the following alloying additions were made:
500 lb. pig iron. 80 lb. ferrosilicon. 9 lb. ferromanganese. 146 lb. 6 per cent carbon ferrochrome.
The furnace was rotated forward to an inclined position and the charge poured into the ladle, from which in turn it was poured into molds.
5:40 P.M.—Heat poured. Analysis: Carbon Silicon Sulphur Phosphorus Manganese Chromium 0.97 0.25 0.014 0.013 0.33 0.70
Ingot weight poured 94.0 per cent Scull 2.7 per cent Loss 3.3 per cent
Total current consumption for the heat, 4,700 kW.-hr. or 710 kw.-hr. per ton.
Electric steel, in fact, all fine steel, should be cast in big-end-up molds with refractory hot tops to prevent any possibility of pipage in the body of the ingot. In the further processing of the ingot, whether in the rolling mill or forge, special precautions should be taken in the heating, in the reduction of the metal and in the cooling.
No attempt is made to compare the relative merits of open hearth and electric steel; results in service, day in and day out, have, however, thoroughly established the desirability of electric steel. Ten years of experience indicate that electric steel is equal to crucible steel and superior to open hearth.
The rare purity of the heat derived from the electric are, combined with definite control of the slag in a neutral atmosphere, explains in part the superiority of electric steel. Commenting on this recently Dr. H. M. Howe stated that "in the open hearth process you have such atmosphere and slag conditions as you can get, and in the electric you have such atmosphere and slag conditions as you desire."
Another type of electric furnace is shown in Figs. 7 and 8. This is the Ludlum furnace, the illustrations showing a 10-ton size. Figure 7 shows it in normal, or melting position, while in Fig. 8 it is tilted for pouring. In melting, the electrodes first rest on the charge of material in the furnace. After the current is turned on they eat their way through, nearly to the bottom. By this time there is a pool of molten metal beneath the electrode and the charge is melted from the bottom up so that the roof is not exposed to the high temperature radiating from the open arc. The electrodes in this furnace are of graphite, 9 in. in diameter and the current consumed is about 500 kw.-hr. per ton.
One of the things which sometimes confuse regarding the contents of steel is the fact that the percentage of carbon and the other alloys are usually designated in different ways. Carbon is usually designated by "points" and the other alloys by percentages. The point is one ten-thousandth while 1 per cent is one one-hundredth of the whole. In other words, "one hundred point carbon" is steel containing 1 per cent carbon. Twenty point carbon, such as is used for carbonizing purposes is 0.20 per cent. Tool steel varies from one hundred to one hundred and fifty points carbon, or from 1.00 to 1.50 per cent.
Nickel, chromium, etc., are always given in per cent, as a 3.5 per cent nickel, which means exactly what it says—3-1/2 parts in 100. Bearing this difference in mind all confusion will be avoided.
CLASSIFICATIONS OF STEEL
Among makers and sellers, carbon tool-steels are classed by "grade" and "temper." The word grade is qualified by many adjectives of more or less cryptic meaning, but in general they aim to denote the process and care with which the steel is made.
Temper of a steel refers to the carbon content. This should preferably be noted by "points," as just explained; but unfortunately, a 53-point steel (containing 0.53 per cent carbon) may locally be called something like "No. 3 temper."
A widely used method of classifying steels was originated by the Society of Automotive Engineers. Each specification is represented by a number of 4 digits, the first figure indicating the class, the second figure the approximate percentage of predominant alloying element, and the last two the average carbon content in points. Plain carbon steels are class 1, nickel steels are class 2, nickel-chromium steels are class 3, chromium steels are class 5, chromium-vanadium steels are class 6, and silico-manganese steels are class 9. Thus by this system, steel 2340 would be a 3 per cent nickel steel with 0.40 per cent carbon; or steel 1025 would be a 0.25 plain carbon steel.
Steel makers have no uniform classification for the various kinds of steel or steels used for different purposes. The following list shows the names used by some of the well-known makers:
Air-hardening steel Chrome-vanadium steel Alloy steel Circular saw plates Automobile steel Coal auger steel Awl steel Coal mining pick or cutter steel Axe and hatchet steel Coal wedge steel Band knife steel Cone steel Band saw steel Crucible cast steel Butcher saw steel Crucible machinery steel Chisel steel Cutlery steel Chrome-nickel steel Drawing die steel (Wortle)
Drill rod steel Patent, bush or hammer steel Facing and welding steel Pick steel Fork steel Pivot steel Gin saw steel Plane bit steel Granite wedge steel Quarry steel Gun barrel steel Razor steel Hack saw steel Roll turning steel High-speed tool steel Saw steel Hot-rolled sheet steel Scythe steel Lathe spindle steel Shear knife steel Lawn mower knife steel Silico-manganese steel Machine knife steel Spindle steel Magnet steel Spring steel Mining drill steel Tool holder steel Nail die shapes Vanadium tool steel Nickel-chrome steel Vanadium-chrome steel Paper knife steel Wortle steel
Passing to the tonnage specifications, the following table from Tiemann's excellent pocket book on "Iron and Steel," will give an approximate idea of the ordinary designations now in use:
Approximate Grades carbon range Common uses
Extra soft 0.08-0.18 Pipe, chain and other welding purposes; (dead soft) case-hardening purposes; rivets; pressing and stamping purposes. Structural (soft) 0.15-0.25 Structural plates, shapes and bars for (medium) bridges, buildings, cars, locomotives; boiler (flange) steel; drop forgings; bolts. Medium 0.20-0.35 Structural purposes (ships); shafting; automobile parts; drop forgings. Medium hard 0.35-0.60 Locomotive and similar large forgings; car axles; rails. Hard 0.60-0.85 Wrought steel wheels for steam and electric railway service; locomotive tires; rails; tools, such as sledges, hammers, pick points, crowbars, etc. Spring 0.85-1.05 Automobile and other vehicle springs; tools, such as hot and cold chisels, rock drills and shear blades. Spring 0.90-1.15 Railway springs; general machine shop tools.
COMPOSITION AND PROPERTIES OF STEEL
It is a remarkable fact that one can look through a dozen text books on metallurgy and not find a definition of the word "steel." Some of them describe the properties of many other irons and then allow you to guess that everything else is steel. If it was difficult a hundred years ago to give a good definition of the term when the metal was made by only one or two processes, it is doubly difficult now, since the introduction of so many new operations and furnaces.
We are in better shape to know what steel is than our forefathers. They went through certain operations and they got a soft malleable, weldable metal which would not harden; this they called iron. Certain other operations gave them something which looked very much like iron, but which would harden after quenching from a red heat. This was steel. Not knowing the essential difference between the two, they must distinguish by the process of manufacture. To-day we can make either variety by several methods, and can convert either into the other at will, back and forth as often as we wish; so we are able to distinguish between the two more logically.
We know that iron is a chemical element—the chemists write it Fe for short, after the Latin word "ferrum," meaning iron—it is one of those substances which cannot be separated into anything else but itself. It can be made to join with other elements; for instance, it joins with the oxygen in the air and forms scale or rust, substances known to the chemist as iron oxide. But the same metal iron can be recovered from that rust by abstracting the oxygen; having recovered the iron nothing else can be extracted but iron; iron is elemental.
We can get relatively pure iron from various minerals and artificial substances, and when we get it we always have a magnetic metal, almost infusible, ductile, fairly strong, tough, something which can be hardened slightly by hammering but which cannot be hardened by quenching. It has certain chemical properties, which need not be described, which allow a skilled chemist to distinguish it without difficulty and unerringly from the other known elements—nearly 100 of them.
Carbon is another chemical element, written C for short, which is widely distributed through nature. Carbon also readily combines with oxygen and other chemical elements, so that it is rarely found pure; its most familiar form is soot, although the rarer graphite and most rare diamond are also forms of quite pure carbon. It can also be readily separated from its multitude of compounds (vegetation, coal, limestone, petroleum) by the chemist.
With the rise of knowledge of scientific chemistry, it was quickly found that the essential difference between iron and steel was that the latter was iron plus carbon. Consequently it is an alloy, and the definition which modern metallurgists accept is this:
"Steel is an iron-carbon alloy containing less than about 2 per cent carbon."
Of course there are other elements contained in commercial steel, and these elements are especially important in modern "alloy steels," but carbon is the element which changes a soft metal into one which may be hardened, and strengthened by quenching. In fact, carbon, of itself, without heat treatment, strengthens iron at the expense of ductility (as noted by the percentage elongation an 8-in. bar will stretch before breaking). This is shown by the following table:
Elastic Ultimate Percentage. Class by use. Class by Per cent limit strength elongation hardness. carbon. lb. per lb. per in 8 inches. sq. in. sq. in. - Boiler rivet steel Dead soft 0.08 to 0.15 25,000 50,000 30 Struc. rivet steel Soft 0.15 to 0.22 30,000 55,000 30 Boiler plate steel Soft 0.08 to 0.10 30,000 60,000 25 Structural steel Medium 0.18 to 0.30 35,000 65,000 25 Machinery steel Hard 0.35 to 0.60 40,000 75,000 20 Rail steel Hard 0.35 to 0.55 40,000 75,000 15 Spring steel High carbon 1.00 to 1.50 60,000 125,000 10 Tool steel High carbon 0.90 to 1.50 80,000 150,000 5
Just why a soft material like carbon (graphite), when added to another soft material like iron, should make the iron harder, has been quite a mystery, and one which has caused a tremendous amount of study. The mutual interactions of these two elements in various proportions and at various temperatures will be discussed at greater length later, especially in Chap. VIII, p. 105. But we may anticipate by saying that some of the iron unites with all the carbon to form a new substance, very hard, a carbide which has been called "cementite." The compound always contains iron and carbon in the proportions of three atoms of iron to one atom of carbon; chemists note this fact in shorthand by the symbol Fe3C (a definite chemical compound of three atoms of iron to one of carbon). Many of the properties of steel, as they vary with carbon content, can be linked up with the increasing amount of this hard carbide cementite, distributed in very fine particles through the softer iron.
SULPHUR is another element (symbol S) which is always found in steel in small quantities. Some sulphur is contained in the ore from which the iron is smelted; more sulphur is introduced by the coke and fuel used. Sulphur is very difficult to get rid of in steel making; in fact the resulting metal usually contains a little more than the raw materials used. Only the electric furnace is able to produce the necessary heat and slags required to eliminate sulphur, and as a matter of fact the sulphur does not go until several other impurities have been eliminated. Consequently, an electric steel with extremely low sulphur (0.02 per cent) is by that same token a well-made metal.
Sulphur is of most trouble to rolling and forging operations when conducted at a red heat. It makes steel tender and brittle at that temperature—a condition known to the workmen as "red-short." It seems to have little or no effect upon the physical properties of cold steel—at least as revealed by the ordinary testing machines—consequently many specifications do not set any limit on sulphur, resting on the idea that if sulphur is low enough not to cause trouble to the manufacturer during rolling, it will not cause the user any trouble.
Tool steel and other fine steels should be very low in sulphur, preferably not higher than 0.03 per cent. Higher sulphur steels (0.06 per cent, and even up to 0.10 per cent) have given very good service for machine parts, but in general a high sulphur steel is a suspicious steel. Screw stock is purposely made with up to 0.12 per cent sulphur and a like amount of phosphorus so it will cut freely.
Manganese counteracts the detrimental effect of sulphur when present in the steel to an amount at least five times the sulphur content.
PHOSPHORUS is an element (symbol P) which enters the metal from the ore. It remains in the steel when made by the so-called acid process, but it can be easily eliminated down to 0.06 per cent in the basic process. In fact the discovery of the basic process was necessary before the huge iron deposits of Belgium and the Franco-German border could be used. These ores contain several per cent phosphorus, and made a very brittle steel ("cold short") until basic furnaces were used. Basic furnaces allow the formation of a slag high in lime, which takes practically all the phosphorus out of the metal. Not only is the resulting metal usable, but the slag makes a very excellent fertilizer, and is in good demand.
SILICON is a very widespread element (symbol Si), being an essential constituent of nearly all the rocks of the earth. It is similar to carbon in many of its chemical properties; for instance it burns very readily in oxygen, and consequently native silicon is unknown—it is always found in combination with one or more other elements. When it bums, each atom of silicon unites with two atoms of oxygen to form a compound known to chemists as silica (SiO2), and to the small boy as "sand" and "agate."
Iron ore (an oxide of iron) contains more or less sand and dirt mixed in it when it is mined, and not only the iron oxide but also some of the silicon oxide is robbed of its oxygen by the smelting process. Pig iron—the product of the blast furnace—therefore contains from 1 to 3 per cent of silicon, and some silicon remains in the metal after it has been purified and converted into steel.
However, silicon, as noted above, burns very readily in oxygen, and this property is of good use in steel making. At the end of the steel-making process the metal contains more or less oxygen, which must be removed. This is sometimes done (especially in the so-called acid process) by adding a small amount of silicon to the hot metal just before it leaves the furnace, and stirring it in. It thereupon abstracts oxygen from the metal wherever it finds it, changing to silica (SiO2) which rises and floats on the surface of the cleaned metal. Most of the silicon remaining in the metal is an excess over that which is required to remove the dangerous oxygen, and the final analysis of many steels show enough silicon (from 0.20 to 0.40) to make sure that this step in the manufacture has been properly done.
MANGANESE is a metal much like iron. Its chemical symbol is Mn. It is somewhat more active than iron in many chemical changes—notably it has what is apparently a stronger attraction for oxygen and sulphur than has iron. Therefore the metal is used (especially in the so-called basic process) to free the molten steel of oxygen, acting in a manner similar to silicon, as explained above. The compound of manganese and oxygen is readily eliminated from the metal. Sufficient excess of elemental manganese should remain so that the purchaser may be sure that the iron has been properly "deoxidized," and to render harmless the traces of sulphur present. No damage is done by the presence of a little manganese in steel, quite the reverse. Consequently it is common to find steels containing from 0.3 to 1.5 per cent.
ALLOYING ELEMENTS.—Commercial steels of even the simplest types are therefore primarily alloys of iron and carbon. Impurities and their "remedies" are always present: sulphur, phosphorus, silicon and manganese—to say nothing of oxygen, nitrogen and carbon oxide gases, about which we know very little. It has been found that other metals, if added to well-made steel, produce definite improvements in certain directions, and these "alloy steels" have found much use in the last ten years. Alloy steels, in addition to the above-mentioned elements, may commonly contain one or more of the following, in varying amounts: Nickel (Ni), Chromium (Cr), Vanadium (Va), Tungsten (W), Molybdenum (Mo). These steels will be discussed at more length in Chapters III and IV.
PROPERTIES OF STEEL
Steels are known by certain tests. Early tests were more or less crude, and depended upon the ability of the workman to judge the "grain" exhibited by a freshly broken piece of steel. The cold-bend test was also very useful—a small bar was bent flat upon itself, and the stretched fibers examined for any sign of break. Harder stiff steels were supported at the ends and the amount of central load they would support before fracture, or the amount of permanent set they would acquire at a given load noted. Files were also used to test the hardness of very hard steel.
These tests are still used to a considerable extent, especially in works where the progress of an operation can be kept under close watch in this way, the product being periodically examined by more precise methods. The chief furnace-man, or "melter," in a steel plant, judges the course of the refining process by casting small test ingots from time to time, breaking them and examining the fracture. Cutlery manufacturers use the bend test to judge the temper of blades. File testing of case-hardened parts is very common.
However there is need of standardized methods which depend less upon the individual skill of the operator, and which will yield results comparable to others made by different men at different places and on different steels. Hence has grown up the art of testing materials.
Strength of a metal is usually expressed in the number of pounds a 1-in. bar will support just before breaking, a term called the "ultimate strength." It has been found that the shape of the test bar and its method of loading has some effect upon the results, so it is now usual to turn a rod 5-1/2 in. long down to 0.505 in. in diameter for a central length of 2-3/8 in., ending the turn with 1/2-in. fillets. The area of the bar equals 0.2 sq. in., so the load it bears at rupture multiplied by 5 will represent the "ultimate strength" in pounds per square inch.
Such a test bar is stretched apart in a machine like that shown in Fig. 9. The upper end of the bar is held in wedged jaws by the top cross-head, and the lower end grasped by the movable head. The latter is moved up and down by three long screws, driven at the same speed, which pass through threads cut in the corners of the cross-head. When the test piece is fixed in position the motor which drives the machine is given a few turns, which by proper gearing pulls the cross-head down with a certain pull. This pull is transmitted to the upper cross-head by the test bar, and can be weighed on the scale arm, acting through a system of links and levers.
Thus the load may be increased as rapidly as desirable, always kept balanced by the weighing mechanism, and the load at fracture may be read directly from the scale beam.
This same test piece may give other information. If light punch marks are made, 2 in. apart, before the test is begun, the broken ends may be clamped together, and the distance between punch marks measured. If it now measures 3 in. the stretch has been 1 in. in 2, or 50 per cent. This figure is known as the elongation at fracture, or briefly, the "elongation," and is generally taken to be a measure of ductility.
When steel shows any elongation, it also contracts in area at the same time. Often this contraction is sharply localized at the fracture; the piece is said to "neck." A figure for contraction in area is also of much interest as an indication of toughness; the diameter at fracture is measured, a corresponding area taken out from a table of circles, subtracted from the original area (0.200 sq. in.) and the difference divided by 0.2 to get the percentage contraction.
Quite often it is desired to discover the elastic limit of the steel, in fact this is of more use to the designer than the ultimate strength. The elastic limit is usually very close to the load where the metal takes on a permanent set. That is to say, if a delicate caliper ("extensometer," so called) be fixed to the side of the test specimen, it would show the piece to be somewhat longer under load than when free. Furthermore, if the load had not yet reached the yield point, and were released at any time, the piece would return to its original length. However, if the load had been excessive, and then relieved, the extensometer would no longer read exactly 2.0 in., but something more.
Soft steels "give" very quickly at the yield point. In fact, if the testing machine is running slowly, it takes some time for the lower head to catch up with the stretching steel. Consequently at the yield point, the top head is suddenly but only temporarily relieved of load, and the scale beam drops. In commercial practice, the yield point is therefore determined by the "drop of the beam." For more precise work the calipers are read at intervals of 500 or 1,000 lb. load, and a curve plotted from these results, a curve which runs straight up to the elastic limit, but there bends off.
A tensile test therefore gives four properties of great usefulness: The yield point, the ultimate strength, the elongation and the contraction. Compression tests are seldom made, since the action of metal in compression and in tension is closely allied, and the designer is usually satisfied with the latter.
Impact tests are of considerable importance as an indication of how a metal will perform under shock. Some engineers think that the tensile test, which is one made under slow loading, should therefore be supplemented by another showing what will happen if the load is applied almost instantaneously. This test, however, has not been standardized, and depends to a considerable extent upon the type of machine, but more especially the size of the specimen and the way it is "nicked." The machine is generally a swinging heavy pendulum. It falls a certain height, strikes the sample at the lowest point, and swings on past. The difference between the downward and upward swing is a measure of the energy it took to break the test piece.
It has been known for fifty years that a beam or rod would fail at a relatively low stress if only repeated often enough. It has been found, however, that each material possesses a limiting stress, or endurance limit, within which it is safe, no matter how often the loading occurs. That limiting stress for all steels so far investigated causes fracture below 10 million reversals. In other words, a steel which will not break before 10,000,000 reversals can confidently be expected to endure 100,000,000, and doubtless into the billions.
About the only way to test one piece such a large number of times is to fashion it into a beam, load it, and then turn the beam in its supports. Thus the stress in the outer fibers of the bar varies from a maximum stretch through zero to a maximum compression, and back again. A simple machine of this sort is shown in Fig. 10, where B and E are bearings, A the test piece, turned slightly down in the center, C and D ball bearings supporting a load W. K is a pulley for driving the machine and N is a counter.
The word "hardness" is used to express various properties of metals, and is measured in as many different ways.
"Scratch hardness" is used by the geologist, who has constructed "Moh's scale" as follows:
Talc has a hardness of 1 Rock Salt has a hardness of 2 Calcite has a hardness of 3 Fluorite has a hardness of 4 Apatite has a hardness of 5 Feldspar has a hardness of 6 Quartz has a hardness of 7 Topaz has a hardness of 8 Corundum has a hardness of 9 Diamond has a hardness of 10
A mineral will scratch all those above it in the series, and will be scratched by those below. A weighted diamond cone drawn slowly over a surface will leave a path the width of which (measured by a microscope) varies inversely as the scratch hardness.
"Cutting hardness" is measured by a standardized drilling machine, and has a limited application in machine-shop practice.
"Rebounding hardness" is commonly measured by the Shore scleroscope, illustrated in Fig. 11. A small steel hammer, 1/4 in. in diameter, 3/4 in. in length, and weighing about 1/12 oz. is dropped a distance of 10 in. upon the test piece. The height of rebound in arbitrary units represents the hardness numeral.
Should the hammer have a hard flat surface and drop on steel so hard that no impression were made, it would rebound about 90 per cent of the fall. The point, however, consists of a slightly spherical, blunt diamond nose 0.02 in. in diameter, which will indent the steel to a certain extent. The work required to make the indentation is taken from the energy of the falling body; the rebound will absorb the balance, and the hammer will now rise from the same steel a distance equal to about 75 per cent of the fall. A permanent impression is left upon the test piece because the impact will develop a force of several hundred thousand pounds per square inch under the tiny diamond-pointed hammer head, stressing the test piece at this point of contact much beyond its ultimate strength. The rebound is thus dependent upon the indentation hardness, for the reason that the less the indentation, the more energy will reappear in the rebound; also, the less the indentation, the harder the material. Consequently, the harder the material, the more the rebound.
"Indentation hardness" is a measure of a material's resistance to penetration and deformation. The standard testing machine is the Brinell, Fig. 12. A hardened steel ball, 10 mm. in diameter, is forced into the test piece with a pressure of 3,000 kg. (3-1/3 tons). The resulting indentation is then measured.
While under load, the steel ball in a Brinell machine naturally flattens somewhat. The indentation left behind in the test piece is a duplicate of the surface which made it, and is usually regarded as being the segment of a sphere of somewhat larger radius than the ball. The radius of curvature of this spherical indentation will vary slightly with the load and the depth of indentation. The Brinell hardness numeral is the quotient found by dividing the test pressure in kilograms by the spherical area of the indentation. The denominator, as before, will vary according to the size of the sphere, the hardness of the sphere and the load. These items have been standardized, and the following table has been constructed so that if the diameter of the identation produced by a load of 3,000 kg. be measured the hardness numeral is found directly.
TABLE FOR BRINELL BALL TEST Diameter of Ball Hardness Number Diameter of Ball Hardness Number Impression, mm. for a Load of Impression, mm. for a Load of 3,000 kg. 3,000 kg. - - - 2.0 946 4.5 179 2.1 857 4.6 170 2.2 782 4 7 163 2.3 713 4.8 156 2.4 652 4.9 149 2.5 600 5.0 143 2.6 555 5.1 137 2.7 512 5.2 131 2.8 477 5.3 126 2.9 444 5.4 121 3.0 418 5.5 116 3.1 387 5.6 112 3.2 364 5.7 107 3.3 340 5.8 103 3.4 321 5.9 99 3.5 302 6.0 95 3.6 286 6.1 92 3.7 269 6.2 89 3.8 255 6.3 86 3.9 241 6.4 83 4.0 228 6.5 80 4.1 217 6.6 77 4.2 207 6.7 74 4.3 196 6.8 71.5 4.4 187 6.9 69
ALLOYS AND THEIR EFFECT UPON STEEL
In view of the fact that alloy steels are coming into a great deal of prominence, it would be well for the users of these steels to fully appreciate the effects of the alloys upon the various grades of steel. We have endeavored to summarize the effect of these alloys so that the users can appreciate their effect, without having to study a metallurgical treatise and then, perhaps, not get the crux of the matter.
Nickel may be considered as the toughest among the non-rare alloys now used in steel manufacture. Originally nickel was added to give increased strength and toughness over that obtained with the ordinary rolled structural steel and little attempt was made to utilize its great possibilities so far as heat treatment was concerned.
The difficulties experienced have been a tendency towards laminated structure during manufacture and great liability to seam, both arising from improper melting practice. When extra care is exercised in the manufacture, particularly in the melting and rolling, many of these difficulties can be overcome.
The electric steel furnace, of modern construction, is a very important step forward in the melting of nickel steel; neither the crucible process nor basic or acid open-hearth furnaces give such good results.
Great care must be exercised in reheating the billet for rolling so that the steel is correctly soaked. The rolling must not be forced; too big reduction per pass should not be indulged in, as this sets up a tendency towards seams.
Nickel steel has remarkably good mechanical qualities when suitably heat-treated, and it is preeminently adapted for case-hardening. It is not difficult to machine low-nickel steel, consequently it is in great favor where easy machining properties are of importance.
Nickel influences the strength and ductility of steel by being dissolved directly in the iron or ferrite; in this respect differing from chromium, tungsten and vanadium. The addition of each 1 per cent nickel up to 5 per cent will cause an approximate increase of from 4,000 to 6,000 lb. per square inch in the tensile strength and elastic limit over the corresponding steel and without any decrease in ductility. The static strength of nickel steel is affected to some degree by the percentage of carbon; for instance, steel with 0.25 per cent carbon and 3.5 per cent nickel has a tensile strength, in its normal state, equal to a straight carbon steel of 0.5 per cent with a proportionately greater elastic limit and retaining all the advantages of the ductility of the lower carbon.
To bring out the full qualities of nickel it must be heat-treated, otherwise there is no object in using nickel as an alloy with carbon steel as the additional cost is not justified by increased strength.
Nickel has a peculiar effect upon the critical ranges of steel, the critical range being lowered by the percentage of nickel; in this respect it is similar to manganese.
Nickel can be alloyed with steel in various percentages, each percentage having a very definite effect on the microstructure. For instance, a steel with 0.2 per cent carbon and 2 per cent nickel has a pearlitic structure but the grain is much finer than if the straight carbon were used. With the same carbon content and say 5 per cent nickel, the structure would still be pearlitic, but much finer and denser, therefore capable of withstanding shock, and having greater dynamic strength. With about 0.2 per cent carbon and 8 per cent nickel, the steel is nearing the stage between pearlite and martensite, and the structure is extremely fine, the ferrite and pearlite having a very pronounced tendency to mimic a purely martensite structure. Steel with 0.2 per cent carbon and 15 per cent nickel is entirely martensite. Higher percentages of nickel change the martensitic structure to austenite, the steel then being non-magnetic. The higher percentages, that is 30 to 35 per cent nickel, are used for valve seats, valve heads, and valve stems, as the alloy is a poor conductor of heat and is particularly free from any tendency towards corrosion or pitting from the action of waste gases of the internal-combustion engine.
Nickel steels having 3-1/2 per cent nickel and 0.15 to 0.20 per cent carbon are excellent for case-hardening purposes, giving hard surfaces and tough interiors.
To obtain the full effect of nickel as an alloy, it is essential that the correct percentage of carbon be used. High nickel and low carbon will not be more efficient than lower nickel and higher carbon, but the cost will be much greater. Generally speaking, heat-treated nickel alloy steels are about two to three times stronger than the same steel annealed. This point is very important as many instances have been found where nickel steel is incorrectly used, being employed when in the annealed or normal state.
Chromium when alloyed with steel, has the characteristic function of opposing the disintegration and reconstruction of cementite. This is demonstrated by the changes in the critical ranges of this alloy steel taking place slowly; in other words, it has a tendency to raise the Ac range (decalescent points) and lower the Ar range (recalescent points). Chromium steels are therefore capable of great hardness, due to the rapid cooling being able to retard the decomposition of the austenite.
The great hardness of chromium steels is also due to the formation of double carbides of chromium and iron. This condition is not removed when the steel is slightly tempered or drawn. This additional hardness is also obtained without causing undue brittleness such as would be obtained by any increase of carbon. The degree of hardness of the lower-chrome steels is dependent upon the carbon content, as chromium alone will not harden iron.
The toughness so noticeable in this steel is the result of the fineness of structure; in this instance, the action is similar to that of nickel, and the tensile strength and elastic limit is therefore increased without any loss of ductility. We then have the desirable condition of tough hardness, making chrome steels extremely valuable for all purposes requiring great resistance to wear, and in higher-chrome contents resistance to corrosion. All chromium-alloy steels offer great resistance to corrosion and erosion. In view of this, it is surprising that chromium steels are not more largely used for structural steel work and for all purposes where the steel has to withstand the corroding action of air and liquids. Bridges, ships, steel building, etc., would offer greater resistance to deterioration through rust if the chromium-alloy steels were employed.
Prolonged heating and high temperatures have a very bad effect upon chromium steels. In this respect they differ from nickel steels, which are not so affected by prolonged heating, but chromium steels will stand higher temperatures than nickel steels when the period is short.
Chromium steels, due to their admirable property of increased hardness, without the loss of ductility, make very excellent chisels and impact tools of all types, although for die blocks they do not give such good results as can be obtained from other alloy combinations.
For ball bearing steels, where intense hardness with great toughness and ready recovery from temporary deflection is required, chromium as an alloy offers the best solution.
Two per cent chromium steels; due to their very hard tough surface, are largely used for armor-piercing projectiles, cold rolls, crushers, drawing dies, etc.
The normal structure of chromium steels, with a very low carbon content is roughly pearlitic up to 7 per cent, and martensitic from 8 to 20 per cent; therefore, the greatest application is in the pearlitic zone or the lower percentages.
A combination of the characteristics of nickel and the characteristics of chromium, as described, should obviously give a very excellent steel as the nickel particularly affects the ferrite of the steel and the chromium the carbon. From this combination, we are able to get a very strong ferrite matrix and a very hard tough cementite. The strength of a strictly pearlitic steel over a pure iron is due to the pearlitic being a layer arrangement of cementite running parallel to that of a pure iron layer in each individual grain. The ferrite i.e., the iron is increased in strength by the resistance offered by the cementite which is the simple iron-carbon combination known to metallurgists as Fe3C. The cementite, although adding to the tensile strength, is very brittle and the strength of the pearlite is the combination of the ferrite and cementite. In the event of the cementite being strengthened, as in the case of strictly chromium steels, an increased tensile strength is readily obtained without loss of ductility and if the ferrite is strengthened then the tensile strength and ductility of the metal is still further improved.
Nickel-chromium alloy represents one of the best combinations available at the present time. The nickel intensifies the physical characteristics of the chromium and the chromium has a similar effect on the nickel.
For case-hardening, nickel-chromium steels seem to give very excellent results. The carbon is very rapidly taken up in this combination, and for that reason is rather preferable to the straight nickel steel.
With the mutually intensifying action of chromium and nickel there is a most suitable ratio for these two alloys, and it has been found that roughly 2-1/2 parts of nickel to about 1 part of chromium gives the best results. Therefore, we have the standard types of 3.5 per cent nickel with 1.5 per cent chromium to 1.5 per cent nickel with 0.6 per cent chromium and the various intermediate types. This ratio, however, does not give the whole story of nickel-chromium combinations, and many surprising results have been obtained with these alloys when other percentage combinations have been employed.
Vanadium has a very marked effect upon alloy steels rich in chromium, carbon, or manganese. Vanadium itself, when combined with steel very low in carbon, is not so noticeably beneficial as in the same carbon steel higher in manganese, but if a small quantity of chromium is added, then the vanadium has a very marked effect in increasing the impact strength of the alloy. It would seem that vanadium has the effect of intensifying the action of chromium and manganese, or that vanadium is intensified by the action of chromium or manganese.
Vanadium has the peculiar property of readily entering into solution with ferrite. If vanadium contained is considerable it also combines with the carbon, forming carbides. The ductility of carbon-vanadium steels is therefore increased, likewise the ductility of chrome-vanadium steels.
The full effect of vanadium is not felt unless the temperatures to which the steel is heated for hardening are raised considerably. It is therefore necessary that a certain amount of "soaking" takes place, so as to get the necessary equalization. This is true of all alloys which contain complex carbides, i.e., compounds of carbon, iron and one or more elements.
Chrome-vanadium steels also are highly favored for case hardening. When used under alternating stresses it appears to have superior endurance. It would appear that the intensification of the properties due to chromium and manganese in the alloy steel accounts for this peculiar phenomenon.
Vanadium is also a very excellent scavenger for either removing the harmful gases, or causing them to enter into solution with the metal in such a way as to largely obviate their harmful effects. Chrome-vanadium steels have been claimed, by many steel manufacturers and users, to be preferable to nickel-chrome steels. While not wishing to pass judgment on this, it should be borne in mind that the chrome-vanadium steel, which is tested, is generally compared with a very low nickel-chromium alloy steel (the price factor entering into the situation), but equally good results can be obtained by nickel-chromium steels of suitable analysis.
Where price is the leading factor, there are many cases where a stronger steel can be obtained from the chrome and vanadium than the nickel-chrome. It will be safe to say that each of these two systems of alloys have their own particular fields and chrome-vanadium steel should not be regarded as the sole solution for all problems, neither should nickel-chromium.
Manganese adds considerably to the tensile strength of steel, but this is dependent on the carbon content. High carbon materially adds to the brittleness, whereas low-carbon, pearlitic-manganese steels are very tough and ductile and are not at all brittle, providing the heat-treating is correct. Manganese steel is very susceptible to high temperatures and prolonged heating.
In low-carbon pearlitic steels, manganese is more effective in increasing ultimate strength than is nickel; that is to say, a 0.45 carbon steel with 1.25 per cent manganese is as strong as a 0.45 carbon steel with 1.5 per cent nickel. The former steel is much used for rifle barrels, and in the heat-treated condition will give 80,000 to 90,000 lb. per square inch elastic limit, 115,000 to 125,000 lb. per square inch tensile strength, 23 per cent elongation, and 55 per cent reduction in area.
Manganese when added to steel has the effect of lowering the critical range; 1 per cent manganese will lower the upper critical point 60 deg.F. The action of manganese is very similar to that of nickel in this respect, only twice as powerful. As an instance, 1 per cent nickel would have the effect of lowering the upper critical range from 25 to 30 deg.F.
Low-carbon pearlitic-manganese steel, heat-treated, will give dynamic strength which cannot be equaled by low-priced and necessarily low-content nickel steels. In many instances, it is preferable to use high-grade manganese steel, rather than low-content nickel steel.
High-manganese steels or austenite manganese steels are used for a variety of purposes where great resistance to abrasion is required, the percentage of manganese being from 11 to 14 per cent, and carbon 1 to 1.5 per cent. This steel is practically valueless unless heat-treated; that is, heated to about yellow red and quenched in ice water. The structure is then austenite and the air-cooled structure of this steel is martensite. Therefore this steel has to be heated and very rapidly cooled to obtain the ductile austenite structure.
Manganese between 2 and 7 per cent is a very brittle material when the carbon is about 1 per cent or higher and is, therefore, quite valueless. Below 2 per cent manganese steel low in carbon is very ductile and tough steel.
The high-content manganese steels are known as the "Hadfield manganese steels," having been developed by Sir Robert Hadfield. Small additions of chrome up to 1 per cent increase the elastic limit of low-carbon pearlitic-manganese steels without affecting the steel in its resistance to shock, but materially decrease the percentage of elongation.
Vanadium added to low-carbon pearlitic manganese steel has a very marked effect, increasing greatly the dynamic strength and changing slightly the susceptibility of this steel to heat treatments, giving a greater margin for the hardening temperature. Manganese steel with added vanadium is most efficient when heat-treated.
Tungsten, as an alloy in steel, has been known and used for a long time. The celebrated and ancient damascus steel being a form of tungsten-alloy steel. Tungsten and its effects, however, did not become generally realized until Robert Mushet experimented and developed his famous mushet steel and the many improvement made since that date go to prove how little Mushet himself understood the peculiar effects of tungsten as an alloy.
Tungsten acts on steel in a similar manner to carbon, that is, it increases its hardness, but is much less effective than carbon in this respect. If the percentage of tungsten and manganese is high, the steel will be hard after cooling in the air. This is impossible in a carbon steel. It was this combination that Mushet used in his well-known "air-hardening" steel.
The principal use of tungsten is in high-speed tool steel, but here a high percentage of manganese is distinctly detrimental, making the steel liable to fire crack, very brittle and weak in the body, less easily forged and annealed. Manganese should be kept low and a high percentage of chromium used instead.
Tools of tungsten-chromium steels, when hardened, retain their hardness, even when heated to a dark cherry red by the friction of the cutting or the heat arising from the chips. This characteristic led to the term "red-hardness," and it is this property that has made possible the use of very high cutting speeds in tools made of the tungsten-chromium alloy, that is, "high-speed" steel.
Tungsten steels containing up to 6 per cent do not have the property of red hardness any more than does carbon tool steel, providing the manganese or chromium is low.
When chromium is alloyed with tungsten, a very definite red-hardness is noticed with a great increase of cutting efficiency. The maximum red-hardness seems to be had with steels containing 18 per cent tungsten, 5.5 per cent chromium and 0.70 per cent carbon.
Very little is known of the actual function of tungsten, although a vast amount of experimental work has been done. It is possible that when the effect of tungsten with iron-carbon alloys is better known, a greater improvement can be expected from these steels. Tungsten has been tried and is still used by some steel manufacturers for making punches, chisels, and other impact tools. It has also been used for springs, and has given very good results, although other less expensive alloys give equally good results, and are in some instances, better.
Tungsten is largely used in permanent magnets. In this, its action is not well understood. In fact, the reason why steel becomes a permanent magnet is not at all understood. Theories have been evolved, but all are open to serious questioning. The principal effect of tungsten, as conceded by leading authorities, is that it distinctly retards separation of the iron-carbon solution, removing the lowest recalescent point down to atmospheric temperature.
A peculiar property of tungsten steels is that if a heating temperature of 1,750 deg.F. is not exceeded, the cooling curves indicate but one critical point at about 1,350 deg.F. But when the heating temperature is raised above 1,850 deg.F., this critical point is nearly if not quite suppressed, while a lower critical point appears and grows enormously in intensity at a temperature between 660 and 750 deg.F.
The change in the critical ranges, which is produced by heating tungsten steels to over 1,850 deg.F., is the real cause of the red-hard properties of these alloys. Its real nature is not understood, and there is no direct evidence to show what actually happens at these high temperatures.
It may readily be understood that an alloy containing four essential elements, namely: iron, carbon, tungsten and chromium, is one whose study presents problems of extreme complexity. It is possible that complex carbides may be formed, as in chromium steels, and that compounds between iron and tungsten exist. Behavior of these combinations on heating and cooling must be better known before we are able to explain many peculiarities of tungsten steels.
Molybdenum steels have been made commercially for twenty-five years, but they have not been widely exploited until since the war. Very large resources of molybdenum have been developed in America, and the mining companies who are equipped to produce the metal are very active in advertising the advantages of molybdenum steels.
It was early found that 1 part molybdenum was the equivalent of from 2 to 2-1/2 parts of tungsten in tool steels, and magnet steels. It fell into disrepute as an alloy for high-speed tool steel, however, because it was found that the molybdenum was driven out of the surface of the tool during forging and heat treating.
Within the last few years it has been found that the presence of less than 1 per cent of molybdenum greatly enhances certain properties of heat-treated carbon and alloy steels used for automobiles and high-grade machinery.
In general, molybdenum when added to an alloy steel, increases the figure for reduction of area, which is considered a good measure of "toughness." Molybdenum steels are also relatively insensible to variations in heat treatment; that is to say, a chromium-nickel-molybdenum steel after quenching in oil from 1,450 deg.F. may be drawn at any temperature between 900 and 1,100 deg.F. with substantially the same result (static tensile properties and hardness).
Silicon prevents, to a large extent, defects such as gas bubbles or blow holes forming while steel is solidifying. In fact, steel after it has been melted and before it has been refined, is "wild" and "gassy." That is to say, if it would be cast into molds it would froth up, and boil all over the floor. A judicious amount of silicon added to the metal just before pouring, prevents this action—in the words of the steel maker, silicon "kills" the steel. If about 1.75 per cent metallic silicon remains in a 0.65 carbon steel, it makes excellent springs.
Phosphorus is one of the impurities in steel, and it has been the object of steel makers for years to eliminate it. On cheap grades of steel, not subject to any abnormal strain or stress, 0.1 per cent phosphorus is not objectionable. High phosphorus makes steel "cold short," i.e., brittle when cold or moderately warm.
Sulphur is another impurity and high sulphur is even a greater detriment to steel than phosphorus. High sulphur up to 0.09 per cent helps machining properties, but has a tendency to make the steel "hot short," i.e., subject to opening up cracks and seams at forging or rolling heats. Sulphur should never exceed 0.06 per cent nor phosphorus 0.08 per cent.
Steel used for tool purposes should have as low phosphorus and sulphur contents as possible, not over 0.02 per cent.
We can sum up the various factors something as follows for ready reference.
The ingredient Its effect
Iron The basis of steel Carbon The determinative Sulphur A strength sapper Phosphorus The weak link Oxygen A strength destroyer Manganese For strength Nickel For strength and toughness Tungsten Hardener and heat resister Chromium For resisting shocks Vanadium Purifier and fatigue resister Silicon Impurity and hardener Titanium Removes nitrogen and oxygen Molybdenum Hardener and heat resister Aluminum Kills or deoxidizes steel
PROPERTIES OF ALLOY STEELS
The following table shows the percentages of carbon, manganese, nickel, chromium and vanadium in typical steel alloys for engineering purposes. It also gives the elastic limit, tensile strength, elongation and reduction of area of the various alloys, all being given the same heat treatment with a drawing temperature of 1,100 deg.F. (600 deg.C.). The specimens were one inch rounds machined after heat treatment.
Tungsten is not shown in the table because it is seldom used in engineering construction steels and then usually in combination with chromium. Tungsten is used principally for the magnets of magnetos, to some extent in the manufacture of hacksaws, and for special tool steels.
TABLE I. PROPERTIES OF ALLOY STEELS Manganese,/ Chromium,/ Elastic Tensile Elongation Reduction Carbon, per /Nickel, per /Vanadium, limit, Strength, in 2 in., of area, per cent per cent per cent lb. per lb. per per cent per cent cent cent sq. in. sq. in. - - - - - - 0.27 0.55 49,000 80,000 30 65 0.27 0.47 0.26 66,000 98,000 25 52 0.36 0.42 58,000 90,000 27 60 0.34 0.87 0.13 82,500 103,000 22 57 0.45 0.50 65,000 96,000 22 52 0.43 0.60 0.32 96,000 122,000 21 52 0.47 0.90 0.15 102,000 127,500 23 58 0.30 0.60 3.40 75,000 105,000 25 67 0.33 0.63 3.60 0.25 118,000 142,000 17 57 0.30 0.49 3.60 1.70 119,000 149,500 21 60 0.25 0.47 3.47 1.60 0.15 139,000 170,000 18 53 0.25 0.50 2.00 1.00 102,000 124,000 25 70 0.38 0.30 2.08 1.16 120,000 134,000 20 57 0.42 0.22 2.14 1.27 0.26 145,000 161,500 16 53 0.36 0.61 1.46 0.64 117,600 132,500 16 58 0.36 0.50 1.30 0.75 0.16 140,000 157,500 17 54 0.30 0.50 0.80 90,000 105,000 20 50 0.23 0.58 0.82 0.17 106,000 124,000 21 66 0.26 0.48 0.92 0.20 112,000 137,000 20 61 0.35 0.64 1.03 0.22 132,500 149,500 16 54 0.50 0.92 1.02 0.20 170,000 186,000 15 45
NON-SHRINKING, OIL-HARDENING STEELS
Certain steels have a very low rate of expansion and contraction in hardening and are very desirable for test plugs, gages, punches and dies, for milling cutters, taps, reamers, hard steel bushings and similar work.
It is recommended that for forging these steels it be heated slowly and uniformly to a bright red, but not in a direct flame or blast. Harden at a dull red heat, about 1,300 deg.F. A clean coal or coke fire, or a good muffle-gas furnace will give best results. Fish oil is good for quenching although in some cases warm water will give excellent results. The steel should be kept moving in the bath until perfectly cold. Heated and cooled in this way the steel is very tough, takes a good cutting edge and has very little expansion or contraction which makes it desirable for long taps where the accuracy of lead is important.
The composition of these steels is as follows:
Per cent Manganese 1.40 to 1.60 Carbon 0.80 to 0.90 Vanadium 0.20 to 0.25
EFFECT OF A SMALL AMOUNT OF COPPER IN MEDIUM-CARBON STEEL
This shows the result of tests by C. R. Hayward and A. B. Johnston on two types of steel: one containing 0.30 per cent carbon, 0.012 per cent phosphorus, and 0.860 per cent copper, and the other 0.365 per cent carbon, 0.053 per cent phosphorus, and 0.030 per cent copper. The accompanying chart in Fig. 13 shows that high-copper steel has decided superiority in tensile strength, yield point and ultimate strength, while the ductility is practically the same. Hardness tests by both methods show high-copper steel to be harder than low-copper, and the Charpy shock tests show high-copper steel also superior to low-copper. The tests confirm those made by Stead, showing that the behavior of copper steel resembles that of nickel steel. The high-copper steels show finer grain than the low-copper. The quenched and drawn specimens of high-copper steel were found to be slightly more martensitic.
HIGH-CHROMIUM OR RUST-PROOF STEEL
High-chromium, or what is called stainless steel containing from 11 to 14 per cent chromium, was originally developed for cutlery purposes, but has in the past few years been used to a considerable extent for exhaust valves in airplane engines because of its resistance to scaling at high temperatures.
Percentage Carbon 0.20 to 0.40 Manganese, not to exceed 0.50 Phosphorus, not to exceed 0.035 Sulphur, not to exceed 0.035 Chromium 11.50 to 14.00 Silicon, not to exceed 0.30
The steel should be heated slowly and forged at a temperature above 1,750 deg.F. preferably between 1,800 and 2,200 deg.F. If forged at temperatures between 1,650 and 1,750 deg.F. there is considerable danger of rupturing the steel because of its hardness at red heat. Owing to the air-hardening property of the steel, the drop-forgings should be trimmed while hot. Thin forgings should be reheated to redness before trimming, as otherwise they are liable to crack.
The forgings will be hard if they are allowed to cool in air. This hardness varies over a range of from 250 to 500 Brinell, depending on the original forging temperature.
ANNEALING can be done by heating to temperatures ranging from 1,290 to 1,380 deg.F. and cooling in air or quenching in water or oil. After this treatment the forgings will have a hardness of about 200 Brinell and a tensile strength of 100,000 to 112,000 lb. per square inch. If softer forgings are desired they can be heated to a temperature of from 1,560 to 1,650 deg.F. and cooled very slowly. Although softer the forgings will not machine as smoothly as when annealed at the lower temperature.
HARDENING.—The forgings can be hardened by cooling in still air or quenching in oil or water from a temperature between 1,650 and 1,750 deg.F.
The physical properties do not vary greatly when the carbon is within the range of composition given, or when the steel is hardened and tempered in air, oil, or water.
When used for valves the following specification of physical properties have been used:
Yield point, pounds per square inch 70,000 Tensile strength, pounds per square inch 90,000 Elongation in 2 in., per cent 18 Reduction of area, per cent 50
The usual heat treatment is to quench in oil from 1,650 deg.F. and temper or draw at 1,100 to 1,200 deg.F. One valve manufacturer stated that valves of this steel are hardened by heating the previously annealed valves to 1,650 deg.F. and cooling in still air. This treatment gives a scleroscope hardness of about 50.
In addition to use in valves this steel should prove very satisfactory for shafting for water-pumps and other automobile parts subject to objectionable corrosion.
TABLE 2. COMPARISON OF PHYSICAL PROPERTIES FOR HIGH-CHROMIUM STEELS OF DIFFERENT CARBON CONTENT C 0.20 C 0.27 C 0.50 Mn 0.45 Mn 0.50 Cr 12.56 Cr 12.24 Cr 14.84 - Quenched in oil from degrees Fahrenheit 1,600 1,600 1,650 Tempered at degrees Fahrenheit 1,160 1,080 1,100 Yield point, pounds per square inch 78,300 75,000 91,616 Tensile strength, pounds per square inch 104,600 104,250 123,648 Elongation in 2 in., per cent 25.0 23.5 14.5 Reduction of area, per cent 52.5 51.4 33.5
TABLE 3.—COMPARISON OF PHYSICAL PROPERTIES BETWEEN AIR, OIL AND WATER-HARDENED STEEL HAVING CHEMICAL ANALYSIS IN PERCENTAGE OF ————————————————————————————————————- Carbon 0.24 Manganese 0.30 Phosphorus 0.035 Sulphur 0.035 Chromium 12.85 Silicon 0.20
- Hardened Elastic Tensile Hardening from, Tempered limit, strength, Elongation Reduction medium degrees at, degrees per lb. lb. Per in 2 in. of area, Fahrenheit Fahrenheit sq. in. sq. in. per cent per cent - - - - - 930 158,815 192,415 13.0 40.5 1,100 99,680 120,065 21.0 59.2 Air 1,650 1,300 70,785 101,250 26.0 64.6 1,380 66,080 98,335 28.0 63.6 1,470 70,785 96,990 27.0 64.7 - - - - - 930 163,070 202,720 8.0 18.2 Oil 1,650 1,100 88,255 116,480 20.0 56.9 1,300 77,950 105,505 25.5 63.8 1,380 88,255 98,785 27.0 66.3 - - - - - 930 158,815 202,050 12.0 34.2 Water 1,650 1,100 90,270 120,735 22.0 59.8 1,300 66,080 102,590 25.8 64.8 1,380 67,200 97,890 27.0 65.2 -
This steel can be drawn into wire, rolled into sheets and strips and drawn into seamless tubes.
CORROSION.—This steel like any other steel when distorted by cold working is more sensitive to corrosion and will rust. Rough cut surfaces will rust. Surfaces finished with a fine cut are less liable to rust. Ground and polished surfaces are practically immune to rust.
When chromium content is increased to 16 to 18 per cent and silicon is added, from 2 to 4 per cent, this steel becomes rust proof in its raw state, as soon as the outside surface is removed. It does not need to be heat-treated in any way. These compositions are both patented.
S. A. E. STANDARD STEELS
The following steel specifications are considered standard by the Society of Automotive Engineers and represents automobile practice in this country. These tables give the S. A. E. number, the composition of the steel and the heat treatment. These are referred to by letter—the heat treatments being given in detail on pages 134 to 137 in Chap. 8. It should be noted that the percentage of the different ingredients desired is the mean, or halfway between the minimum and maximum.
TABLE 4. CARBON STEELS S. A. E. Carbon Manganese Specification (minimum and (minimum and Phosphorus Sulphur Heat no. maximum) maximum) (maximum) (maximum) treatment - - - - - 1,010 0.05 to 0.15 0.30 to 0.60 0.045 0.05 Quench at 1,500 1,020 0.15 to 0.25 0.30 to 0.60 0.045 0.05 A or B 1,025 0.20 to 0.30 0.50 to 0.80 0.045 0.05 H 1,035 0.30 to 0.40 0.50 to 0.80 0.045 0.05 H, D or E 1,045 0.40 to 0.50 0.50 to 0.80 0.045 0.05 H, D or E 1,095 0.90 to 1.05 0.25 to 0.50 0.040 0.05 F
TABLE 5. SCREW STOCK - S. A. E. Carbon Manganese Phosphorus Sulphur Specification no. (maximum) - 1,114 0.08 to 0.20 0.30 to 0.80 0.12 0.06 to 0.12 -
TABLE 6. NICKEL STEELS - S. A. E. Phosphorus Specification (maximum) no. / Carbon Manganese Sulphur Nickel Heat (minimum and (minimum and (maximum) (minimum and treatment maximum) maximum) maximum) - - - 2,315 0.10 to 0.20 0.50 to 0.80 0.04 0.045 3.25 to 3.75 G, H or K 2,320 0.15 to 0.25 0.50 to 0.80 0.04 0.045 3.25 to 3.75 G, H or K 2,330 0.25 to 0.35 0.50 to 0.80 0.04 0.045 3.25 to 3.75 H or K 2,335 0.30 to 0.40 0.50 to 0.80 0.04 0.045 3.25 to 3.75 H or K 2,340 0.35 to 0.45 0.50 to 0.80 0.04 0.045 3.25 to 3.75 H or K 2,345 0.40 to 0.50 0.50 to 0.80 0.04 0.045 3.25 to 3.75 H or K -
TABLE 7. NICKEL-CHROMIUM STEELS - S. A. E. Phosphorus Sulphur Specification (maximum) (maximum) Heat no. treatment Carbon Manganese Nickel Chromium (minimum and (minimum and (minimum and (minimum and maximum) maximum) maximum) maximum) - - 3,120 0.15 to 0.25 0.50 to 0.80 0.04 0.045 1.00 to 1.50 0.45 to 0.75* G,H or D 3,125 0.20 to 0.30 0.50 to 0.80 0.04 0.045 1.00 to 1.50 0.45 to 0.75* H,D or E 3,130 0.25 to 0.35 0.50 to 0.80 0.04 0.045 1.00 to 1.50 0.45 to 0.75* H,D or E 3,135 0.30 to 0.40 0.50 to 0 80 0.04 0.045 1.00 to 1.50 0.45 to 0 75* H,D or E 3,140 0.35 to 0.45 0.50 to 0.80 0.04 0.045 1.00 to 1.50 0.45 to 0.75* H,D or E 3,220 0.15 to 0.25 0.30 to 0.60 0.04 0.040 1.50 to 2.00 0.90 to 1.25 G,H or D 3,230 0.25 to 0.35 0.30 to 0.60 0.04 0.040 1.50 to 2.00 0.90 to 1.25 H or D 3,240 0.35 to 0.45 0.30 to 0.60 0.04 0.040 1.50 to 2.00 0.90 to 1.25 H or D 3,250 0.45 to 0.55 0.30 to 0.60 0.04 0.040 1.50 to 2.00 0.90 to 1.25 M or Q X3,315 0.10 to 0.20 0.45 to 0.75 0.04 0.040 2.75 to 3.25 0.60 to 0.95 G X3,335 0.30 to 0.40 0.45 to 0.75 0.04 0.040 2.75 to 3.25 0.60 to 0.95 P or R X3,350 0.45 to 0.55 0.45 to 0.75 0.04 0.040 2.75 to 3.25 0.60 to 0.95 P or R 3,320 0.15 to 0.25 0.30 to 0.60 0.04 0.040 3.25 to 3.75 1.25 to 1.75 L 3,330 0.25 to 0.35 0.30 to 0.60 0.04 0.040 3.25 to 3.75 1.25 to 1.75 P or R 3,340 0.35 to 0.45 0.30 to 0.60 0.04 0.040 3.25 to 3.75 1.25 to 1.75 P or R - * Another grade of this type of steel is available with chromium content of 0.15 per cent to 45 per cent. It has somewhat lower physical properties.
TABLE 8. CHROMIUM STEELS - S. A. E. Specification no. - Carbon Manganese Chromium (minimum and (minimum and Phosphorus Sulphur (minimum and Heat maximum) maximum) (maximum) (maximum) maximum) treatment - - - 5,120 0.15 to 0.25 * 0.04 0.045 0.65 to 0.85 B 5,140 0.35 to 0.45 * 0.04 0.045 0.65 to 0.85 H or D 5,165 0.60 to 0.70 * 0.04 0.045 0.65 to 0.85 H or D 5,195 0.90 to 1.05 0.20 to 0.45 0.03 0.03 0.90 to 1.10 M, P or R 51,120 1.10 to 1.30 0.20 to 0.45 0.03 0.03 0.90 to 1.10 M, P or R 5,295 0.90 to 1.05 0.20 to 0.45 0.03 0.03 1.10 to 1.30 M, P or R 52,120 1.10 to 1.30 0.20 to 0.45 0.03 0.03 1.10 to 1.30 M, P or R - Two types of steel are available in this class, one with manganese 0.25 to 0.50 per cent (0.35 per cent desired), and silicon not over 0.20 per cent; the other with manganese 0.60 to 0.80 per cent (0.70 per cent desired), and silicon 0.15 to 0.50 per cent.
TABLE 9. CHROMIUM-VANADIUM STEELS - S. A. E. Phosphorus Sulphur Vanadium Specification (maximum) (maximum) (minimum) no. / - Carbon Manganese Chromium Heat (minimum and (minimum and (minimum and treatment maximum) maximum) maximum) - - - - 6,120 0.15 to 0.25 0.50 to 0.80 0.04 0.04 0.80 to 1.10 0.15 S 6,125 0.20 to 0.30 0.50 to 0.80 0.04 0.04 0.80 to 1.10 0.15 S or T 6,130 0.25 to 0.35 0.50 to 0.80 0.04 0.04 0.80 to 1.10 0.15 T or U 6,135 0.30 to 0.40 0.50 to 0.80 0.04 0.04 0.80 to 1.10 0.15 T or U 6,140 0.35 to 0.45 0.50 to 0.80 0.04 0.04 0.80 to 1.10 0.15 T or U 6,145 0.40 to 0.50 0.50 to 0.80 0.04 0.04 0.80 to 1.10 0.15 U 6,150 0.45 to 0.55 0.50 to 0.80 0.04 0.04 0.80 to 1.10 0.15 U 6,195 0.90 to 1.05 0.20 to 0.45 0.03 0.03 0.80 to 1.10 0.15 U -
TABLE 10. SILICO-MANGANESE STEELS - S. A. E. Specification no. - Carbon Manganese Silicon (minimum and (minimum and Phosphorus Sulphur (minimum and Heat maximum) maximum) (maximum) (maximum) maximum) treatment - - - 9,250 0.45 to 0.55 0.60 to 0.80 0.045* 0.045 1.80 to 2.10 V 9,260 0.55 to 0.65 0.50 to 0.70 0.045* 0.045 1.50 to 1.80 V - * Steel made by the acid process may contain maximum 0.05 phosphorus.
LIBERTY MOTOR CONNECTING RODS
The requirements for materials for the Liberty motor connecting rods are so severe that the methods of securing the desired qualities will be of value in other lines. The original specifications called for chrome-nickel but the losses due to the difficulty of handling caused the Lincoln Motor Company to suggest the substitution of chrome-vanadium steel, and this was accepted by the Signal Corps. The rods were accordingly made from chromium-vanadium steel, containing carbon, 0.30 to 0.40 per cent; manganese, 0.50 to 0.80 per cent; phosphorus, not over 0.04 per cent; sulphur, not over 0.04 per cent; chromium, 0.80 to 1.10 per cent; vanadium, not less than 0.15 per cent. This steel is ordinarily known in the trade as 0.35 carbon steel, S. A. E., specification 6,135, which provides a first-rate quality steel for structural parts that are to be heat-treated. The fatigue resisting or endurance qualities of this material are excellent. It has a tensile strength of 150,000 lb. minimum per square inch; elastic limit, 115,000 lb. minimum per square inch; elongation, 5 per cent minimum in 2 in.; and minimum reduction in area, 25 per cent.