THE HISTORY OF ALLOY STEELS
Alloy steel was a development of the very late 19th and the 20th centuries. It could not occur until steel itself was readily available and that did not take place until after about 1880. The only truly alloy steel before this time was a tool steel invented by Robert A. Mushet in 1868. Mushet's steel was very high in both carbon(2% or more) and alloy(7% tungsten and 2% manganese) and was considered a unique material not a forerunner of modern-day engineering alloy steels.
The idea of alloying, of course, was old. It had been practiced since ancient times with copper and tin to make bronze, with gold and silver or copper and many other metals. Most mineral sources available contained combinations of metals so that alloys were the natural output of many of these ancient metalworking activities. Although it is not emphasized the carbon in cast-iron and steel automatically make them alloys. It was only wrought iron as made for hundreds of years that could be called unalloyed iron. In fact carbon is considered by the metallurgist to be the single most potent alloying element added to iron. Amounts as small as 0.05% have profound effects on the behavior of iron, and 0.10 to 0.25% additions are sufficient to make mild steel. Most heat-treated alloy steels contain 0.30 to 0.40% carbon, and some modern alloy steel applications contain as little as 0.10 to 0.15%.
Carbon has two characteristics which account for the powerful effects of such small amounts-- it is very low in density, therefore, there are a great number of atoms in these small amounts of alloy additions by weight, and the atoms are small in size compared to iron so they do not substitute for an iron atom in the crystal lattice but take up a unique position in the holes between the iron atoms. It is the interstitial position of the carbon atoms in the iron lattice, along with the crystal lattice transformation from face-centered-cubic to body-centered-cubic, that makes steel that marvelous material for construction, for power transmission and for tools in our 20th century technology.
The first alloy steel was patented in 1865(Patent No. 49495) by an American named George Baur and manufactured by the Chromium Steel Co. of Brooklyn, NY. This alloy steel was selected by Eads for use as the major structural material in his famous bridge across the Mississippi at St. Louis, Mo. (1869). This was a daring and a costly decision for Eads. Daring because no one had yet used steel for such a large structure let alone used a totally unknown alloy addition of chromium, and costly because Eads used crucible steel--a process primarily for making tool steels. This steel cost Eads about $300 per ton compared with $80 for wrought iron. This first use of steel, and an alloy steel at that, for bridge building makes Eads a direct technical descendent of the Darby’s. For over a 140 years Ead’s bridge has stood the test of time.
The publicity surrounding Ead's bridge and his use of a chromium alloy steel prompted a French metallurgist by the name of Henri-Ami Brustlein to become interested in making chromium alloy steels. Brustlein soon learned that to alloy chromium with steel, he first needed to refine the chromium ore to produce what is called ferrochromium or a master alloy of iron-chromium-carbon. This master alloy would readily dissolve into the melt of the crucible process, otherwise the recovery of chromium would be too erratic to control the alloy content. Brustlein produced and sold chromium alloy steels for tools, cannon shells and armor plate over a period of about 15 to 20 years before anyone else. For his work in developing ferrochromium, alloy steel, heat-treatments and applications, Brustlein deserves to be called "The Father of Alloy Steels". In addition, he replaced the manganese with chromium in Robert Mushet's Special Tool Steel to improve this world-famous steel. He brought samples of this new tool steel to the United States where he interested Fredrick Taylor at Midvale Steel Co. in their use. Taylor went on to develop the first high- speed steel which revolutionize the machining industry. Thus Brustlein laid the groundwork for both engineering alloy steels and modern tool steels.
During the period that Brustlein was developing chromium steels, other French metallurgist were learning to smelt nickel containing ore from New Caledonia, a French possession, in the South Pacific. The resulting ferronickel was then used to add nickel to steel. The production of nickel steel was observed in France in 1888 by James Riley an Englishman who made arrangements for similar steels to be made at The Steel Company of Scotland in 1889. He immediately tested these steels and reported their properties in the Journal of the Iron and Steel Institute. One of his steels containing about 0.2% carbon and 5% nickel developed strength properties of considerable interest for many different structural and machine applications. This steel as processed by rolling and annealing was about 40% stronger than similar steel without nickel. Almost immediately nickel steels became of interest for armor plate in France (5% nickel) and in Germany (7% nickel). The United States Navy arranged a now-famous shoot-out in 1890 at Annapolis where 8-inch armor-piercing shells were fired at close range to compare carbon steel, nickel steel and a special British compound-carbon steel. The nickel steel was the clear winner. Shortly thereafter, Bethlehem Iron Company produced nickel steel in forged armor plate and other forged parts including the shaft for George Ferris's famous wheel which he built for the Chicago Columbian Fair in 1893. This first Ferris wheel was not the small carnival type we are used to seeing, but a giant machine carrying 1200 people in 30 cars. Nickel steels were used that same year for shafts on several German-built ships for the North Atlantic.
The first alloy steel used in regular industrial production in the United States was a 5% nickel steel for bicycle chain (1898) by the Whitney Manufacturing Company of Hartford, Connecticut; followed the next year by bicycle tubing by the Pope Manufacturing Company. The first use of an alloy steel in the infant auto industry was a 5% nickel steel axle by Haynes and Apperson. Somewhat later nickel steels (3.5%) became popular for the structural members of large bridges-- these included the Manhattan, Queensborough and George Washington Bridge in New York. In the year 1900 about 3000 tons of alloy steel was produced in the United States, with 90% being simple nickel steels. Nickel steels not only offered improved properties for engineering designers; they also served to increase the interest and trust in steel in general over the much lower-strength but reliable wrought iron.
Shortly after nickel steels came into use, more complex alloy steels, containing both chromium and nickel were being tried by Krupp in Germany and by the Comagnie des Forges de la Marine in France. These nickel-chromium steels could be hardened in large sections by heat treating so they became very popular for armor and large forgings. After the turn of the century the straight nickel steels rapidly declined in use in favor of the nickel-chromium steels and the newly developed chromium-vanadium steels.
As noted previously, the first use of production alloy steel was in bicycles. It was the automobile, however, that introduced the age of alloy steel. The first attempt at alloy steel specifications was by a group known as the Association of Licensed Automobile Manufacturers. In 1910 they issued specifications written by Henry Souther for a nickel steel and two nickel-chromium steels along with three carbon steels. Elwood Haynes had reported earlier on using a nickel-chromium steel for gears. His comments seem quaint today but show the state-of-the-art in 1907--" gears were made of the best kind of tool steel without success because the ends of the teeth would break off. However, gears made from nickel-chromium steel, if properly heat treated, would run for an entire season without the breakage or serious injury of a single tooth".
The early motorist was accustomed to the annual replacement of gears, axles, bearings and other highly stressed parts. The auto would be the force behind the development of modern 20th century alloy steels and their heat treatment. By 1912 both the new Society of Automotive Engineers (SAE) and the American Society for Testing Materials (ASTM) developed standard specifications for alloys containing nickel, nickel-chromium, and nickel-chromium-vanadium. These two societies would continue to develop alloy steel specifications under their respective symbols (SAE/ASTM), not only for the automotive industry, but for industry in general. These specifications were usually written by committees assembled by the societies representing individuals from both the suppliers and the users of the various alloys. Alloy steel specifications are never just single compositions in chemistry, but represent a range for each element, which is a compromise between what the user would like to have and what is practical for the producer to make. They also put limits on carbon, silicon, phosphorus, sulfur and sometimes other elements which are added or are considered impurities. A typical specification for an alloy steel is shown below:
SAE 4340
Carbon,% 0.38/0.43
Manganese,% 0.60/0.80
Nickel,% 1.65/2.00
Chromium,% 0.70/0.90
Molybdenum,% 0.20/0.30
Silicon,% 0.20/0.30
Sulfur &Phosphorus,% Max. 0.40
This specification states that each element can vary within the range shown and that the impurities, sulfur and phosphorus, must be no more than 0.40% maximum each. This alloy can be ordered anywhere in the world to the SAE/ASTM Specification and it will be furnished to this composition range. Tables of all the carbon steels and the so-called engineering alloy steels under the SAE/ASTM system are published and used by all of American industry and many foreign countries.
The number of alloy steels used in the auto industry increased slowly through the decade of the 1910-1920. Walter Jominy a metallurgist from the University of Michigan who worked for the Studebaker Car Co. published in1920 a list of 12 alloy steels that he stated filled all the needs for building automobiles.
The use of alloy steel increased from the 3000 tons of mostly nickel steel in 1900 to about 150,000 tons by 1910. Of this larger quantity 40% was still nickel steel, about 30% was nickel-chromium steel and about 25% was chromium-vanadium steel, which was introduced in 1905. WWI provided added emphasis on the use of alloy steels and on the process of heat treatment. The quantity of alloy steel made in the United States reached over 1 million tons in 1918. Nickel steels had declined to only 25%, nickel-chromium steels were still at 30% and chromium-vanadium steels had increased to about 35%. The remaining 10% was composed of a variety of alloy compositions containing only silicon or chromium or manganese, except for a new type of alloy with nickel-chromium-molybdenum. The addition of molybdenum was found to suppress an embrittlement that occurred during the heat treating operation.
The 1920s saw a continuing increase in the use of alloy steels, mainly because of the increasing production of autos, trucks, tractors and other consumer goods. In fact the 1920s was the first blooming of the great American consumer revolution fueled by that other great American innovation " buy-now pay-later". By 1929 auto production reached 5 million cars, a number that still looms large even now in these first years of the 21st century. While the cars and other vehicles of the 1930s seem crude to us now, they had become much more durable and reliable than their counterparts of the prewar time. Much of this improvement was due to the greater use of heat-treated, alloy steel in suspension systems, steering, power transmission and engine components. This along with the general improvement of engineering designs, the fully enclosed body( made possible by the invention of the continuous rolling mill ), improvements in hydraulic shock absorbers and brakes and the introduction of six-cylinder engines allowed the production of the vehicle that is a direct ancestor of the cars of today.
The use of alloy steels brought about a significant need for the process of heat treating. The term heat treating has many meaning, but the principal one for alloy steels is the heating to a high temperature, rapidly quenching to ambient temperature and reheating(called tempering) to produce a set of properties suitable for the intended application. This treatment for steel has been used for hundreds of years as practiced as an art. The Damasks, Toledo and Japanese swords are examples of the highest art work of their time. The average blacksmith practiced the same treatment to make steel cutting edges for knives, axes and other tools. The art was closely guarded and generally passed down from father to son. As would be expected the results varied considerably. The control of the variables, carbon content, temperature, and quenching media was beyond the technology of the times. This would gradually change as the industrial revolution produced the need for machines and tools.
The large use of alloy steel increased the need for acquiring and sharing of information that had been only a guarded art. William Park Woodside, a former blacksmith, started meetings in Detroit to exchange information among heat treaters in the early auto industry. These meetings lead to the formation of a formal group called the Steel Treaters Club, which later was called the American Society for Steel Treating and eventually the American Society for Metals. These early groups formed chapters in various industrial regions around the country and published data sheets on the technical aspects of heat treating. Soon they provided a vehicle called the Transactions for serious researcher to publish their papers. In 1930 the American Society for Steel Treating published a magazine called Metals Progress. It became the most popular source of information in all of metal working, not just heat treating. The society changed it’s name again in the early 1930s to the American Society for Metals and is located in Cleveland, Ohio. As ASM they still publish a vast amount of information on all aspects of metallurgy, organize conventions and symposia and conduct training classes for metallurgist and nonmetallurgist in the technology of metal processing.
Early researchers in science, especially chemists, who were developing ideas and techniques for determining the composition of matter speculated on the difference between iron and steel. By the late 18th century Bergman in Sweden proposed that iron was allotropic (had more than one crystal form) and that steel contained carbon which made it different from iron. Later such well-known scientists as the Englishmen Davy and Michael Faraday devoted some studies to iron and steel (Faraday examined a number of exotic alloy additions to steel), but little real understanding came from this work. At about this same time (1820-1830) Karsten showed that wrought iron, steel and pig iron differed principally in the amount of carbon they contained; and Liebig developed a method for measuring the amount of carbon present. Gradually over the next half century the role of carbon was more widely understood and steelmakers began offering steels of controlled carbon analysis. It was the age of the craftsman, however, because the technical understanding of hardening of steel was non-existent. The individual skills of the steel-maker determined the success of the product. Science had little to offer and few men of scientific stature were interested in such mundane affairs as the making and heat treating of steel. There was much more interesting work in the world of chemistry and physics.
The first stirrings of technical interests in examining the nature of steel came just past the midpoint of the 19th century. Henry Clifton Sorby of Sheffield, England examined polished and etched surfaces of meteorites and several commercial steels during the period 1863-1866. Sorby discovered that the microstructure of steel was complex and he found an area that he called “pearly”. However, no interest developed at the time even though he reported his findings in his home town of Sheffield, England, which had been a tool making center for centuries. Several other developments occurred just after Sorbys work which are important to our history of alloy steel.
The very beginning of any scientific or technical field is always difficult to determine because there is usually some knowledge or activity that can be shown to predate whatever beginning is selected. Thus while the period 1885 to 1890 can be shown as the beginning of the studies of the internal structures of metals, some previous work had been done by Sorby , Tchernof, Gore, Osmond, Martens and others.
It was the first attempts by Osmond and Martens to publish their examination of polished and etched surfaces of steel in the Journal of the Iron and Steel Institute of Great Britain that reawakened Sorby's interest in an area he had worked on over 20 years before. Sorby immediately set about a new examination of the microstructure of steels. He presented this work in 1886 and published it in the British Journal of the Iron and Steel Institute in 1887. This new work by Sorby along with that of Osmond and Martens is considered the real beginning of the field of Metallography, the study of the internal structure of metals. From this point in history, the ever-increasing research in how the behavior of metals relate to their structure has been the foundation of our modern technological age. From this point there was never again any lull in the technical activity to understand and improve the properties and usefulness of metals.
Shortly a new temperature measuring technique for high temperatures was developed by Le Chatelier of France with the use of Platinum-platinum/ rhodium thermocouples. Osmond, who worked with Le Chatelier for many years at the Sorbonne, Immediately put the new thermocouple to use in measuring the so-called critical temperatures in steel. These temperatures where changes were noted in the rates of cooling or heating were first pointed out by the Russian metallurgist Tchernoff. He stated that steel could not be hardened upon quenching until it was first heated above the uppermost critical temperature. These critical temperatures were believed to represent important internal changes in steel--but the nature of these changes was beyond the ability of these early pioneers in the science of metallurgy to decipher with their limited knowledge and crude tools. In fact the final solution to these questions on the internal changes during the heat treatment of steel lay over 40 years in the future and a continent away--but these first few metal makers in their intense desire to understand the mystery would devote the rest of their working careers adding bit by bit to the knowledge, but the understanding would elude them until their students or even their students students would finally unravel this complex problem.
In all of metal making nothing has exceed the technical importance, the scientific complexity and the human curiosity of the hardening of steel. To convey the magnitude of the available strength properties in hardened alloy steel, a comparison with ordinary structural steel, which has a yield strength of about 40,000 pounds per square inch of cross-section to 150,000 to 200,000 psi. for alloy steel with ductility and toughness, to as high as 250,000 to 300,000 psi. for special application and to 400,000 psi and higher for tools, bearings and other really severe uses. Thus, hardened, alloy steel is a metal of enormous versatility-- nature's bountiful gift to mankind for the technological age and all of this at a reasonable economic cost.
A discovery far beyond the field of metallurgy occurred during these early times. An American research professor named Joshua Willard Gibbs at Yale University developed a theory called “The Equilibrium of Heterogeneous Substances”. It received very little notice in the field of science and none in metallurgy. A generation later a Dutch metallurgist named Rooseboom applied Gibb’s theory to the addition of carbon to iron. This was the first quantitative experiment to explain what the metallurgist were seeing in the microscope. The polished and etched surface showed a white phase which was nearly pure iron. With the addition of carbon a new phase occurred which Sorby had called “pearly”. In honor of Sorby the metallurgist named this phase pearlite. Both Gibb’s theory and metallography showed that pearlite was a mixture of iron and iron-carbide and the higher the carbon content the more pearlite appeared in the microstructure. At about 0.80% carbon the steel microstructure was all pearlite.
The work of Rooseboom started a trend of measuring the phase relationships in a many alloy systems. This work was performed especially in Germany and England. With the light microscope, the high-temperature thermocouple, and Willard Gibb’s theory, the metals researcher had some powerful tools to finally explain some of the behavior of metals.
From the mid-1880s new knowledge was being developed in several countries on the heat treatment of steel and the study of internal structures-Floris Osmond and LeChatelier in France. Martin and later Wedding in Germany. Sorby. Arnold. Stead, Roberts-Austen, Hadfield and others in England; and Howe and Sauvuer in the United States. These beginnings initiated a burst of technical activity which saw the establishment of "Metallurgy" as a curriculum in a number of universities and colleges where some of the above workers with associated colleagues and students carried on research in metallurgy much as the chemists and physicists had been doing in their fields for many years.
After the decade of the 1890s with its enthusiastic and productive beginnings of the metallography of steel in England, France Germany, Russia, Japan and the United States, along with the totally unproductive bickering over the hardening mechanism of steel, a period of rather quiet consolidation occurred in the early 20th century. Most of the pioneers joined or set up Metallurgy Departments or Metallographic Sections within older Mining or Chemical Engineering Departments throughout the industrial world. Henry Marion Howe, Americas earliest metals researcher, joined Columbia University in 1897 to become their first full-time Professor of Metallurgy. Howe was the son of the famous Julie Howe who wrote "The Battle Hymn of the Republic". He was a graduate of MIT and had worked in various metals industries. By 1905 he had a new laboratory and a staff of five, including William Campbell, a Scotsman who had come to America to study under Howe and remained to serve a lifetime career teaching young Americans in metallography. We will encounter several of these students of Campbell's, whose careers were greatly influenced by this quiet Scotsman. Albert Sauveur, another American metals scientist, began teaching metallurgy at Harvard in 1898, when he was totally unsuccessful in finding employment in industry after loosing his first position at the Illinois Steel Company.
Within a decade or so from the founding of Metallurgical Departments and the first teaching of metallography as a major branch of metal knowledge some remarkable students of this new technology began to emerge from American universities. They came from Mining, Chemistry, Chemical Engineering and other branches of technology; but they had a common interest in pursuing work in metallography after having been exposed to one or more courses in the subject.
These young researchers came from many places and directions. One of the first and most influential of the group was Zay Jeffries from a cross road town in South Dakota; a graduate in a class of 11 out of a total enrollment of 45 at the South Dakota School of Mines in 1911. There was Edgar Bain, Ohio State University, class of 1912 in Chemical Engineering, who became intrigued by viewing slides of steel microstructures. He went on to Columbia to study metallography under Campbell. In this same group was a graduate from MIT named Marcus Grossman who would spend his career studying the hardening of steel.
Several of these newly emerging metallographers obtained advanced degrees studying under the well-known pioneers in Europe. They were following in the footsteps of John Mathews of Crucible Steel who studied under the famous Roberts-Austen at the Royal School of Mines in London. Samuel Hoyt, Mining Engineer, from the University of Minnesota (1909) was an early student to be influenced by William Campbell at Columbia to continue his education in Europe. Hoyt studied for two years at the Royal Institute of Technology at Charlottenburg, Germany under such famous German scientist as Adolph Marten (after whom Osmond named "martensite") and William Guettler who had taught at MIT. Hoyt was joined at Charlottenburg by another American student, Paul Merica. Merica was born and educated in Indiana. He then took a degree in chemistry at the University of Wisconsin. From there he was recruited to teach chemistry for the next two years in China. It was after this tour in China that he enrolled in Berlin for graduate studies in chemistry. Merica was to become world famous in a few years for his research into a new metals hardening mechanism. Another of this new generation of American metallurgist, one who figures prominently in this history of alloy steels, was Marcus Grossman. He graduated from MIT in 1911. Thus he followed in the footsteps of Howe and Sauveur from a school that later would become a well-known graduate training ground for Metallurgical Engineers.
The first of these young metallurgist from the classes of 1909-1912 to become located professionally was Zay Jeffries. He followed the president of his alma mater to the Case School of Applied Science in Cleveland, Ohio, where he became Instructor of Metallurgy in the fall of 1911. After a few years of teaching he found a fruitful field of consulting in the Cleveland area, which was rapidly becoming one of Americas leading industrial cities. He developed long-range relationships with two of his clients- General Electric, Cleveland Wire Co. and the Aluminum Casting Company(later acquired by Alcoa). Jeffries gave up his teaching post to pursue full time consulting; meanwhile, he obtained a PhD from Harvard under Sauveur during WW1. The work of Jeffries and others at the G. E. Wire Company on tungsten wire for lamps was at the frontier of metallurgical development during the decade of the 1910s. While every school child learns that Thomas Edison invented the electric light bulb, his filament of carburized thread was not a long-term solution to the problem.
Jeffries reputation and the challenging work at both the G. E. Wire Company and Alcoa attracted others to join him. Among these were Edgar Bain, Samuel Hoyt and Robert Archer. Thus a school of metallurgical talent developed around Jeffries. A similar group developed at the United States Bureau of Standards around G. K. Burgess. These included Paul Merica, Howard Scott, Herbert French and others who worked on metallurgical problems of interest to the war effort. Out of these two groups came many of the leading researchers who would play prominent roles over the succeeding 20 years in developing the alloys, science and technology to permit the United States to become the dominant technological power during and after WWII. The individual members of these groups served their careers in government or in industrial laboratories not in universities, except for brief periods at the beginning.
The theory of the hardening mechanism of heat treated steel saw little progress during the first two decades of the new century. While this lack of scientific advance did not deter the extremely rapid use of heat treated alloy steel, it lead to a hodge-podge of alloy types with emphases on special alloy compositions providing superior properties. No two manufacturing companies seemed to use the same alloy steel for the same application. It was qualitative rather than quantitative, and it was wasteful of expensive alloying elements.
The first research published that brought the older ideas into question was a paper in 1919 by a well-known French metallurgist, A. Portevin and his co-author M. Garvin. They showed for the first time that the transformation to hard martensite did not occur until the steel being quenched had cooled to temperatures well below that where pearlite, formed. A few years later (1922) an American production metallurgist by the name of W. R. Chapin showed that a carbon tool steel could be quenched to 570 F. and still be austenitic; and further with slow cooling below 570 F. it could be studied as it gradually transformed to martensite on falling temperature. These two early studies apparently did not change the thinking of most established metal researchers, certainly not Sauveur who clung to the ideas he helped formulate 25 years previously. But they were of exciting interest to Zay Jeffries, Edgar Bain and Marcus Grossman.
One of the truly great achievements of the scientific world, which had a large impact on the field of metals, occurred just prior to WWI. This was the discovery that x-rays could be used to measure the structure of crystalline solids. The concept was really very simple, but profound. Ever since the discovery of x-rays, scientists had theorized regarding their nature. Eventually it was proposed that x-rays exhibit some properties of waves, such as light, but with wave lengths ( the distance from any point on the wave curve to the same point on the next cycle ) much shorter than any for the visible spectrum of light and its many colors. The German Max Von Laue proposed that perhaps the wave lengths of x-rays could be diffracted and reflected by the planes of atoms making up the structure of crystalline solids just as man-made diffraction gratings were used with light waves. This was an elegant scientific proposal which was proven by Laue when he aimed a beam of x-rays at thin slices of crystalline material. Photographic film mounted behind his samples picked up the main beam coming through the sample, but it also recorded spots from diffracted radiation.
This diffraction phenomenon was reduced to a useful quantitative tool by the brilliant father and son team of English physicists, W. H. and W. L. Bragg. For their work on the mathematical solution to x-ray diffraction of crystalline solids, the Bragg's received the Nobel Prize for Physics in 1915. Shortly thereafter, a technique of passing x-rays through powder samples was developed simultaneously by Hull in the United States and Debye and Scherrer in Germany
At last the metallurgists had a powerful new tool for determining the internal structure of metals, and it is frequently new tools which lead to technical breakthroughs. Among the earliest Americans to apply x-ray diffraction to the study of metals was Edgar C. Bain. He showed that steel heated to the hardening temperature (austenite, named after Sir Rober-Austen) had a face-centered-cubic crystal structure; whereas, quench-hardened steel (martensite) was body-centered-cubic. Austenite has a large capacity to fit the carbon atoms into the face=centered-cubic crystal structure, whereas, body-centered-cubic iron(called ferrite) has essentially no ability to fit carbon into the crystal lattice. Thus, for austenite to transform into ferrite the carbon has to migrate into the areas of iron carbide to form pearlite.
The next major scientific advance in hardened steel was the discovery through precision x-ray diffraction by Fink and Campbell that martensite was not a simple body-centered-cubic structure as is iron but a distorted structure called tetragonal. Professor E.D.Campbell of the University of Michigan was a chemist turned metallographer, who was blinded early in his career by a laboratory accident. The prestigious annual Campbell Memorial Lecture was named in his honor by the ASM. At last after 35 years of studying martensite there was now some rationale for its great hardness; unlike iron which has no carbon or pearlite which is layers of iron and iron carbide, the martensite has the carbon trapped within its crystal structure- trapped on an atomic scale.
In 1929 an Englishman named J. R. Robertson published a study on quenching small wires to various temperatures and examining the structures that formed. He discovered a structure which was unlike the previously known ones. This structure formed at temperatures below that for pearlite and above martensite.
The next year (1930) Edgar C. Bain and E. S. Davenport of the United States Steel Corporation, Research Laboratory at Kearny, New Jersey published their world-famous paper on " The Transformation of Austenite at Constant Subcritical Temperature". What Bain and Davenport did that was totally new was to quench a series of thin samples from the hardening temperature to a transformation temperature and hold them at this temperature for various times before water quenching to martensite. By the use of metallography and thermal expansion measurements, they were able to follow the progress of formation of the new transformation product-whither it was ferrite, pearlite, iron carbide, or a product they continued to call trootsite, from the very beginning , through the course of the reaction to the end. They plotted individual transformation curves of percent transformed as a function of time at each transformation temperature.
At last the sequence of events occurring at decreasing temperatures could be watched, accurately described and quantitatively measured as the austenite transformed to a variety of structures depending on the transformation temperature. The diagram resulting from plotting the beginning and end of transformation was called a time-temperature-transformation diagram, or a TTT diagram for short; and hundreds of these diagrams were determined for as many steels in the years since the original work of Bain and Davenport. Even though the TTT Diagram is not the most useful tool for practical heat treating, it satisfies the deeper desires of the steel metallurgist for a more basic understanding of the transformation of austenite.
Bain's studies showed that austenite transformed to ferrite and\or coarse pearlite at the higher temperatures of 1200-1300F and a finer pearlite at intermediate temperatures of 900\1000. At still lower temperatures a unique structure formed which was not the troostite or sorbite of Howe and Sauvuer, but the structure first reported by Robertson just the year before Bain's work was published. Later Bain's fellow workers at United States Steel Corp. honored him by calling this new structure, bainite. This nomenclature was officially adopted-thus Edgar C. Bain, perhaps America's most outstanding metallurgist, was the only native who’s name has passed into the daily nomenclature of the metal maker. (A fine biography of Edgar C. Bain has been written by James B. Austin for the National Academy of Sciences. It can be found under Bain's name on the internet.)
The transformation of austenite to pearlite was a subject of intensive study during the late 1930s at the Carnegie Institute of Technology (now the Carnegie-Mellon Institute)in Pittsburgh under the direction of Professor Robert F. Mehl. Mehl was in the process of developing an outstanding graduate school of metallurgy on a par with the one at MIT. A group of graduate students under Mehl published a series of papers on the mechanisms and rates of formation of pearlite nodules. This work culminated in a mathematical solution developed by W. A. Johnson and R. F. Mehl which could describe the pearlite reaction. Later Mehl would be the first American to use the electron microscope for metallography.
Throughout the 1920s and 1930s other American metallurgical investigators were working on the more practical aspects of heat treating steel. Howard Scott and H. J. French at the Bureau of Standards and later in industry ( Scott at Westinghouse and French at the International Nickel Company) worked on quenching round bars and measuring the depth of the martensite layers. A major breakthrough came when Walter P. Jominy and A. Boeghold at the Buick Motor Division of General Motors developed a test for measuring the "hardenability" of any particular steel or heat of steel. This test consisted in heating a 1 in. round by 3 in. long bar of alloy steel to the heat treating temperature, inserting it into a fixture and directing a stream of water onto one end of the hot bar until the entire sample was cooled from the hardening temperature. Two flats were then ground on opposite sides of the bar. One side was placed on the anvil of a hardness testing machine and a series of hardness readings are taken every 1/16th of an inch on the other flat side starting at 1/16th of an inch from the quenched end. These hardness readings were then plotted as a function of the distance from the quenched end. The resulting curve is known as the "end-quench hardenability curve" or simply the "hardenability curve". It is the metallurgical equivalent of the "fingerprint" of a steel. It is the permanent identification of a given composition for a given set of heat treating variables. Alloy steels are bought and sold today based on their "hardenability" as well as their chemical composition, and the “Jominy Hardenability Test” is performed hundreds of times daily in steel mills and manufacturing plants all over the world to ensure their proper heat treat performance. (Hardenability should not be confused with hardness. Hardness is the resistance to indentation by a special tool and is used as a measure of strength. Hardenability is the depth within a bar or part which will retain maximum hardness. Hardness is effected only by carbon content whereas hardenability is effected by all alloying elements. Each element has it’s own effect on the depth of hardening and combining elements has a multiplying effect. Thus multiple alloying additions are generally preferred when high hardenability is required.)
At the same symposium in 1938 where Jominy reported on his test for "hardenability", a paper was presented by Marcus Grossman, Morris Asimow and S. F. Urban on "Hardenability and Its Relation to Quenching". This research paper developed a mathematical analysis which allowed the true hardenability of a steel to be separated from the severity of the quench. Much later Bain in his autobiographical book stated that "This monumental undertaking that grew out of the collaboration of the eminent metallurgist, Grossman, aided by the experimental research expert, Urban, with the distinguished mathematician, Asimow, stands as one of the few superlative scientific contributions to the science of metals during the middle half-century ( about 1915 to 1965 )".
The heat treating of alloy steels requires a reheating after quenching to a low temperature(usually 500F to 900F) called tempering. The as-quenched martensite is too brittle for any engineering use. Tempering, again, is a complex process that has required considerable scientific study to reveal the underlying principles. The effect of alloying elements can alter the properties, especially toughness, available for any given set of tempering conditions. This subject is explored in more detail in the Chapter on Tool Steels.
Hardened alloy steel serves mankind in all those applications where real muscle is needed--ie, sinews of steel. While ordinary steel, which industry calls "mild steel" or "carbon steel" can be used for bridges, buildings, ships and most static structures, our machines require ever increasing muscle as more and more power is being transmitted in ever smaller packages. The breakthrough in understanding alloy steels has been very important to the development of the American technological age. There were three principal parts to this breakthrough with many minor activities between. The first part was Joshua Willard Gibb's scientific work on heterogeneous equilibria, although this work was so basic that it was not understood by metallurgist for a generation. The second principal part was Davenport and Bain's study on the isothermal transformation of austenite to the lower temperature phases. The third part was the work of Grossman et al on calculating hardenablity from the chemical composition and the other variables involved in heat treating and quenching. All of this work plus Jominy’s hardenability test was in place in time for America to take full advantage of it for the massive use of alloy steels in WWII.
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Occasionally modern technology catches up with history. This happened to Ead’s bridge. In 1988 a river boat hit the bridge knocking off one of the segments of the arch. It fell onto the ship’s deck and was recovered for examination by the Neuter Company of St Louis. Chemical analyses showed a carbon content of 0.60% and a chromium content of 0.59%. Metallographic examination showed a very large grain size and a coarse set of microparticles. The carbon was extremely high for a structural application and the chromium was too low for any major effect on the properties. The steel was rolled at too high a temperature for grain size control and the cooling was too slow for transformation to a suitable microstructure. Baur’s chromium steel would be unacceptable in any application today.
However, Eads was a good practicing engineer and in the tradition of the Darbys he did not subject the steel in his bridge to tension or bending loads. He built an arch bridge and the sections were made of numerous segments. Each segment was assembled by inserting carefully machined bars of steel into large, circular tubes of wrought iron. Each segment was like a massive barrel. These barrel segments were placed end to end with wrought iron clamps holding them at each joint--thus producing arches that spanned the piers he had sunk into the bedrock of the Mississippi River. Eads used an unacceptable steel by modern standards, but he used excellent engineering to ensure that this new concept of using steel would provide a safe solution to his great advance in bridge building.
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WHAT THE METALLOGRAPHERS SAW-
THE ART AND SCIENCE OF MICROSTRUCTURES
For the readers not familiar with the microstructure of iron and steel, some examples are shown here. Samples are cut from larger metal pieces by water-cooled abrasive wheels to prevent overheating. Usually the samples are mounted in plastic and then are ground and polished to a mirror finish. The appropriate chemical is selected to slightly attack the polished surface. The sample is then examined under a reflecting microscope at magnifications usually of 100X to 500X.
The first sample is a cast iron such as Abraham Darby would have used. The long, thin, black phase is graphite which is called flakes. It is these flakes that render cast iron brittle. They act as sharp notches where any tensile or bending stresses will cause failure. The background phase is pearlite, parallel laminations of ferrite and iron carbide. While cast iron cannot be worked hot or cold because of this brittleness, it has a big role today in applications that take advantage of its attractive characteristics. Casting is an easy way to make complex parts, such as, internal combustion engines and large bases for machine tools. It is easy to machine as the flakes provide lubricant for the cutting tool. The flakes also provide vibration dampening which is valuable in engines and tool bases.
The next sample is a low-carbon or mild steel. Mild steel is made in very large quantities for most structural uses, such as, bridges, high-rise buildings, trucks, railroad equipment, tanks, etc. The microstructure shows the white phase of ferrite and the areas of pearlite. These are the structures that Sorby and all the other early metallographers would have seen. They may not have understood how these structures formed or what they signified as a science, but they studied the properties and learned how to judge the quality of the product.
A higher carbon, 0.80% and above, microstructure is shown next. Slow cooling from a heat treating temperature allows the sample to transform to all pearlite. This sample is shown at a higher magnification to illustrate the layers of ferrite and iron carbide which make up pearlite.
A rapidly quenched alloy steel is shown in the next photomicrograph. This was a big mystery to the early metallographers but they knew it was the bases for making high-strength steel. Only much later would the metals researchers learn that unlike ferrite and pearlite, the very hard phase formed by a complex shear reaction which trapped the carbon atoms in a body-centered tetragonal crystal structure.
The as-quenched martensite is brittle and must be reheat to allow the carbon to precipitate as fine carbides. This sample was tempered at 900F which would produce reasonable strength with superior toughness.
The reflecting, light microscope is one of the most valuable tools available to the metallurgist. It is used in research, quality control, failure analysis and general metals examination. It is the first tool used to judge failures in aircraft, automotive, trains, bridges, cranes, tanks, and machine tools.
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