Early progress in the Melting of iron.
Last revision: March 12, 2000
Originally written by V.H.Patterson and M.J.Lalich as part of their paper "Fifty years of progress in the inoculation of cast irons.". This paper was presented at the 44th International Foundry Congress, held in Florence in 1977.
Early progress in the Melting of Iron.
According to history, cast iron was first produced succesfully by the Chinese 800-700 B.C.(1) Even though iron was produced many centuries before, it apparently could not be cast because the furnaces were incapable of producing the required temperatures. However, the Chinese, as pointed out by Simpson(1), "had developed melting equipment capable of producing greater draft than hitherto had been possible".
Another reason for the succes of the Chinese in being able to produce cast iron, as mentioned by Simpson 1), was that they reduced iron oxide by heating in the presence of an exess amount of carbon, apparently in the form of charcoal. This procedure resulted in a soft, pure iron with a melting point of 15300C (27860F). The iron was then carburized, reducing its melting point to about 11700C (21380F) thereby making it easier to melt in their high draft furnaces.
Additional references indicate that the Chinese used some high phosphorus coal along with high phosphorus iron ore as charge materials (1,2). These materials, by lowering melting temperatures, reduced the amount of blast needed to melt the iron.
From these early beginnings, the interest in cast iron continued to grow. Many applications for this "new cast metal" were made possible by improvements in melting equipment and techniques as well as great progress in the art of molding. Several engineering applications employed cast iron from time to time, including iron chain suspension bridges, the first of which were constructed by the Chines in 56 A.D. (2) However, iron was not generally cast in what might be called "substantial quantities" in Europe untill the fourteenth century A.D. (1).
The Development of the Blast Furnace
Although the early furnaces for melting iron were probably a very crude form of blast furnace, the development of the Catalan forge in Spain in the eight century A.D. was most likely the forerunner of the blast furnace (1). In the Catalan forge, iron ore and charcoal were charged vertically in the top, resulting in a "loupe" or ball of iron which was "hooked out and hammered into a bloom".(1) By modifying this simple furnace, the Swiss made an improved melting unit which was vertical, above the ground, and charged with alternate layers of ore and charcoal. The next improvements leading to the development of the true blast furnace were made by German and Swedish craftsmen in about 1000 A.D.(1).
During the next 300 years these early blast furnaces were improved and made larger. In 1325 A.D., water driven bellows, which delivered sufficient draft to make hot molten metal directly from the blast furnace, were introduced. Development of these bellows led to the production of substantial amounts of pig iron in Europe by 1400 A.D.(1) and marked the beginning of modern iron foundry practice.
Early Improvements in the Quality of Cast Iron through the Use of Fluxes.
When the famous Spanish Armade attempted to invade England in the sixteenth century, an important step in improving the quality of cast iron was discovered(3). In his historical book "Full Fathom Five" about an expedition organized to recover the buried wrecks of the "invincible" Armada off the coast of England, Colin Martin (3) indicates that the cast iron cannons, shot and anchors of the Spanish fleet were inferior to those used by the British. Martin cites this as an important reason why the British were able to defeat the Spanish and thus prevent the conquest of England.
Even though the Spaniards possessed a good quality hematite ore, they produced poor quality iron guns, anchors and shot due to their lack of knowledge of the behavior of cast iron.
The historical evidence indicates that in the smelting and fluxing of the ore, the refining after smelting, and in the molding and casting techniques, the Spanish were years behind the English. Practically all of their iron castings contained slag. The inferior quality and brittle nature of the shot, coupled with the explosive force of the potent "black powder" caused the shot to crack and partially disintegrate prior to hitting its target. Similarly, many of the cast iron guns exploded during the firing, indicating poor strength and poor ability to absorb shock and vibration. For the same reasons Spanish anchors broke under the stresses of heavy seas and were the cause of many shipwrecks.
What were the reasons for the superiority of the English cast iron, which was the envy of their continental competitors? Martin points out that there was no magic formula. All of the practices of the 16th century founders of the Weald of Sussex, the seat of the English iron industry at that time, are known to us. The practice of weathering the ore for several months washed out many impurities. The ore was then crushed and washed again. Fossilized gray shells inherent in the ore resulted in a high degree of fluxing during smelting, allowing the removal of surface dross and other impurities.
The advanced knowledge of the British founders during this period is demonstrated by the fact that the gun and shot molds were dried and warmed prior to the casting of the iron. The metal, in turn, was poured each time at as even a temperature as possible. This practice minimized what we refer to today as "undercooling" and established close to equilibrium conditions of solidification.
After pouring, the castings were allowed to cool gradually in the molds to room temperature. This procedure minimized the stresses in the finished castings. The Spanish, on the other hand, as pointed out by Martin, cooled the castings as quick as possible in order to expedite production. Their practice often involved water quenching the castings, which contributed to stresses and cracking.
For several years after the defeat of the Spanish Armada, iron founders on the continent attempted to determine the reasons for the better quality of the British castings. In 1619, a Dutchman, Jan Andries Moerbeck, proved that he was on to something new and revolutionary in the art of iron founding, by applying for and obtaining a twelve year patent involving the use of iron ore from the Weald of Sussex. By comparing the English ore having build-in flux, with their flux free, but otherwise good quality hematite ore, the Dutch developed the use of limestone for fluxing. This new technique spread rapidly across the continent to Germany, France and, eventually, Spain, and should be credited as a major contribution in the development of engineering cast irons.
Refinements in the Process of Making cast Iron.
The next significant development, credited to an English iron-founder named Darby in 1730, was the discovery and production of coke which lowered the cost of producing cast iron. This development encouraged experiment for better quality cast iron with improved mechanical properties. As a result, the French founders tried remelting pig iron in separate, smaller furnaces. This type of refining resulted in more uniform iron with respect to chemistry and was another big step toward the development of engineering grade cast iron. Untill this time, apparently most iron castings were poured from iron directly from the blast furnace.
The improved quality iron produced by remelting pig iron in separate furnaces made it possible for James Watt to build the first steam engine in 1765. Watt's steam engine, in turn, was used to provide the air blast for operating the first cupola build in 1794 by John Wilkinson (1). The controlled air blast plus the higher melting temperatures in the cupola further improved the quality of the cast iron. As a result, designers, engineers, builders and others became more interested in cast iron as an engineering material.
Applications for the steam engine in such fields as land and sea transportation, agricultural equipment and, later, electrical poweer generation, created a demand for large quantities of high quality gray cast iron. As this demand grew, so did the need for higher strength and better quality iron requiring more efficient melting equipment, improved charge materials, and closer control of the melting operations.
The Early Use of Ferrosilicon in Cast iron.
About 1810, Bergelius, a Swedish chemist, and, Stromeyer, a German physicist, operating independently, produced ferrosilicon (1). A mixture of silica, carbon and iron fillings was melted in a sealed crucible. Stromeyer produced several grades of ferrosilicon by this method.
Although there appears to be no record as to how the ferrosilicon was used, it was probably added to the melting furnace. Most likely the iron founders became interested in a source of silicon because of the differences in silicon content in the various pig irons produced by the different furnaces due to varying silica content in the iron ores used. The advantages of higher silicon in making softer and less brittle irons were obvious. By adding silicon to the furnaces along with charge materials consisting of scrap and pig iron, the foundrymen were able to make consistently good quality cast iron. They soon learned that it was advantageous to have the silicon low in thick section castings and high in thin section. It is not known that ferrosilicon was added to the ladles in the early to middle 19th century.
In 1885, Turner (4) ran a number of experiments in which ferrosilicon was added to white iron to produce high quality gray iron castings. It is reasonable to assume that the ferrosilicon was added to the iron in the ladle. If so, this would be an indication that some of the early investigators recognized the chill reducing potential of adding ferrosilicon to the ladle.
In 1920, G.Schury (5) discussed the use of ferrosilicon briquettes in the cupola. A discusser of the paper indicated that he had added ferrosilicon to molten iron as early as 1890 for improving cast iron properties.
The knowledge of silicon control would trigger another series of improvements on cast iron structures and mechanical properties, a process that in fact is continuing up till the present day.
References:
(1) Bruce L.Simpson: History of the Metal Casting Industry.
(2) Clyde A.Sanders and Dudley C. Gould: History Cast in Metal.
(3) Colin Martin: Full Fathom Five, pp. 247-261.
(4) T.Turner: Metallurgy of Iron, London, Griffin, 1895.
(5) G.Schury: Giesserei, 1920, Vol. 7, pp. 241-244.