Crucible Process

crucible process

Technique for producing cast or tool steel. It was invented in Britain c. 1740 by Benjamin Huntsman, who heated small pieces of carbon steel in a closed fireclay crucible placed in a coke fire. This was the first process used in Europe in which the temperature (2,900°F, or 1,600°C) was high enough to melt the steel, producing a homogeneous metal of uniform composition. After 1870 the Siemens regenerative gas furnace replaced the coke-fired furnace. Capable of producing even higher temperatures, the Siemens furnace had a number of combustion holes, each holding several crucibles, and heated as many as 100 crucibles at a time. All high-quality tool steel and high-speed steel was long made by the crucible process. In the 20th century the electric furnace has replaced it in countries with inexpensive electric power. See also wootz.

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Crucible Process 

a process for melting metals and alloys in pots, or crucibles, made of refractory materials. The crucible process is the oldest known method for melting metals (copper, bronze). It was described by Aristotle in the fourth century B.C. and was well known in India, Persia, and Syria. Crucible steel was used for producing sharp knives, strong tools, and such steel weapons as damask blades. In later centuries, the secret of the process was lost.

The crucible process was rediscovered in Europe in 1740 by the British inventor B. Huntsman. According to his method, carbon steel made by the cementation process invented in Sweden was remelted under a layer of flux of green glass in a crucible set in a furnace with a natural draft and a coke fire. The product was a pure steel of uniform composition suitable for knives, razor blades, clock and watch springs, and pendulums. Most of the development work on the crucible process occurred in the first half of the 19th century.

Although the basic principles of the crucible process remained unchanged, continued research led to a number of improvements. Changes were made in the composition of the charge, manufacture of the crucible, and structure of the furnace. The British metallurgist R. Mushet discovered the desirable effect on steel of manganese and began to add manganese oxides to the crucible charge, thus introducing the production of killed steel (1801). S. I. Badaev proposed a furnace with separate cementation and crucible compartments that operated in sequence. He introduced a production method in 1808 that involved the cementation of iron followed by the melting of the steel produced. In 1837, P. P. Anosov, working on the production of damask steel, discovered gas carburizing of iron during the crucible process. This simultaneous realization of both processes—cementation and melting—shortened the production time to nine or ten hours from the several days needed to produce the steel in the cementation process. The main feature of the method proposed in 1857 by P. M. Obukhov was the use of iron ore, which resulted in steel of the same composition even with varying carbon contents in the raw materials. In Russia, Obukhov’s method was used in the major steel foundries built in Zlatoust (1860), Perm’ (1863), and St. Petersburg (1865).

Although crucible steel was expensive and could be produced only in small quantities, it remained for a long time the only material suitable for making certain critical tools and mechanical parts. The crucible process remained the most important method for producing high-quality steel until the emergence of the electric-furnace process. Sweden continued to manufacture crucible steel until the mid-20th century. The main use of crucible steel has been the production of high-quality tools. The crucible process is also used in nonferrous metallurgy, mainly in small foundries and workshops, for producing alloys of nonferrous metals and for melting metals and alloys prior to casting.

REFERENCES

Lipin, V. N. Metallurgiia chuguna, zheleza i stali, vol. 2, part 1. Leningrad, 1930.
Mezenin, N. A. Povest’ o masterakh zheleznogo dela. Moscow, 1973.

N. A. MEZENIN