METALLURGY

 

    The great importance of metals in the history of civilization is reflected by the names given to entire historic eras in accordance with the metals mainly used then, e.g. Bronze Age, Iron Age.  The utilization of metals is unrivalled in its significance for cultural development.  Thus the improvement of letterpress printing by Johannes Gutenberg of Mainz, achieved after the year 1400, was made possible only by mass production of cast printing letters made of alloys of tin and zinc with a low melting point. 

    The first known metals were probably gold and copper, occurring in pure form.  They were worked into jewelry and decorative items (some finds are about 7,000 years old).  The production of larger metal parts from gold or copper was possible only after the invention of pottery, which made fireproof crucibles for melting and casting available.

    The beginnings of metal extraction from ores are unknown to us.  The oldest traces of smelting processes date back to 6,000 years ago.  The Egyptians reduced malachite from the Sinai Mountain to copper using shaft furnaces of about one meter in height and charcoal.  Copper was the first metal used by man to a large extent.  By simultaneous melting of oxidic tin or zinc ores, the first tin bronzes were produced some 5,000 years ago.  The first brass production took place about 4,000 years ago.  The first extraction of iron from ores was about 4,000 years ago and is ascribed to the Hittites in Asia Minor.  The historic extraction methods were based upon experience.  The recipes were often carefully kept secrets and passed on from generation to generation.

    The ore had to be transformed in a way unintelligible to man.  Since it was possible to produce only relatively low temperatures up to 1,100 degrees Celsius, metallurgy was limited to metals occurring in solid form, like gold, silver, copper and mercury, and easily reducible oxides of the elements copper, lead, antimony and iron.

    The evolution of analytic Chemistry, about 200 years ago, and of physical Chemistry, about 100 years ago, was the basis for a scientific approach in metallurgy.  Towards the end of the 19th century the distinction between iron and steel metallurgy and non-ferrous metallurgy familiar in our times, was introduced.  The iron and steel industry, producing larger quantities, required a highly differentiated professional training.

    Today there are two widely differing ore-extraction principles: pyro-metallurgy works with high temperatures; and hydrometallurgy uses chemical solution and precipitation processes at low temperatures (up to about 300 degrees Celsius).  The link between these two processes is fusion electrolysis, used for aluminum production, for example.  Pyrometallurgical and hydrometallurgical processes can be distinguished in metal extraction and refining just as with ore enrichment.

    Whereas the production of iron will probably remain a field reserved exclusively for pyro-metallurgy (the metal is mostly extracted in liquid form), the non-ferrous metallurgy increasingly gives room to hydro-metallurgy (solution of the metals by acids or lyes with subsequent recovery by wet chemical or wet electrolytic processes).

    The department of metallurgy is divided into the following sections: non-ferrous metallurgy, iron and steel metallurgy, forming methods and finally forming and casting.

 

NON-FERROUS METALLURGY

    The extraction of non-ferrous metals from the raw materials generally takes place in two steps: reduction to the raw materials and its refining to a purity degree of 99% or even 99.9% or more.  Heat, chemical or electric energy is needed for these steps.  Preparatory metallurgical processes used for sulphur-bearing ores are enrichment, roasting (oxidizing or sulphating) and sintering.  Extraction methods in use during the 16th century were in a parting, refining and smelting room.  The Freiberg assaying furnace or Plattner’s furnace of the 19th century illustrates methods used for the determination of noble metal contents in minerals.  Sectional models of electrolytic cells demonstrate the production of aluminum and magnesium.  A graduated bank shows the various forms in which the products of metallurgical plants are marketed.

 

Roasting during the 16th century

The metals bound to sulfur in pyrite-ferrous ores require an additional preparatory process to enrichment: roasting.  In the 16th century the comminuted ore was heated in heaps and sheds to a temperature beneath the melting point.  The uncontrolled escape of calcinations gases with their content of sulfur dioxide seriously damaged the surrounding vegetation.

 

 

Metallurgy of the 16th century: parting room

The lightened silver, i.e. the gold-bearing raw silver obtained by cupellation, was separated into pure silver and gold by the nitric acid process, i.e. by quartation, developed at Goslar in 1493.  Silver was dissolved into silver nitrate by the nitric acid (aqua fortis); a fine gold mud remained and was melted in the crucible.

 

 

IRON AND STEEL METALLURGY

    The transformation of iron from ore to semi-finished steel products is explained in its historic evolution and in the following production steps: extraction of pig-iron, steel production and forming.  Frequent fundamental terms such as iron, ferrous products and steel are explained here:

Iron is the name of the element Fe and of the working material with a degree of purity between 99.8% and 99.9% Fe; ferrous products are all metal alloys, the average iron percentage weight of which is higher than that of all the other elements.

    Steel includes all ferrous products which are generally suitable for hot forming.  Steel has a maximum carbon content of about 2% (except for some steel with rich chrome contents).

Pig-iron and cast-iron have a carbon content of approximately over 2% and can be formed only by casting.

 

Production of Pig-Iron and Iron Sponge

    Historical displays of bloomery fires and the evolution of furnaces lead to the blast furnace processes and their products.  The extraction of liquid pig-iron from ores takes place in the blast furnace by reduction (separation of oxygen from iron) with coke.  In the steel works the liquid pig-iron is refined to steel by means of oxygen (decrease of carbon and other unwanted admixtures to defined percentages).  The product of direct reduction is iron sponge in solid form, from which steel is melted in electric steel plants.  The dominant manufacturing sequence- blast furnace or oxygen blowing steel works- continuous casting, is completed by the secondary line, direct reduction- electric steel plant- continuous casting.

 

Production of Steel from Pig-Iron and Scraps

    Refining hearths and puddling works produce steel in a pasty form called wrought iron.  Steel in liquid state, called ingot steel, is the product of the Bessemer, Martin-Siemens, Thomas and crucible steel processes.  The distinction between wrought iron and ingot steel become obsolete with the disappearance of puddling works (early 20th century).  Today steel is produced by electric or oxygen steel-making plants.

 

FORMING

    The modern classification of forming processes is based upon the active strains in the deformation zone.  This is for example forming under compressive, tensile and compressive, tensile conditions, by bending and under shearing conditions.  Forging, rolling and drawing prevail

 

    Forging has the longest tradition.  In the beginnings, the blacksmiths were also miners, charcoal burners and melters.  Opendie or drop forging, using hammers or presses, was done.

 

    Rolling is the dominant forming process today; over 90% of the world’s entire raw steel production undergoes plastic deformation by hot-rolls.

 

     Drawing has a long history in wire production.  The most common products of modern drawing are wires, bars of round or profiled cross-section and pipes.

 

MOULDING AND CASTING

    Casting produces geometrically designed parts with defined properties.  Besides forging, casting probably is the most ancient production method and has been used for more than 5000 years.  This reached a high standard of quality very early.

    Today the manifold moulding and casting techniques are called archetypes.

 

Bloomery fire of the Siegerland, La Tene period, after 500 B.C.

Since 500 B.C., in order to extract forgeable iron and steel from rich manganiferous iron ores, the Celts used numerous bloomery fires with natural drought on the heights and slopes of the Siegerland.  In shaft furnaces of about 1.7 meters in height, alternating layers of charcoal and comminuted ore were charged on a charcoal fire.  The daily output was about 25 kg of bloomery iron and steel.

 

 

Bessemer converter of 1874

In 1855, Henry Bessemer (1813-1898) was awarded a patent for his process which blows air into liquid pig-iron and thus converts it to steel.  This was the beginning of large-scale steel production.  The converter on display here worked for 30 years.  The charge weight was 6.5 tons.  Blowing took 14 minutes and the complete melting process 25 minutes.  After 20 to 24 melting operations the bottom had to be replaced.

 

 

Blast furnace and hot blast stove, of 1951

The main product of the blast furnace is pig-iron extracted from the ores by refining and melting.  The sectional model shows the construction type of a blast furnace and of a hot blast stove.  The flow chart in the shaft visualizes the processes.  Two to five hot blast stoves are connected with each blast furnace and preheat the combustion air for the ore reduction and melting processes to about 1200 degrees Celcius.

 

Sythe forge from the Black Forest of about 1800

The many steps of scythe manufacture are illustrated by samples.  The hammer is of the tail hammer type, belonging to the chop hammers just like front hammers and tilt hammers.  The turning arbor of the hammer, mostly driven by an undershot water wheel, was fitted with cams depressing the hammer end with a reinforcement ring.

           

 

 

         taken from- Deutsches Museum: Guide through the Collections