Metallurgy of Stainless Steel and Cobalt – Chromium alloys

BESSEMER PROCESS

 

Is a process for converting pig iron (high carbon iron from the blast furnace) into steel (low carbon iron alloy). Air is blown through the molten iron; the oxygen of the air converts the carbon in the iron into carbon dioxide, which escapes. This reaction produces heat which keeps the iron molten. The principle was known to Chinese and Japanese ironmasters by the 17th-c, and to US ironmaster William Kelly (1811-88) as early as 1846, but the process is named after Henry Bessemer, who introduced it successfully into Britain in 1856.

 

It was an invention of the greatest importance, because it led to many industrial processes being greatly changed or inaugurated by the easy availability of steel. It did not work well with some ores, for which the open-hearth process was devised.

 

OPEN-HEARTH PROCESS      

 

A steel-making process devised in 1860 by William Siemens (1823-83) and first successfully operated in 1864 by Pierre Emile Martin (1824-1915). The molten pig iron from which the steel is to be made is not in direct contact with the fuel providing the heat, but only with the hot flames from combustion which play on a shallow hearth containing pig-iron, scrap, and a flux. The steel can be withdrawn continuously. It will operate with iron derived from ores, with which the Bessemer process is ineffective.

 

HEROULTS ELECTRIC-ARC FURNACE

 

An electric furnace used particularly in steel making, where a very high temperature is generated in the ‘arc’ or discharge between two electrodes, the current passing through material evaporated from the electrodes. In the original process devised in 1870 by Siemens, the arc was struck beneath the crucible, heating the metal indirectly.

Later, in the type invented by French metallurgist Paul Heroult (1863-1914) the arc was struck between electrodes and the metal itself. In the type devised in 1898 by Italian metallurgist Enrico Stassano, the arc is struck between electrodes above the metal, heating being by radiation.

CARBON STEELS

Steels are iron based alloys that usually contain less than 1.2% carbon.

Stainless steels are the major alloy system used in orthodontics. However the metallurgy and terminology of these alloys are intimately connected to the binary iron carbon alloy system and to carbon steel alloys.

Carbon steel is not an orthodontic alloy because of its corrosion problems. Carbon steel is mainly iron, but small amt of carbon is added to the alloy. These steels are actually a mixture of pure iron, called ferrite, and iron carbide called cementite.  Ferrite the pure iron is a soft and ductile material without much structural strength. Cementite is hard enough to scratch glass and is comparably brittle; there fore used by itself; it would also be a poor structural material. Steel is a mixture in which these two mediocre materials support and reinforce one another, much like glass fibers and plastic. The physical properties of steel depend almost entirely on the proportions of ferrite and cementite and on the way in which they are intermixed with one another.

 

HEAT TREATMENT OF CARBON STEEL     

 

The different classes of steels are based on three possible lattice arrangements of iron.

Pure iron at room temperature has a body centered cubic structure and is referred to as ferrite.                     At the high end of temperature scale [1400-1500°F or 750-800°C], steel is a homogenous material with all of the carbon in solid solution in iron. The iron carbide [cementite] is completely decomposed at this temperature. Steel in this form of iron is face centered cubic structure [FCC] called austenite, named after the British metallurgist – Robert Austen. In carbon steel austenite is stable only at these high temperatures.

If the austenite is cooled rapidly, by quenching, it will undergo a spontaneous diffusion less transformation to a body centered tetragonal [BCT] structure called Martensite. The lattice is highly distorted and strained resulting in an extremely hard, strong and brittle alloy named after the German metallurgist Adolf Marten. This is almost pure cementite the hardest and the most brittle form of the iron –carbon combination.

Between these high and low extremes of temperature, many intermediate phases are formed.

When steel is quenched in water the resulting martensite is so brittle that the steel is not suitable for most mechanical applications. This is remedied by reheating to intermediate temperature ranges under carefully controlled conditions to permit a partial transformation in to softer forms. This reheating is called tempering.