Metallurgical Terms
O
O
Chemical symbol for oxygen.
Occlusion
The retention or absorption of gases or liquids by solid substances. The
term is applied particularly to the absorption or entrapment of gases by metals.
Palladium will absorb nearly 1,000 times its volume of hydrogen, whilst gold
absorbs 46 times and nickel 15 times their respective volumes of hydrogen. Other
metals absorb gases in varying volumes. When the metals are in powder form the
quantity may be very large.
O.H.
(a) Oil hardened.
(b) Open hearth.
Oersted
The c.g.s. unit of magnetizing force. (See Magnetic Flux Density).
Open Hearth Furnace
The open hearth process is being superseded by basic oxygen and electric are
processes. The open hearth furnace consists of a shallow rectangular hearth,
enclosed by walls, the front wall being provided with a number of charging
doors. A tapping hole, which in stationary furnaces is plugged with refractory
materials, is situated in the middle of the back wall. Air and fuel gas or oil
are admitted independently through ports at either end of the furnace and these
ports, which are connected by vertical hues with regenerator chambers, act
alternately as inlet and exhaust ports. Combustion takes place across the
furnace above the hearth, and the spent gases escape through the ports at the
opposite end of the furnace, heating up the checker brickwork of the second pair
of regenerators on their way to the stack. Every 10 minutes or so the flow is
reversed so that the gas and air enter through the checker work just heated by
exhaust gases. In the basic open hearth process the hearth is lined with rammed
magnesite or dolomite and the slag is basic, i.e., it consists preponderantly of
lime. In this way sulphur and phosphorus can be largely eliminated from the
steel, passing into the slag. The charge consists of pig iron often in the
molten condition (Hot Metal Process), steel scrap, and considerable quantities
of limestone. In the acid open hearth process, the furnace is built of acid
materials, i.e., the walls, roof, hearth and ports are built of silica brick
whilst ganister and silica sand form the bed of the hearth Since neither sulphur
nor phosphorus is removed in this process, high quality raw materials are
essential. Pig iron and steel scrap are charged cold and limestone and fluorspar
are added as required to promote the fluidity of the slag in both processes.
Iron ore additions are made, firstly to oxidize the silicon and manganese, these
passing into the slag, which is essentially a lime-iron-manganese silicate, and
finally to oxidize the carbon which escapes from the molten bath in the form of
carbon monoxide giving the bath the appearance of being on the boil. The time
occupied in the open hearth process from the commencement of charging to tapping
is of the order of nine to twelve hours, whilst the furnaces vary in capacity up
to about 30 tonnes.
Orange Peel Effect
An effect which arises from the roughening of the surface due to the coarse
grain size, when steel sheets are stretched beyond (heir elastic limit. Also
applied to the roughening of the surface of hard rolls arising from differential
wear.
Orefng Down
(See Pigging Back).
Orientation
The direction of the axis of a crystal in relation to a certain surface or
line; for example, the polished surface of a cleavage plane.
Os
Chemical symbol for osmium.
Overheating
Heating a metal or alloy below the temperature at which burning occurs, but
to such a high temperature that deterioration of properties results. The
condition is characterized by the fracture which shows dull facets and by the
appearance of a characteristic network on etching polished sections with
sulphuric/nitric acid reagent. Steels which have been overheated cannot be
rectified by any commercially applicable method.
Oxy-Acetylene Welding
(See Welding).
Oxygen (O)
A chemically active gas used by the steel industry in large quantities for
steelmaking, welding and cutting. Oxygen may be injected into the liquid steel
to oxidize carbon and other elements. (See also Basic Oxygen Process and A.O.D.).
(See Welding).
Oxy-propane Welding
(See Welding).