Page 360 - Introduction to Marine Engineering

.S32 Engineering materials

Normalising. The steel is heated to a temperature of 850-950°C,
depending upon its carbon content, and is then allowed to coo! in air. A
hard strong steel with a refined grain structure is produced.
Annealing. Again the steel is heated to around 850-950°C, but it is cooled
slowly, either in the furnace or an insulated space. A softer, more
ductile, steel than that in the normalised condition, is produced.
Hardening. The steel is heated to 850-950°C and is then rapidly cooled
by quenching in oil or water. The hardest possible condition for the
particular steel is thus produced and the tensile strength is increased.
Tempering. This process follows the quenching of steel and involves
reheating to some temperature up to about 680°C. The higher the
tempering temperature the lower the tensile properties of the material,
Once tempered the metal is rapidly cooled by quenching.
Controlled rolling. This is sometimes described as a thermomechanical
treatment. In two-stage controlled rolling an initial rough rolling is first.
carried out at 950-1100°C. The first controlled rolling stage is carried
out at 850-920°C. The second stage is completed at about
700—730°C. The process is designed to achieve fine grain size, improve
mechanical properties and toughness, and enhance weldability.

Material forming

In the production of engineering equipment various different processes
are used to produce the assortment of component parts. These forming
or shaping processes can be grouped as follows:

1. Casting.
2. Forging.
3. Extruding.
4. Sintering.
5. Machining.
Casting is the use of molten metal poured into a mould of the desired
shape. A wooden pattern, slightly larger in dimensions than the desired
item, to allow for shrinkage, may be used to form a mould in sand. Entry
and exit holes, the gate and riser, are provided for the metal in the sand
mould. Alternatively a permanent metal mould or 'die' may be made in
two parts and used to make large quantities of the item. This method is
called 'die casting'. The molten metal may be poured into the dies or
forced in under pressure.
Forging involves shaping the metal when it is hot but not molten. In
the manufacturing process of forging a pair of die blocks have the hot
metal forced into them. This is usually achieved by placing the metal on
the lower half die and forcing the top half down by a hydraulic press.