Page 361 - Introduction to Marine Engineering

Engineering materials 333

Extrusion involves the shaping of metal, usually into a rod or tube cross
section, by forcing a block of material through appropriately shaped
dies. Most metals must be heated before extrusion in order to reduce the
extruding pressure required.
Sintering is the production of shaped parts from metal powder. A
suitable metal powder mixture is placed in a die, compressed and heated
to a temperature about two thirds of the material melting point. This
heating process results in the powder compacting into a metal in the
required shape.
Machining of one type or another is usually carried out on all metal
items. This may involve planing flat surfaces, drilling holes, grinding
rough edges, etc. Various equipment, such as milling machines, drilling
machines, grinders, lathes, etc., will be used. Many of these machines are
automatic or semi-automatic in operation and can perform a number of
different operations in sequence.

Common metals and alloys

Some of the more common metals met in engineering will now be briefly
described. Most metals are alloyed in order to combine the better
qualities of the constituents and sometimes to obtain properties that
none of them alone possesses. The various properties, composition and
uses of some common engineering materials are given in Table 16.1,

Steel
Steel is an alloy of carbon and iron. Various other metals are alloyed to
steel in order to improve the properties, reduce the heat treatment
necessary and provide uniformity in large masses of the material.
Manganese is added in amounts up to about 1.8% in order to improve
mechanical properties. Silicon is added in amounts varying from 0.5% to
3.5% in order to increase strength and hardness. Nickel, when added as
3 to 3.75% of the content, produces a finer grained material with
increased strength and erosion resistance. Chromium, when added,
tends to increase grain size and cause hardness but improves resistance
to erosion and corrosion. Nickel and chromium added to steel as 8% and
18% respectively produce stainless steel. Molybdenum is added in small
amounts to improve strength, particularly at high temperatures.
Vanadium is added in small amounts to increase strength and resistance
to fatigue. Tungsten added at between 12 and 18%, together with up to
5% chromium, produces high speed steel.