slowly at or below room temperature (so slowly, in
which is usually considered to be a nonplastic
material, is plastic if an external force is applied to it
fact, that years are required to complete a single creep
test), the importance of this type of plastic
very slowly. If you want to demonstrate this to
deformation has not been recognized until fairly
yourself, take a sheet of glass and lay it in a horizontal
recent. Creep-resisting steel is now used in most
position in such a way that it is supported only at the
modem naval ships for high-temperature piping.
ends. Then put a weight in the middle of the glass.
After several days (or possibly weeks, depending upon
the kind of glass you use), you will be able to observe
a visible deformation of the glass.
The substance known as "Silly Putty" is an even
metal will fracture if it is bent or struck a sharp blow.
better example of the relative nature of the property of
A brittle material is one that fractures before
exhibiting any noticeable permanent deformation.
Putty," it is more plastic than chewing gum; throw it
Most cast iron is very brittle.
against the floor and it may either bounce like a rubber
ball or break into pieces; hit it sharply with a hammer,
and it will shatter almost like glass.
Before these properties are studied in detail, the
Corrosion resistance is the property that enables a
welder should have an understanding of the effect of
material to resist entering into chemical combination
carbon on the properties of steel and a knowledge of
with other substances. A high degree of corrosion
alloys in general.
resistance would be very desirable in all metals used
aboard ship. Most metals are easily corroded,
DUCTILITY AND MALLEABILITY
however, as shown by the fact that pure metals occur
only rarely in nature.
alloying elements, may greatly alter the corrosion
that makes it possible for a material to be drawn out
resistance of a metal. For example, the zinc that is
into a thin wire or, in other words, it is the property
known as "commercially pure" contains a small
that enables the material to withstand extensive
amount of impurities; this grade of zinc corrodes about
permanent deformation from TENSION. Ductility is
10,000 times as fast as zinc that is chemically pure. On
the ability of a metal to be stretched. A very ductile
the other hand, many alloys have been developed for
metal such as copper or aluminum may be pulled
the particular purpose of increasing the corrosion
resistance of the material. For example, pure iron
that makes it possible for a material to be stamped,
would be entirely unsuitable for use in boilers because
hammered, or rolled into thin sheets; a malleable
it has very poor resistance to corrosion, particularly at
material is one that can withstand extensive permanent
high temperatures; yet alloys composed primarily of
deformation from COMPRESSION.
iron are used successfully for this service.
Most metals that exhibit one of these properties
also exhibit the other. However, this is not always true.
WELDABILITY AND MACHINABILITY
Lead, for example, is very malleable (it can be
permanently deformed in compression without
Although not strictly properties, in the sense of the
breaking), but it is not ductile (it cannot be
other properties we have discussed, weldability and
permanently deformed in tension to any great extent).
machinability are important practical considerations
in the fabrication or repair of any metal part.
Weldability refers to the relative ease with which a
metal may be welded. Machinability describes the
The term creep describes a special kind of plastic
ease with which a metal may be turned, planed, milled,
or otherwise shaped in the machine shop. Some metals
deformation that occurs very slowly at high
temperatures when the material is under a constant
are not easily machined because they are too hard.
stress. It is interesting to note that this stress may be
Some soft metals are not easily machined because they
are too tough. Both weldability and machinability are
considerably less than the yield point of the material
at room temperature. Because creep occurs very
really based upon the combination of other properties