Application of Thermal Expension

Application Considerations

With respect to temperature, the magnitude ofthe CTE increases with rising temperature .Thermal expansion of pure metals has been wellc up to their melting points, but datafor engineering alloys at very high temperatures may be limited. In general, CTE values for metals fall between those of ceramics (lower values)apolymers (higher values). Common values

for metals and alloys are in the range of 10 to

30 × 10–6/K (5.5 to 16.5 × 10–6/°F). The lowest

expansion is found in the iron-nickel alloys such

as Invar. Increasing expansion occurs with sili-

con, tungsten, titanium, silver, iron, nickel, steel,

gold, copper, tin, magnesium, aluminum, zinc,

lead, potassium, sodium, and lithium.

Low-expansion alloys are materials with di-

mensions that do not change appreciably with

temperature. Alloys included in this category are

various binary iron-nickel alloys and several ter-

nary alloys of iron combined with nickel-

chromium, nickel-cobalt, or cobalt-chromium

alloying. Low-expansion alloys are used in ap-

plications such as rods and tapes for geodetic

surveying, compensating pendulums and bal-

ance wheels for clocks and watches, moving

parts that require control of expansion (such as

pistons for some internal-combustion engines),

bimetal strip, glass-to-metal seals, thermostatic

strip, vessels and piping for storage and trans-

portation of liquefied natural gas, superconduct-

ing systems in power transmissions, integrated-

circuit lead frames, components for radios and

other electronic devices, and structural compo-

nents in optical and laser measuring systems.

Aluminum and Aluminum Alloys. The di-

mensional change of aluminum and its alloys

with a change of temperature is roughly twice

that of the ferrous metals. The average CTE for

commercially pure metal is 24 × 10–6/K (13 ×

10–6/°F). Aluminum alloys are affected by the

presence of silicon and copper, which reduce ex-

pansion, and magnesium, which increases it. Its

high expansion should be considered when alu-

minum is used with other materials, especially in

rigid structures, although the stresses developed

are moderated by the low elastic modulus of alu-

minum. If dimensions are very large, as for ex-

ample in a light alloy superstructure on a steel

ship or where large pieces of aluminum are set on

a steel framework or in masonry, then slip joints,

plastic caulking, and other stress-relieving de-

vices are usually needed. In the aluminum inter-

nal-combustion engine piston that works in an

iron or steel cylinder, differential expansion is

countered by the employment of low-expansion

iron cylinder linings, or by split piston skirts and

nonexpanding struts cast into the piston.

Steels. Plain chromium stainless steel grades

have an expansion coefficient similar to carbon

(mild) steels, but that of the austenitic grades is

about 11

⁄2 times higher. The combination of high

expansion and low thermal conductivity means

that precautions must be taken to avoid adverse

effects. For example, during welding of

austenitic grades use low heat input, dissipate

heat by use of copper backing bars, and use ad-

equate jigging. Coefficient of thermal expansion

must be considered in components that use a

mixture of materials such as heat exchangers

with mild steel shells and austenitic grade tubes.

Welding. The coefficient of thermal expan-

sion is an important factor when welding two

dissimilar base metals. Large differences in the

CTE values of adjacent metals during cooling

will induce tensile stress in one metal and com-

pressive stress in the other. The metal subject to

tensile stress may hot crack during welding, or it

may cold crack in service unless the stresses are

relieved thermally or mechanically. This factor

is particularly important in joints that will oper-

ate at elevated temperatures in a cyclic tempera-

ture mode. A common example of this is

austenitic stainless steel/ferritic steel pipe butt

joints used in energy-conversion plantsl.