Thermal Expansion
A further property that can be relevant in high temperature applications is the thermal expansion of the particular material. The coefficient of thermal expansion is expressed in units of proportional change of length for each degree increase in temperature, usually x10-6/°C, μm/m/°C, or x10-6cm/cm/°C, all of which are identical units. The increase in length (or diameter, thickness, etc) can be readily calculated by multiplying the original dimension by the temperature change by the coefficient of thermal expansion. For example, if a three metre long Grade 304 bar (coefficient of expansion 17.2 μm/m/°C) is heated from 20°C to 200°C, the length increases by:

3.00 x 180 x 17.2 = 9288 μm = 9.3 mm

The coefficient of thermal expansion of the austenitic stainless steels is higher than for most other grades of steel, as shown in the following table.

Table 2. Coefficient of thermal expansion - average values over 1-100°C


Coefficient of Thermal Expansion
(x10-6/°C)

Carbon Steels
12

Austenitic Steels
17

Duplex Steels
14

Ferritic Steels
10

Martensitic Steels
10

* or micrometres/metre/°C


This expansion coefficient not only varies between steel grades, it also increases slightly with temperature. Grade 304 has a coefficient of 17.2 x 10-6/°C over the temperature range 0 to 100°C, but increases above this temperature

The effect of thermal expansion is most noticeable where components are restrained, as the expansion results in buckling and bending. A problem can also arise if two dissimilar metals are fabricated together and then heated; dissimilar coefficients will again result in buckling or bending. In general the quite high thermal expansion rates of the austenitic stainless steels mean that fabrications in these alloys may have more dimensional problems than similar fabrications in carbon or low alloy steels, in ferritic, martensitic or duplex stainless steels.

The non-austenitic stainless steels also have somewhat higher thermal conductivities than the austenitic grades, which may be an advantage in certain applications.

Localised stresses from expansion during heating and cooling can contribute to stress corrosion cracking in an environment which would not normally attack the metal. These applications require design to minimise the adverse effects of temperature differentials such as the use of expansion joints to permit movement without distortion and the avoidance of notches and abrupt changes of section.



Source: Atlas Steels Australia



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