Page 312 - Chemical engineering design

287
MATERIALS OF CONSTRUCTION
7.3.6. Creep
Creep is the gradual extension of a material under a steady tensile stress, over a prolonged
period of time. It is usually only important at high temperatures; for instance, with steam
and gas turbine blades. For a few materials, notably lead, the rate of creep is significant
at moderate temperatures. Lead will creep under its own weight at room temperature and
lead linings must be supported at frequent intervals.
The creep strength of a material is usually reported as the stress to cause rupture in
100,000 hours, at the test temperature.

7.3.7. Effect of temperature on the mechanical properties
The tensile strength and elastic modulus of metals decrease with increasing temperature.
For example, the tensile strength of mild steel (low carbon steel, C < 0.25 per cent)
2
Ž
Ž
is 450 N/mm at 25 C falling to 210 at 500 C, and the value of Young’s modulus
2
Ž
2
Ž
200,000 N/mm at 25 C falling to 150,000 N/mm at 500 C. If equipment is being
designed to operate at high temperatures, materials that retain their strength must be
selected. The stainless steels are superior in this respect to plain carbon steels.
Creep resistance will be important if the material is subjected to high stresses at elevated
temperatures. Special alloys, such as Inconel (International Nickel Co.), are used for high
temperature equipment such as furnace tubes.
The selection of materials for high-temperature applications is discussed by Day (1979).
Ž
At low temperatures, less than 10 C, metals that are normally ductile can fail in a
brittle manner. Serious disasters have occurred through the failure of welded carbon steel
vessels at low temperatures. The phenomenon of brittle failure is associated with the
crystalline structure of metals. Metals with a body-centred-cubic (bcc) lattice are more
liable to brittle failure than those with a face-centred-cubic (fcc) or hexagonal lattice. For
low-temperature equipment, such as cryogenic plant and liquefied-gas storages, austenitic
stainless steel (fcc) or aluminium alloys (hex) should be specified; see Wigley (1978).
V-notch impact tests, such as the Charpy test, are used to test the susceptibility of
materials to brittle failure: see Wells (1968) and BS 131.
The brittle fracture of welded structures is a complex phenomenon and is dependent on
plate thickness and the residual stresses present after fabrication; as well as the operating
temperature. A comprehensive discussion of brittle fracture in steel structures is given by
Boyd (1970).

7.4. CORROSION RESISTANCE
The conditions that cause corrosion can arise in a variety of ways. For this brief discussion
on the selection of materials it is convenient to classify corrosion into the following
categories:

1. General wastage of material uniform corrosion.
2. Galvanic corrosion dissimilar metals in contact.
3. Pitting localised attack.
4. Intergranular corrosion.