Materials of construction for low-temperature and cryogenic processes: Part 1: to succeed in materials selection for subzero temperatures, appreciate the importance of impact strength.(Cover story)

Trends in the worldwide chemical process industries, especially a steep rise in the compression and liquefaction of gases to lower the costs of their storage and transportation, have heightened the engineer's need to have a working familiarity with chemical-process operation at low (nominally, below 0[degrees]C) and cryogenic (below -100[degrees]C) temperatures. In addition to natural gas, numerous other fluids, such as hydrogen, argon, oxygen, nitrogen, carbon dioxide, ethylene, ammonia and LPG are commonly liquefied.

One key engineering consideration is the choice of materials of construction for such frigid temperatures. Aside from chemical compatibility with and corrosion resistance to the stored liquid, a key mechanical property in the assessment of a given metal or other material for such applications is its toughness, namely, its impact strength. Indeed, toughness can be more important than the tensile strength, yield strength and elongation that play so major a role in specification of materials for more conventional temperatures.

Behavior at low temperatures

At more-conventional operating temperatures, steels are normally considered to be ductile and thus to yield under loads, deforming before an ultimate failure. At subzero temperatures, by contrast, the material exhibits sudden brittleness, making it prone to unexpected fractures. In such a condition, the material is subject to failure without any plastic flow, deformation or other warning. (This strange behavior of steels was first observed in the catastrophic failure of hull sections of warships in the Arctic during World War II.)

The temperature at which the material becomes transformed from the ductile to the brittle phase as it cools is called Nil Ductility Transition Temperature (NDTT). The material's consequent susceptibility to rupture is called its notch brittleness, because the presence of any metallurgical or mechanical notch, such as a crack, can trigger premature and sudden failure as just described. Conversely, the notch toughness (or, simply, toughness) of the material can be defined as the ability of the material to resist fracture in the presence of any mechanical or metallurgical notch.

Notch toughness is a function of the impact strength: a notch-tough, and accordingly ductile, material possesses better impact strength than does a brittle one. The impact strength of a material varies with the …