Page 382 - Forensic Structural Engineering Handbook

STEEL STRUCTURES 11.13

FIGURE 11.7 Localized, through-wall pitting corrosion in a steel member.

suspension bridges. The high strength wires are susceptible to SCC in moist environments
where H can be generated.
2

Creep
As a result of atomistic thermal activation at elevated temperatures, plain carbon and alloy
steels slowly deform inelastically under load even if the service stress is substantially below
the yield strength. This damage accumulation, known as creep, is time-dependent and is a
function of temperature and applied stress. For structures that are exposed to a constant load
(e.g., boiler tubes, turbine blades, process piping) and can undergo unrestricted deformation,
creep results in the continuous accumulation of inelastic deformation and eventual fracture.
However, creep does not usually occur in typical structural steel applications, except possi-
bly, in a long burning fire.

Metallurgical Degradation
Metallurgical degradation arises from changes in steel’s microstructure due to elevated
temperature exposure. Changes in microstructure, such as unwanted precipitation or exces-
sive grain growth, can affect strength, fracture toughness, ductility, weldability, and corro-
sion resistance. For most steels, thermal aging occurs either during elevated temperature
service, such as steam cooled piping in a power plant, or during welding, as might occur
during the fabrication of a heavy section truss. Such degradation can be identified by
mechanical property testing and/or microstructural evaluation.