Mechanical and Metallurgical Failure Modes
Mechanical failure modes
Mechanical failures come from the wrong combination of stress and resistance. Overload produces either ductile fracture (dimples, shear lip, visible deformation) or brittle fracture (cleavage, no deformation, rapid). Brittle fracture needs a stress riser, a thick or hard section, and a temperature below the ductile-to-brittle transition; it commonly occurs during startup, hydrotest, or cold weather.
Fatigue is cyclic-stress driven and initiates at stress concentrations - weld toes, keyways, thread roots, corrosion pits. Mechanical fatigue shows beach marks and ratchet marks on the fracture; vibration fatigue is high cycle, low stress, and often found on small-bore connections and tubing. Thermal fatigue comes from cyclic temperature gradients rather than applied load and usually shows multiple parallel cracks at the surface. Corrosion fatigue has no fatigue limit; the environment removes the protective threshold.
Creep needs elevated temperature plus sustained stress; look for bulging, dimensional growth, and intergranular cavitation, and remember the creep range for carbon steel begins in the vicinity of 700 to 800 deg F. Erosion, cavitation, and fretting also remove metal but with distinct appearance: erosion is directional, cavitation produces clustered rough pits where bubbles collapse, fretting is fine debris at mated surfaces.
Metallurgical embrittlement
Temper embrittlement, sigma phase, and 885 deg F embrittlement reduce toughness without obvious metal loss; suspect them when a Cr-Mo or stainless component cracks in a brittle manner at low or ambient temperature after long-time exposure.
Trap and example
The classic trap is confusing fatigue, thermal fatigue, and SCC. Worked example: a pump impeller shows smooth, rounded, localized pitting on the low-pressure side of the vanes with a spongy texture. That is cavitation; erosion would be preferential grooving at the leading edges, and corrosion would be more uniform.