There are various types of metallurgical damages which are associated with the operation at high temperature in aggressive environment. Almost all of the metallurgical degradations take place progressively depending upon operating temperature, length of exposure, and metallurgy of exposed metal. Unless the damaging environment is not restricted in one area, the damage in all of the metallurgical degradations is usually wide spread over large areas. Most of the damages can be predicted with timely inspection using appropriate inspection techniques like eddy currents, back scattered ultrasonic spectroscopy, and metallographic replicas. Out of all the inspection techniques the metallurgical replica is the most reliable techniques to accurately assess extent of damage. There are numerous kinds of metallurgical degradation damages which highly depend on the process conditions and compatibility of the affected metal to these conditions. Following text outlines most commonly found metallurgical damages in process industry. 1. Creep/Stress Rupture. Creep is the metallurgical damage caused due to exposure of the metal under tensile stress, at high temperature for prolonged period of time. Micro- voids are formed at the location of the stress at high temperature. The failure takes place well below the minimum yield strength of the material due to the micro‐voids. The main controlling factors in creep deformation are …
Stress Corrosion Cracking also known as Environmental Cracking. It is a group of damage mechanisms in which cracks are initiated and propagate under the simultaneous effect of a tensile stress and corrosion due to environment. Stress Corrosion Cracking Explanation. The tensile stresses originate from processes like cold forming, welding, heat treatment, machining and grinding, while corrosive environment is introduced by various process conditions. Because most of the surface of affected material remains un‐attacked, it is very difficult to detect or predict the failure. Fine cracks develop and penetrating into the material at accelerated rate. These cracks can have an intergranular or a trans granular morphology. All of the SCC mechanisms can result in catastrophic failure with minimal overall material loss. Macroscopically, stress corrosion cracking fractures have a brittle appearance. There are mainly two mechanisms of crack propagation in stress corrosion cracking. One is called as “Active Path Dissolution” in which crack propagates as the metal dissolves at the tip of the crack due to corrosion. The second mechanism is “Hydrogen Embrittlement”, in which hydrogen gas produced by electrochemical process is absorbed into the metal and breaks…
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