Since superheater and reheater tubes, headers and pipes are exposed to high temperature and high pressure, they will be damaged by creep. The heat exchanger tubes may burst mainly by creep damage due to overheating. The headers and the pipes suffer damage mainly at their weldments parts subjected to stress concentration, creating cavities and cavity-to-cavity coupling cracks due to the creep damage.
As creep damage evaluation methods, the following no.1 to 4 are available. By these methods, internal cracks are detected using ultrasonic wave and creep damage is evaluated by transferring the micro structure of metal to make a replication using grain deformations and cavities as damage parameters.
The electronic focus sector scan ultrasonic testing (ELFOSS-UT) developed by Babcock-Hitachi K.K. detects internal cracks with high precision and measures their sizes. It can be used for evaluation of weldments of large diameter pipes.
Crack detection
Features
On materials with relatively high strength in grains (weldments and austenitic stainless steel), strain is concentrated along the grain boundary due to creep damage, resulting in creation of voids (cavities, vacancies). On the weldments of CrMo steel, a void creation starts at the medium stage of damage and then it grows and the number of voids increases as the creep damage progresses. As a method to quantify a void, the void area fraction method is provided. In this method, the area fraction of the void to an area to be observed is used as a damage parameter. When evaluating an actual unit, a replication of the micro structure of the subject part is made, and observed with the scanning electron microscope (SEM) to measure the void area fraction and estimate a creep life fraction based on the master data.
Since the Cr-Mo base metal is provided with high ductility in grains, the grains deform in the direction of stress as the creep damage progresses. In the MDG method, the degree of deformation of the grains due to the creep damage is quantified to evaluate the creep damage.
Generally as for alloy steels, hardness falls by aging. Furthermore, if stress applies, precipitation and growth of carbide and dislocation density,etc. will be acceleration and hardness will fall further. Creep damage is evaluated using this phenomenon.
