Prediction Of Creep Crack Initiation In Pipes With Embedded Spherical Defects

For energy conservation and the reduction of greenhouse gas emissions, clean and efficient ultra-supercritical(USC) thermal power units are being developed actively all over the world. However, defects usually exist in large components during manufacturing process and cracks, pores, slags and incomplete penetration are usually detected after welding process. These defects severely affect the service life of large components such as steam pipes. Therefore, the integrity of pipes containing defects must be evaluated accurately to maintain safety and save money.Some researchers have conducted many studies on creep crack initiation and growth and the current integrity assessment methods R5, RCC-MR code, Two-Criteria-Diagram, and ASME code are relatively mature for creep crack assessment. However, there are no specialized assessment methods for volume defects under creep conditions. According to BS 7910, if non-planar flaws in steels meet certain requirements, they could be free from assessment. However, this may be improper since it does not consider creep damage accumulation. Some non-planar flaws deemed to be acceptable may be dangerous and could not be ignored. On the other hand, it is excessively conservative to treat non-planar flaws as if they are planar, using the methods of assessing planar flaws to determine the appropriate dimensions. Therefore, supervision departments of power plants may make improper maintenance decisions due to the lack of appropriate assessment methods for volume defects.Based on damage mechanics theory, numerical simulation technique is applied to study the creep damage mechanism of pipes containing embedded spherical defects, and then life prediction model is established. The main research work and conclusions are summarized as follows:(1) A new parameter–the average equivalent stress is proposed. Within the creep crack initiation time, ti, the equivalent stress is integrated over time and then divided by ti. The average equivalent stress can represent the stress level of the entire creep damage process, which is of physical significance.(2) Based on the average equivalent stress method, creep crack initiation laws of P92 steel pipes with embedded spherical defects are investigated and the reason of the laws is further studied in terms of stress and constraint. The creep crack initiation position and time prediction functions of P92 pipes containing embedded spherical defects are proposed.(3) The creep crack initiation position and time prediction functions of P92 pipes containing embedded spherical defects are improved and extended by considering different material properties and service temperatures. Therefore, the creep crack initiation position and time prediction models are established, which can be applied to various materials at different temperatures. The creep crack initiation position and time results can be calculated by these prediction models according to the geometric factors and loading factors, which would save much time consumed by finite element methods and data processing.