PROCESS EQUIPMENT UPDATE
MATERIAL COMPOSITION Eesan Vamadevan, senior metallurgist at Sulzer, explains: “Chemical analysis and testing of the mechanical properties of the components involved in the failure is very important. Te alloy (ASTM A470 Grade C) used in the composition of the rotor disk was confirmed using optical emission spectroscopy. Te mechanical properties were also tested and only the tensile strength was found to be out of specification; in fact, it was higher than the maximum value specified and typically this can lead to increased susceptibility to corrosion.” Further investigation was carried out using energy dispersive spectroscopy (EDS) to determine the chemical composition of the material at the fracture surface. Aside from the elements that were expected to be found in the base metal alloy, the EDS identified sodium, magnesium, tin and chlorine. Te most likely source is the steam used to power the turbine.
DESIGN ANALYSIS To address all of the possible causes for the cracks in the rotor disk, an investigation into the stresses using finite element analysis (FEA) was carried out. Tis involved creating a 3D computer-generated model of the disk with the blades fitted. Te stress analysis looked at the maximum values generated while the rotor operated at maximum continuous operating speed (MCOS). In this case, the equivalent stresses in the disk at the short blade root exceeded 100 ksi all the way along the root axial width, however, the maximum stress value is 114 ksi, which is less than the measured yield strength of the material, 125 ksi. Tis provided further evidence that the stress alone was not sufficient to cause the material to yield, rather, that stress corrosion cracking was the primary cause of the failure.
A 3D CAD model of the blade was created by Sulzer to perform the finite element analysis
Even with the largest projects, attention to detail is crucial to delivering a long-lasting solution
disk housing. In this case, a total of seven roots with cracks were identified using a magnetic particle inspection process. Te next step is to determine the cause of the cracks and several inspections were carried out. Initially, four cracks were opened mechanically and the fracture surfaces were examined using a scanning electron microscope (SEM), which showed evidence of intergranular cracking. Tere was no evidence that other fracture mechanisms, such as fatigue, had played a role in the failure. In addition, optical metallography was used to examine a section of the cracked area and found branched cracking immediately below the fracture surface. In conjunction with the findings from the SEM observations, the cause of the cracks was confirmed as stress corrosion cracking.
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