Quality inspection of reconditioning, depositing and spraying technologies
It is impossible to assure quality inspection of modern reconditioning, depositing and spraying technologies for worn-out power equipment units and components without the inspection of the metal’s state before and after the execution of the above-mentioned technologies. The capabilities of the conventional methods used for this purpose (selective hardness measurement, metallographic investigation of the surface layer) cannot ensure the complete and reliable control of the metal’s state. The problems of metal inspection occur during the execution of the welding-thermal repair technologies and the recovery heat treatments (including the high-temperature ones) of welded joints of collectors, steam pipelines, case-shaped components, drums, rotor blades of steam turbines and other units.
There is a possibility to solve the above-indicated problem of the metal’s state control and, accordingly, of the reconditioning technologies quality by application of the new non-destructive testing method based on the use of the metal magnetic memory effect.
In the course of industrial and laboratory investigations it was established that it is unpractical to carry out any type of a reconditioning technology in presence of irreversible changes in the structure of the repaired unit’s metal, and if this unit is in the pre-failure state.
The metal magnetic memory (MMM) method, reflecting the product’s structural and process history, and the appropriate inspection instruments, which do not require any preparatory works, allow ensuring the 100% quick inspection of the entire surface and detecting units and components, on which it is unpractical to execute the reconditioning technology.
Inspection of the metal’s state using the MMM method on power equipment units and components before and after execution of reconditioning repair technologies on them allows evaluating the quality of these technologies, the structural changes, the level and the distribution of the residual stresses.
The techniques and the inspection instruments based on the use of the magnetic memory of metal are certified in Russia and are in the process of standardization at ISO. They are available at Energodiagnostika Co. Ltd. (Moscow).
The similar problem with the residual stress control occurs during carrying out the repair deposits in crack development zones in various structures.
Fig.1 shows the fragment of the results of the blade #84 inspection by the MMM method before (fig.1, a) and after (fig.1, b) application of the wear-resistant coating on the input flange. The powder coating was carried out in a special furnace at the metal heating to about 300°С.
Fig.1a. The results of the blade inspection before the application of the input flange wear-resistant coating.
Fig.1b. The results of the blade inspection after the application of the input flange wear-resistant coating.
It can be seen from the comparison of fig.1, a, and fig.1, b, that in the application area (on the segment from 350 to 550 mm) the magnetic Hp field and its gradient dHp/dx decreased and became more uniform. Using the available technique, it is possible to carry out the quantitative assessment of the reduction level of the radial component of residual stresses in the deposit area.
Fig.2, a shows the fragment of the results of the Hp magnetic field’s normal component inspection along the deposited metal in the crack formation area on one of the bridge construction units. Inspection was carried out according to the arrow indicated in fig.2, b before treatment with ultrasound (the red graph) and after treatment (the blue graph). Treatment with ultrasound was performed in order to reduce the residual stresses in the deposit area.
Fig.2a. The results of the Hp magnetic field’s normal component inspection along the deposited metal in the crack formation area.
Fig.2b. The scheme of the deposited metal inspection in the crack formation area: 1 - the crack.
It is seen in fig.2, a, that after the ultrasonic treatment the normal component of the Hp field and, accordingly, the normal component of the residual stresses decreased, and their distribution became more uniform.