Ultrasound Laminography

Project Number 208

The VGB Executive Board has granted a new budget from the VGB research fund in order to continue the previous research project no. 171, "Non-destructive proof of damage". The investigations will be carried out in the period from July 1, 2000 to December 31, 2004 by a research group at the Institute for Materials and Processing at TU Munich under Professor Schmitt-Thomas. The VGB Technical Committee "Materials and Quality Supervision" will organise and monitor the project.

Power plant components, like main steam pipes, reheaters that get hot and also the high pressure steam turbine are operated in the high temperature range and, after operating for more than 100,000 hours, reveal damage in the form of micropores at the grain boundaries. This process is accelerated as the number of hours increases and results in the formation of cracks that can destroy the components. The German Steam Boiler Ordinance (TRD) requires a mathematical fatigue analysis and often a structure investigation for material damage after 100,000 hours. The latter investigation is a complicated metallographic procedure at a pre-determined point of the component. In view of the complicated method, we limit ourselves today to just a few points of the order of square millimetres, without precise knowledge about the point of maximum damage; the area investigated is not necessarily characteristic of the maximum damage of the component.

Therefore, for some time now, operators and superintendents have been interested in a non-destructive test method with which the point with the maximum damage can be located and analysed quickly, cheaply and reliably. In the past, magnetic processes and the tracing of pores were investigated without success using US pulse/echo methods and also laser thermography. To determine this damage, one process that is promoted by the VGB Research Foundation and was developed at TU Munich uses US surface waves to determine the change in the modulus of elasticity as a consequence of creep damage.

In the meantime, more than 12 damaged elbows that are in service have been investigated using this method and the damage was found to correspond well to the local structure investigation. It was also found that the measuring results include influences from the chemical composition and the introduction of heat. In order to be able to quantify the previous qualitative results, taking into account approximately 8 different materials, it is necessary to expand the database. The very different damage behaviour of the new steels compared with the traditional grades will also be investigated by flaring and ovality measurements using US laminography.

The results up to now show that this method is the only one known that can detect creep damage with just slight surface abrasion reliably and quickly. The method offers considerable cost-cutting potential, because the time and cost consuming process of preparing the surface is no longer necessary.