Layer thickness measurement on composite tubes

Skogh, Lars-Ove; Sandvik Materials Technology; Sweden

Skogh, L.-O.; Sandvik Materials Technology; Sweden

ID: ECNDT-0450-2018
Session: Eddy Current-Techniques 3
Room: H2
Date: 2018-06-14
Time: 16:40 - 17:00

Sandvik manufactures composite steel tubes consisting of two different alloys. They are used in applications where the conditions outside and inside the tube require material properties that cannot be met by one material only. In the following example the inside component is an approved pressure vessel carbon steel alloy. The outside component is a corrosion resistant stainless steel alloy. In the production process the two alloys are metallurgically bonded together to achieve good thermal transfer properties. Because of the bonding total wall thickness is easily measured with conventional ultrasonic pulse-echo technique. However, it is also important to know the thicknesses of the two layers. The corrosion resistant outside layer must be thick enough to withstand the harsh environment in which it is used. The inside component must be thick enough to withstand the required pressure. Due to the similar acoustic impedances of the two materials no interface echo appears, and therefore an ultrasonic technique cannot be used for this measurement. In order to achieve continuous automatic measurement of the layer thickness of the two tube components, an eddy current technique is used to measure the layer thickness of the outside stainless steel component. The thickness of the remaining carbon steel component is then calculated as the difference between the total thickness and the thickness of the carbon steel component. This paper describes the hardware used, and explains how it has been designed to achieve robust measurements.