Quality assurance measures during tightening of various bolts at Wendelstein 7-X

The central part of W7-X is the main magnet system, consisting of 50 non-planar and 20 planar superconducting coils which are fixed to a rigid central support structure. For further stiffening of the magnet system various inter-coil connection elements are used which are either welded or bolted to the coils. The bolts and screws from M20 to M90 are made from high-strength Inconel 718 as well as the tightening elements called Superbolts. In order to cope with the very large mechanical loads at 4 K due to the Lorentz-forces and to provide a secure connection, all bolts are pre-tensioned close to the elastic yield limit. Therefore the measurement of the preload must be done very carefully with an accuracy of better than 5%. This measurement is made by using the ultrasonic method. Since parameters like Young's modulus, speed of sound, temperature, etc., influence this measurement; an individual calibration must be performed for each specific charge of material and each specific dimension. Special attention must be paid to the short bolts with small grip length. Their stretching under full load often is only 1/10 mm. The paper will describe the procedure of tensioning, the structure of the database for the calibration data, the experiences with the reproducibility and accuracy of the measurement.