Die Scheiteldruckprüfung korrosionsbeanspruchter Hoch- und Höchstdruckrohre zur Untersuchung des Bauteil-Rißwachstumsverhaltens unter Modus II-Schwingungsrißkorrosion

External Crown Fatigue Loading of High and Ultrahigh Pressure Tubes Subjected to Corrosion – a Highly Informative Test Predicting the Crack Growth Behaviour of Tubular Components under Mode II Corrosion Fatigue Conditions Stressing high and ultrahigh pressure tubes by external static or fatigue loads has been qualified as a convenient method to simulate the load case “internal pulsating pressure” by analysing the stress state of thick walled tubes when loaded by internal pressure and external crown loads, respectively. The results of different analytical calculations were compared with that of a Finite-Element-Computation demonstrating, for tubes with nominal pressures in the range of 325–3600 bar, an excellent correspondence. Tests with 86 tube cuttings of steel X 6 CrNiMoTi 17 12 2 (W.-No. 1.4571; ASTM UNS S 31635; BS 320531) showed the following results: In air, pulsating pressures of 325 bar (corresponding to the maximum allowable operating pressures) are sustained indefinitely. Under mode II-corrosion fatigue in 0,1 N H₂SO₄ (30°C) failure occures after 3,8 · 10⁷ mode cycles. A twentyfold H₂SO₄ concentration will lower the number of cycles to fracture to one tenth of this value without leaving mode II. Under mode II corrosion fatigue crack growth will propagate faster in radial direction than in air, so that leak-before-break under internal pressure will be likely. Crack growth rates in radial direction increased with increasing acid concentration so that the probability for leak-before-break will further increase. Highest priority for the surveillance strategy of components loaded in mode II CF has, however, the prove that crack initiation in this environment is commencing much earlier than in air, and definitely earlier than found for compact specimens tested in a mode II pulsating fatigue or rotating bending test.