Fatigue of thick-walled pipes from soft martensitic and semi-austenitic chrome-nickel steels under pulsating internal pressure

Thick‐walled components subjected to pulsating internal pressure are widely applied in high‐pressure technology and in manufacturing processes such as fluid‐jet cutting and high‐pressure cleaning, mainly in conjunction with reciprocating pumps. Corrosive fluids require high‐strength and tough chrome‐nickel steels with soft martensitic or semi‐austenitic structure. This contribution reports on the fatigue of thick‐walled plain and cross‐bored pipes made from high alloy chrome‐nickel steels such as X5 CrNiMoCu 21 8 and X5 CrNiMo 16 5. The speciments, uniaxial standard form and thick‐walled pipes, were cut from forged blocks in the three axial directions. For loading with pulsating pressure, a suitable, high‐frequency piston pulsation machine has been developed. The fatigue tests on pipe specimens show typical Woehler characteristics with only slight scatter and relatively good isotropy. The surprisingly large admissible pulsating pressure can be explained for the applied steels by dynamic generation of residual stresses as a result of shake‐down effects. Presentation in a Smith digram explains the occuring dynamic shake‐down and its favourable results in comparison to the more brittle highly tensile steels. It also reveals that heat treatment to higher tensile strength does not always yield an increase in the admissible pulsating pressure. It will be shown that static autofretting and shake‐down affect the fatigue strength of thick‐walled pipe specimens in the same way. Tests with internal liners in the tube specimens provide indications on the sensitivity of material failures towards fluids. The investigation aids the understanding of the fatigue behaviour and the design of components made of modern high‐strength corrosion resistant steels.