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

Abstract

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 highpressure 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 X5CrNiMoCu 21 8 and X5CrNiMo 16 5. The specimens, 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 occurring 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.

Finite-element modelling of low-temperature autofrettage of thick-walled tubes of the austenitic stainless steel AISI 304 L: Part I. Smooth thick-walled tubes

“…When autofrettaged with an internal pressure (p) applied on the inner surface of the tube (figure 2(a)), a plastic deformation can take place in a region (r = r p ) between the inner surface (r = r int ) and the outer surface (r = r ext ) as shown in figure 2(c) where the diameter ratio r ext /r int = 3. In order to evaluate the degree of autofrettage quantitatively, one defines [7,10,11,13,14] an 'autofrettage degree' as the ratio of (r p − r int )/(r ext − r int ).…”

“…The latter is essentially based on the (repeated) application of a high internal pressure in a thick-walled tube, causing partial yielding of the tube from the inner bore into the cross section. After autofrettage, a beneficial residual triaxial stress state whose most important component is a tangential (hoop) compressive stress is retained in the inner part of the tube (see, e.g., [1][2][3][4][5][6][7][8][9][10][11][12][13][14]).…”

“…Different mathematical treatments have been employed, such as analytic approaches [1,3,6,8,12,13], the finite-element (FE) method [16][17][18][19] and 0965-0393/98/010051+19$19.50 c 1998 IOP Publishing Ltd the boundary element method [20,21]. Furthermore, the enhancement of the fatigue resistance by the autofrettage technique has been studied by many investigators, see, for example [10,11,13,14,[22][23][24][25]. Quite generally, most earlier experimental and theoretical investigations were focused on the autofrettage of ferritic medium-to high-strength steels at room temperature, except for the work of Vetter et al [10,11] and Mischorr [7] on soft-martensitic and semi-austenitic steels, again at room temperature.…”

“…Furthermore, the enhancement of the fatigue resistance by the autofrettage technique has been studied by many investigators, see, for example [10,11,13,14,[22][23][24][25]. Quite generally, most earlier experimental and theoretical investigations were focused on the autofrettage of ferritic medium-to high-strength steels at room temperature, except for the work of Vetter et al [10,11] and Mischorr [7] on soft-martensitic and semi-austenitic steels, again at room temperature. Considering the increasing importance of thick-walled components made of austenitic stainless steels, it is timely to study the autofrettage behaviour of austenitic stainless steel in more detail.…”

Load path sensitivity and fatigue life estimation of 30CrNiMo8HH

“…Several studies on this material have been reported in the literature. Vetter et al [15] reported a set of fatigue experiments on a thick-walled plain and cross-bored pipes made from high alloy chrome-nickel steels under pulsating internal pressure. They showed that the improvement in the admissible pulsating pressure is not always achievable by the heat treatment to higher strength materials.…”