Low‐temperature Autofrettage: An Improved Technique to Enhance the Fatigue Resistance of Thick‐walled Tubes Against Pulsating Internal Pressure

Abstract

It is shown that autofrettage at low temperatures is superior to autofrettage at room temperature in enhancing the fatigue resistance of thick-walled tubes against pulsating internal pressure. The physical reason is based on the well-known temperature dependence of the mechanical behaviour of metals and alloys which generally exhibit an enhancement of both the yield stress and strain hardening behaviour at lower temperatures. As a consequence, significantly larger compressive residual hoop stresses can be introduced during pressurization at low temperatures than at room temperature. Experimental data obtained on thick-walled tubes of the metastable austenitic stainless steel AISI 304 L which were subjected to pulsating internal pressure at room temperature after autofrettage at temperatures between -110°C and room temperature are presented. These data demonstrate convincingly the advantages offered by low-temperature autofrettage in enhancing both the fatigue life in the finite-life region and the fatigue endurance limit in comparison with autofrettage at room temperature. In conclusion, some specific materials requirements for optimum low-temperature autofrettage performance are discussed. NOMENCLATUREAF = autofrettage c = degree of autofrettage d = inner diameter of tube d, = diameter of circular plastic zone in tube D = outer diameter of tube p = internal autofrettage pressure plmX = autofrettage pressure for 100% autofrettage (c = 100%) T, RT = temperature and room temperature, respectively AuraF = sum of two distinct stress increments at temperature TAF A U~, ' .~ = stress increment: difference between yield stresses uO,' at temperatures T and RT T,, = (low) temperature of autofrettage = stress increment: difference between flow and yield stresses at a given temperature T E = plastic strain ut.p =tangential (hoop) stress under an internal pressure p u ,~.~ = von Mises equivalent stress under an internal pressure p u:~, uFs = axial and radial residual stress after unloading respectively urs = compressive residual tangential (hoop) stress after unloading 4; = von Mises equivalent residual stress after unloading u~,~ = 0.1% proof stress (yield stress) uuss = flow stress at fracture: ultimate tensile strength C T~, , ,~ =yield stress, 0.1% proof stress at a temperature T oE,* = flow stress after strain hardening at a temperature T u~.~, u~.~, u~~.~ = flow stress after I%, 3% and 18% plastic strain, respectively 595 596 H. MUGHRABI et al.

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

“…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]).…”

High cycle fatigue analysis in the presence of autofrettage compressive residual stress

“…Experimental studies have shown that fatigue strength can be greatly increased through application of autofrettage processes. Application of a low temperature autofrettage process was found to result in a greater than 40% increase in fatigue limit . Other autofrettage studies have reported fatigue strength increase in excess of 60% .…”

Generation of deformation-induced martensite when cryogenic turning various batches of the metastable austenitic steel AISI 347

“…Component surfaces that are exposed to high tribological loads, such as shaft counter surfaces for radial shaft seals, must have high hardness in the surface layer and at the same time a sufficient bulk toughness to guarantee their functionality [2]. For non-brittle materials like stainless austenitic steel, a high hardness in the workpiece surface layer also results in a longer service life of cyclically stressed components, because the crack initiation is hampered and the crack propagation rate is reduced [3][4][5]. Usually, workpieces are post-processed after machining by a subsequent heat treatment or a mechanical hardening process in order to increase the hardness of the workpiece surface layer.…”