The stresses and strains introduced by low-temperature autofrettage of smooth thick-walled tubes made of the austenitic stainless steel AISI 304 L were modelled by the finite-element (FE) method. The objective was to show that low-temperature autofrettage is much more efficient than autofrettage at room temperature in enhancing the fatigue resistance by introducing a higher beneficial tangential (hoop) residual compressive stress at the inner part of the tube. Attention was paid to the influences of the autofrettage temperature and pressure, the work hardening and the reverse yielding on the residual stress components and on the total strain components of the tube. The FE calculations confirmed that more beneficial residual stress patterns can be attained by autofrettage at low rather than at room temperature. From the quantitative calculations, the optimal autofrettage temperature and pressure of the tube were concluded to be about and 4000 bar, respectively. The results of the calculations were shown to be in good agreement with recently measured data.

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.

No abstract

No abstract

Kickel base alloq 706 has estensi~e application historq for rotating parts in land based gas turbines combining high strength and toughness o\ er the \\hole range of the s e n ice temperatures. Thls material is tqpicallj finish forged in a closed die press. The size of parts needed for the land-based gas turbines is Lrerq large requiring the use of the largest forging presses in the norld. Finish forging of alloq 706 and other nickel-based alloj s for rotating parts in gas turbines require homogenous structure and properties throughout the forging. Manj evperts considered that open die forging process u ould not be able to achieve such uniformlt>. ,411 experiment mas done to determine if a \\ell-controlled open die forging process could achie\ e the necessarj uniformit! in microstructure and properties for a large gas turbine part made of a l l o~ 706 I'his paper mill describe the details of the forging and heat treatment process. Results of mechanical tests and metallographic examination \+ill be also discussed. Issues with process control and repeatability for open die forging are also discussed.