LOW-TEMPERATURE AUTOFRETTAGE - AN IMPROVED TECHNIQUE TO ENHANCE THE FATIGUE RESISTANCE OF THICK-WALLED TUBES AGAINST PULSATING INTERNAL-PRESSURE

It is shown that autofrettage at low temperatures is superior to autof rettage at room temperature in enhancing the fatigue resistance of thi ck-walled tubes against pulsating internal pressure. The physical reas on 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 temp eratures. As a consequence, significantly larger compressive residual hoop stresses can be introduced during pressurization at low temperatu res than at room temperature. Experimental data obtained on thick-wall ed tubes of the metastable austenitic stainless steel AISI 304 L which were subjected to pulsating internal pressure at room temperature aft er autofrettage at temperatures between -110 degrees C and room temper ature are presented. These data demonstrate convincingly the advantage s offered by low-temperature autofrettage in enhancing both the fatigu e life in the finite-life region and the fatigue endurance limit in co mparison with autofrettage at room temperature. In conclusion, some sp ecific materials requirements for optimum low-temperature autofrettage performance are discussed.