Effect of forging ratio and grain size on tensile and fatigue strength of pure titanium forgings

Abstract

Pure titanium is expected to be used in high-speed ships in light of its corrosion resistance, high specific strength, ability to reduce ship-dismantling costs, and recyclability. Titanium rudder forgings, each of which is an integrated combination of a rudder plate and a rudder stock, have been used for high-speed ships. Fatigue fractures have often been observed in the rudder stocks. In the context of these fractures, tensile and fatigue tests were performed using forgings with forging ratios of 2 and 4 and different grain sizes. Typical results obtained are summarized as follows : (1) 0.2% proof stresses in the tensile test agreed with the Hall-Petch equation; (2) their slope was approximately equal to that for low-carbon steels ; (3) S-N curves showed that forgings with a forging ratio of 4 were obviously superior in fatigue strength to those with a forging ratio of 2; (4) fatigue crack initiation was observed at grain boundaries between large grains, indicating that the decrease in fatigue strength is caused by an inhomogeneous microstructure; and (5) the forging ratio had an obvious effect on fatigue strength rather than on tensile properties.