Effect of Forging Ratio and Grain Size on Tensile and Fatigue Strength of Pure Titanium Forgings
Yuki KOBAYASHI, Yoshiteru TANAKA, Kazuyoshi MATSUOKA, Kazuhiro KINOSHITA, Yoshiyuki MIYMOTO and Hidenori MURATA
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. Key Words:Pure titanium forgings, Forging ratio, Grain size, Hall-Petch equation, Fatigue, S-N curve, Crack initiation