Tadashi Kawasaki1, Hisashi Izumi2, Kasaburo Yoshimura3 and Masashi Kamata3
1Department of Mechanical Engineering, Faculty of Engineering, Tohoku University, Sendai
The strength of the high temperature boiler alloy AN 31 was investigated by means of a fatigue test and some other mechanical tests at temperatures ranging from 600°C to 750°C. The structural changes associated with the fatigue were also examined by an optical microscope.
The fatigue strength of the AN 31 alloy was much higher than that of the 18-8 austenitic steel and also than the proof stress of this alloy. The fatigue strength was improned by understressing followed by a process of gradually increasing the amplitude of the alternating stress in small increament.
Considerable carbide precipitations along the slip bands and grain boundaries were observed in the fatigued specimens.
A possible explanation for the observed high fatigue strength of this alloy and for the coaxing effect in fatigue may be given as follows: The niobium carbide can precipitate rapidly from the matrix under a plastic deformation under a repeated stress, and then the fine precipitates harden the matrix, causing the grains to resist fatigue damage.
While a sharp peak was observed in the hot hardness vs temperature curve at about 700°C accompanying a marked drop of the impact value, no peak was observed for the specimen which had been fatigued before the hardness test. This implies that most of the solute niobium in the matrix precipitated out during the fatigue test.
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