Attempts were made to produce microduplex structures in 32%Cr-8%Ni steel with ferrite matrix and stable austenite phase by thermomechanical treatments. The specimens having 0.8 to 26.9μ grain size of austenite were obtained and their fatigue behaviors were studied. The main results obtained are as follows;
(1) The microduplex structures having small austenite grain sizes up to 0.8μ were produced simply by quenching from the ferrite region, cold rolling and subsequent annealing at the two phase region.
(2) Various appearances of fatigue cracks and various types of propagation were observed by changing the grain size and stress amplitude. The appearance of fatigue cracks was different at different locations of a specimen. Those were considered to be caused by the variation in crystallographic orientation, morphology and length of free path of ferrite and austenite.
(3) Although the appearance of cracks depended on the grain size, it was observed by using an electron microscope (replica) that the initiation process of the cracks had similar features in both large and small grained specimens. In fine grained specimens fine slip band cracks initiated close to the ferrite-austenite interphase or inclusions during the initiation process of ferrite-austenite interphase or inclusion cracks, while in large grained specimens slip band cracks initiated within ferrite and austenite phases.
(4) The fatigue strength followed the Petch-type relation. This may be caused by the fact that, as mentioned above, cracks initiate at the slip band irrespective of the grain size but the length of slip band cracks depends on the grain size.