Microstructure, Mechanical Properties and Fracture of Austenitic Steel EK-164 After Warm Deformation at 600–900°C

The effect of the modes of thermomechanical treatment (TMT), involving warm plastic deformation (e = 2) by rolling in the temperature interval of 600 – 900°C, on the austenitic steel microstructure is studied. Using the methods of transmission electron microscopy, it is shown that this plastic deformation results in the formation of a highly defective fragmented microstructure with a high density of low-angle misorientation boundaries. Numerous grains/subgrains flattened in the rolling plane and elongated in the rolling direction are revealed. It is shown that as the TMT temperature is increased, the fraction of low-angle boundaries decreases and so does the dislocation density. It is noted that the average transverse subgrain size varies from 112 nm (TMT-1) to 306 nm (TMT-4). These peculiarities of the steel microstructure ensure advanced strength properties as a result of all TMT modes. The optimal mechanical properties are achieved in this steel after TMT-1, in which case the resulting microstructure is highly homogeneous with a high dislocation density retained; the yield strength values are 808 MPa at 20°C and 516 MPa at 650°C, with the elongation to failure of about 7%. The principal fracture mechanism of the steel samples is that of ductile dimple transgranular fracture.