PSI - Issue 16

Andrii Kotliarenko et al. / Procedia Structural Integrity 16 (2019) 223–229 Andrii Kotliarenko et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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piston stops. On the diagram (Fig. 2b), it can be determined the initial range of the rapid growth of strain and the range of the monotonic growth of strain to the value of 0.93%. To evaluate changes in mechanical properties of the metal that have changed after exposure at stress at + 650 °C , the specimen C012 was then tested in tension. The obtained data was compared with the results of a tension test at the same temperature of the specimen (C06) in the initial condition. The value of σ в is 599.7 MPa for specimen C06 and 583.6 MPa for specimen C012. Other mechanical properties of material also do not differ. Thus, we can conclude that relaxation at + 650 °C does not have a significant effect on the mechanical properties of hot-work steel at a given temperature.

3.3. Deformation of specimens with different strain rates test

Stress-strain diagrams of tension tests of specimens at different loading rates are shown in Fig. 3a  max for the specimen C011 258 MPa at  = 0.664%, and for the specimen C010 171.4 MPa at  = 0.788%. The specimens C010 and C011 after the test are presented in Fig. 3b. а b

v = 0,0001 mm/s

v = 0,01 mm/s

Fig. 3. (a) Stress-strain diagrams of hot-worked steel and (b) specimens after tests at 650  С at different deformation rates.

The absence of the neck and other notable signs of plastic strain of specimen C010 indicates the brittle fracture. Unlike specimen C010, fracture of the specimen C011 has been ductile with significant plastic strain. A characteristic feature of deformation is the lack of a stage of uniform plastic elongation – after the elastic region comes the moment of maximum tensile stress. Obviously, this feature in the specimens localizes the zone of fracture: – in the specimen C011 ( v 011 = 0,01 mm/s) in the form of the neck; – in the specimen C010 ( v = 0.0001 mm/s) a crack occurs, although the diagram is smooth and the characteristic jump of stress on it is absent. A large number of cracks can be observed on the surface of the metal near the fracture of the specimen C010 (Fig. 4). Their main direction is circumferential, across the action of tensile stresses. а b

 100

 25

Fig. 4. Images of fracture of the specimen C010 with a magnification (а) × 100 and (b) × 25.