Effect of thermomechanical treatment, including extrusion, on the mechanical properties of steel

Conclusions

• 1. The optimum combination of mechanical properties results from thermomechanical treatment and tempering at 100°C. After ordinary quenching and tempering at 100°C σ_b=200 kg/mm², while after high-temperature thermomechanical treatment σ_b=215 kg/mm², and after low-temperature thermomechanical treatment σ_b=238 kg/mm² (ε=80%). Under the same conditions, the yield strength changes from 160 to 170 and 185 kg/mm². The strength increases from 232 to 317 and 330 kg/mm², mainly as the result of the great increase in reduction of the section after thermomechanical treatment (ψ changes from 30 to 52 and 42% respectively). The relative elongation increases from 9 to 11.5% (high-temperature thermomechanical treatment) and 10.8% (low-temperature thermomechanical treatment), and the impact strength increases from 5.3 to 6.5 and 8.1 kgm/cm² respectively.

• 2. The highest ductility and impact strength result from deformation at 700–850°C, while the highest strength results from deformation at 550–850°C. The increase in ductility apparently depends on the process of recovery in austenite.

• 3. An increase in the degree of plastic deformation from 30 to 80% increases the ductility, while the strength remains the same. Apparently this is due to heating of the sample induced by deformation and the processes occurring as the result of recovery.

• 4. The recrystallization resulting from thermomechanical treatment (deformation 70–80% at 850°C) leads to considerable increase in ductility without decreasing the strength. Thus, recrystallization has a favorable effect on the results of thermomechanical treatment under the conditions tested.

• 5. It was shown that the relationship between the hardness and the ultimate strength of the steel ceases to exist as the degree of plastic deformation increases. Therefore, hardness is not a characteristic of the degree of strengthening induced by thermomechanical treatment.