Fundamentale Mechanismen zur Verbesserung der mechanischen Eigenschaften von Kohlenstoffstählen durch kontrollierte Nanostrukturbildung
Final Report Abstract
The aim of this work was to investigate (i) the coupled mechanisms of nanostructure formation and phase transformations (nanocrystallisation and concomitant carbide dissolution against the thermodynamic equilibrium) in a medium carbon steel C45 during severe plastic deformation (SPD) using high pressure torsion (HPT) at room and elevated temperatures, and (ii) the relationship between the nano-scale structure / local phase composition achieved and the resulting mechanical properties. The research was performed using state of the art analytical techniques (high resolution and orientation- and phase mapping TEM) and new mechanical testing equipment (e.g. in-situ deformation measurements in a FE-SEM and mechanical testing for miniature tensile samples). The investigations conducted showed that room temperature HPT processing of C45 steel was effective in grain refinement, but did not provide flaw-free samples consequently. Furthermore, even good samples demonstrated together with very high strength also very limited ductility. Most likely the problem was related with room temperature processing, which inevitably leaded to formation of macro- and microcracks, and to high level of macrostresses due to inhomogeneous strain distribution during HPT. Annealing of as-HPT samples resulted in notable improvement of ductility but unfortunately also to a decrease of strength for two times (from 2064.5 MPa in as-HPT samples to 1071.5 MPa in annealed ones). In contrast, severe plastic deformation at elevated temperatures such as warm HPT presented a promising method for processing carbon steels to generate steels with enhanced mechanical properties. A record combination of strength and ductility was obtained (yield strength of ~1500 MPa, ultimate strength up to 2100 MPa and uniform elongation of 3.7%). This level of ultimate tensile strength is greater than that of some grades of the advanced high strength steel and even martensitic steels. Furthermore, the obtained in warm HPT-processed C45 steel values of strength and ductility are higher than these values obtained after room temperature HPT in steels with similar composition. However the full potential of this treatment has not been realized in the present project. Despite their strong refinement, the majority of cementite particles were located at ferrite grain boundaries and their contribution to the strength and strain hardening capacity was only minor. Further attempts should be undertaken to obtain a better intragranular distribution of cementite particles, for example by varying the initial state and/or by deformation ageing. In this respect the investigation of mechanical properties of quenched (with martensitic structure) and/or tempered carbon steels after warm HPT should be performed.
Publications
- The obtaining of a homogeneous Fe-C nanostructure from a ferritic-pearlitic dual-phase steel by high pressure torsion. NanoSPD 5, Nanjing, China, March 21-25, 2011
J.L. Ning, Yu. Ivanisenko, D. Wang, M. Murashkin, H.–J. Fecht
- “Continuous recrystallization and mechanical properties of a C45 steel after high pressure torsion”. DPG conference, Dresden, March 13-18, 2011
M. Haddad, J. Ning, U. Hörmann, M. Murashkin, Yu. Ivanisenko, H. Fecht
- “The obtaining of a homogeneous Fe-C nanostructure from a ferritic-pearlitic dual-phase steel by high pressure torsion”. Materials Science Forum Vols. 667-669 (2011) pp 199-204
J.L. Ning, Yu. Ivanisenko, D. Wang, M. Murashkin, H.–J. Fecht
- Processing of high strength carbon steels by warm severe plastic deformation. Nano 2012, Rhodes, Greece, August 26-31, 2012
Yu. Ivanisenko
- “Deformation and fracture behavior of high strength nanocrystalline medium carbon steel investigated by in-situ SEM tensile tests” E-MRS conference, Warsaw, September 16-20, 2012
M. Haddad, Yu. Ivanisenko, H.-J. Fecht
- “Grain Boundary Segregation in UFG Alloys Processed by Severe Plastic Deformation”, Adv. Eng. Materials, Vol. (2012) 14, p. 968–974
X. Sauvage, A. Ganeev, Yu. Ivanisenko, N. Enikeev, M. Murashkin, R. Valiev
(See online at https://doi.org/10.1002/adem.201200060) - “Tensile properties and work hardening behaviors of ultrafine grained carbon steel and Armco iron processed by warm high-pressure torsion”. Mat. Sci. Eng. A581 (2013) 8-15
J.L. Ning, E. Courtois-Manara, L. Kurmanaeva, A.V. Ganeev, R.Z. Valiev, C. Kübel, Yu. Ivanisenko
(See online at https://doi.org/10.1016/j.msea.2013.05.008)
