Abstract: By using TEM strain-induced precipitation of Nb(CN) during deformation of undercooled austenite was investigated in Nb-microalloyed steel. The results showed that at 1200°C all of Nb were dissolved and there were no Nb(CN) precipitates formed during cooling until down to 760°C; During deformation enhanced ferrite transformation Nb(CN) of dynamic precipitation required an incubation
period, but compared with isothermal transformation it reduced significantly. Only when the strain increased to 0.69, Nb(CN) began to precipitate on dislocation nodes and grain boundaries. Furthermore the volume fraction of Nb(CN) precipitation increased with increasing strain but their coarsening wasn’t significant. Results showed that the measured grain size is in good agreement with
the calculated value.
Abstract: TEM, EDS and STEM were used to investigate distribution of Mo element in new CrMo steel ADF1 which is applied to 1300MPa high strength bolts with superior delayed fracture resistance. Research results reveal that Mo easily concentrates at grain boundaries. According to refined EDS point and mapping analysis, the region of Mo segregation at grain boundary is about several nanometers wide, and distribution of Mo along the grain boundary is not uniform. The average content of Mo at grain boundary is about double of that in grain area. Concentration of Mo
can increase the bonding force of grain boundary, and this is of advantage to the improvement of strength and delayed fracture resistance of the new CrMo steel.
Abstract: The intragranular ferrite, which renders fabrication of fine-microstructure and improves toughness of welds in ultra-fine grain steels, is often observed to nucleation on non-metallic inclusion. The mechanism of this nucleation is related to the interfacial energy between austenite, ferrite with inclusions, the solute depletion zone around the inclusions and the strain energy due to different thermal coefficients between matrix and inclusions, et al. The interfacial energy of iron
with nitrides and carbides is crucial to promote the ferrite nucleation on such as VN. On the other hand, the composition change in local austenite is probably the control reason for ferrite on MnS and Ti2O3. The thermal strain energy is calculated to be far less than the driving force for phase transformation and not effective to promote ferrite nucleation unless at very small undercoolings.
Abstract: It has been well known that Hadfield steel behaviors excellent wear resistance under high impact energy. Up to now there exist many theories to explain the wear mechanism of Hadfield steel. In this research subsurface microstructure evolution process of Hadfield steel was investigated after high energy impact experiments. It was shown from high resolution electron microscope (HRTEM) examination of subsurface microstructure that nanocrystallized austenite grains have been formed in the procedure of the reaction and rearrangement of high density dislocations under the heavy plastic deformation, sub-grains as a transitional structure and, finally, the formation of nano austenite grains. On the other side, the interactions of twins and stack faults or dislocations and stack faults make austenite crystals transform to amorphous solid. With increasing impact cycles the sizes of nano-grains were decreased and the amorphous volumes were increased further. A
large amount of nano-sized grains embedded in bulk amorphous matrix were fully developed, which will dominate the wear of the steel. In the subsurface no martensitic transformation was observed.
Abstract: Cooled in water after isothermal relaxation of deformed austenite for different time, a
Nb-bearing microalloyed steel always exhibited synthetic microstructures of bainitic ferrite, granular bainite and acicular ferrite. When these samples were reheated to and held at 650°C or 700 °C, the non-equilibrious microstructures tended to evolve into equilibrious ones, accompanied by obvious change of hardness. The rate of microstructures evolution was closely related to relaxation time of deformed austenite. The sample relaxed for 60s displayed the highest thermal stability, while microstructure evolution was quickest in the sample relaxed for 1000s even though it was softest before reheating. By hardness measurement, it was found that softening was not only process occurring during reheating, in which hardness fluctuated with time. There were two peaks in hardness-time curve of each sample having undergone relaxation, while single peak occurred in the curve of the sample not being relaxed. These results indicate that thermal stability of microstructures is determined by their history of formation.
Abstract: After bainitic transformation, the dislocations formed in deformed austenite remained to be pinned by the precipitates so that thermostability of the bainitic ferrite was improved. Coarsening of the precipitates accompanied by their distribution density change occurred during reheating. After long reheating, further precipitates nucleated in bainite. Dislocations inside laths getting rid of pinning of precipitates and their polygonization play the precursor to the evolution of microstructures, in which lath boundaries disappeared gradually.
Abstract: By the aid of optical microscope, Hardness measurement, SEM, TEM and the chemical quantitative phase analysis technique, the influence of copper, niobium, and chromium on the aging hardness has been investigated. The aging precipitation behavior and the interaction between various precipitates have also been discussed. The results indicate that since there are multi aging-hardening elements in the steel the aging hardening behaviors are complicated. During the aging the ε－copper， and carbides containing iron and chromium will precipitate. Also, new
niobium carbonitride precipitation occurs. These precipitation processes cause marked hardening effect and various hardness peaks. The ε－copper can precipitate at dislocation and lath boundaries preferentially. During the aging the bainite lath united and became wider and then gradually vanished.
Abstract: In this paper, the circulation rolling plastic deformation(CRPD) surface nanocrystallization technology is proposed based on the idea that the severe plastic deformation can induce grain refinement. The equipment of CRPD is designed and manufactured. A nanocrystallization surface layer was successfully obtained in a column sample of low carbon steel. The average grain size in the top surface layer is about 18 nm, and gradually increases with the distance from the surface. The
hardness increases gradually from about 200HV0.1 in the matrix to about 600HV0.1 in the surface layer.
Abstract: Nanostructured layers were fabricated on the surface of 0.4C-1.0Cr and 1.0C-1.5Cr low alloy steels by using an ultrasonic particulate peening (USPP) technique. The microstructures and mechanical properties of the nanocrystallized layers were characterized by means of transmission electron microscopy, and nano-indentation test. Results showed that the average grain size in the surface nanocrystallized layer of 0.4C-1.0Cr and 1.0C-1.5Cr low alloy steel was about 5nm and 10nm, respectively. The nano-indentation hardnesses of the surface nanocrystallized layer were enhanced significantly and reach upwards of 8.0 GPa and 12.5 GPa, respectively.
Abstract: The design of base chemistry and optimization of rolling schedule are the two important factors that influence large strain accumulation in multi-pass rolling in order to obtain ultra-fine grain size by dynamic recrystallization. A base chemistry of 0.03C-0.003N-0.08Nb-0.015Ti-1.8Mn (all in weight %) of HTP steel design was chosen in order to control the time evolution of strain induced precipitation of NbC and the strain accumulation through precipitate interaction with recovery and
recrystallization at short inter-pass times characteristic of strip rolling. Experimental data on the critical strain for static and dynamic recrystallisation for HTP steel are used in a quantitative model to predict strain accumulation pass by pass and to achieve grain refinement by dynamic recrystallisation through large strain accumulation. The model is used to optimize the time-temperature-deformation
schedule to prevent static recrystallization during the inter-pass times and to target ultra-fine grain size through dynamic recrystallization by large strain accumulation. The model predictions are validated by simulation of strip rolling of HTP steel on the thermo-mechanical simulator (WUMSI) to obtain a uniform ultra-fine ferrite grain size of about 1.5 micrometer diameter in final ferrite microstructure.