АВТОРЕФЕРАТ диссертации на соискание ученой степени кандидата технических наук, 2013 г. Специальность 05.16.02 – Металлургия черных, цветных и редких металлов
We investigated the thermodynamics and kinetics of carbide precipitation in a cold-rolled Fe-7Mn-0.1C-0.5Si medium manganese steel during low temperature tempering. The material was annealed up to 24 h at 450 C in order to follow the... more
We investigated the thermodynamics and kinetics of carbide precipitation in a cold-rolled Fe-7Mn-0.1C-0.5Si medium manganese steel during low temperature tempering. The material was annealed up to 24 h at 450 C in order to follow the kinetics of precipitation. Using thermodynamics and kinetics simulations , we predicted the growth of M 23 C 6 carbides according to the local-equilibrium negligible partition (LENP) mode where carbide growth is controlled by the diffusion of carbon, while maintaining local chemical equilibrium at the interface. Atom-probe tomography (APT) measurements performed on samples annealed for 1, 6 and 24 h at 450 C confirmed that LENP is indeed the mode of carbide growth and that Mn segregation is necessary for the nucleation. Additionally, we observed the heterogeneous nucleation of transition carbides with a carbon content between 6 and 8 at% at segregated dislocations and grain boundaries. We describe these carbides as a complex face-centered cubic transition carbide type (CFCC-TmC phase) obtained by the supersaturation of the FCC structure by carbon that will act as precursor to the more stable g-M 23 C 6 carbide that forms at the dislocations and grain boundaries. The results suggest that the addition of carbon does not directly favor the formation of austenite, since Mn is consumed by the formation of the carbides and the nucleation of austenite is thus retarded to later stages of tempering as every FCC nucleus in the initial stages of tempering is readily converted into a carbide nucleus. We propose the following sequence of transformation: (i) carbon and Mn co-segregation to dislocations and grain boundaries; (ii) formation of FCC transition carbides; (iii) growth controlled according to the LENP mode and (iv) austenite nucleation and growth.
In this paper, resistance spot weldability of high-Mn steels were investigated in order to get high reliability in welded joints of automotive components. Microstructural characterizations, cross-tensile test (CTT), microhardness tests of... more
In this paper, resistance spot weldability of high-Mn steels were investigated in order to get high reliability in welded joints of automotive components. Microstructural characterizations, cross-tensile test (CTT), microhardness tests of spot welded parts were conducted. The effects of weld current on the microstructural characteristics, mechanical properties, and fracture modes were investigated using optical microscopy (OM) and scanning electron microscopy (SEM). The hardness in the weld nugget was observed to be lower than that in the base metal (BM). In CTT, the failure initiation was observed to occur at the boundary of the weld nugget. Also welding imperfections of welded parts were investigated. Liquation cracking in heat affected zone (HAZ), porosity, and shrinkage cavity were found most common welding defects in welded parts. Furthermore, the effects of welding imperfections on weld quality and failure criteria were identified and discussed.
For 5000 years, metals have been mankind’s most essential materials owing to their ductility and strength. Linear defects called dislocations carry atomic shear steps, enabling their formability. We report chemical and structural states... more
For 5000 years, metals have been mankind’s most essential materials owing to their ductility and strength. Linear defects called dislocations carry atomic shear steps, enabling their formability. We report chemical and structural states confined at dislocations. In a body-centered cubic Fe–9 atomic percent Mn alloy, we found Mn segregation at dislocation cores during heating, followed by formation of face-centered cubic regions but no further growth. The regions are in equilibrium with the matrix and remain confined to the dislocation cores with coherent interfaces.The phenomenon resembles interface-stabilized structural states called complexions. A cubic meter of strained alloy contains up to a light year of dislocation length, suggesting that linear complexions could provide opportunities to nanostructure alloys via segregation and confined structural states.
tructural defects such as interfaces or dislocations in crystalline solid solutions are disturbed regions and attract solute segregation when diffusion is enabled (1–5). According to the Gibbs isotherm, the driving force is the reduction of the system’s energy. Extending this concept also to nonisostructural cases suggests that local structural transformations can occur if the chemical composition and stress at a defect reach a level sufficient for stabilizing a state different from that of the matrix (6, 7). The concept of interface complexions (8–17) extends the classical isothermto interface-stabilized states that have a structure and composition different from that of the matrix and remain confined in the region where they form. We observed such a phenomenon also at linear defects—edge dislocations—in a binary Fe–9 atomic % Mn model alloy in which a stable face-centered cubic (fcc; austenitic) confined structure forms in an otherwise body-centered cubic (bcc; martensitic) crystal. This is a phenomenological one-dimensional (1D) analog of the previously observed complexions that were observed at planar defects (8). We homogenized the Fe–9 atomic%Mn alloy at 1100°C and then quenched and cold-rolled it to 50% reduction for increasing the dislocation density. Subsequent annealing at (i) 400°C for 336 hours (2weeks); (ii) 450°C for 6 hours, 18 hours, and 336 hours; and (iii) 540°C for 6 hours enabled Mn diffusion (18). To characterize structure and composition at the same positions, we conducted correlative scanning transmission electron microscopy–atom probe tomography (STEM-APT) analysis (19–23).We identified structural defects using STEM and cross-correlated it with solute decoration observed with APT (Fig. 1). Two grain boundaries and a single dislocation line are highlighted in Fig. 1 by blue arrows in the STEMmicrograph and in the 3D atommap, in which they are visible asMn-enriched regions. The correlative STEM experiments clearly identify the linear Mn-enriched features in the APT volumes as dislocations. As evident from the STEM micrograph, not all dislocations attract solute segregation high enough to be detectable with APT (Fig. 1A, red arrow “1”). We obtained 1D compositional profiles along cylindrical regions with 1 nm diameter at individual locations (Fig. 1E). Profile 1 shows a concentration of 25 ± 2 atomic % Mn at the dislocation core, which corresponds to an enrichment factor of 2.7 compared with the matrix concentration of Mn (9.1 atomic %). The average thickness of the Mn-enriched zone is ~1 nm. Besides the high Mn content at the dislocation cores, the composition along the dislocation line (profile 2) reveals periodic Mn changes— enriched and depleted zones alternating with a spacing of ~5 nm, resembling a nano-sized pearl necklace.
The present paper analyzes the effect of the chemical composition and heat treatments on the microstructure and wear resistance of manganese steels. The studied steels are melted in an electric arc furnace. Alloying elements (Cr ? Ni ?... more
The present paper analyzes the effect of the chemical composition and heat treatments on the microstructure and wear resistance of manganese steels. The studied steels are melted in an electric arc furnace. Alloying elements (Cr ? Ni ? Nb) are added as ultra-fine powder in a well-heated ladle. Samples are subjected to two heat treatments: one at 1100°C and the other at 1050°C, and then quenched in water. Optical microscopy, scanning electron microscopy and X-ray diffraction are used to evaluate the microstructural changes. Hardness and microhardness measurements, mass loss and friction coefficient were also performed to determine the wear behavior of the studied steels. The results indicated that the microstructure of the manganese steel in the as-cast state consists of an auste-nitic matrix and cementite alloyed with manganese and chromium ((Fe,Mn,Cr) 3 C). Increasing chromium content increases the size of the alloyed cementite (Fe,Mn,Cr) 3 C. Addition of niobium leads to the apparition of new secondary carbide (NbC). The heat-treated microstructures consist of martensite, retained austenite and a small quantity of precipitates. Increasing in heat treatment temperature and addition of alloying elements (Cr ? Ni ? Nb) increase the hardenability of the studied steel and favor the martensitic transformation. As a result, addition of niobium and increasing in chromium and nickel contents improve hardness and wear resistance of the studied manganese steel.
In this study, the effect of heat treatments on the microstructure and wear resistance of Hadfield steel with different Cr + Ni contents is investigated. Hadfield steel with 1.2% wt. C and 12% wt. Mn is melted in an electric arc furnace.... more
In this study, the effect of heat treatments on the microstructure and wear resistance of Hadfield steel with different Cr + Ni contents is investigated. Hadfield steel with 1.2% wt. C and 12% wt. Mn is melted in an electric arc furnace. Added elements (Cr and Ni) are crushed and added as ultra-fine powders of ferro-alloyed composition in a well heated ladle. Two series of heat treatments are applied: one at 1100°C and the other at 1050°C. The micro-structure of these steels is analysed using optical microscopy, scanning electron microscopy and X-ray diffraction. The Rockwell C hardness and the Vickers microhardness are measured at ambient temperature. The wear behaviour of all samples in as-cast and heat treated states is studied using pin-on-disk wear tests. The obtained results show that the microstructure of the as-cast Hadfield steel samples consists of an austenitic matrix and complex carbides precipitated at the grain boundaries. Increase in the Cr + Ni content refines the structure that improves the hardness and the wear resistance. In the heat-treated state, the microstructure reveals two distinct phases: mar-tensite and retained austenite. The increase of heat treatment temperature favours the martensitic transformation, which positively affects the hardness and wear behaviour of studied steels.
A High Temperature Synchrotron Radiation X-Ray Powder Diffraction experiment was performed to determine the manganese compounds formed during the heating and cooling of 70 wt% PbO – 30 wt% SiO2 mixture or the equivalent glass plus 10 wt%... more
A High Temperature Synchrotron Radiation X-Ray Powder Diffraction experiment was performed to determine the manganese compounds formed during the heating and cooling of 70 wt% PbO – 30 wt% SiO2 mixture or the equivalent glass plus 10 wt% of MnO. The effect of adding calcite, dolomite and kaolinite were also studied. All mixtures were fired between 690°C and 1020°C in oxidizing conditions and analysed by Scanning Electron Microscopy. A sequence of manganese phases are formed during firing: bixbyite (Mn2O3), barysilite ((Pb,Mn)Si2O7), kentrolite (Pb2Mn2Si2O9) and braunite (Mn7SiO12). Kentrolite and braunite crystallise with different crystal habits during the heating and the cooling. If dolomite is present diopside ((Ca,Mg,Mn)2Si2O6) is formed. If calcite is present, ganomalite (Pb3(CaMn)2Si3O11), margarosanite (Pb(Ca,Mn)2Si3O9) and wollastonite ((Ca,Mn)SiO3) are also formed. Wollastonite can incorporate enough manganese to transform into bustamite ((Mn,Ca)3Si3O9) at high temperatures. This leaves less manganese available for the crystallisation of kentrolite and braunite.
