First principle calculations have been performed to study the crystalline, electronic, and magnetic structures of three iron-carbide systems: θ-Fe3C, χ-Fe5C2, and η-Fe2C. The Kohn-Sham equations were solved by applying the full-potential linearized augmented plane wave method. The generalized gradient approximation in the Perdew-Wang formalism was used to the exchange and correlation energy functional. The internal positions of atoms within the unit cell were optimized and the ground state properties such as lattice parameter and bulk modulus were calculated. The results are compared with experimental data when available. Comparison of the two metastable systems χ-Fe5C2 and η-Fe2C shows that the last one has lower formation energy; this is corroborated by the formation sequence observed during tempering. The electronic structures of the three carbides were then studied and the magnetic moments calculated by means of electronic spin-resolved density of state calculations at their equilibrium lattice consta...

Crystalline, electronic, and magnetic structures of θ-Fe3C, χ-Fe5C2, and η-Fe2C from first principle calculation

Microstructural features induced by a high magnetic field in a hypereutectoid steel during austenitic decomposition

Grain boundary engineering by magnetic field application

Molybdenum carbide precipitation in an Fe-C-Mo alloy under a high magnetic field

Laser surface remelting (LSM) experiments were conducted on surface of 304 stainless steel using a 2 kW CW Nd: YAG laser and the effects of LSM on the intergranular corrosion (IGC) resistance of 304 stainless steel were reassessed from view point of grain boundary engineering (GBE). LSM could make the sensitized microstructures locally desensitize, and could improve the IGC resistance. The improved IGC resistance of the laser-surface-melted specimens could be attributed in part to Cr redistribution at the boundaries of the cells and grains and in part to existence of a large amount of low energy Σ(1 ≤ Σ ≤ 29) boundaries and the formation of 〈0 0 1〉(1 0 0) texture. However, the laser-surface-melted specimens became much more susceptible to IGC in the sensitization temperature region, and the corrosion rate of the resensitized specimen was even higher than that of the base materials under the same sensitization condition. A subsequent annealing treatment changed the grain boundary character distribution (GBCD) remarkably and the IGC resistance of the processed specimens was improved.

Reassessment of the effects of laser surface melting on IGC of SUS 304

The effect of low temperature magnetic tempering on carbide precipitation in a medium carbon steel, 42CrMo, has been investigated. As-quenched steel specimens were tempered at 200°C for 60 min without and with a 14-T magnetic field. Results show that under the magnetic field processing there forms the relatively high-temperature monoclinic χ-Fe5C2 carbide with a denser distribution and smaller sizes, as compared to the usual orthorhombic η-Fe2C carbide obtained without the magnetic field. The impact of the external magnetic field refers to a change in the precipitation sequence of the transition carbides by effectively lowering the Gibbs free energy of the high magnetization phases. The denser distribution and smaller size of χ-Fe5C2 precipitates are attributed to the increased nucleation rate and the weaker diffusion capacity required for growth as the formation temperature is lower. This offers additional dispersion strengthening to compensate for the decrease in strength and hardness due to the loss of supersaturation of carbon atoms in the matrix and raises the toughness of the steel.

Low temperature tempering of a medium carbon steel in high magnetic field

The Weiss model is extended by substituting the molecular field coefficient lwith a short-range-ordering coefficient g valid around and above Tc. The temperature variations of susceptibility of fcc Fe are calculated with a band model. Based on the above models, the phase equilibrium diagram of Fe-C binary system under a magnetic field is calculated.

Calculation of Magnetization and Phase Equilibrium in Fe-C Binary System under a Magnetic Field

The influence of a high magnetic field on the formation of ferrite in high purity Fe-0.52C (mass%) during austenite to ferrite transformation was studied. Results show that magnetic field can reduce the amount of Widmanstatten ferrite by enhancing the transformation driving force and make the ferrite grains elongate and align along the field direction.

Effect of a High Magnetic Field on The Formation of Widmanstatten Ferrite in Fe-0.52C

High magnetic field is applied with the field direction parallel to the rolling direction during annealing of a cold rolled IF steel sheet. Results of X-ray ODF analysis show that, magnetic field annealing retards the normal recrystallization texture evolution for the IF steel sheet. It is worth noting that an abnormal increase of orientation intensity at {100} is found after magnetic annealing for 25min at 650°C. When the magnetic field strength is increased from 10 Tesla to 14 Tesla, the a-fiber is further strengthened, especially the {100} component. Combined with EBSD analysis results, it is considered that the magnetic field does not change the mechanism of recrystallization texture evolution for the IF steel sheet in the present case.

Yu Dong Zhang

Papers

Low temperature tempering of a medium carbon steel in high magnetic field

Calculation of Magnetization and Phase Equilibrium in Fe-C Binary System under a Magnetic Field

Effect of a High Magnetic Field on The Formation of Widmanstatten Ferrite in Fe-0.52C

Characteristics of Recrystallization Texture Evolution in High Magnetic Field for Interstitial Free (IF) Steel Sheet

Revealing essence of magnetostructural coupling of Ni-Co-Mn-Ti alloys by first-principles calculations and experimental verification