Nailu Chen

Effect of Partitioning Treatment on the Mechanical Property of Fe-0.19C-1.47Mn-1.50Si Steel with Refined Martensitic Microstructure

In order to understand the effect of microstructural features on the mechanical property, quenching and partitioning (Q&P) and quenching and tempering (Q&T) treatments were carried out on a cold-rolled low-carbon Fe-0.19C-1.47Mn-1.50Si steel sheet. It has been shown that because of the rolling in advance, the grain size of prior austenite was dramatically reduced, which resulted in a great decrease in martensite packet/block size and an increase in dislocation density in martensite in the as-quenched state. However, there was no obvious change in average lath size. Different from Q&T treatment, Q&P not only stabilized a large amount of retained austenite, but also led to a serious carbon depletion in martensite as revealed by X-ray diffraction and three-dimensional-atom-probe analyses. In Q&T and Q&P samples, refining martensitic microstructure improves both the strength and impact toughness markedly but does not affect the elongation very much. Compared with Q&T sample, Q&P one is softer due to the existence of considerable amount of retained austenite and less carbon content in martensite, i.e., it has higher elongation and impact toughness but lower strength. Analyses indicated that the strength loss caused by carbon depletion in martensite is critical which has even completely covered up the strengthening effect of microstructural refinement. On the other hand, the carbon depletion in martensite is more essential in improving impact toughness, comparing the role of microstructural refinement and the existence of more retained austenite. Through a combination of rolling and Q&P processes, the refined Q&P microstructure was achieved for a greatly improved product of strength and elongation and a much lower ductile-to-brittle transition temperature.

Finite Element Simulation and Experimental Verification of Internal Stress of Quenched AISI 4140 Cylinders

The study of internal stress in quenched AISI 4140 medium carbon steel is of importance in engineering. In this work, the finite element simulation (FES) was employed to predict the distribution of internal stress in quenched AISI 4140 cylinders with two sizes of diameter based on exponent-modified (Ex-Modified) normalized function. The results indicate that the FES based on Ex-Modified normalized function proposed is better consistent with X-ray diffraction measurements of the stress distribution than FES based on normalized function proposed by Abrassart, Desalos and Leblond, respectively, which is attributed that Ex-Modified normalized function better describes transformation plasticity. Effect of temperature distribution on the phase formation, the origin of residual stress distribution and effect of transformation plasticity function on the residual stress distribution were further discussed.

Influence of Transformation Plasticity on the Distribution of Internal Stress in Three Water-Quenched Cylinders

Based on the hardenability of three medium carbon steels, cylinders with the same 60-mm diameter and 240-mm length were designed for quenching in water to obtain microstructures, including a pearlite matrix (Chinese steel mark: 45), a bainite matrix (42CrMo), and a martensite matrix (40CrNiMo). Through the combination of normalized functions describing transformation plasticity (TP), the thermo-elasto-plastic constitutive equation was deduced. The results indicate that the finite element simulation (FES) of the internal stress distribution in the three kinds of hardenable steel cylinders based on the proposed exponent-modified (Ex-Modified) normalized function is more consistent with the X-ray diffraction (XRD) measurements than those based on the normalized functions proposed by Abrassart, Desalos, and Leblond, which is attributed to the fact that the Ex-Modified normalized function better describes the TP kinetics. In addition, there was no significant difference between the calculated and measured stress distributions, even though TP was taken into account for the 45 carbon steel; that is, TP can be ignored in FES. In contrast, in the 42CrMo and 40CrNiMo alloyed steels, the significant effect of TP on the residual stress distributions was demonstrated, meaning that TP must be included in the FES. The rationality of the preceding conclusions was analyzed. The complex quenching stress is a consequence of interactions between the thermal and phase transformation stresses. The separated calculations indicate that the three steels exhibit similar thermal stress distributions for the same water-quenching condition, but different phase transformation stresses between 45 carbon steel and alloyed steels, leading to different distributions of their axial and tangential stresses.

A chemical-structural model for coherent martensite/parent interface in Mn-based antiferromagnetic shape memory alloys

The martensite/parent coherent interface of Mn-based shape memory alloys (SMAs) is a significant part in the research of their martensitic transformation, reversible shape memory effect and magnetic shape memory effect. In the present work, a chemical-structural model was proposed to calculate the martensite/parent coherent interfacial energy of Mn-X (X = Cu, Fe) alloys. In this model, the coherent heterophase interfacial energy consists of chemical and structural parts. Resulting from the formation process of the heterophase interface, the chemical interfacial energy is expressed as the incremental value of bond energy, while the structural part is obtained by calculating the interfacial strain energy. The results show that the structural interfacial energy plays the chief role in the total interfacial energy, and the total interfacial energy decreases as the temperature rises when the alloy composition is fixed. In addition, the preferred orientation has noteworthy influence on the total interfacial energy. Using the proposed model, interfacial energy, interfacial entropy, interfacial enthalpy and interfacial heat capacity are found to be correlated with temperature and interface preferred orientation. Furthermore, the influences of alloy composition, modulus softening, and the index of the habit plane on the results were discussed.

Phase-Field Study of Microstructure and Plasticity in Polycrystalline MnNi Shape Memory Alloys

AbstractThe evolution of microstructure and plasticity in polycrystalline MnNi shape memory alloys was simulated by a phase-field method considering plastic deformation. Plastic strain was found to occur around grain boundaries and intersections between martensitic bands. In addition, the accumulation of plastic strain would keep on continuously occurring during the thermal or stress cycling process. The occurring of plastic strain could be caused by the lattice incompatibility between austenite and martensite near the tip of martensite plate.