Investigation On Phase Transformation During Severe Plastic Deformation And Aging In Eutectoid Steels

Solid phase transformation and deformation are the core basis for improving mechanical properties of steels, especially in developing high strength steel. The eutectoid steel is known as one of the natural composite material with pearlitic lamellar consisting of cementite and ferrite, which demonstrates high tensile strength after an appropriate treatment. The tensile strength of a heavily cold drawn eutectoid steel wire has ever been reported as high as 5.7GPa, even higher than that of martensite steels. It is well known that such a high strength of heavily cold drawn eutectoid steel (HCDES) is closely associated with the refining effect of the pearlitic structure all down to the nano scale. During the last three decades, many scholars have carried out detail researches. However, some important issues such as the phase stability during the severe plastic deformation (SPD), the distribution of carbon after the deformation and the mechanism of the increase of strength for post aging are still controversial. In this thesis, the evolution of the microstructure during heavily cold drawn, a surface mechanical attrition treatment (SMAT) and the post aging were studied by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), M?ssbauer spectroscopy. Changes of element distribution were recorded by three dimension atom probe (3DAP). Moreover, the stability of cementite and the mechanism of the post aging of HCDES have been discussed in associated with changes of microstructure, mechanical properties and elements distribution. The main conclusions are summarized as follows.Tensile tests of HCDES at different true strain indicated that the tensile strength and nominal yield strength increased with the increase of the true strain and the pearlitic lamellar was observed refined and cementite lamellar was observed plastic deformation in the SPD process. Moreover, the orientation of cementite changed. Based on the calculation of the crystallography, there are enough slip systems to be initiated after the cementite phase turned different orientation. About a half of the original cementite phase was dissolved by means of M?ssbauer spectroscopy on HCDES up toε=2.89. In addition, carbon atoms in ferrite after the deformation ofε=2.89 was detected as high as to 1.02 at.% by 3DAP which is much higher than the equilibrium solubility of carbon in ferrite, i.e.,0.1at.%. Meanwhile, carbon atoms with the concentration as high as 4.62at.% was dumped into the interfaces between ferrite and cementite.It seems that the dislocation plays significant roles in cementite dissolution in a HCDES by the combination of M?ssbauer spectroscopy and 3DAP analysis: the dissolution of cementite was mainly driven by minimizing the energy of the system and redistribution of elements. Decomposed carbon atoms were brought away maily through dislocations. Non-stoichiometric cementite structure was recorded by M?ssbauer spectroscopy. Meanwhile, the experiments also shed a light on the facilitating effect of manganese on the cementite dissolution. 3DAP experiment showed that the cementite was prone to decompose and change to be a b.c.c structure in the positions where manganese atoms segregated. There were three sites to accumulate the carbon atoms: phase interfaces between ferrite and cementite, dislocations and subgrain boundaries in ferrite. By calculating the maximum number of carbon atoms segregated per unit length of a dislocation and analyzing the thermodynamic of segregation of carbon atoms in the ferrite/cementite interfaces and the kinetic of carbon diffused into ferrite, it is concluded that the majority of carbon atoms solute in ferrite to form the supersaturated ferrite, as well as in the interfaces between cementite and ferrite.A nanostructural surface layer was synthesized on the eutectoid steel by surface mechanical attrition treatment (SMAT), and its microstructure and evolution were characterized by means of XRD and TEM. The results revealed that the original structure of pearlite consists of alternate thin plates of ferrite and cementite. After subjected to SMAT treatment, the ferrite size was reduced to nanometer scale because of well known dislocation activities under SPD: dislocation tangle, dislocation cells formed and finally led to division of ferrite grains; while the cementite lamellar underwent bending, fracture and finally dissolved into b.c.c ferrite and graphite during such a severe plastic deformation. Interestingly, f.c.c austenite phase during severe deformation at room temperature which is normally stable at high temperature was found and approved by means of TEM and back-scattering M?ssbauer spectroscopy. On the basis of the thermodynamic analysis ofα→γreverse martensite transformation, the mechanism ofα→γreverse martensite transformation was discussed: the existence of a large number of carbon atoms supersaturated in ferrite and the increase of interfacial energy caused the increase of the total free energy of the system. Although the free energy of austensite with the same composition also increases under the condition of equilibrium, the difference of free energy between ferrite and austensite at the same temperature under SPD reduced. At the same time, after SMAT, the effect of compression stress at the boundaries of nano-size grains further decreases the difference of two phases'free energy, and made free energy of austenite lower than that of ferrite near the room temperature, resulting that the reverse transformation temperature decreases dramatically. The above two factors lead to the appearance of austensite after severe plastic deformation at room temperature.Effects of aging on the microstructure and mechanical property of a drawn eutectoid carbon steel with true strain 2.89 has been studied by means of TEM and electrical resistance measurement. The results indicated that aging at 200℃for 1h increased the tensile strengths from 2245MPa to 2448MPa. Better combinated property of strength and ductility was observed after aging at 400℃for 1h in association with the precipitation of fine carbide dissolution in the drawing. Further increase of aging temperature deteriorated the property possibly due to the recrystalization triggered at high temperature.The post aging of HCDES wires has been studied by means of electrical resistivity measurement. The kinetic of the early stage of the aging after being drawn to true strain 2.89 is described by a Johnson-Mehl equation. The results showed that there were two stage of aging:in the temperature range of 80℃～200℃(3600s), the electron resistivity increased firstly, then decreased with isothermal times; however, in the temperature range of 200℃～298℃,the electron resistivity decreased with isothermal time. The values of Avrami exponent n, temperature constant K and the phase transformation activation energy E have been obtained, too. The mechanism of aging of heavily cold drawn eutectoid steel was discussed. Since there are many dislocations existed in the interfaces between ferrite and cementite and ferrite phase, high concentration of carbon atoms are supersaturated in ferrite. Thus, driving force for the formation of carbide is provided by the supersaturated ferrite. High energy interfaces between ferrite and cementite supply the nucleation sites. The transition from theε-carbide toθ-carbide and the growth of precipitations are controlled by the long-rang diffusion of carbon atoms. Due to the processes of ageing and precipitation for HCDES were similar to the processes of ageing and tempering for martensite steels, the dynamic model of severe plastic eutectoid iron interface ageing and precipitation was proposed based on the nucleation and growth model of Becker-D?ring. This model could be used to analyse the dynamic of cementite precipitation in severe plastic eutectoid iron.