Nucleation of Proeutectoid Ferrite on Complex Precipitates in Austenite

Abstract

Effect of various intragranular inclusions or precipitates (MnS, VC and V(C,N)) on the microstructure and kinetics of intragranular ferrite transformation at the temperatures between 973 and 823 K was studied using various Fe-2Mn-(0.13, 0.2)C(mass%) alloys with the small addition of sulfur, vanadium, and nitrogen. In Fe-2Mn-0.13C-50ppmS and Fe-2Mn-0.2C-470ppmS alloys, MnS particles, mostly incoherent in austenite, do not act as effective nucleation sites of ferrite. V addition slightly improves the potency of MnS as ferrite nucleation site by forming MnSϩVC complex precipitates. The addition of both V and N largely enhances the intragranular nucleation of ferrite idiomorph on the MnSϩV(C,N) complex precipitate. It is considered that two factors, i.e., (1) the advantage in the balance of interphase boundary energy and (2) the increase in the fraction of V(C,N) precipitate by the addition of nitrogen, are mainly responsible for the promotion of intragranular ferrite formation on the MnSϩV(C,N) complex precipitate.KEY WORDS: phase transformation; precipitation; steel; austenite; ferrite; carbide; nitride; sulfide; inclusion; crystallography; interphase boundary. contain similar dispersion of MnS as that of Steel C. In Steel D, VC precipitates in austenite with the addition of 0.3 mass% V. V(C,N) precipitation occurs in Steel E due to the addition of both V and N. The solution temperatures of inclusion/precipitate phases in austenite were calculated by using the solubility product equation proposed by Turkdogan 9) for MnS and Thermo-Calc for VC and V(C,N). Figure 1 shows the heat treatments employed in the present study. Austenitizing was made at 1 473 K for 0.6 ks after homogenizing at 1 473 K for 43.2 ks of hot-rolled plates. The average grain sizes of austenite after this treatment were 520 mm for Steel A, 450 mm for Steel B, and nearly equal to 150 mm for all of Steels C-E. After austenitizing at 1 473 K, the precipitation treatments of VC and V(C,N) at 1 173 K for various periods were performed for Steels D and E, followed by isothermal holding in the temperature range between 973 and 823 K to promote proeutectoid ferrite transformation.Microstructures of the transformed specimens were observed by means of optical, scanning and transmission electron microscopy (SEM and TEM). Volume fractions of ferrite were determined by point counting in optical micrographs. Electron probe microanalyzer (EPMA) and TEM-EDS (energy dispersive X-ray spectroscopy) analyses were made for identifying precipitate phases which act as ferrite nucleation sites. For optical and SEM observations, specimens were etched with 5 % nital. TEM thin foil specimens, 3 mm in diameter, were prepared by mechanical thinning followed by Argon ion thinning. TEM observation was performed by using Joel JEM-200CX and Philips CM200, CM200FEG operated at 200 kV. Figure 2(a) shows the optical microstructure of the specimen water-quenched after austenitizing in Steel C containing 470 ppm of S. Because the specimens were hot-rolled, MnS particles are aligned ...

On the Role of Non-metallic Inclusions in the Nucleation of Acicular Ferrite in Steels

“…These pure MnS inclusions are not active nucleants for acicular ferrite, because they are incoherent in austenite. 53) Moreover, they also have the least difference in thermal expansion coefficient from austenite, as shown in Fig. 10.…”

“…Miyamoto et al 87) and Furuhura et al 53,54) observed that the addition of about 0.3 % V and 0.01 % N to a low-carbon steel makes it more effective in nucleating AF. This was due to the formation of MnS-V(C, N) complex precipitates.…”

“…49) Grong et al 50) also suggested that the reduction of the energy barrier is the primary cause of nucleation. In addition, Zhang and Farrar, 51) Lee et al 52) and Furuhara et al 53,54) reported that the surfaces of inclusions acted as inert substrates, with the ability to reduce the free energy barrier to nucleation.…”

“…In the open literature, the following mechanisms have been suggested to cause nucleation of acicular ferrite at inclusions: -a reduction in the interfacial energy for simple heterogeneous nucleation on a surface of inclusions, [49][50][51][52][53][54] -an epitaxial nucleation on the inclusions, which have a good coherency with ferrite, [55][56][57] -a nucleation arising from the thermal strains at the inclusions, which is associated with the different thermal expansion coefficients of the inclusions and steel matrix, 58,59) -a nucleation arising from solute depletion of elements in the matrix near inclusions. 10,12,46,[60][61][62][63][64][65] It should be pointed out that it is, in many cases, difficult to decide which mechanism that is most important for formation of acicular ferrite.…”

Effect of Nano Size TiO<sub>2</sub> Particles on Mechanical Properties of AWS E 11018M Type Electrode

“…Recently, He and Edmonds [24] speculated that the formation of Fe-V clusters could act as nucleation sites for acicular ferrite in addition to inclusions. Furthermore, Furahara et al [25] showed that addition of V and N to C-Mn steel containing MnS inclusions led to the formation of acicular ferrite. Above discussion probably suggests that increase in hardenability elements in weld deposits with nano-size TiO 2 allows for greater under cooling as well as favorable inclusion characteristics for greater possibility of acicular ferrite formation.…”