Effects of Ferrite Growth Rate on Interphase Boundary Precipitation in V Microalloyed Steels

Abstract

The interphase boundary precipitation behavior of vanadium carbide during isothermal ferrite transformation, which is the important phenomena for the hot forged medium carbon steels, was investigated and modeled. It was found that the intersheet spacing of interphase boundary precipitation of VC decreased with a decrease of ferrite growth rate during isothermal transformation. As a major factor affecting such an interphase boundary precipitation behavior, it was deduced that the vanadium segregation on migrating austenite/ferrite interphse boundary is quite important to understand the repeated precipitation of VC. In numerical simulation of VC precipitation, the influence of the vanadium concentration on the precipitation behavior at austenite/ferrite interface during isothermal transformation was examined based on a time-dependent solute drag model incorporated with parabolic growth rate. It was found that the change of the intersheet spacing during ferrite growth can be simulated by the newly proposed model for interphase boundary precipitation of alloy carbide.KEY WORDS: interphase boundary precipitation; vanadium carbide; medium carbon steel.

Interphase Precipitation – An Interfacial Segregation Model

“…Figure 10 shows the correlation between the predicted ferrite growth rate and the experimentally measured ferrite half thickness, a slightly improved match from the original Fig. 8 in Murakami et al 14) The significantly larger interfacial energy required in Alloy 1 (0.55 Jm ) when fitting the literature experimental results of Murakami et al 14) as reducing the bulk C alloying in an alloy is well known to increase the interfacial energy of the incoherent interphase boundary. 38) Manganese is an austenite stabilizing element intrinsically reducing the driving force, ΔG γ→α , for ferrite growth at a given transformation temperature.…”

“…This finding is contradictory to the results of many models previously proposed. Figure 9 shows the correlation between the proposed model and the experimental results of Murakami et al, 14) it is shown how the model is capable of predicting the experimentally observed refinement of inter-sheet spacing. The interfacial energy was found to best match the experimental results when σ = 0.17 Jm produces a good fit of the results near the ferrite center and over-predicts measurements further into the ferritic grain, where an interfacial energy of σ ≈ 0.14 Jm − 2 yields a good fit.…”

“…ity of interphase precipitates; Murakami et al 14) found that the inter-sheet spacing decreases as the interface velocity reduces during an isothermal transformation at 948 K. Gray and Yeo 17) also showed that the inter-sheet spacing decreases with the proceeding of austenite to ferrite transformation during continuous cooling. This decreasing inter-sheet spacing however, is the combined effect of ferrite growth and transformation temperature.…”

“…However, as Murakamai et al 14) pointed out the pinning force from precipitates as small as 1 nm (in the order of 6 J mol − 1 ) would be relatively insignificant compared to the driving force for ferrite transformation. The relative insignificance of the pinning force of precipitates during the γ→α + MC transformation by the formation of carbide precipitates is further evidenced by the recent 3D atom probe tomography studies by Mukherjee et al 25,26) Where, in the case of a Ti-Mo micro-alloyed steel, only a few fully developed interphase precipitates were found in the microstructure quenched immediately after the transformation.…”

“…3) Many models have been proposed in attempt to explain and provide guidance for the exploitation of interphase precipitation. [8][9][10][11][12][13][14][15][16] One branch of models including the studies of Rios,10,11) Lagneborg and Zajac 12) proposed ledge based models, using the diffusion of micro-alloying elements such as V to compute the distance when solute concentration enabled nucleation of the precipitates. Okamoto and Ågren built upon these studies incorporating the effect of solute drag in the case of an Nb microalloyed steel.…”