Crystal Orientation Relationships between Acicular Ferrite, Oxide, and the Austenite Matrix

Abstract

Crystal orientation relationships between acicular ferrite (AF), oxide and the austenite matrix have been investigated in low carbon steel weld by the submerged arc welding process. In particular, this study focused on the formation mechanism of the crystal orientation relationships. The AF microstructure was observed in weld metal containing titanium. Oxide particles were composed of MnTi₂O₄, amorphous phase and TiO₂. The AF nucleated on MnTi₂O₄ having the Baker-Nutting (B-N) orientation relationship with the MnTi₂O₄ and Kurdjumov-Sachs (K-S) orientation relationship with the austenite matrix. This result implies that the MnTi₂O₄ had a rational orientation relationship with the austenite matrix. The orientation relationship is considered to be (001)_MnTi2O4//(111)_γ, [100]_MnTi2O4//[211]_γ from the viewpoint of the lattice coherency. It is supposed that the MnTi₂O₄ can be formed within oxide particles having this orientation relationship with the austenite matrix at high temperature during welding process. This mechanism allows the coexistence of both B-N and K-S orientation relationships, which lowers the AF/MnTi₂O₄ and AF/austenite interfacial energies. This results in the decrease of the activation energy for AF nucleation.

1. Introduction

Acicular ferrite (AF) is a fine microstructure formed in low alloy steel weld metals or heat affected zones and has favorable effects on toughness.^1,2,3,4) It is known that some inclusions, e.g. oxides,⁵⁾ nitrides^3,4,6,7) and sulfides^8,9) act as nucleation sites for AF. In particular, many studies have reported that titanium oxides, such as TiO,^2,10,11,12) TiO₂,¹³⁾ Ti₂O₃,¹²⁾ MnTi₂O₄^14,15) promote AF formation.

However, there are still unclear points regarding the formation mechanism of AF. Many authors have proposed formation mechanisms, summarized as follows;¹⁶⁾

– Lowering of the interfacial energy between the inclusion and the AF^11,17)

– Change in chemical composition of austenite matrix local to the inclusion, e.g. manganese depleted zone^3,12,13)

– Stress due to the difference of the thermal expansion between the inclusion and austenite matrix

In addition, Dowling et al. reported that inclusions just act as inert substrates¹⁸⁾ and the kind of inclusion has less influence on the AF nucleation. In recent years, some studies which support the interfacial energy theory have been published.^10,14,19) In these studies, it is revealed that the Baker-Nutting (B-N) orientation relationship is formed between AF and oxide, which achieves good lattice coherency and decreases the interfacial energy between AF and oxide. Moreover, it has been pointed out that AF satisfies the Kurdjumov-Sachs (K-S) orientation relationship with the austenite matrix in addition to the B-N orientation relationship with oxide.^14,20) This implies that a rational orientation relationship is formed between the oxide and the austenite matrix. However, the formation mechanisms of these orientation relationships still remain to be clarified.

The purpose of this paper is to investigate the orientation relationships between AF, oxides and the austenite matrix and to improve the understanding of the formation mechanism of these orientation relationships. Especially, we discuss the orientation relationship between oxides and the austenite matrix.

2. Experimental Procedure

Two multi-pass weld specimens were deposited in a groove by submerged arc welding process to examine the influence of titanium addition on microstructure. Carbon steel plates of 25 mm thickness were prepared and a 10-degree V-groove was cut with a 15 mm root gap. The welding current, voltage, speed and interpass temperature were 425 A, 30 V, 5.8 mm/s and 453–473 K, respectively. The chemical compositions of the specimens are shown in Table 1. The specimen TI contains 510 ppm of titanium so that Mn–Ti oxide is formed. In contrast, the concentration of titanium in the specimen NT is 10 ppm. Phosphorus and sulfur concentrations of both specimens are less than 80 ppm and 30 ppm respectively.

Table 1. Chemical compositions of the weld metal specimens (mass%).

	C	Si	Mn	Al	Ni	Cr	Mo	Ti	O	N	Fe
TI	0.13	0.13	1.65	0.005	1.60	0.28	0.77	0.051	0.025	<0.010	bal.
NT	0.11	0.09	1.45	0.007	1.54	0.28	0.77	0.001	0.037	<0.010	bal.

Microstructures were observed with light optical microscopy (OM) at the as-welded zones (final-pass). The cross-sections of the weld metals normal to the welding direction were polished and etched with 3% nital solution (3% nitric acid and 97% methanol) before observation. The AF formation rate P_AF which is defined by the following equation was measured at a 1000 times magnification.   

$$P_{AF}=N_{AF}/N_S\times 100$$(1)

where N_AF is the number density of oxides which act as AF nucleation sites and N_S is the total number of oxides per unit area. N_AF and N_S were measured in regard to oxides with circle equivalent diameters of over 0.5 μm. The total area for measurement was 0.060 mm² per specimen.

Crystal orientation of the as-welded zone was analyzed by electron backscatter diffraction (EBSD) using a JEOL JSM-6500F. Selected area electron diffraction (SAED) pattern and energy dispersive X-ray spectroscopy (EDS) by transmission electron microscopy (TEM) analyses were made for identifying the oxides which act as AF nucleation sites. TEM observation was performed by using JEOL JEM-2010F operated at 200 kV. Specimen for TEM analysis was fabricated by focused ion beam. The orientation relationship between the oxide and the AF was evaluated by Kikuchi pattern analysis.²¹⁾

3. Results

3.1. Microstructure and Identification of Oxide Promoting AF

The OM images of the as-welded zones are shown in Fig. 1. The AF microstructure is observed over the entire surface of the specimen TI. As arrows indicate, oxide particles act as nucleation sites for AF in the specimen TI. The N_AF and N_S are 514 and 1556 mm^–2 respectively. Thus, the P_AF is 33%. Some AFs are not in apparent contact with oxides. It is considered that these AFs were formed on oxides inside the specimen or sympathic-nucleated on preformed AFs.²²⁾ Considering that oxide particles in a weld metal have similar composition,¹⁴⁾ it is supposed that oxide particles which did not act as nucleation site for AF were surrounded by AF formed on other oxide particles before AF nucleation. In the specimen NT, a coarse bainite structure was formed and the AF microstructure is not observed. As many authors pointed out,^{2,10,11,12,13,14,15)} the effect of titanium addition on the formation of the AF is clear.

Fig. 1.

Optical micrograph at the as-welded zone of the specimens; (a) TI and (b) NT.

Figures 2, 3, 4 shows bright field image, EDS mapping and SAED patterns of the oxide at the as-welded zone of the specimen TI. In Fig. 2 two AF crystals (AF1 and AF2) are shown on a single oxide particle. Based on the EDS data in Fig. 3, the oxide particle is divided into three areas; Si rich, Ti rich, and both Mn and Ti rich. All three area were identified as amorphous phase, TiO₂ and MnTi₂O₄ respectively by the SAED patterns. MnS,⁸⁾ TiN^3,4,6) and TiO,^2,10,11,12) which have been also reported as AF nucleation sites, are not observed. Apparently the AF1 formed on the MnTi₂O₄, while AF2 is adjacent to both the TiO₂ and MnTi₂O₄. Considering that MnTi₂O₄ is known as a good nucleation site for AF,^14,15) it is supposed that the MnTi₂O₄ also promoted the formation of the AF in the present weld metal. Then, we focus on the AF1 in order to analyze the orientation relationships between AF, MnTi₂O₄ and the austenite matrix.

Fig. 2.

TEM bright field image of the oxide and AF at the as-welded zone of the specimen TI.

Fig. 3.

a) TEM bright field image of the oxide observed in Fig. 2 and b)–d) EDS mapping of Si, Mn, and Ti.

Fig. 4.

SAED patterns of the oxide particle observed in Fig. 2.

3.2. Orientation Relationships between AF, MnTi₂O₄, and the Austenite Matrix

The stereographic projections for the AF1 and the MnTi₂O₄ observed in Fig. 2 are shown in Fig. 5 including the {001} poles for both phases and the <110> poles for the MnTi₂O₄ crystal. There exist a near-B-N orientation relationship between the AF1 and the MnTi₂O₄:

(001)_AF//(001)_MnTi2O4, [100]_AF//[110]_MnTi2O4

This result agrees with previous work.¹⁴⁾

Fig. 5.

Stereographic projections for the AF1 and the MnTi₂O₄.

Figure 6 shows a crystal orientation map of the as-welded zone of the specimen TI containing the AF1. The (001)_α pole figure of the corresponding area is shown in Fig. 7. The AF microstructure is approximately satisfied with the character expected for the K-S orientation relationship:

(110)_AF//(111)_γ, [111]_AF//[011]_γ

The crystal orientations of points 1 and 2 indicated in Fig. 6 are shown in Fig. 7 by arrows. The crystal orientation of point 1 (just after nucleation) is slightly different from the K-S orientation relationship.

Fig. 6.

Inverse pole figure map of the as-welded zone of the specimen TI containing the AF1.

Fig. 7.

(001)_α pole figure of the area corresponding to Fig. 6.

Figure 8 shows the change of misorientation angle from point 1 to 2 in Fig. 6. Note all measurements between points 1 and 2 are within one grain.²³⁾ The misorientation angle increases continuously and indicates the presence of localized crystallographic rotation within the sub-grain, then reaches approximately constant value beyond 3 μm from point 1. The misorientation angle between points 1 and 2 is 5.5 degrees. Considering the stereographic projection (Figs. 6 and 7), it seems that the crystal orientation rotates to match more closely the K-S orientation relationship from point 1 to 2. Moreover, according to the result of TEM analysis, it is considered that AF1 nucleated having the B-N orientation relationship with MnTi₂O₄ and approximately the K-S orientation relationship with the austenite matrix, then rotated to satisfy the ideal K-S orientation relationship through the growing process. This result is the same as previous work about AF nucleation on TiO.²⁴⁾

Fig. 8.

Change of misorientation angle between points 1 and 2 within the AF1 in Fig. 6.

4. Discussion

4.1. Orientation Relationship between MnTi₂O₄ and the Austenite Matrix

It has been revealed that the AF which has the K-S orientation relationship with the austenite matrix can nucleate on MnS+V(C,N) precipitates without a specific crystal orientation relationship with the precipitate.²⁵⁾ In addition, Dowling et al. showed there are no rational orientation relationship between AF and inclusions.¹⁸⁾ However, the AF1 here has both the B-N orientation relationship with MnTi₂O₄ and the K-S orientation relationship with the austenite matrix, which is the same result as reported by Okazaki et al.¹⁴⁾ Thus, it is considered that most AFs satisfy both the B-N and K-S orientation relationships in this study. One hypothesis which can explain this difference is that both the B-N and K-S orientation relationships were formed accidentally. It is probable that some AFs nucleate on oxide with the B-N orientation relationship and the AFs of which crystal orientation is near the K-S orientation relationship can keep growing more easily, as is indicated by Takada et al.²⁴⁾ To evaluate this hypothesis, the probability that an AF satisfies both the B-N and K-S orientation relationships, P_A, is evaluated based on Grong’s method.¹⁷⁾

First, (110)_AF planes of AF and inferred (111)_γ planes of the austenite matrix are considered. The numbers of equivalent plane of (110)_AF and (111)_γ are 6 and 4 respectively. When AF is formed having a random orientation relationship with the austenite matrix, the probability that the deviation angle between at least one (110)_AF plane and (111)_γ plane is less than θ (degree), P_P, is written as   

$$P_P=6\times 4\times 2\times \omega$$(2)

where ω is   

$$\omega =\frac{1}{4\pi }\times 2\pi \times {\int }_0^{\theta }sin\theta d\theta =\frac{1}{2}\left(1-cos\theta \right)$$(3)

Next, a [111]_AF direction on the (110)_AF plane and a [011] direction on the (111)_γ plane are considered. Provided a (110)_AF plane is parallel to a (111)_γ plane, the probability by chance that at least one [111]_AF direction lies within a region where the deviation angle between [111]_AF and [011]_γ is less than θ, P_D, is given by   

$$P_D=2\times 3\times 2\times \frac{2\theta }{360}$$(4)

Thus, the probability that an AF nucleus accidentally has a crystal orientation which differs from the ideal K-S orientation relationship with a misorientation angle of less than θ, P_K, is described as   

$$P_K=P_P\times P_D=\frac{8}{5}\theta \left(1-cos\theta \right)$$(5)

Note that the B-N orientation relationship has three variants. Then, the P_A is written by   

$$P_A=3\times P_K=\frac{24}{5}\theta \left(1-cos\theta \right)$$(6)

The misorientation angle between the AF1 crystal just after nucleation and the K-S orientation relationship is measured to be about 6 degrees. The associated P_A is calculated to be 15.8% by substituting 6 degrees for θ. Apparently, this value is lower than the experimental AF formation rate (P_AF), 33%. Therefore, it is suggested that the coexistence of the B-N and K-S orientation relationships occurred not by accident.

Coexistence of the B-N and K-S orientation relationships implies that the MnTi₂O₄ (MTO) had a rational orientation relationship with the austenite matrix (A) denoted as below;

MTO-A: (hkl)_MnTi2O4//(h’k’l’)_γ, [uvw]_MnTi2O4//[u’v’w’]_γ

It is probable that this orientation relationship achieved good coherency between the MnTi₂O₄ and the austenite matrix. Taking the lattice parameter of MnTi₂O₄, 0.8628 nm,²⁶⁾ into account, one of the following pairs of crystal directions ([uvw]_MnTi2O4//[u’v’w’]_γ) is likely to have formed.

[110]_MnTi2O4//[111]_γ

[111]_MnTi2O4//[110]_γ

[100]_MnTi2O4//[211]_γ

One dimensional misfits²⁷⁾ of these vector pairs show a low value, 3.5%.

Now we estimate the MTO-A orientation relationship from the viewpoint of the crystal direction pairs above and the misorientation angle of the AF from the K-S orientation relationship. In a coordinate system where the x, y and z axes are parallel to <100>, <010> and <001> directions of the austenite matrix respectively, the orientation matrix of MnTi₂O₄ based on the MTO-A orientation relationship, M is described as   

$$\mathbf{M}=\left(\begin{array}{ccc}{u}^{\prime }/U^{\prime }& a^{\prime }& h^{\prime }/H^{\prime }\\ v^{\prime }/U^{\prime }& b^{\prime }& k^{\prime }/H^{\prime }\\ w^{\prime }/U^{\prime }& c^{\prime }& l^{\prime }/H^{\prime }\end{array}\right)\times {\left(\begin{array}{ccc}u/U& a& h/H\\ v/U& b& k/H\\ w/U& c& l/H\end{array}\right)}^{-1}$$(7)

where h, k, l, u, v, w, h’, k’, l’, u’, v’, and w’ are mirror indexes of parallel planes and directions of the MTO-A orientation relationship, U’, H’, U, H, a’, b’, c’, a, b, and c are given by   

$$U^{\prime }=\sqrt{{u^{\prime }}^2+{v^{\prime }}^2+{w^{\prime }}^2}$$(8)

  

$$H^{\prime }=\sqrt{{h^{\prime }}^2+{k^{\prime }}^2+{l^{\prime }}^2}$$(9)

  

$$\text{U}=\sqrt{u^2+v^2+w^2}$$(10)

  

$$H=\sqrt{h^2+k^2+l^2}$$(11)

  

$$\left(\begin{array}{c}{a}^{\prime }\\ b^{\prime }\\ c^{\prime }\end{array}\right)=\left(\begin{array}{c}{h}^{\prime }/H^{\prime }\\ k^{\prime }/H^{\prime }\\ l^{\prime }/H^{\prime }\end{array}\right)\times \left(\begin{array}{c}{u}^{\prime }/U^{\prime }\\ v^{\prime }/U^{\prime }\\ w^{\prime }/U^{\prime }\end{array}\right)$$(12)

  

$$\left(\begin{array}{c}a\\ b\\ c\end{array}\right)=\left(\begin{array}{c}h/H\\ k/H\\ l/H\end{array}\right)\times \left(\begin{array}{c}u/U\\ v/U\\ w/U\end{array}\right)$$(13)

respectively. Provided an AF nucleates have the B-N orientation relationship with the MnTi₂O₄, the orientation matrix of the AF, F, is written as   

$$\mathbf{F}=\mathbf{M}\times {\mathit{M}}_{\mathit{B}\mathit{N}}\times {\mathit{F}}_{\mathit{B}\mathit{N}}^{-1}$$(14)

${\mathit{M}}_{\mathit{B}\mathit{N}}\times {\mathit{F}}_{\mathit{B}\mathit{N}}^{-1}$ describes the B-N orientation relationship, e.g.   

$${\mathit{M}}_{\mathit{B}\mathit{N}}\times {\mathit{F}}_{\mathit{B}\mathit{N}}^{-1}=\left(\begin{array}{ccc}0.707& -0.707& 0\\ 0.707& 0.707& 0\\ 0& 0& 1\end{array}\right)\times {\left(\begin{array}{ccc}1& 0& 0\\ 0& 1& 0\\ 0& 0& 1\end{array}\right)}^{-1}$$(15)

The orientation matrix of ferrite satisfying the K-S orientation relationship, F_KS, is described in 24 ways corresponding to 24 variants.²⁸⁾

Then, the rotation angle between F and F_KS is regarded as the misorientation angle between the AF satisfying the B-N orientation relationship with the MnTi₂O₄ which has the MTO-A orientation relationship and the ideal K-S orientation relationship. According to this method, the calculated rotation angle shows a minimum value, 5.3 degrees when the following orientation relationship is assumed,

MTO-A:(001)_MnTi2O4//(111)_γ, [100]_MnTi2O4//[211]_γ

This value agrees approximately with the experimental result of the misorientation angle within the AF1 sub-grain, which nucleated on MnTi₂O₄ having the B-N orientation relationship and then, rotated to satisfy the K-S orientation relationship (see Fig. 8). This result supports the formation of the MTO-A orientation relationship estimated above between the MnTi₂O₄ and austenite matrix.

4.2. Formation Mechanism of Orientation Relationships between AF, MnTi₂O₄ and the Austenite Matrix

It has been considered that inclusions in weld metal may not satisfy rational orientation relationships with the austenite matrix since inclusions are formed in molten iron.²⁹⁾ In contrast, it is revealed that the MnTi₂O₄ had the MTO-A orientation relationship with the austenite matrix in the present study. One explanation for the observed MTO-A orientation relationship is that austenite grains having the MTO-A orientation relationship nucleated on MnTi₂O₄. In this case, it is expected that fine and equiaxed austenite grains might be formed since a high number density of oxide particles dispersed uniformly; N_S is 1556 mm^–2. However, the observed austenite grains have coarse elongated shapes. This indicates that the austenite grains nucleated at grain boundaries of δ ferrite, which nucleated on the interface between liquid weld metal and base metal, and then grew elongatedly in a dendrite shape. Therefore, the nucleation of austenite grains on the MnTi₂O₄ cannot explain the formation of the MTO-A orientation relationship.

The other possibility is that the MnTi₂O₄ crystalized on cooling from amorphous or liquid oxide entrapped within a solid austenite matrix at the austenite temperature range. Blais et al. reported that the solidification temperature of MnTi₂O₄ is below that of the matrix.¹⁵⁾ Therefore, it is considered that the oxide existed as a liquid phase at high temperature in the present weld metal during the welding process, and then the MnTi₂O₄ nucleated having the MTO-A orientation relationship with the surrounding austenite matrix within the oxide particle. The planer disregistry of the MTO-A orientation relationship is 8.6%. It is reported that carbide or nitride of which planer disregistry is less than 12% can be an effective nucleation agent in liquid iron.³⁰⁾ Therefore, it is considered that this low disregistry has positive effect on the formation of the MTO-A orientation relationship. It is also probable that the formation of the MTO-A orientation relationship promote the crystallization of MnTi₂O₄ in molten oxide.

Thus, the formation mechanism of the crystal orientation relationships between AF, MnTi₂O₄ and the austenite matrix during the welding process is considered as below;

– MnTi₂O₄ can be formed having the MTO-A orientation relationship with the surrounding austenite matrix from liquid oxide particle at high temperature.

– The formation of the MTO-A orientation relationship would permit AF nucleation having the B-N orientation relationship with MnTi₂O₄ and the near-K-S orientation relationship with the austenite matrix.

– The Crystal orientation of the AF rotates to satisfy the K-S orientation relationship during growth.

4.3. Formation Mechanism of AF

Assuming that AF crystal nucleates on planer surface of the oxide, the activation energy for AF nucleation, ΔG* is described as   

$$\mathrm{\Delta }G*=\frac{16\pi {\mathrm{\sigma }}_{\alpha \gamma }^3}{3{\left(\mathrm{\Delta }{G}_V+\mathrm{\Delta }{G}_S\right)}^2}\cdot f\left(\theta \right)$$(16)

where σ_αγ is the interfacial energy of AF/austenite matrix, ΔG_v is the driving force of transformation, ΔG_S is the strain energy, f(θ) is the shape factor, and θ is the contact angle of AF nuclei. The θ is determined by the austenite matrix/MnTi₂O₄ interfacial energy (σ_γX), AF/MnTi₂O₄ interfacial energy (σ_αX), and σ_αγ.³¹⁾ According to this equation, higher σ_γX, lower σ_αγ and σ_αX can lower the ΔG*. The presence of the B-N orientation relationship indicates that the planer disregistry between the AF1 and MnTi₂O₄ is small, 6.4%.³²⁾ Thus it is considered that low σ_αX derived from the good coherency has a favorable influence on the nucleation of AF. The formation of the K-S orientation relationship also lowers the σ_αγ and achieves the decrease of the ΔG*. The effect of the σ_γX should be examined carefully. Since the MTO-A orientation relationship may cause the lowering of σ_γX, which results in the increase of ΔG*. Formation of the MTO-A orientation relationship apparently has a tendency to interrupt the nucleation of AF. However, it should be noted that the formation of the MTO-A orientation relationship allows the coexistence of the B-N and K-S orientation relationships. In particular, the lowest interfacial energy between ferrite and austenite for crystals which satisfy the K-S orientation relationship has been calculated to be 0.27 J/mm², which is about a half of the average ferrite/austenite interfacial energy for all interface orientatinos.^33,34) Thus, ΔG* is calculated here to evaluate the influence of the MTO-A, B-N, and K-S orientation relationships on the AF nucleation in regard to following situations;

ΔG*₁: No rational orientation relationships are formed between AF, MnTi₂O₄ and the austenite matrix.

ΔG*₂: Only the B-N orientation relationship is formed.

ΔG*₃: Only the K-S orientation relationship is formed.

ΔG*₄: The MTO-A, B-N and K-S orientation relationships are formed.

According to Merwe,³⁵⁾ the structural interfacial energy between phases i and j, σ_ij can be written as   

$${\mathrm{\sigma }}_{ij}=\frac{\mu c}{4{\pi }^2}\left\{1+\beta -\sqrt{1+{\beta }^2}-\beta ln\left[2\beta \sqrt{1+{\beta }^2}-2{\beta }^2\right]\right\}$$(17)

where μ is rigidity modulus, c is a constant obtained by lattice parameters, and β is a constant calculated from rigidity modulus, Poisson ratio, and Young’s modulus. Based on these equations, the interfacial energy of MnTi₂O₄/AF having the B-N orientation relationship, σ_α X^BN can be estimated, ignoring chemical effects. In this calculation, it is assumed the Poisson ratio and Young’s modulus of MnTi₂O₄ are the same as TiO₂, 0.28 and 489 GPa respectively. Thus the interfacial energy is calculated to be σ_α X^BN=0.64 J/m². It is difficult to apply this equation to the calculation of the interfacial energy of MnTi₂O₄/austenite matrix having the MTO-A orientation relationship, σ_γX^MA, since the equation above is based on the interface where atoms are arranged tetragonally. However, it is valid to treat the σ_γX^MA as larger than σ_α X^BN because of the poorer planer disregistry. According to an earlier study,³⁶⁾ interfacial energies of partly coherent interface range between 0.2–0.8 J/m². Therefore, it is supposed that the σ_γX^MA is between 0.64 and 0.8 J/mm². In this study, the σ_γX^MA is assumed to be 0.7 J/m². Furthermore, in case of a random orientation relationship, it is assumed that the AF/austenite matrix, MnTi₂O₄/AF, and MnTi₂O₄/austenite matrix interfacial energies have the same value, 0.8 J/m².

Then the ΔG*₁–ΔG*₄ are evaluated based on following assumption:³⁷⁾

ΔG_v = 50×10⁶ J/m³

ΔG_S = 0 J/m³

The results are

ΔG*₁ = 1.7×10^–15 J

ΔG*₂ = 1.2×10^–15 J

ΔG*₃ = 6.6×10^–17 J

ΔG*₄ = 4.4×10^–17 J

Both ΔG*₃ and ΔG*₄ values are smaller than Morikage’s calculation for AF nucleation on TiN.³⁷⁾ In particular, the observation that ΔG*₄ is the lowest value for the assumed potential nucleation events indicates that formation of the MTO-A, B-N, and K-S orientation relationships are effective for the AF nucleation. Thus, it is considered that the formation of the MTO-A orientation relationship would permit both the B-N and K-S orientation relationships and result in enhancement of AF nucleation, though the formation of the MTO-A orientation relationship itself has a negative effect on AF nucleation. Moreover, the formation of the K-S orientation relationship may have a favorable effect on the growth of the AF in addition to nucleation. It is notable that the activation energy for ferrite nucleation at prior-austenite grain boundary is lower than that for AF nucleation on oxide particle.³⁸⁾ Nevertheless, AF microstructure is observed over the entire surface in the present study. Considering that prior-austenite grain size is large (over 100 μm), it is supposed that coarse austenite grains result in small amount of ferrite nucleation at prior-austenite grain boundaries.

In addition to the ΔG* calculations, direct measurements of Mn profiles across oxide-matrix interface provides further insight into the critical nucleation events. Figure 9 shows the measured profile along the bold line in Fig. 3(a) and indicates the high Mn content in the oxide and an essentially uniform composition in the matrix without the presence of a Mn-depleted zone reported by others^3,12) as a requisite for AF nucleation. The lack of the depleted zone further supports the proposed events leading to the calculation of ΔG*₄. So it is believed that interfacial energy effects are responsible for the preferential nucleation on MnTi₂O₄, rather than chemical effects.

Fig. 9.

Mn concentration profile along the bold line in Fig. 3(a).

Therefore, it is concluded that the AF formation on MnTi₂O₄ is mainly caused by the lowering of the σ_αγ and σ_αX based on the K-S and B-N orientation relationships. In addition, Blas et al. considered that MnTi₂O₄ promote the nucleation of AF by strains produced at MnTi₂O₄/γ interface and reaction between MnTi₂O₄ and the surrounding austenite matrix.¹⁵⁾ Further research is necessary to clarify the quantitative performance of each effect.

5. Conclusions

(1) MnTi₂O₄ promotes AF formation. The AF has the Baker-Nutting (B-N) orientation relationship with MnTi₂O₄ and the Kurdjumov-Sachs (K-S) with the austenite matrix.

(2) The coexistence of the B-N and K-S orientation relationships implies that the MnTi₂O₄ had the following orientation relationship with the austenite matrix.

(001)_MnTi2O4//(111)_γ, [100]_MnTi2O4//[211]_γ

(3) It is considered that the MnTi₂O₄ was formed having the above orientation relationship with the austenite matrix within oxide particle at the high temperature during welding process. Subsequently, AF can nucleates on the MnTi₂O₄ with the B-N and K-S orientation relationships.

(4) Formation of both B-N and K-S orientation relationships lower the AF/MnTi₂O₄ and AF/austenite interfacial energies, which causes the AF nucleation.

Acknowledgements

The authors would like to thank Professor John G. Speer and Professor David K. Matlock of the Advanced Steel Processing and Products Research Center, Colorado School of Mines for their helpful advice.

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