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Observations of cleavage initiation at (Ti,V)(C,N) particles of heterogeneous composition in microalloyed steels
Scripta Materialia 50 (2004) 371–375 www.actamat-journals.com
Observations of cleavage initiation at (Ti,V)(C,N) particles of heterogeneous composition in microalloyed steels M.J. Balart *, C.L. Davis, M. Strangwood School of Engineering, Department of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK Received 30 April 2003; received in revised form 24 September 2003; accepted 9 October 2003
Abstract The mechanism of cleavage fracture initiated by microcracking of (Ti,V)(C,N) particles of heterogeneous composition has been investigated in Ti–V–N and V–N microalloyed forging steels. Preferential microcracking of the (Ti,V)(C,N) particles was observed to occur across the interface which separates regions richer in V from lower V content regions. 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Impact test; Energy dispersive X-ray spectroscopy (EDS); Steels; Fracture; Inclusions
1. Introduction Microalloyed steels commonly contain small amounts of titanium, added to form finely dispersed TiN precipitates, in order to control austenite grain size during processing at high austenitisation temperatures such as those experienced during hot forging or in the heat affected zones (HAZ) of weldments during thermal cycling [1–8]. However, depending on the levels of Ti and N and the process parameters, it is possible for coarse TiN particles to form during solidification. These coarse particles can act as potent cleavage initiation sites. Generally, cracking of TiN particles initiates a microcrack [5,7,8], although cleavage initiation by TiN debonding has also been observed [4]. The microcrack propagates at first across the particle–matrix interface, and finally as a transgranular cleavage crack across matrix–matrix interfaces. The critical event for cleavage fracture at 77 K in Ti– N microalloyed steels is usually the propagation of a TiN particle-sized microcrack into the matrix (propagation control) [9], although nucleation of a TiN particlesized microcrack by a fibre loading mechanism has also been observed (nucleation control) [10]. The significant findings of those studies on the mechanism of cleavage
*
Corresponding author. Tel.: +44-121-4143265; fax: +44-1214145232. E-mail address: [email protected] (M.J. Balart).
initiation at cracked TiN particles and particle–matrix propagation control are as follows: • The onset of cleavage fracture is triggered by TiN particles of a critical size, which is dependent on the matrix strength level. This has been ascribed to a Weibull volume effect [9]. • TiN particles are well bonded to the matrix and are local stress raisers [9]. Because of the different thermal expansion coefficients of TiN particles and the matrix, thermal residual (tessellated) stresses develop during cooling [11]. The particle is subjected to Ôradial’ compression and the matrix to both Ôradial’ compression and circumferential tension, which in turn results in a strongly bonded particle–matrix interface [8]. A strong TiN particle–matrix bond allows transferability of high local stress from the matrix to the particle, to initiate a microcrack, and from the cracked particle to the matrix, to propagate. This behaviour has been reported previously for weldments [2,12,13]. • Cleavage initiation facets tend to exhibit low misorientation angles with respect to the notch plane (in 80% of the examined cases <10 and in all cases 6 25) [9,14], i.e. the initiations facets are nearly normal to the maximum tensile stress. Moreover, of key importance is the crystallographic relationship between the particle and matrix in propagating cleavage fracture from the cracked particles to the surrounding matrix. For pure TiN particle, there is
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a simple cube-on-cube orientation relationship between a-Fe and TiN. According to the heterogeneous nucleation theory of a-Fe (a ¼ 0:286 nm) on a pure TiN substrate (a ¼ 0:424 nm), the orientation relationship {1 0 0}fcc TiN kf1 0 0gbcc a-Fe and h1 1 0ifcc TiNkh1 0 0ibcc a-Fe has a low misfit of 4.6% [15]. Such orientations of particle–matrix cleavage facet are essential for the transference of high stress.
2. Experimental procedure The medium carbon Ti–V–N and V–N microalloyed forging steels were prepared as 50 kg air melted laboratory cast ingots with compositions as given in Table 1. The cast ingots were forged and hot rolled to 50 or 25 mm diameter, then re-austenitised at 1200 C for 1 h and air cooled to simulate the thermal history of a typical hot forged sample. Specimens are represented by steel number and bar diameter, e.g. specimen 1/50 indicates steel 1 rolled to 50 mm diameter. The mechanical properties and the resulting microstructures of the Ti– V–N and V–N microalloyed steels used in the present investigation have been reported previously [20]. Fractography of 5 room temperature Charpy impact specimens was carried out using JEOL 5410 and 6300 scanning electron microscopes (SEMs) operating at 20 kV. EDS qualitative microanalysis was performed in different regions within the inclusion. Because the Ka line of V overlaps the Kb line of Ti, qualitative assessment of the Ti and V contents was determined as the ratio of peak intensities of Ti Ka to [Ti Kb + V Ka ], after background subtraction by linear interpolation [21].
Vanadium additions are also made to Ti–N microalloyed steels in order to increase strength levels through precipitation strengthening. In Ti–V–N microalloyed steels, complex Ti, V carbonitrides form. In general, the microchemistry and stability of a complex carbonitride depends on alloy composition and processing parameters [16–18]. A TEM study [19] on the precipitation behaviour in a Ti–V–N microalloyed steel at different stages during processing showed that: • Ti and V concentration gradients can exist within the (Ti,V)(C,N) particle. The particle core being more Vrich (less Ti) compared to the outer particle region which is more Ti-rich (less V). • Dissolution of vanadium from a mixed (Tix V1 x )N particle into austenite occurs, according to thermodynamic and kinetic models [19]. This is in agreement with a large difference in the equilibrium solubility of TiN and VN in austenite. • It was predicted, using the Hillert–Staffanson equilibrium model, that phase separation of the (Ti,V)(C,N) precipitate into TiN-rich nitride and VC-rich carbide could occur in austenite at temperatures close to the Ae3 temperature.
3. Results and discussion The failure mechanism for the room temperature Charpy impact samples was identified as being cleavage fracture for all compositions and bar diameters. The fracture initiation sites caused by inclusions were identified in a previous study as being from single phase (Ti,V)(C,N) inclusion or complex multiphase inclusions [containing (Ti,V)(C,N), Al2 O3 and/or MnS] where the common component to all initiating inclusions was (Ti,V)(C,N) [20]. Fracture in specimen 1/25 and steels 2 and 3 was initiated by a single inclusion event. The Titreated steels 2 and 3 contained Ti-rich (Ti,V)(C,N) particles. Conversely, the Ti-free steel 1 contained V-rich (V,Ti)(C,N) particles, the Ti content arising from residual Ti in the steel. Some examples of matching images of the main cleavage initiation sites from Ti-rich (Ti,V)(C,N); V-rich (V,Ti)(C,N) and (Ti,V)(C,N)/Al2 O3 / MnS particles are given in Fig. 1. The numbers in Fig. 1 represent the ratio of peak intensities Ti Ka to [Ti Kb + V Ka ] from EDS spectra as described earlier. EDS spectra corresponding to the ratio of intensities 4.21 and 2.75 in Fig. 1(a) and (b) are shown in Fig. 2(a) and (b); 0.26#
In a previous study on the effects of matrix microstructure and non-metallic inclusions on cleavage fracture mechanisms in Ti–V–N and V–N microalloyed steels used in the present investigation [20], it was reported that the cleavage initiation mechanism at the (Ti,V)(C,N)-bearing particles involved (Ti,V)(C,N) particle fracture. The present study focuses on an investigation of the mechanism of microcracking in (Ti,V)(C,N) particles of heterogeneous composition compared with the results reported in the literature for supposedly homogeneous TiN particles. The mechanism by which a coarse particle initiates cleavage fracture will be affected by the fracture stress of the particle.
Table 1 Chemical composition (wt.%) of the steels investigated Steels
C
Si
Mn
P
S
Cr
Mo
Ni
Al
Cu
N
Ti
V
O
1 2 3
0.41 0.37 0.37
0.53 1.05 0.53
1.43 1.48 1.44
0.013 0.013 0.012
0.026 0.025 0.025
0.12 0.12 0.12
0.03 0.03 0.03
0.1 0.1 0.1
0.026 0.034 0.035
0.21 0.21 0.21
0.020 0.021 0.021
<0.002 0.022 0.022
0.16 0.16 0.16
0.0053 0.0043 0.0047
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Fig. 1. SEM matching micrographs of the main cleavage initiation site in Charpy impact specimens at room temperature from (a) and (b) steel 3/50, the marked regions indicate the following phases (A) (Ti,V)(C,N); (B) Al2 O3 and (C) MnS; (c) and (d) steel 3/25 (Ti,V)(C,N) particle; (e) and (f) steel 2/25 (Ti,V)(C,N) particle and (g) and (h) steel 1/25 (V,Ti)(C,N) particle. The numbers represent the ratio of peak intensities Ti Ka to [Ti Kb + V Ka ] from EDS spectra. Figures (c) and (g) were slightly etched showing pearlitic areas.
and 0.31 of Fig. 1(g) and (h) are shown in Fig. 2(c) and (d), respectively. It is important to note that, for the fractured (Ti,V)(C,N) particle in Fig. 1(a) and (b), the more V-rich half presented a higher intensity of Fe Ka peak, Fig. 2(b), compared to the more Ti-rich half, Fig. 2(a), the significance of this observation will be discussed below. The X-ray generation range R (lm) was calculated from Kanaya–Okayama relationship [21]: R¼
0:0276AðE01:67 Ec1:67 Þ Z 0:889 q
ð1Þ
where E0 is the incident beam energy (keV), Ec is the critical ionisation energy for the characteristic X-rays (keV), A is the target atomic weight (g/mol), Z is the target atomic number and q is the target density (g/ cm3 ). From Eq. (1) after substitution of A (Ti), Z (Ti) and q (TiN) of 47.9, 22 and 5.3, respectively, and E0 ¼ 20 keV, the X-ray range for Ti K (Ec ¼ 4:965 keV) in TiN was 2.15 lm. This indicates that the EDS results are likely to include a contribution from the surrounding matrix for the size of particles in this study.
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Fig. 2. EDS spectra corresponding to the ratio of intensities (a) and (b) from Fig. 1(a) and (b) 4.21 and 2.75; and (c) and (d) from Fig. 1(g) and (h) 0.26# and 0.31, respectively.
Fractographic observations of the mixed (Ti,V)(C,N) particles initiating cleavage reported previously [20,22] showed the broken particles to have fairly rough surfaces, as shown in Fig. 1. Comparatively, TiN particles initiating cleavage have been shown to have relatively flat surfaces [7,8]. This implies that a different mechanism for particle microcracking generating a rough surface, might operate for mixed (Ti,V)(C,N) particles rather than simple cleavage fracture generally on a single plane in a particle with a homogeneous TiN composition. From the Ti Ka to [Ti Kb + V Ka ] intensity ratios given in Fig. 1, it can be seen that Ti and V concentration gradients exist within the mixed (Ti,V)(C,N) particles. Both Ti-rich (Ti,V)(C,N) and V-rich (V,Ti)(C,N) particles can be observed where for each particle type both halves of the broken particles are Ti-rich or V-rich, respectively. However, it can also be seen that for both types of particle the two broken halves of the same particle show different levels of Ti and V, i.e. one half may be richer in Ti (or V) than the other half. EDS spectra on the more V-rich half of the fractured (Ti,V)(C,N) particle type consistently showed a high Fe Ka peak intensity, an example is shown in Fig. 2(b), compared with the matching more Ti-rich half of the fractured (Ti,V)(C,N) particle (Fig. 2(a)). This suggests that a significant percentage of the electron microprobe
sampling may have come from matrix underneath the particle. This suggests that the more V-rich half is thinner than the corresponding matching more Ti-rich half. The more V-rich half was estimated to be thinner than the X-ray range, which is of the order of 2.15 lm. It is of interest that the particles in steel 1 are generally smaller in size than those in steels 2 and 3. An example of a cleavage initiating particle for (V,Ti)(C,N) is shown in Fig. 1(g) and (h). In the EDS spectra shown in Fig. 2(c) and (d) there is a low, but significant, intensity of Fe Ka peaks on both matching halves implying that the smaller particles break approximately into equal thickness halves. The presence of the Fe Ka peaks at a relatively low level indicates that the particles are reasonably thick. This makes it difficult to compare the size of the fractured particle with the Xray range. Prikryl et al. [19] characterised precipitates in a Ti–V– N microalloyed steel with a similar composition to that of the Ti–V–N microalloyed steels of the present investigation. It was shown that reheating the as-cast bloom specimens at 1100 and 1350 C for different times (up to 235 h), followed by quenching, resulted in the formation of coring in the (Ti,V)(C,N) precipitates, with the core being more V-rich and the outer region being V-depleted and with the precipitate becoming progressively more Ti-rich overall and coarsening. V is partioning out of the (Ti,V)(C,N) particle, due to the higher solubility of VN compared to TiN and greater diffusivity of V in austenite than Ti. Moreover, they demonstrated theoretically, using the Hillert–Staffanson equilibrium model, that phase separation of the (Ti,V)(C,N) precipitate into TiN-rich nitride and VC-rich carbide could occur in austenite at temperatures close to the Ae3 temperature in the Ti–V–N microalloyed steel. By contrast, in the present investigation, the effect of coring during reheating the samples cannot account for the experimental observation that the outer regions of the (Ti,V)(C,N) particles in steels 2 and 3 are more V-rich. It is possible that, during air-cooling of the steel bars, the evolution of the reaction to the thermodynamic stable phases––TiNrich nitride and VC-rich carbide––may have taken place at temperatures close to the Ae3 temperature. Studies on the brittle fracture initiation mechanism from non-metallic inclusions in microalloyed steels [6] and a ferritic stainless steel [23] reported the importance of a low misorientation angle between particle–matrix cleavage facets (the former) and the crystallographic orientation, of the cleavage initiating particle and matrix facet, relative to the applied tensile stress (the later). Nevertheless, in the present study, preferential microcracking of (Ti,V)(C,N) particles initiating cleavage took place across the interface which separates regions richer in V from lower V content regions, which in turn could account for the rough surface appearance of the cracked (Ti,V)(C,N) particle.
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4. Conclusions EDS analysis of (Ti,V)(C,N) inclusions initiating cleavage fracture in microalloyed forging steels has shown that the particles crack resulting in Ti-richer and V-richer parts on the matching cleavage facets. The EDS results indicate that the (Ti,V)(C,N) inclusions have a Vricher surface, possibly due to evolution of the phase separation reaction of the (Ti,V)(C,N) precipitate into thermodynamic stable TiN-rich nitride and VC-rich carbide phases, rather than partioning of V during the heat treatment of the samples at 1200 C for 1 h.
Acknowledgements Corus Group Swinden Technology Centre is acknowledged for the provision of Charpy impact test samples. Thanks are due to Professor I.R. Harris for the provision of research facilities at the University of Birmingham and Professor J.F. Knott for helpful discussions during the realisation of this work.
References [1] Baker TN, editor. Titanium technology in microalloyed steels. London, UK: The Institute of Materials; 1997. [2] Vassilaros MG. PhD Thesis, The University of Cambridge; 1991. [3] Zhang LP, Davis CL, Strangwood M. Met Trans 1999;30A: 2089.
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[4] Zhang LP, Davis CL, Strangwood M. Met Trans 2001;32A:1147. [5] Linaza MA, Romero JL, Rodrıguez-Ibabe JM, Urcola JJ. Scr Metall Mater 1993;29:451. [6] Linaza MA, Romero JL, Rodrıguez-Ibabe JM, Urcola JJ. Scr Metall Mater 1993;29:1217. [7] Fairchild DP, Howden DG, Clark WAT. Met Trans 2000;31A:641. [8] Fairchild DP, Howden DG, Clark WAT. Met Trans 2000;31A:653. [9] Linaza MA, Rodrıguez-Ibabe JM, Urcola JJ. Fatigue Fract Eng Mater Struct 1997;20:619. [10] Echeverrıa A, Rodrıguez-Ibabe JM. Scr Metall Mater 1999;41:131. [11] Brooksbank D, Andrews KW. JISI 1972;210:246. [12] Knott JF. Cleavage fracture: George R. Irwin symposium, TMS, Warrendale, PA, USA; 1997. [13] Tweed JH, Knott JF. Acta Metall 1987;35:1401. [14] Linaza MA, Romero JL, Rodrıguez-Ibabe JM, Urcola JJ. Scr Metall Mater 1995;32:395. [15] Mills AR, Thewlis G, Whiteman JA. Mater Sci Technol 1987;3:1051. [16] Loberg B, Nordgren A, Strid J, Easterling KE. Met Trans 1984;15A:33. [17] Strid J, Easterling KE. Acta Metall 1985;33:2057. [18] Suzuki S, Weatherly GC, Houghton DC. Acta Metall 1987;35:341. [19] Prikryl M, Kroupa A, Weatherly GC, Subramanian SV. Met Trans 1996;27A:1149. [20] Balart MJ, Davis CL, Strangwood M. Mater Sci Eng A 2000;284:1. [21] Goldstein JI, Newbury DE, Echlin P, Joy DC, Fiori C, Lifshin E. Scanning electron microscopy and X-ray microanalysis. 2nd ed. London: Plenum Press; 1992. pp. 89, 372. [22] Balart MJ, Davis CL, Strangwood M. Mater Sci Eng A 2002;328:48. [23] Veistinen MK, Lindroos VK. Scr Metall 1983;17:1147.
Observations of cleavage initiation at (Ti,V)(C,N) particles of heterogeneous composition in microalloyed steels M.J. Balart *, C.L. Davis, M. Strangwood School of Engineering, Department of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK Received 30 April 2003; received in revised form 24 September 2003; accepted 9 October 2003
Abstract The mechanism of cleavage fracture initiated by microcracking of (Ti,V)(C,N) particles of heterogeneous composition has been investigated in Ti–V–N and V–N microalloyed forging steels. Preferential microcracking of the (Ti,V)(C,N) particles was observed to occur across the interface which separates regions richer in V from lower V content regions. 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Impact test; Energy dispersive X-ray spectroscopy (EDS); Steels; Fracture; Inclusions
1. Introduction Microalloyed steels commonly contain small amounts of titanium, added to form finely dispersed TiN precipitates, in order to control austenite grain size during processing at high austenitisation temperatures such as those experienced during hot forging or in the heat affected zones (HAZ) of weldments during thermal cycling [1–8]. However, depending on the levels of Ti and N and the process parameters, it is possible for coarse TiN particles to form during solidification. These coarse particles can act as potent cleavage initiation sites. Generally, cracking of TiN particles initiates a microcrack [5,7,8], although cleavage initiation by TiN debonding has also been observed [4]. The microcrack propagates at first across the particle–matrix interface, and finally as a transgranular cleavage crack across matrix–matrix interfaces. The critical event for cleavage fracture at 77 K in Ti– N microalloyed steels is usually the propagation of a TiN particle-sized microcrack into the matrix (propagation control) [9], although nucleation of a TiN particlesized microcrack by a fibre loading mechanism has also been observed (nucleation control) [10]. The significant findings of those studies on the mechanism of cleavage
*
Corresponding author. Tel.: +44-121-4143265; fax: +44-1214145232. E-mail address: [email protected] (M.J. Balart).
initiation at cracked TiN particles and particle–matrix propagation control are as follows: • The onset of cleavage fracture is triggered by TiN particles of a critical size, which is dependent on the matrix strength level. This has been ascribed to a Weibull volume effect [9]. • TiN particles are well bonded to the matrix and are local stress raisers [9]. Because of the different thermal expansion coefficients of TiN particles and the matrix, thermal residual (tessellated) stresses develop during cooling [11]. The particle is subjected to Ôradial’ compression and the matrix to both Ôradial’ compression and circumferential tension, which in turn results in a strongly bonded particle–matrix interface [8]. A strong TiN particle–matrix bond allows transferability of high local stress from the matrix to the particle, to initiate a microcrack, and from the cracked particle to the matrix, to propagate. This behaviour has been reported previously for weldments [2,12,13]. • Cleavage initiation facets tend to exhibit low misorientation angles with respect to the notch plane (in 80% of the examined cases <10 and in all cases 6 25) [9,14], i.e. the initiations facets are nearly normal to the maximum tensile stress. Moreover, of key importance is the crystallographic relationship between the particle and matrix in propagating cleavage fracture from the cracked particles to the surrounding matrix. For pure TiN particle, there is
1359-6462/$ - see front matter 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2003.10.009
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a simple cube-on-cube orientation relationship between a-Fe and TiN. According to the heterogeneous nucleation theory of a-Fe (a ¼ 0:286 nm) on a pure TiN substrate (a ¼ 0:424 nm), the orientation relationship {1 0 0}fcc TiN kf1 0 0gbcc a-Fe and h1 1 0ifcc TiNkh1 0 0ibcc a-Fe has a low misfit of 4.6% [15]. Such orientations of particle–matrix cleavage facet are essential for the transference of high stress.
2. Experimental procedure The medium carbon Ti–V–N and V–N microalloyed forging steels were prepared as 50 kg air melted laboratory cast ingots with compositions as given in Table 1. The cast ingots were forged and hot rolled to 50 or 25 mm diameter, then re-austenitised at 1200 C for 1 h and air cooled to simulate the thermal history of a typical hot forged sample. Specimens are represented by steel number and bar diameter, e.g. specimen 1/50 indicates steel 1 rolled to 50 mm diameter. The mechanical properties and the resulting microstructures of the Ti– V–N and V–N microalloyed steels used in the present investigation have been reported previously [20]. Fractography of 5 room temperature Charpy impact specimens was carried out using JEOL 5410 and 6300 scanning electron microscopes (SEMs) operating at 20 kV. EDS qualitative microanalysis was performed in different regions within the inclusion. Because the Ka line of V overlaps the Kb line of Ti, qualitative assessment of the Ti and V contents was determined as the ratio of peak intensities of Ti Ka to [Ti Kb + V Ka ], after background subtraction by linear interpolation [21].
Vanadium additions are also made to Ti–N microalloyed steels in order to increase strength levels through precipitation strengthening. In Ti–V–N microalloyed steels, complex Ti, V carbonitrides form. In general, the microchemistry and stability of a complex carbonitride depends on alloy composition and processing parameters [16–18]. A TEM study [19] on the precipitation behaviour in a Ti–V–N microalloyed steel at different stages during processing showed that: • Ti and V concentration gradients can exist within the (Ti,V)(C,N) particle. The particle core being more Vrich (less Ti) compared to the outer particle region which is more Ti-rich (less V). • Dissolution of vanadium from a mixed (Tix V1 x )N particle into austenite occurs, according to thermodynamic and kinetic models [19]. This is in agreement with a large difference in the equilibrium solubility of TiN and VN in austenite. • It was predicted, using the Hillert–Staffanson equilibrium model, that phase separation of the (Ti,V)(C,N) precipitate into TiN-rich nitride and VC-rich carbide could occur in austenite at temperatures close to the Ae3 temperature.
3. Results and discussion The failure mechanism for the room temperature Charpy impact samples was identified as being cleavage fracture for all compositions and bar diameters. The fracture initiation sites caused by inclusions were identified in a previous study as being from single phase (Ti,V)(C,N) inclusion or complex multiphase inclusions [containing (Ti,V)(C,N), Al2 O3 and/or MnS] where the common component to all initiating inclusions was (Ti,V)(C,N) [20]. Fracture in specimen 1/25 and steels 2 and 3 was initiated by a single inclusion event. The Titreated steels 2 and 3 contained Ti-rich (Ti,V)(C,N) particles. Conversely, the Ti-free steel 1 contained V-rich (V,Ti)(C,N) particles, the Ti content arising from residual Ti in the steel. Some examples of matching images of the main cleavage initiation sites from Ti-rich (Ti,V)(C,N); V-rich (V,Ti)(C,N) and (Ti,V)(C,N)/Al2 O3 / MnS particles are given in Fig. 1. The numbers in Fig. 1 represent the ratio of peak intensities Ti Ka to [Ti Kb + V Ka ] from EDS spectra as described earlier. EDS spectra corresponding to the ratio of intensities 4.21 and 2.75 in Fig. 1(a) and (b) are shown in Fig. 2(a) and (b); 0.26#
In a previous study on the effects of matrix microstructure and non-metallic inclusions on cleavage fracture mechanisms in Ti–V–N and V–N microalloyed steels used in the present investigation [20], it was reported that the cleavage initiation mechanism at the (Ti,V)(C,N)-bearing particles involved (Ti,V)(C,N) particle fracture. The present study focuses on an investigation of the mechanism of microcracking in (Ti,V)(C,N) particles of heterogeneous composition compared with the results reported in the literature for supposedly homogeneous TiN particles. The mechanism by which a coarse particle initiates cleavage fracture will be affected by the fracture stress of the particle.
Table 1 Chemical composition (wt.%) of the steels investigated Steels
C
Si
Mn
P
S
Cr
Mo
Ni
Al
Cu
N
Ti
V
O
1 2 3
0.41 0.37 0.37
0.53 1.05 0.53
1.43 1.48 1.44
0.013 0.013 0.012
0.026 0.025 0.025
0.12 0.12 0.12
0.03 0.03 0.03
0.1 0.1 0.1
0.026 0.034 0.035
0.21 0.21 0.21
0.020 0.021 0.021
<0.002 0.022 0.022
0.16 0.16 0.16
0.0053 0.0043 0.0047
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Fig. 1. SEM matching micrographs of the main cleavage initiation site in Charpy impact specimens at room temperature from (a) and (b) steel 3/50, the marked regions indicate the following phases (A) (Ti,V)(C,N); (B) Al2 O3 and (C) MnS; (c) and (d) steel 3/25 (Ti,V)(C,N) particle; (e) and (f) steel 2/25 (Ti,V)(C,N) particle and (g) and (h) steel 1/25 (V,Ti)(C,N) particle. The numbers represent the ratio of peak intensities Ti Ka to [Ti Kb + V Ka ] from EDS spectra. Figures (c) and (g) were slightly etched showing pearlitic areas.
and 0.31 of Fig. 1(g) and (h) are shown in Fig. 2(c) and (d), respectively. It is important to note that, for the fractured (Ti,V)(C,N) particle in Fig. 1(a) and (b), the more V-rich half presented a higher intensity of Fe Ka peak, Fig. 2(b), compared to the more Ti-rich half, Fig. 2(a), the significance of this observation will be discussed below. The X-ray generation range R (lm) was calculated from Kanaya–Okayama relationship [21]: R¼
0:0276AðE01:67 Ec1:67 Þ Z 0:889 q
ð1Þ
where E0 is the incident beam energy (keV), Ec is the critical ionisation energy for the characteristic X-rays (keV), A is the target atomic weight (g/mol), Z is the target atomic number and q is the target density (g/ cm3 ). From Eq. (1) after substitution of A (Ti), Z (Ti) and q (TiN) of 47.9, 22 and 5.3, respectively, and E0 ¼ 20 keV, the X-ray range for Ti K (Ec ¼ 4:965 keV) in TiN was 2.15 lm. This indicates that the EDS results are likely to include a contribution from the surrounding matrix for the size of particles in this study.
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Fig. 2. EDS spectra corresponding to the ratio of intensities (a) and (b) from Fig. 1(a) and (b) 4.21 and 2.75; and (c) and (d) from Fig. 1(g) and (h) 0.26# and 0.31, respectively.
Fractographic observations of the mixed (Ti,V)(C,N) particles initiating cleavage reported previously [20,22] showed the broken particles to have fairly rough surfaces, as shown in Fig. 1. Comparatively, TiN particles initiating cleavage have been shown to have relatively flat surfaces [7,8]. This implies that a different mechanism for particle microcracking generating a rough surface, might operate for mixed (Ti,V)(C,N) particles rather than simple cleavage fracture generally on a single plane in a particle with a homogeneous TiN composition. From the Ti Ka to [Ti Kb + V Ka ] intensity ratios given in Fig. 1, it can be seen that Ti and V concentration gradients exist within the mixed (Ti,V)(C,N) particles. Both Ti-rich (Ti,V)(C,N) and V-rich (V,Ti)(C,N) particles can be observed where for each particle type both halves of the broken particles are Ti-rich or V-rich, respectively. However, it can also be seen that for both types of particle the two broken halves of the same particle show different levels of Ti and V, i.e. one half may be richer in Ti (or V) than the other half. EDS spectra on the more V-rich half of the fractured (Ti,V)(C,N) particle type consistently showed a high Fe Ka peak intensity, an example is shown in Fig. 2(b), compared with the matching more Ti-rich half of the fractured (Ti,V)(C,N) particle (Fig. 2(a)). This suggests that a significant percentage of the electron microprobe
sampling may have come from matrix underneath the particle. This suggests that the more V-rich half is thinner than the corresponding matching more Ti-rich half. The more V-rich half was estimated to be thinner than the X-ray range, which is of the order of 2.15 lm. It is of interest that the particles in steel 1 are generally smaller in size than those in steels 2 and 3. An example of a cleavage initiating particle for (V,Ti)(C,N) is shown in Fig. 1(g) and (h). In the EDS spectra shown in Fig. 2(c) and (d) there is a low, but significant, intensity of Fe Ka peaks on both matching halves implying that the smaller particles break approximately into equal thickness halves. The presence of the Fe Ka peaks at a relatively low level indicates that the particles are reasonably thick. This makes it difficult to compare the size of the fractured particle with the Xray range. Prikryl et al. [19] characterised precipitates in a Ti–V– N microalloyed steel with a similar composition to that of the Ti–V–N microalloyed steels of the present investigation. It was shown that reheating the as-cast bloom specimens at 1100 and 1350 C for different times (up to 235 h), followed by quenching, resulted in the formation of coring in the (Ti,V)(C,N) precipitates, with the core being more V-rich and the outer region being V-depleted and with the precipitate becoming progressively more Ti-rich overall and coarsening. V is partioning out of the (Ti,V)(C,N) particle, due to the higher solubility of VN compared to TiN and greater diffusivity of V in austenite than Ti. Moreover, they demonstrated theoretically, using the Hillert–Staffanson equilibrium model, that phase separation of the (Ti,V)(C,N) precipitate into TiN-rich nitride and VC-rich carbide could occur in austenite at temperatures close to the Ae3 temperature in the Ti–V–N microalloyed steel. By contrast, in the present investigation, the effect of coring during reheating the samples cannot account for the experimental observation that the outer regions of the (Ti,V)(C,N) particles in steels 2 and 3 are more V-rich. It is possible that, during air-cooling of the steel bars, the evolution of the reaction to the thermodynamic stable phases––TiNrich nitride and VC-rich carbide––may have taken place at temperatures close to the Ae3 temperature. Studies on the brittle fracture initiation mechanism from non-metallic inclusions in microalloyed steels [6] and a ferritic stainless steel [23] reported the importance of a low misorientation angle between particle–matrix cleavage facets (the former) and the crystallographic orientation, of the cleavage initiating particle and matrix facet, relative to the applied tensile stress (the later). Nevertheless, in the present study, preferential microcracking of (Ti,V)(C,N) particles initiating cleavage took place across the interface which separates regions richer in V from lower V content regions, which in turn could account for the rough surface appearance of the cracked (Ti,V)(C,N) particle.
M.J. Balart et al. / Scripta Materialia 50 (2004) 371–375
4. Conclusions EDS analysis of (Ti,V)(C,N) inclusions initiating cleavage fracture in microalloyed forging steels has shown that the particles crack resulting in Ti-richer and V-richer parts on the matching cleavage facets. The EDS results indicate that the (Ti,V)(C,N) inclusions have a Vricher surface, possibly due to evolution of the phase separation reaction of the (Ti,V)(C,N) precipitate into thermodynamic stable TiN-rich nitride and VC-rich carbide phases, rather than partioning of V during the heat treatment of the samples at 1200 C for 1 h.
Acknowledgements Corus Group Swinden Technology Centre is acknowledged for the provision of Charpy impact test samples. Thanks are due to Professor I.R. Harris for the provision of research facilities at the University of Birmingham and Professor J.F. Knott for helpful discussions during the realisation of this work.
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