3. Microstructure and hardness

by heat treatment after the forming. It is reported that the dispersion of fine carbides such as VC, NbC, TiC and Mo2C is effective for strengthening of low carbon ferritic and martensitic

From the above background, some researches on precipitation-hardened medium carbon steels have been also undertaken. Murase et al. have investigated the precipitation-hardening behavior of medium carbon ferritic steels with single or multiple element additions of V and Cu [4] and have found that precipitation hardening of medium carbon steels is obviously improved by the multiple element addition of V and Cu rather than single additions of each element. Grange et al. precisely examined the effect of alloying elements (such as Mn, P, Si, Ni, Cr, Mo and V) on precipitation-hardening behavior in low and medium carbon steels with tempered martensite [5] and reported that the estimated hardening HvEstimated could be evalu-

HvEstimated ¼ Hv þ ΔHvMn þ ΔHvP þ ΔHvSi þ ΔHvNi þ ΔHvCr þ ΔHvMo þ ΔHvV, (1)

where Hv and ΔHvMn, ΔHvP, ΔHvSi, ΔHvNi, ΔHvCr, ΔHvMo and ΔHvV are the initial hardness of Fe–C alloy before element addition and the increments in hardness due to the individual element additions. On the other hand, Kosaka et al. investigated the precipitation-hardening behavior in 0.1 C–2.0 Mn steels (in mass %) with tempered martensite and revealed that the increase in precipitation hardening due to complex-composition carbide, that is, (Mo, V)C, is larger than those by carbides with simple compositions, that is, Mo2C and VC [6]. They have also shown that the effect of carbides with simple compositions on hardening becomes larger in the order of Mo2C, VC, TiC and NbC. The amount of precipitation hardening would be, therefore, considerably and complicatedly changed depending on

Medium carbon steels with bainite are well known for their good balance of mechanical properties of strength and ductility [7]. They are expected to be one of the most useful steels for machinery parts in the future. However, as far as the authors know, few studies on precipitationhardening behavior and the strengthening mechanisms due to Nb, Ti and V additions in medium carbon steels with bainite have been carried out. Here, Nb, Ti and V are the most commonly employed elements for strengthening of low carbon steels as already shown above. In the present study, the effects of compositions of carbides on the precipitation-hardening behavior of medium

Various kinds of medium carbon steels with different compositions, shown in Table 1, were vacuum induction melted and cast into 15 kg ingots. Steel A is the base sample. In steels B, C and D, elements of Nb, Ti and V, respectively, were further added to the base sample. In steel E, Nb, Ti and V were all added. Mn as well as Mo was added to all the samples to attain bainite microstructure. The casted samples were then hot forged at 1523 K into bars with a diameter of 55 mm. The bars were solution treated at 1523 K for 30 min followed by air cooling down to

carbon bainitic steel and the strengthening mechanisms are precisely investigated.

ated by simple summation of the individual hardening effects as follows:

steels [1–3].

26 New Uses of Micro and Nanomaterials

the composition of carbide.

2. Experimental procedure

Figure 1 shows the optical micrographs of the microstructure developed in steels D and E after solid-solution treatment at 1523 K for 30 min followed by air cooling, that is, before aging. It can be seen in Figure 1 that a typical acicular bainite microstructure was almost uniformly evolved in both samples. A completely similar microstructure was observed to develop in all the other samples, while they are not displayed here.

The samples were then aged at various temperatures for 120 min and the change in the hardness was measured. Figure 2 shows the summarized results of the amount of precipitation hardening (ΔHV) as a function of aging temperature. In steels A, B and C, the maximum values of ΔHV were approximately 30. This result indicates, therefore, precipitation hardening due to the additions of 0.019% Nb or 0.039% Ti (see Table 1) is quite small. On the other hand, the ΔHV values in steels D and E after aging at 873 K exceeded 40. Moreover, in steel E, the maximum ΔHV value was approximately 90. It should be noted, therefore, that the amount of precipitation hardening in steel E, ΔHV = 90, due to multiple element addition, is significantly larger than those in steels B, C and D with the single element additions. When the values of ΔHV at 873 K are compared, it in steel E is larger than the summation of the ΔHV values of

Figure 1. Typical microstructure of (a) steel D and (b) steel E after solution treatment at 1523 K for 30 min followed by air cooling.

steel E, slightly finer disk precipitates of approximately 5 nm in diameter were observed (Figure 3(b)). The size of precipitates in steel E looks more homogeneous than that in steel D. The moiré fringes due to the lattice misfit between the precipitates and matrix were also observed. High-resolution TEM observation of the precipitates in steel E revealed the presence of misfit strain at the edges of them indicating coherency of precipitate/matrix interface (Figure 3(c)). Figure 3(d) shows selected-area-diffraction pattern taken along direction parallel to the [001] axis of the matrix of steel E. Based on the analyses by the diffraction pattern and EDS, the fine precipitates were identified to be MC-type carbide having the Baker and Nutting crystallographicorientation relationship, that is, (100)MC//(100)α, [010]MC//[011]α, and [001]MC//[011]<sup>α</sup> [8]. The EDS analysis indicated the compositions of carbides in steels D and E as VC and (Nb, Ti, V)C,

Figure 3. TEM micrographs of steels D and E after aging at 873 K: (a) Bright-field image of steel D, (b) Bright-field image

Mechanisms of Significant Precipitation Hardening in a Medium Carbon Bainitic Steel by Complex Nanocarbides…

http://dx.doi.org/10.5772/intechopen.80273

29

of steel E, (c) magnified image of (b), and (d) diffraction pattern taken from the ferrite and MC carbide in (b).

It is reported that the critical size of MC-type carbides to keep coherency with matrix is approximately 5 nm and the loss of coherency derives a decrease in shear stress for dislocation to bypass the precipitates [3]. Even while the precipitates in steels A, B and C were not observed using TEM, the lower hardness of these steels than those of D and E (see Figure 2) should be affected by coherency loss of coarse MC-type carbides as observed in steel D.

respectively. Mo2C carbides were not detected in all the samples.

Figure 2. Change in the amount of precipitation hardening (ΔHv) depending on aging temperature and added elements.

steels B, C and D, i.e., ΔHV = 73. This result indicates that the amount of age-hardening is not a simple summation of individual hardening as described in Eq. (1).

The developed precipitates were examined by TEM. Figure 3 shows the typical TEM photographs of steels D and E after aging at 873 K for 120 min. In steel D, fine disk-shaped precipitates of 8 nm in diameter in average were distributed (Figure 3(a)). On the other hand, in

Mechanisms of Significant Precipitation Hardening in a Medium Carbon Bainitic Steel by Complex Nanocarbides… http://dx.doi.org/10.5772/intechopen.80273 29

Figure 3. TEM micrographs of steels D and E after aging at 873 K: (a) Bright-field image of steel D, (b) Bright-field image of steel E, (c) magnified image of (b), and (d) diffraction pattern taken from the ferrite and MC carbide in (b).

steel E, slightly finer disk precipitates of approximately 5 nm in diameter were observed (Figure 3(b)). The size of precipitates in steel E looks more homogeneous than that in steel D. The moiré fringes due to the lattice misfit between the precipitates and matrix were also observed. High-resolution TEM observation of the precipitates in steel E revealed the presence of misfit strain at the edges of them indicating coherency of precipitate/matrix interface (Figure 3(c)). Figure 3(d) shows selected-area-diffraction pattern taken along direction parallel to the [001] axis of the matrix of steel E. Based on the analyses by the diffraction pattern and EDS, the fine precipitates were identified to be MC-type carbide having the Baker and Nutting crystallographicorientation relationship, that is, (100)MC//(100)α, [010]MC//[011]α, and [001]MC//[011]<sup>α</sup> [8]. The EDS analysis indicated the compositions of carbides in steels D and E as VC and (Nb, Ti, V)C, respectively. Mo2C carbides were not detected in all the samples.

It is reported that the critical size of MC-type carbides to keep coherency with matrix is approximately 5 nm and the loss of coherency derives a decrease in shear stress for dislocation to bypass the precipitates [3]. Even while the precipitates in steels A, B and C were not observed using TEM, the lower hardness of these steels than those of D and E (see Figure 2) should be affected by coherency loss of coarse MC-type carbides as observed in steel D.

steels B, C and D, i.e., ΔHV = 73. This result indicates that the amount of age-hardening is not a

Figure 2. Change in the amount of precipitation hardening (ΔHv) depending on aging temperature and added elements.

Figure 1. Typical microstructure of (a) steel D and (b) steel E after solution treatment at 1523 K for 30 min followed by air

cooling.

28 New Uses of Micro and Nanomaterials

The developed precipitates were examined by TEM. Figure 3 shows the typical TEM photographs of steels D and E after aging at 873 K for 120 min. In steel D, fine disk-shaped precipitates of 8 nm in diameter in average were distributed (Figure 3(a)). On the other hand, in

simple summation of individual hardening as described in Eq. (1).
