1. Introduction

For machinery parts, such as bolts and gears, medium carbon steels are regularly employed. These steel parts are generally formed to prescribed shapes and designs through thermomechanical processes of hot and/or cold forging, and machining followed by heat treatments for further strengthening. Recently, demands for high strength steels, with their reduction of cost in addition to superior balance of mechanical properties, are increasing.

The dispersion of hard particles should be one of the most effective methods for strengthening. For this purpose, microalloyed steels with dispersed fine carbides are developed and actually employed for machinery parts because of their good balance of formability and strengthening

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 steels [1–3].

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

$$\Delta \mathbf{H} \mathbf{v}\_{\text{Estimated}} = \mathbf{H} \mathbf{v} + \Delta \mathbf{H} \mathbf{v}\_{\text{Mn}} + \Delta \mathbf{H} \mathbf{v}\_{\text{P}} + \Delta \mathbf{H} \mathbf{v}\_{\text{Si}} + \Delta \mathbf{H} \mathbf{v}\_{\text{Ni}} + \Delta \mathbf{H} \mathbf{v}\_{\text{Cr}} + \Delta \mathbf{H} \mathbf{v}\_{\text{Mo}} + \Delta \mathbf{H} \mathbf{v}\_{\text{V}} \tag{1}$$

room temperature. Thereafter, some of the bars were aged for 120 min at various temperatures

Table 1. Chemical compositions in mass% of tested medium carbon bainitic steels.

C Si Mn Mo Ni Nb Ti V N Fe

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

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

Steel A 0.26 0.69 1.59 0.50 0.35 ——— 0.003 Bal. Steel B 0.26 0.71 1.59 0.49 0.35 0.019 — — 0.003 Bal. Steel C 0.26 0.70 1.58 0.49 0.35 — 0.039 — 0.003 Bal. Steel D 0.25 0.71 1.58 0.49 0.35 — — 0.30 0.003 Bal. Steel E 0.25 0.71 1.58 0.49 0.35 0.020 0.038 0.31 0.003 Bal.

Microstructural observations were carried out using optical microscopy and transmissionelectron microscopy (TEM) on the cross-sections normal to the longitudinal direction of bars and at the positions of the half radius of the bars. Samples for the optical microscopic observation were prepared by mechanical polishing and etching using the nital solution of 3% nitric acid and 97% ethanol. TEM specimens were also prepared by mechanical polishing followed by electrolytic polishing using a solution of 5% perchloric acid and 95% acetic acid. The size, distribution and composition of precipitated carbides were examined by TEM equipped with energy-dispersive X-ray spectroscopy (EDS). The hardness was measured using a micro-Vickers hardness tester. The amount of precipitation hardening (ΔHv) was estimated using

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 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

ΔHV ¼ ðHardness after agingÞ � ð Þ Hardness before aging (2)

between 673 and 973 K.

the following equation:

3. Microstructure and hardness

the other samples, while they are not displayed here.

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 the composition of carbide.

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 carbon bainitic steel and the strengthening mechanisms are precisely investigated.

### 2. Experimental procedure

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

Mechanisms of Significant Precipitation Hardening in a Medium Carbon Bainitic Steel by Complex Nanocarbides… http://dx.doi.org/10.5772/intechopen.80273 27


Table 1. Chemical compositions in mass% of tested medium carbon bainitic steels.

room temperature. Thereafter, some of the bars were aged for 120 min at various temperatures between 673 and 973 K.

Microstructural observations were carried out using optical microscopy and transmissionelectron microscopy (TEM) on the cross-sections normal to the longitudinal direction of bars and at the positions of the half radius of the bars. Samples for the optical microscopic observation were prepared by mechanical polishing and etching using the nital solution of 3% nitric acid and 97% ethanol. TEM specimens were also prepared by mechanical polishing followed by electrolytic polishing using a solution of 5% perchloric acid and 95% acetic acid. The size, distribution and composition of precipitated carbides were examined by TEM equipped with energy-dispersive X-ray spectroscopy (EDS). The hardness was measured using a micro-Vickers hardness tester. The amount of precipitation hardening (ΔHv) was estimated using the following equation:

$$
\Delta\text{H}\_{\text{V}} = \text{(Hardness after again)} - \text{(Hardness before again)}\tag{2}
$$
