Effects of Rare-earth and Microalloying Elements on the Microstructure Characteristics of Hypereutectoid Rails

DAI Yuheng, ZHAO Wenqian, BAO Xirong*, CHEN Lin*

(1. School of Material and Metallurgy, Inner Mongolia University of Science & Technology, Baotou 014010, China; 2. Beijing Beiye Functional Materials Corporation, Beijing 100000, China)

Abstract: We performed thermal simulation experiments of double-pass deformation of hypereutectoid rails with different microalloying elements at a cooling rate of 1°C/s and deformation of 80% to explore the influence of rare-earth and microalloying elements on the structure of hypereutectoid rails and optimize the composition design of hypereutectoid rails. Scanning electron microscopy, transmission electron microscopy,X-ray diffraction, and other characterization techniques were employed to quantitatively analyzed the effects of different microalloying elements, including rare-earth elements, on pearlite lamellar spacing, cementite characteristics, and dislocation density. It was found that the lamellar spacing was reduced by adding various microalloying elements. Cementite lamellar thickness decreased with the refinement of pearlite lamellar spacing while the cementite content per unit volume increased. Local cementite spheroidization, dispersed in the ferrite matrix in granular form and thus playing the role of dispersion strengthening, was observed upon adding cerium(Ce). The contributions of dislocation density to the alloy strength of four steel sheet samples with and without the addition of nickel, Ce, and Ce–copper (Cu) composite were 26, 27, 32, and 37 MPa, respectively, indicating that the Ce–Cu composite had the highest dislocation strengthening effect. The Ce–Cu composite has played a meaningful role in the cementite characteristics and dislocation strengthening, which provides a theoretical basis for optimizing the composition design of hypereutectoid rails in actual production conditions.

Key words: microalloying elements; rare earth; hypereutectoid rail; cementite; dislocation

1 Introduction

As the performance of traditional eutectoid rails approaches a plateau, hypereutectoid rail has attracted scientific attention[1-2]. The higher density of cementite in hypereutectoid rail makes it have higher hardness,wear resistance and surface damage resistance. It is suitable for laying in small radius curve and has broad application prospects[3].

In addition to heat treatment, alloying elements can improve the performance of hypereutectoid rails,similar to traditional eutectoid rails. For example,according to previous studies[4-8], adding chromium(Cr) and niobium (Nb) can improve the hardenability,strength, and toughness of rails and refine the lamella spacing of pearlite. Adding nickel (Ni) and copper (Cu)improves the strength of steel through the interaction between them, and adding cerium (Ce), a rare-earth element, can affect the formation order of inclusions in steel, resulting in the inclusions’ deterioration.

Fine pearlite lamellar spacing improves the fracture toughness and fatigue properties of steel[9]. In addition, the thickness and quantity of cementite lamellae change with thinning of lamellar spacing. Remarkably,an increase in the number of cementite lamellae inhibits the propagation of fatigue cracks, and fine cementite lamella thickness can effectively improve fracture toughness[10,11]. For hypereutectoid steel, high carbon content implies more cementite content per unit volume. Thus, studying the changes in cementite characteristics in hypereutectoid steel is important.

Rolling deformations also play an important role in microstructural transformation in the actual production. Cenet al[12]have shown that stress accelerates the transformation of pearlite microstructure in rails, and dislocations retained by rolling deformation in steel influence dislocation strengthening, which affects the service performance of rails. Sano and Hong[13,14]reported that the lower the strain, the easier the dislocation entanglement, and the higher the strain, the more likely dislocations exist independently and the greater the dislocation density. The strength of metals is closely related to the dislocation density because dislocations are retained in the microstructure of a material after they are entangled. Furthermore, the dislocation configuration and distribution can be directly observed via transmission electron microscopy (TEM)[15], the dislocation density information in the microregion of the material can be reflected, and the dislocation density in the macroregion of the material can be calculated by X-ray diffraction (XRD) linear analysis.

Therefore, exploring the changes in cementite characteristics and dislocation configurations after adding microalloying elements under actual production conditions is important. Herein, the cementite characteristics and dislocation density evolution of experimental steels to which different microalloying elements were added by developing a heat treatment and deformation process simulating field production were observed and compared by scanning electron microscopy (SEM), TEM, and XRD. We quantitatively analyzed the effects of different microalloying elements on the microstructure and dislocation density of hypereutectoid rails to determine the optimal composition design of experimental steel field production conditions, providing a theoretical basis for the design and production of high carbon hypereutectoid rails.

2 Experimental

Hypereutectoid steel rails smelted in the laboratory were used as the reference test steel, labeled 1#.Based on 1#, Ni, Ce, and Ce–Cu composite additives were added to the steel and labeled 2#, 3#, and 4#,respectively. The initial state of the experimental steel was forged, and the specific chemical compositions are listed in Table 1.

Table 1 Chemical compositions of experimental steel/wt%

Double-pass deformation experiments were conducted using Gleeble 1500D thermal simulation experimental machine, which mainly simulates on-site rolling processes. The sample size wasФ8×15 mm.Fig.1 shows the relationship between the loading stress and strain, and Fig.2 depicts the thermal simulation process. First, the samples were heated to 1 250 ℃,held for 5 min, and cooled to 1 100 ℃ at 5 ℃/s for the first compression deformation (deformation rate is 1/s, deformation is 50%) and then to 950 ℃ at 5 ℃/s for the second compression deformation (deformation rate is 10/s, deformation is 30%). Finally, they were continuously cooled to room temperature at a cooling rate of 1 ℃/s.

Fig.1 Relationship between loading stress and strain

Fig.2 Thermal simulation process

After thermal simulations, the samples were observed via TEM. The samples were cut following thermal simulations to 300 μm. The thickness direction was the compression direction, and the samples were ground to 80 μm. Then, they were punched into aФ3 mm disk using a punching tool and further ground to 50 μm. The processed wafer sample was thinned via double spraying, and the microstructure of the steel samples was examined through field-emission TEM.The observation surface was parallel to the compression direction. Using digital micrograph and nanomeasurement software, the effects of different alloying elements on the cementite content and lamellar thickness per unit volume in the steel samples were investigated.

Furthermore, the dislocation configuration and distribution in the micro and macroregions were observed via TEM and XRD, respectively. The sample preparation method was the same as that of the metallographic sample. The test surface was parallel to the compression direction, and Cu-Kα was used. The scanning angle was 30°-120°, and the scanning speed was 2°/min. Using data from the crystal plane index and half-width of the diffract following peak, the dislocation density of the samples with different alloying elements and the effects of dislocation density on the strength were evaluated.

3 Analysis and discussion

3.1 Effect of alloying elements on cementite characteristics

3.1.1 Variation of pearlite morphology

Fig.3 depicts the effect of cooling rate on the pearlite structure under 80% deformation. The pearlite lamellar spacing in the samples supplemented with microalloying elements was refined to varying degrees.Combining nanomeasurements and statistics software,the lamellar spacing of samples 1#-4# was 99, 95, 86,and 93 nm, respectively. Hence, the refinement degree of Ni was 4.0%, that of 0.011% Ce was 9.5%, and that of 0.004% Ce and 0.49% Cu was 6.1%.

Fig.3 Pearlite structure and morphology of samples at a cooling rate of 1 ℃/s under 80% deformation for samples: (a) 1#, (b) 2#,(c) 3#, and (d) 4#

3.1.2 Variation of cementite morphology

Fig.4 shows the morphology of cementite lamellae of samples 1#-4# obtained at a cooling rate of 1℃/s after 80% deformation. The cementite in the microstructure of samples with different microalloying elements showed different degrees of torsion and fracture. The cementite lamellae in sample 1# were bent and fractured in some areas to withstand deformation(Fig.4(a)), and there was no obvious directionality between adjacent pearlite lamellae. The cementite and ferrite of samples 2# and 4# deformed harmoniously,and some flat cementite lamellae showed directionality and certain plastic deformation (Figs.4(b) and 4(d)).Furthermore, as the pearlite lamellar spacing decreased,the cementite lamellae thinned, decreasing fracture lamellae. In the local fracture area, spheroidization was observed along the original cementite lamellae. In sample 3#, cementite lamellar fracture and pearlite lamellar junctions with different orientations gradually turned to granular cementite (Fig.3(c)). Based on these results,the addition of microalloying elements can significantly refine matrix structures under the same deformation conditions, promote the transformation of austenite into degraded pearlite, and improve the spheroidization efficiency and cementite distribution uniformity.

Fig.4 Cementite structure and morphology at a cooling rate of 1 ℃/s under 80% deformation for samples: (a) 1#, (b) 2#, (c) 3#,and (d) 4#

To further investigate the morphological characteristics of cementite in the microregions, the morphology of cementite in samples 1#-4# was characterized via TEM. The cementite layer in sample 1# was relatively thick and was broken and bent under 80% deformation (Fig.5(a)). The cementite lamellae in samples 2# and 4# were thin, and a small number of short rod cementite lamellae were spheroidized and regularly arranged in the direction of the original cementite lamellae (Figs.5(b) and 5(d)). Analyzing the diffraction spot diagram presented in Fig.5(c), we found that A is the cementite structure of the orthorhombic system, and B is the ferrite matrix of a body-centered cubic structure.Moreover, short and rod cementite was dispersed in the matrix of sample 3#, indicating a transformation from short rod to granular cementite. These results show that rare-earth elements can refine the size of cementite,improve its aggregation and distribution, reduce its driving force, and make it disperse evenly in a matrix.Furthermore, the finer the pearlite lamella, the easier it is to dissolve the cementite. Thus, rare-earth elements refine the pearlite lamella and promote the transition from lamellar to granular cementite.

Fig.5 Cementite morphology at a cooling rate of 1 ℃/s under 80%deformation for samples: (a) 1#, (b) 2#, (c) 3#, and (d) 4#

3.1.3 Variation of cementite content per unit volume and cementite lamellar thickness

Fig.6 shows TEM images of the pearlite structures of samples 1#-4#; the dark lamella indicates a cementite structure. The cementite lamella with microalloying elements was refined to varying degrees. We selected at least 20 complete areas of the cementite layer and measured the thickness of the cementite layer using nanomeasurement software, and the thicknesses of the cementite layer of samples 1#-4# was 25.2, 19.6, 15.5,and 16.3 nm, respectively. Analyzing the images using digital micrograph software, we found that the contents of cementite per unit volume of samples 1#-4# were 13.64%, 15.88%, 18.58%, and 16.79%, respectively.The cementite thickness, cementite content per unit volume, and lamellar spacing are listed in Table 2.

Table 2 Variation of pearlite and cementite

Fig.6 Cementite lamella morphology at a cooling rate of 1 ℃/s under 80% deformation: (a) 1#, (b) 2#, (c) 3#, and (d) 4#

As shown in Table 2, adding Ni, Ce, and Ce-Cu could promote the spheroidization of cementite and reduce the thickness of cementite lamella to improve the lamellar structure of pearlite and the mechanical properties of the samples. Furthermore, the greater the cementite content per unit volume, the thinner the cementite lamella. This is primarily because the pearlite lamella is large when the cementite lamella thickness is large, and the arrangement is loose. Therefore, the finer the pearlite lamella, the thinner the cementite lamella,and the more closely arranged they are in the ferrite matrix, the higher the cementite content per unit volume.

3.2 Effects of alloying elements on dislocation

3.2.1 Dislocation patterns

Fig.7 depicts the transmission of the dislocation configuration of samples 1#–4# at a cooling rate of 1℃/s and total deformation of 80%.

Fig.7 Dislocation patterns of samples: (a) 1#, (b) 2#, (c) 3#, and(d) 4# after thermal simulation experiments fabricated at a cooling rate of 1 ℃ and 80% deformation

The dislocation lines in the ferrite lamella of sample 1# were arranged at a certain angle with the lamella(Fig.7(a)). The two ends of the straight dislocation lines were pinned at the interface between ferrite and cementite. As shown in Fig.7(b), the dislocation lines are entangled in ferritic lamellae of sample 2#, and “bow”dislocation lines appear because of pinning at both ends of dislocation lines after the reduction of ferritic lamellae. For sample 3#, the dislocation lines arranged between ferritic lamellae were reduced (Fig.7(c)).The formation of a dislocation wall is attributed to the increase in dislocation density and entanglement of dislocation lines. The formation of the dislocation wall reduces the system energy and alleviates the stress concentration caused by dislocation entanglement, allowing plastic deformation to continue. As shown in Fig.7(d), there were many tangled dislocations in the ferritic lamellae of sample 4# and dislocation lines running through the two cementite lamellae. In addition,flat dislocations were arranged in parallel between the lamellae, and the dislocation density further increased.Finally, following the addition of Ni, Ce, and Ce-Cu,the pearlite lamellae were refined. The reduction of pearlite lamella, in contrast, provided a corresponding driving force for dislocation proliferation and increased the dislocation density. Furthermore, the greater the number of the second phase particles in steel, the greater the impediment to dislocation movement, increasing dislocation density.

3.2.2 Dislocation density

Since TEM can only reflect dislocation densities in microregions, Williamson and Hall[16], in the 1950s,hypothesized that the broadening of diffraction peaks was caused by grain size changes and microdistortions,which can quantitatively represent the dislocation density. Microdistortion broaden diffraction peak is expressed as follows:

whereβsis the diffraction peak broadening,εthe microscopic strain, andθis the diffraction peak angle.

The diffraction peak broadening caused by grainsize refinement is expressed as follows:

whereβdis the diffraction peak broadening caused by grain refinement,Dis the grain size, andλis the X-ray wavelength.

Krilet al[17]stated thatD/1 is very small when the grain size exceeds 100 nm, and the diffraction peak broadening caused by grain refinement is negligible.The dislocation density, according to Williamson and Hall’s method, is expressed as

Renxu[18]calculated the macro dislocation density through experiments and mathematical models to quantitatively evaluate the material dislocation density.To obtain an expression for the dislocation density, the half-height width of each diffraction peak in the XRD spectrum of the material is extracted and is expressed as follows:

whereDis the half-height width of the diffraction peak andbis the Berger vector.

Herein, XRD was performed on samples 1#-4#after thermal simulations. The experimental data were analyzed and processed using Jade 6.0 to obtain the XRD diffraction patterns of the four samples (Fig.8).

Fig.8 XRD pattern of samples 1#–4#

The diffraction peaks of (110), (200), (211), (220),and (310) crystal planes were clearly observed with high intensities in samples 1#-4# (Fig.8). Moreover,the relative strength of the (110) crystal plane is higher than the other four crystal planes, and the shape of the diffraction peak is high and narrow, indicating that the grains prefer orientation on the (110) crystal plane.

Next, the half-height width of the diffraction peak in the XRD pattern was extracted, and the data were substituted into Eq. (4). The dislocation densities of samples 1#-4# were 3.2×1014, 3.6×1014, 4.9×1014, and 6.6×1014m−2, respectively, and the histogram of each crystal plane and total dislocation density are shown in Fig.9.

Fig.9 Dislocation density distribution histogram for samples 1#-4#

After adding Ni, 0.011% Ce, 0.004% Ce, and 0.49% Cu, the total dislocation density and dislocation density of each crystal plane increased sequentially. In sample 4#, the dislocation density of the (200) crystal plane sharply increased because the lattice in the (200)crystal plane was distorted and slid, increasing the dislocation density. Thus, the total dislocation density increased because the alloying elements refined the lamellar spacing. As a result, both ends of the dislocation line were pinned, and the dislocations continued to grow in response to an external force. Furthermore, rare earth elements and Cu can increase the number of the second phase particles in steel, thereby preventing dislocation movement and increasing dislocation density.The contribution of dislocation density to the strength of hypereutectoid steel can be calculated as follows[19]:

whereMis the Taylor factor (with a value of 3),Gis the shear modulus (80 000 MPa),bis the Berger vector(0.25 nm), andαis a constant (0.24).

The contributions of dislocation density to the strength of samples 1#-4# were 26, 27, 32, and 37 MPa, respectively (Fig.10). The contribution of dislocation density to the strength of sample 2# after adding Ni was 1 MPa higher than that of sample 1#. After adding rare earth elements (sample 3#), the strength contribution increased by 6 MPa, compared with that of 1#,and the strength contribution in sample 4# (after Ce-Cu composite addition) increased by 11 MPa, indicating that dislocation strengthening was most significant in sample 4#.

Fig.10 Contribution of dislocation density to the strength of samples 1#-4#

4 Conclusions

a) With 80% deformation observed at a cooling rate of 1 ℃/s, the morphology of cementite changed after the addition of different alloying elements. In samples 2# (Ni addition) and 4# (Ce-Cu addition), the cementite lamellae were thinner. A small amount of short and rod cementite was spheroidized and arranged in the direction of the original cementite lamellae. For sample 3# (0.011% Ce), cementite was spheroidized along the cementite fracture boundary. The cementite was dispersed in the ferrite matrix in a granular and short rod shape. Thus, adding rare earth elements better promotes the transformation from sheet to granular cementite.

b) The spacing of pearlitic lamellae in samples 1#-4# steel was 99, 95, 86, and 93 nm, the thicknesses of cementite lamellae were 25.2, 19.6, 15.5, and 16.3 nm, and the cementite contents per unit volume were 13.64%, 15.88%, 18.58%, and 16.79%, respectively.The smaller the pearlite lamellar spacing, the finer the corresponding cementite lamellar, and the higher the cementite content per unit volume. Adding 0.011% Ce showed the greatest refining effect on cementite, followed by the Ce-Cu composite, and the refining effect of only Ni was relatively weak. The addition of rare earth Ce significantly increased the cementite content per unit volume.

c) Adding alloying elements reduced the lamellar spacing of pearlite. Rare-earth elements and Cu can both promote the formation of second-phase particles in steel, increasing dislocation density. The dislocation density of samples 1#-4# under 80% deformation was 3.2×1014, 3.6×1014, 4.9×1014, and 6.6×1014m−2, and the contributions of dislocation density to the strength were 26, 27, 32, and 37 MPa, respectively. The Ce-Cu composite showed the highest dislocation strengthening effect, followed by 0.011% Ce, and Ni showed the least effect. The Ce-Cu composite played a significant role in the improvement of the cementite characteristics and dislocation strengthening.

Conflict of interest

All authors declare that there are no competing interests.