Effect of (Ti, Mo)_xC Particle Size on Wear Performance of High Titanium Abrasion-resistant Steel

Abstract

TiC is a widely used reinforcement in wear-resistant materials. In recent years, many researchers have studied the effect of TiC on wear resistance, and the size of TiC particles is often perceived to influence the wear resistance. However, when the size of TiC particles changes, the composition or hardness of the experimental steel also changes. In this study, (Ti, Mo)_xC-reinforced steels with the same composition were prepared though melt solidification processing using stepped molds of variable thicknesses to exclude the influence of the composition and hardness of the steel on the abrasion resistance. The solidification rate was varied by the thickness of billets, which directly affected the nucleation and growth rates of the (Ti, Mo)_xC particles. The size of the (Ti, Mo)_xC particles in the (Ti, Mo)_xC-reinforced steels varied between 1.88 and 3.20 µm. The three-body abrasive wear behavior of the (Ti, Mo)_xC-reinforced steels was determined using a wet sand/rubber wheel testing machine and the wear morphology was observed using scanning electron microscopy. The results indicated that the three-body abrasive wear mechanism of the (Ti, Mo)_xC-reinforced steels was mainly pits and micro-cuttings. As the size of the (Ti, Mo)_xC increased, the wear resistance of the (Ti, Mo)_xC-reinforced steels decreased. Larger stress occurred owing to different thermal expansion coefficients between the coarse particle and the surrounding matrix, which was more conducive to crack initiation and propagation. The optimum abrasion resistance of the (Ti, Mo)_xC-reinforced steel was 1.76 times that of traditional low alloy wear-resistant steel with similar hardness.

1. Introduction

Particle-reinforced steels containing a high volume fraction of carbide, nitride, boride, and/or oxide particles have garnered increasing interest, because of their high specific modulus and strength, thermal stability, and excellent wear resistance.^1,2,3,4) Similarly, TiC is attractive as a reinforcing material in particle-reinforced steel because of its low density (~4.93 g/cm³), high melting point (~3430 K), extreme hardness (2859–3200 HV), high Young’s modulus (~440 GPa), and high resistance to oxidation and wear.^5,6,7,8)

There are two main ways to obtain a high volume fraction of TiC particles in an alloy: through liquid state processing or solid state processing. Solid state processing includes powder metallurgy, self-propagation high-temperature synthesis, mechanical alloying, and carbo-thermal reduction.^{9,10,11,12,13,14)} Compared with solid state processing, liquid state processing has the advantage of ease and enabling the fabrication of larger ingots. The melt solidification processing methods that have been reported previously include the addition of TiC into Fe–C, addition of ferrotitanium into molten Fe–C, addition of C into Fe–Ti, and addition of Ti into molten Fe–C.^{15,16,17,18,19,20,21,22)} Additionally, TiC has been incorporated into the surface region of Fe-based alloys through laser cladding or welding.^{23,24,25,26,27)} Currently, the majority of particle-reinforced steels are produced using solid state processing. The influence of volume fraction and size of TiC particles in white iron matrix composites, fabricated using hot isostatic pressing, has been studied using dry sand/rubber wheel testing machines; the results have demonstrated that the abrasion wear resistance increases with TiC volume fraction, and fine particles provide increased wear resistance than coarse particles does.²⁸⁾ Doğan et al.²¹⁾ demonstrated that finer TiC particles provided better wear resistance in the low-stress abrasion environment (dry sand/rubber wheel testing machine), coarser TiC particles were more effective in the high-stress environment (pin-on-disc wear-testing machine), and the TiC particles were dispersed in various steels and nickel matrices using a powder metallurgy process. TiC-reinforced stainless steels (grade 316L) have been developed using solid state processing; the sliding wear resistance demonstrated that finer TiC particles exhibit better wear resistance and hardness.²²⁾ The reported processing methods for producing TiC-reinforced steel include the addition of Ti into molten Fe–C.¹⁹⁾ A new low alloy wear-resistant steel reinforced with (Ti, Mo)_xC particles was reported in our previous study;²⁹⁾ the abrasion resistance of the experimental steel was studied using a dry sand/rubber wheel wear test. The abrasion resistance was 1.6 and 1.8 times that of traditional low alloy wear-resistant steel under applied loads of 130 and 45 N, respectively. Meanwhile our results demonstrated that the wear resistance of (Ti, Mo)_xC-reinforced steels increased with the volume fraction of particles.³⁰⁾ In addition, the comparison study of the abrasive resistance of (Ti, Mo)_xC-reinforced steels under dry and wet sand conditions was reported in our previous research.³¹⁾ The above results indicated that TiC particles were beneficial in improving the wear resistance of the steel, and the wear resistance was influenced by the size of the particles. However, when the size of the particles changed, the composition or hardness of the experimental steel also changed.

In this study, (Ti, Mo)_xC-reinforced steels were produced through liquid state processing, the addition of Ti into molten Fe–C, which was subsequently poured into stepped molds of variable thicknesses. The composition of all the billets was identical, the solidification rate of the billets was changed with different thicknesses, and the nucleation and growth rates of the particles were affected. The particles were regulated by hot rolling with different compression ratios, and the size of the particles was measured. Xu³²⁾ and He et al.³³⁾ observed that spheroidal carbides did not increase steel’s wear resistance to micro-cutting because of the effect of their shape. Springer used area as the statistical standard for the size of irregular TiB2 particles,^34,35) but area cannot comprehensively describe the characteristics of particles. Rana³⁶⁾ provided a comprehensive description of particle characteristics, including diameter, length, aspect ratio, area, number density, and volume fraction. In this study, the length and aspect ratio of particles were measured to provide a comprehensive description of particle characteristics. Because the composition of all billets was identical, the volume fraction of the particles was considered as basically the same; hence, the number density was related to the area of the particles, which could be obtained indirectly by length and aspect ratios. The three-body wear behavior of the (Ti, Mo)_xC-reinforced steels was studied using a wet sand/rubber wheel abrasive wear-testing machine under an applied load of 170 N. The morphology of the samples after wear was observed using scanning electron microscopy (SEM), and the mechanism of wear was analyzed.

2. Experimental Procedure

2.1. Materials and Heat Treatment

The chemical compositions of the (Ti, Mo)_xC-reinforced steels and NM450 (traditional low alloy wear-resistance steel) are listed in Table 1. Ti easily combines with oxygen and nitrogen to form titanium oxide and titanium nitride, which cause Ti burning loss; therefore, the (Ti, Mo)_xC-reinforced steels were melted under a vacuum, and then poured into stepped molds of variable thicknesses, which were developed independently. A schematic representation of the obtained stepped billets with variable thicknesses is shown in Fig. 1. Two stepped billets with variable thickness of 5 + 30 mm and 10 + 60 mm, and a single billet of 90 mm equal thickness were fabricated. The three billets had identical length and composition. The solidification rates were different for the billets of different thicknesses and decreased systematically from 5 to 90 mm. The average cooling rate of the billets varied between 160 and 400°C/min. The solidification rate directly affected the nucleation and growth rates of the (Ti, Mo)_xC particles; thus, it affected the size, shape, and distribution of (Ti, Mo)_xC particles in the (Ti, Mo)_xC-reinforced steels. The billets were heated to 1200°C for 2 h for solution treatment, were subsequently hot rolled by two-stage controlled rolling,³⁷⁾ and were air-cooled to room temperature. Different thickness reduction were designed to study the effects of the solidification rate and thickness reduction on the particles and properties of the (Ti, Mo)_xC-reinforced steels. The hot rolling parameters are shown in Table 2. The hot rolled plates were austenitized at 900°C for 30 min in a box-type resistance furnace, followed by water quenching to ambient temperature, and then reheated to 200°C for tempering; the tempering time (in minutes) was three times of the plate thickness, e.g., the tempering time of the 6-mm thick plate was 18 min.

Table 1. Main chemical compositions of (Ti, Mo)_xC-reinforced steels and NM450 (wt.%).

Materials	C	Si	Mn	Ti	Mo	B
(Ti, Mo)xC-reinforced steels	0.30	0.25	0.50	0.60	0.30	0.0015
NM450	0.21	0.70	1.60	0.016	0.25	0.0020

Fig. 1.

Schematic diagram of a stepped billet with variable thickness.

Table 2. Hot rolling process of (Ti, Mo)_xC-reinforced steels.

	Initial hot rolling temperature/°C	Thickness of billets/mm	Thickness of plates/mm	Thickness reduction/%
Different thickness of billets	1200	30	6	80
	1200	60	12	80
	1200	90	18	80
Different compression ratios	1200	90	12	87
	1200	90	30	67

2.2. Examination of Microstructure and Brinell Hardness

Metallographic samples were cut from the billets and plates using electro-discharge machining. After heat treatment, the samples were polished using standard metallographic procedures. The morphology of the particles was observed using no etched samples and the microstructure was studied after etching with 4% nitric acid alcohol solution (4 ml HNO₃ + 96 ml ethanol) using a Zeiss Ultra-55 SEM operating at 15 kV. The surface scan was conducted on a JEOL JXA-8530F field emission electron probe microscopic analyzer (FE-EPMA) operating at 20 kV. Brinell hardness was conducted at a load of 3000 kg using a KB3000BVRZ-SA hardness tester. Ten hardness measurements were conducted and an average was determined.

2.3. Wet Sand Rubber Wheel Abrasion Wear Test

The abrasive wear samples with a size of 75 mm × 25.5 mm × 7 mm were obtained from the (Ti, Mo)_xC-reinforced steels along the rolling direction. The experiment was conducted at room temperature using a wet sand/rubber wheel wear-testing machine (model MLS-225), which is the same one that we had used in previous study.³¹⁾ A modified ASTM standard test method G105 was used.³⁸⁾ The test parameters were as follows: the load acting against the specimen was 170 N, the rotational speed was 250 rpm, the hardness of the rubber wheel was 60 Shore A hardness, the diameter of the rubber wheel was 178 mm, the sliding distance was 1118 m, and the abrasive slurry used in the test consisted of a mixture of 0.94 kg of deionized water and 1.50 kg of quartz sand with sizes in the range of 212–300 μm. In this study, three test specimens were conducted for each type of steel and an average was determined. The wear samples were cleaned using an ultrasonic cleaning machine with alcohol for 15 min to remove the attached abrasive particles and impurities before and after the test. The weight of the samples was measured by electronic balance, which is accurate to 1 mg. The morphology of the samples after wear was observed using SEM.

3. Results and Discussion

3.1. Analysis of Billet Particles

The morphology and distribution of particles in the billets of different thicknesses are shown in Fig. 2. The (Ti, Mo)_xC particles observed to form in the billets mainly exhibited columnar, polygonal, granular, and cuboid shapes, with uneven distribution. The length, aspect ratio and volume fraction of the particles were measured using the software Image-Pro Plus (IPP; ten SEM photos were measured for each steel; covered at least 1300 particles). The detail data are shown in Table 3. The average size of the (Ti, Mo)_xC particles of the billets of different thicknesses varied between 2.20 and 3.58 μm and increased with billet thickness. Meanwhile, the aspect ratios of the particles were observed to be similar, at approximately 2.6. The volume fraction of particles in the billets of different thicknesses was similar at approximately 1.20%.

Fig. 2.

Morphology of particles in the billets of different thicknesses: (a) 30 mm; (b) 60 mm; (c) 90 mm.

Table 3. Size of particles in the billets of different thicknesses.

Thickness of billets	Length/μm	Aspect ratio	Volume fraction/%
30	2.20	2.59	1.19
60	2.60	2.58	1.19
90	3.58	2.62	1.20

The surface scan maps of the particles are shown in Fig. 3; the particles were observed to mainly contain Ti, Mo, and C elements. In addition, the Mo and C elements were uniformly distributed in the particles, while the Ti elements had a non-uniform distribution. The Ti content in the core of the particles was high, and the surface scan maps indicated almost no N elements in the particles; hence, the particle was concluded to be (Ti, Mo)_xC. The ideal chemical ratio of Ti and C was about 4, but the EDX analysis of particle indicated that the chemical ratio was 3.73, because the substitutional atom Mo can be observed.

Fig. 3.

The chemical composition of the particles in the (Ti, Mo)_xC-reinforced steel: (a) BSE image; (b) C element; (c) Ti element; (d) Mo element; (e) N element; (f) EDX analysis of particle highlighted in BSE image. (Online version in color.)

3.2. Effect of Hot Rolling Processes on the Particles

The billets of different thicknesses were hot rolled with the same thickness reduction, and the morphology of the particles in the (Ti, Mo)_xC-reinforced steels after hot rolling was observed using SEM, which is shown in Fig. 4. Hot rolling can modify the distribution situation of particles and enable a homogeneous distribution situation. The average size of the particles after hot rolling was obtained using IPP, and the detail data are shown in Table 4. The average size of the (Ti, Mo)_xC particles in the plates varied between 1.88 and 3.20 μm after hot rolling and was smaller than those in the billets by approximately 30%. However, the change in particle size in the plates was consistent with that in the billets, which also had a tendency to increase with the billet thickness, but the aspect ratio of particles was similar at approximately 2.4. The volume fraction of particles was similar at approximately 1.15%, which slightly less than that in billet, because after hot rolling the small particles which can not be statistic increased.

Fig. 4.

Particle morphology of billets with different thickness after hot rolling: (a) 30 mm; (b) 60 mm; (c) 90 mm.

Table 4. Average size of particles after hot rolling and macro-hardness after heat treatment in the (Ti, Mo)_xC-reinforced steels.

	Thickness of billets/mm	Thickness of plates/mm	Length/μm	Aspect ratio	Volume fraction/%	Hardness/HBW
Different thickness of billets	30	6	1.88	2.35	1.15	441
	60	12	2.23	2.35	1.15	436
	90	18	2.65	2.40	1.16	431
Different compression ratios	90	12	2.60	2.39	1.16	433
	90	30	3.20	2.51	1.18	421

The particle morphology of 90-mm thick billets after hot rolling with different thickness reduction was observed using SEM and is shown in Fig. 5. The average size of the particles after hot rolling was obtained using IPP, and the detail data are shown in Table 4. As the thickness reduction decreased, the size of particles tended to increase; the smaller the thickness reduction was, the larger the size of the particles was. Meanwhile, the size and aspect ratios of the particles after hot rolling was observed to be similar with the thickness reduction of 80% and 87%, while the average size with the thickness reduction of 67% was 1.2 times larger than that of the former two. The volume fraction of particles was similar, and the plate with thickness reduction of 67% was slightly larger than that of with thickness reduction of 80% and 87%. The detailed analysis of particle data (Table 4) revealed that the aspect ratio was similar; hence, the length has become the main factor of particles affecting the mechanical properties of (Ti, Mo)_xC-reinforced steels.

Fig. 5.

Particle morphology of 90-mm thick billets after hot rolling with different thickness reduction: (a) 87%; (b) 80%; (c) 67%.

3.3. Microstructure and Brinell Hardness

The microstructure of the heat-treated (Ti, Mo)_xC-reinforced steels using an EPMA indicated lath martensite with particles, as shown in Figs. 6 and 7. The Brinell hardness of the heat-treated samples is shown in Table 4. As the thickness of the billets increased with the same thickness reduction, the hardness of the heat-treated samples decreased slightly. Because the thickness of the billets decreased, the cooling speed increased, the nucleation rate increased, the grain size slightly decreased, and the hardness slightly increased. As the thickness reduction of the 90-mm thick billets increased, the hardness increased slightly after heat treatment.

Fig. 6.

Microstructure of billets with different thickness after heat treatment: (a) 30 mm; (b) 60 mm; (c) 90 mm.

Fig. 7.

Microstructure morphology of 90-mm thick billets after heat treatment with different thickness reduction: (a) 87%; (b) 80%; (c) 67%.

3.4. Abrasive Wear and Surface Morphology

The relationship between the average size of particles, hardness, and wear mass loss is shown in Fig. 8. Wear resistance is often noted to increase with hardness;³⁹⁾ however, the mass loss of the (Ti, Mo)_xC-reinforced steels with similar hardness differed. The hardness is only one factor that influences the abrasion resistance of (Ti, Mo)_xC-reinforced steel, the size, the volume fraction and the specie of particle also can influence the abrasive resistance. The wear resistance of a series of (Ti, Mo)_xC-reinforced steels with different hardness under wet sand conditions³¹⁾ is shown in Fig. 9. According to Fig. 9, when the hardness of the (Ti, Mo)_xC-reinforced steels increased from 421 to 441 HBW, the wear resistance increased to 1.16 times, but the wear resistance increased to 1.49 times as shown in Fig. 8. So the results indicate that hardness was not the determinant of the wear resistance of the (Ti, Mo)_xC-reinforced steels; the average size of particles affected the wear resistance. As the average particle size increased, the mass loss of the (Ti, Mo)_xC-reinforced steels increased. In the wet sand/rubber wheel wear test, coarser particles were unfavorable to the improvement of wearing capacity. In this study, the hardness of the comparison sample (a traditional low alloy wear-resistant steel) was 459 HBW, which was slightly higher than that of the (Ti, Mo)_xC-reinforced steels. The mass loss of the NM450 sample was 0.9109 g, and the (Ti, Mo)_xC-reinforced steels have better wear resistance than NM450, indicating that particles improved wear resistance under a certain condition. Our studies have proved that (Ti, Mo)_xC-reinforced steels can significantly improve wear resistance using wet sand/rubber-wheel abrasive wear-testing machines.³¹⁾ The particles in the experimental steel prevented abrasives from embedding in the matrix and hindered the sand tips from sliding on the surface.

Fig. 8.

The relationship between the average size of particles, hardness, and mass loss of (Ti, Mo)_xC-reinforced steels. (Online version in color.)

Fig. 9.

Wear performance of (Ti, Mo)_xC-reinforced and conventional low-alloy abrasion-resistant steels under wet sand condition. (Online version in color.)

The average size of particles was around 2.96 μm in previous work,³¹⁾ which was in range of 1.88–3.20 μm in current work. Figure 9 reveals that the wear resistance result of the (Ti, Mo)_xC-reinforced steels in this paper had a high coherence with the previous report. When the average size of particles was large than 2.96 μm, the relative wear resistance was lower than that in Ref. [31], and vice versa. Meanwhile, Fig. 9 also indicates particle refinement can improve the wear resistance.

The wear morphology of (Ti, Mo)_xC-reinforced steels after the wet sand/rubber wheel abrasive wear test is shown in Fig. 10, which reveals general pits and micro-cutting. The pits could cause more serious mass loss. The number of micro-cuttings was low and their depth was shallow, because the liquid had some lubricating action that reduced the formation of furrows and micro-cuttings, which was shown in good agreement with the previous reported.³¹⁾ The worn surface of the NM450 was characterized by more micro-cutting than the (Ti, Mo)_xC-reinforced steels because the particles in the latter prevented the abrasive from embedding in the matrix and hindered the sand tips from sliding on the surface. The sizes of the pits were approximately 4 μm in the wear morphology of the experimental steel with an average particle size of 1.88 μm. The sizes of pits varied between 8 and 10 μm in the wear morphology of the experimental steel with an average particle size of 2.23 μm; meanwhile, the space between the two adjacent pits gradually decreased. The wear morphology of the experimental steel with an average particle size of 3.20 μm exhibited a significant increase in the size of the pits. The spacing between multiple pits likely gradually decreased until they connected to each other, leading to a larger observed pit size.

Fig. 10.

Wear morphology of the (Ti, Mo)_xC-reinforced steels: (a) NM450; (b) 1.88 μm; (c) 2.23 μm; (d) 2.60 μm; (e) 2.65 μm; (f) 3.20 μm.

3.5. Wear Mechanism

After the wet sand/rubber wheel abrasive wear test was conducted, the longitudinal sectional surface morphology of the (Ti, Mo)_xC-reinforced steels was scanned using SEM (Fig. 11). There was a clear deformation region and crack on the longitudinal section. In the deformation region, the matrix was deformed, but the particle was hardly deformed; therefore, cracks occurred at the interface between the matrix and particles, as shown in Figs. 11(a) and 11(b). As the particle was a hard phase with an irregular shape, stress occurred due to different thermal expansion coefficients between the particle and the surrounding matrix.⁴⁰⁾ The larger the size of particle was, the greater the stress, which was more conducive to the initiation of cracks at the stress concentration zone, and ultimately to overcome the resistance of the particle-matrix interface and extend the matrix. The resistance to crack propagation through the particle-matrix interface can be expressed by the Griffith-Orowan formula:⁴¹⁾   

$${\sigma }_{pm}={\left[\frac{\pi E{\gamma }_{pm}}{(1-v^2)a}\right]}^{1/2}$$(1)

where σ_pm is the resistance to crack propagation through the particle-matrix interface, E is the elastic modulus, γ_pm is the effective interfacial energy of the particle-matrix interface, v is the Poisson ratio, and a is the size of particles. The resistance to crack propagation through the particle-matrix interface at the same temperature is mainly determined by the size of the particle: larger particle size is more conducive to crack growth.

Fig. 11.

SEM micrographs of longitudinal sections of the (Ti, Mo)_xC-reinforced steels. (Online version in color.)

The crack may be propagated by the shear stresses; fluid will be forced into the crack by the load, thus prizing the faces of the crack apart. The crack growth rate is faster under the action of hydraulic pressure mechanisms.^42,43,44,45) When the crack extends through the interface between the matrix and particle, the crack propagation accelerates, as shown in Fig. 11(c), because of interfacial stress.

The wear mechanism of (Ti, Mo)_xC-reinforced steels in the wet sand/rubber wheel wear test was proposed as shown in Fig. 12. Figure 12(a) illustrates the initiation of the crack on the surface under contact loading, at which time the liquid will enter the crack (Fig. 12(b)). Crack closing by a passing load will accelerate the crack growth rate by pressurizing the fluid (Fig. 12(c)). The crack growth rate is faster under the action of fluid pressurization than under the action of shear force only; hence, more cracks and pits can be observed on the longitudinal section of the wet sand/rubber wheel wear samples. When the crack is near a particle, as shown in Fig. 12(d), it will propagate through the particle-matrix interface under the action of fluid pressurization. In addition, owing to the particle being a hard phase, the inconsistent deformation, and concentration at the particle-matrix interface, many cracks can be observed in the deformation region (Fig. 12(e)). The above two types of cracks extend to the surface or merge, causing the matrix to peel off and increasing the mass loss of the (Ti, Mo)_xC-reinforced steel. The larger the particle size, the greater the stress, and the easier it is for the crack to propagate through the particle-matrix interface. This is in accordance with the experimental results: as particle size increases, the wear resistance of (Ti, Mo)_xC-reinforced steels decreases.

Fig. 12.

Wear mechanisms of (Ti, Mo)_xC-reinforced steels: (a) crack initiation; (b) hydraulic pressure mechanism; (c) entrapment and pressurization of fluid; (d) crack growth and spalling; (e) crack growth in deformation region.

4. Conclusions

(1) The particles of billets shown an uneven distribution in the matrix, the average particles size of the billets increased with increasing billet thickness, and the average size of the particles varied between 2.20 and 3.58 μm for the billets with different thicknesses.

(2) After hot rolling, the uneven distribution of the particles was eliminated and the average particle size in the plates increased with increasing billet thickness under the same compression ratio. For the same billet thickness, an increase in the compression ratio decreased the average size of particles in the plates. After heat treatment, the structure of the (Ti, Mo)_xC-reinforced steels was lath martensite with particles, the hardness was approximately 450 HV, and the average particle sizes of different (Ti, Mo)_xC-reinforced steels varied between 1.88 and 3.20 μm.

(3) The wear resistance was influenced by particle size, and the mass loss decreased with increasing average size of particles with the same composition and similar hardness levels, indicating that the increase in particle size had an adverse effect on the improvement of wear resistance. The optimal wear resistance of experimental steel in this study was 1.76 times that of traditional low alloy wear-resistant steel with similar hardness.

(4) The wear morphology of (Ti, Mo)_xC-reinforced steels after wet sand/rubber-wheel abrasive wear test was generally pits and micro-cutting, and many micro-cuttings appeared in the wear morphology of NM450. The particles in the (Ti, Mo)_xC-reinforced steels prevented abrasives from embedding in the matrix and hindered the sand tips from sliding on the surface; thus, the size and number of micro-cuttings reduced, which benefited the wear resistance. However, stress concentration occurred between the particle and the matrix, which was more conducive to crack initiation and propagation in the process of wear, which in turn was unfavorable to wear resistance.

Acknowledgements

The authors acknowledge support from the National Basic Research Program, China (No. 2017YFB0305100), National Natural Science Foundation of China (No. 51874089, U1960112, 51504064), and Fundamental Research Funds for the Central Universities, China (No. N180715002, N2007002).

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