3.2. Mechanical Properties of H13 Steels Manipulated by Trace Nanoparticles
Table 2 gives the tensile and impact toughness values of different types and contents of nanoparticle-reinforced H13 steel at room temperature. The hardness, yield strength, maximum tensile strength, fracture strain, uniform elongation and product of strength plasticity of H13 steel are 46.9 HRC, 1023 MPa, 1325 MPa, 14.8%, 5.3% and 16,348 MPa %, respectively. For the H13 steel with 0.02 wt.% single-phase TiC nanoparticles and the H13 steel with 0.01 wt.% dual-phase TiC + TiB
2 nanoparticles, these properties were slightly improved. However, the hardness, yield strength, maximum tensile strength, fracture strain, uniform elongation and product of strength plasticity of H13 steel with 0.02 wt.% dual-phase TiC + TiB
2 nanoparticles increased by 3.8%,11.6%, 7.6%, 14.2%, 64.2% and 26.4%, respectively, compared to H13. Therefore, the hardness, yield strength, maximum tensile strength and product of strength plasticity of dual-phase TiC + TiB
2 nanoparticle-reinforced H13 steel can be strengthened with the increase of the content of nanoparticles and the fracture strain as well as uniform elongation, which are also significantly improved compared to unreinforced H13 steel. Furthermore, the H13 steel manipulated by dual-phase nanoparticles enhances the uniform elongation of H13 steel, which promotes H13 steel to have more uniform and stable plastic deformation.
In addition, the non-notched and U-notched impact toughness of H13 steel without nanoparticles reached 332.9 J/cm2 and 30.94 J/cm2, respectively. After adding 0.02 wt.% single-phase TiC, 0.01 wt.% dual-phase TiC + TiB2 and 0.02 wt.% dual-phase TiC + TiB2 nanoparticles to H13 steel, the non-notched and U-notched impact toughness were 406.8 J/cm2 and 33.93 J/cm2, 419.6 J/cm2 and 38.46 J/cm2, 449.3 J/cm2 and 41.39 J/cm2, respectively, which increased by 22.2% and 9.6%, 26.0% and 24.3%, 35.0% and 33.8%, respectively, compared with H13 steel without nanoparticles. The H13 steel with 0.02 wt.% dual-phase nanoparticles has larger impact toughness and a more uniform and finer microstructure, thus it has a better performance during plastic deformation. Furthermore, the cracks caused by the reduction in grain size consume more energy during propagation.
Adding single-phase TiC and dual-phase TiC + TiB
2 nanoparticles can improve the performance of H13 steel to a certain extent. Especially, the addition of 0.02 wt.% dual-phase TiC + TiB
2 nanoparticles enhances the yield strength and tensile strength of H13 steel the most, simultaneously with the highest impact toughness and the best wear resistance. Adding nanoparticles to H13 steel makes the grain size of the refined and the distribution of grains more uniform and dense. While the H13 steel is subjected to external force, the better coordination between finer grains can disperse the stress and effectively prevent stress concentration, thereby improving the performance of the steel. By comparison, the dual-phase TiC + TiB
2 nanoparticles have a more obvious strengthening effect on steel. H13 steel manipulated by dual-phase TiC + TiB
2 nanoparticles has finer lath martensite and denser microstructure than that of single-phase TiC nanoparticles, so that it has good mechanical properties and impact toughness. The added nanoparticles can be used as nucleation sites to promote nucleation and inhibit the growth of martensite, which makes the microstructure of H13 steel finer and the grain boundaries increase, thus playing a role in strengthening. Similar studies have been reported in the literature [
5,
25,
42,
64,
65]. The existence of uniformly dispersed carbides can effectively hinder the movement of dislocations and the grain boundaries of the refined H13 steel increase, which is not conducive to the expansion of cracks. These synergistic effects improve the mechanical properties of the H13 steel with different types and contents of nanoparticles. Overall, the main strengthening mechanisms can be attributed to grain refinement strengthening and second-phase strengthening.
3.3. Abrasive Wear Behavior of H13 Steel Manipulated by Trace Nanoparticles
Figure 6 and
Table 3 show the volumetric wear rate of H13 steels without and with different types and contents of nanoparticles under sandpaper with abrasive particle size of 28 μm and the loads of 15 N and 25 N at room temperature. It can be seen that the volumetric wear rate of the different materials increases with the increase in load. When the load is 15 N, the volumetric wear rate of H13 steel with 0.02 wt.% TiC, 0.01 wt.% TiC + TiB
2 and 0.02 wt.% TiC + TiB
2 is reduced by 48.3%, 35.6% and 52.9%, respectively, compared with H13 steel without nanoparticles. Further, when the load is 25 N, the volumetric wear rate of H13 steel with 0.02 wt.% TiC, 0.01 wt.% TiC + TiB
2 and 0.02 wt.% TiC + TiB
2 are respectively reduced by 38.3%, 28.9% and 44.5% compared to H13 steel without nanoparticles. Compared to single-phase nanoparticles, dual-phase nanoparticles have a better effect on improving wear resistance.
Figure 7 shows the SEM micrographs of the abrasive worn surface of H13 steel without and with different types and contents of nanoparticles under the loads of 15 N and 25 N. It can be seen from
Figure 7a that many deeper and wider furrows on the surface wear of H13 steel without nanoparticles are observed. However, the width of the furrow on the surface wear of H13 steel with 0.02 wt.% single-phase TiC nanoparticles is significantly reduced (
Figure 7b). Compared with H13 steel with 0.02 wt.% single-phase TiC nanoparticles, the surfaces of H13 steel with 0.01 wt.% dual-phase TiC + TiB
2 nanoparticles have more furrows and a greater degree of surface wear but this is much less severe than that of the H13 steel matrix (
Figure 7c). Surprisingly, the H13 steel with 0.02 wt.% dual-phase TiC + TiB
2 nanoparticles has the mildest surface wear and the smallest depth and width of furrow (
Figure 7d). While under a load of 25 N, as shown in
Figure 7e–h, the wear characteristic of different types and contents of the steel is similar to that under the load of 15 N. However, it is obvious that the degree of wear of different materials under this load are more severe than that under the load of 15 N. The increase in load increases the interaction strength between the steel and the sandpaper, which leads to an increase in the degree of wear. In general, trace nanoparticle-reinforced H13 steel can effectively reduce the depth and width of the furrows on the surface of the steel and enhance the wear resistance of the steel. The enhancement effect of 0.02 wt.% single-phase TiC nanoparticles on the wear resistance of H13 steel is greater than that of 0.01 wt.% dual-phase TiC + TiB
2 nanoparticles. Furthermore, the H13 steel with 0.02 wt.% dual-phase TiC + TiB
2 nanoparticles has the best wear resistance, indicating that the enhancement effect of dual-phase nanoparticles on the wear resistance of steel is much greater than that of single-phase nanoparticles at the same content. The reason is that the hardness, strength and toughness of bidirectional nanoparticle-strengthened steel are much higher than that of H13 steel, which guarantee its higher wear resistance.
Figure 8 and
Table 4 show the white light interference results of H13 steel without and with different contents of nanoparticles under the loads of 15 N and 25 N. It can be seen from
Figure 8(a
1) that the worn surface of H13 steel without nanoparticles at 15 N is covered with deep furrows with large height differences. However, the worn surfaces of H13 steel with 0.02 wt.% single-phase TiC and 0.01 wt.% dual-phase TiC + TiB
2 nanoparticles are relatively flat and the depth of furrow is relatively uniform (
Figure 8(b
1,c
1)). The wear resistance of H13 steel with 0.02 wt.% dual-phase TiC + TiB
2 nanoparticles is the best accompanied by the flattest surface, the smallest furrow depth and the flattest surface (
Figure 8(d
1)).
Figure 8(a
2,a
3) show that the depth of furrows on the surface of H13 steel without nanoparticles is large and the degree of abrasive wear is obvious. The depth of the worn surface is in the range of −3720 nm–3260 nm, and the maximum depth difference and the width of the wear scar on the worn surface can reach 6980 nm and 165 μm. Compared with H13 steel without nanoparticles, the surface abrasion of H13 steel with 0.02 wt.% single-phase TiC nanoparticles is reduced. The worn surface depth of the steel ranges from −2800 nm to 2900 nm, the maximum difference of worn surface depth is 5700 nm. Furthermore, the maximum width of the wear scar is 175 μm and the overall width of the wear scar is uniform except for the larger ones (
Figure 8(b
2,b
3)). The depth of the worn surface of H13 steel with 0.01 wt.% dual-phase TiC + TiB
2 nanoparticles ranges from −3250 nm to 2500 nm, the maximum difference of the depth can reach 5750 nm and the maximum width of the wear scar is 135 μm. Compared to H13 steel with 0.02 wt.% single-phase TiC nanoparticles, the depth of the furrow of H13 steel with 0.01 wt.% dual-phase TiC + TiB
2 nanoparticles increases slightly, but the maximum width of wear scar is smaller (
Figure 8(c
2,c
3)). The depth of the surface of H13 steel with 0.02 wt.% dual-phase TiC + TiB
2 nanoparticles ranges from −3400 nm to 2200 nm, the maximum difference of the depth of the worn surface is 5600 nm, and the depth of wear scar is reduced by 19.8%. The maximum width of the wear scar is 125 μm, and the depth is reduced by 24.2% (
Figure 8(d
2,d
3)). The surface of H13 steel with 0.02 wt.% dual-phase TiC + TiB
2 nanoparticles has the best abrasive wear performance.
It can be seen from the
Figure 8(e
1–h
1) that increasing load can increase the depth of furrow and the degree of surface wear of the different materials. The deep furrows in
Figure 8(e
1–e
3) show that the surface of H13 steel without nanoparticles at the load of 25 N is worn seriously. The range of the worn surface depth is −4750–3900 nm, the maximum difference of the depth is 8650 nm, and the wear scar reaches 165 μm. However, the ranges of worn surface depth of H13 steel with 0.02 wt.% single-phase TiC, 0.01 wt.% dual-phase TiC + TiB
2, and 0.02 wt.% dual-phase TiC + TiB
2 nanoparticles are −4200–2500 nm, −4250–3750 nm and −3800–2250 nm and the maximum worn surface depth difference is 6700 nm, 8000 nm and 6050 nm, respectively (
Figure 8(e
1–h
2)). The maximum width of the wear scars is 115 μm, 150 μm and 90 μm, respectively, which is reduced by 30.3%, 9.1% and 45.5%, respectively (
Figure 8(e
3–h
3)). In summary, the degree of worn surface of the steel is reduced and the depth of the surface furrow is reduced due to the finer lath martensite of H13 steel manipulated by nanoparticles. Under the loads of 15 N and 25 N, the anti-abrasive wear order of different steels is H13 steel with 0.02 wt.% TiC + TiB
2 > H13 steel with 0.02 wt.%TiC > H13 steel with 0.01 wt.%TiC + TiB
2 > H13 steel. The H13 steel reinforced by dual-phase nanoparticles has stronger resistance to abrasive wear than that of the H13 steel reinforced by single-phase nanoparticles. The addition of nanoparticles refines the microstructure of H13 steel, especially the H13 steel with 0.02 wt.% dual-phase TiC + TiB
2, which has finer martensite and greater hardness. Therefore, it can effectively resist the intrusion of abrasive particles and reduce the generation of grooves. A similar phenomenon was also reported by Coronado et al. [
76].
Figure 9 and
Table 5 show the volumetric wear rate of H13 steels without and with different types and contents of nanoparticles under the load of 25 N using the sandpaper with different abrasive particle size. Rough sandpaper can increase the volume wear rate of various materials. Compared with H13 steel without nanoparticles, the volumetric wear rate of H13 steel with 0.02 wt.% TiC, 0.01 wt.% TiC + TiB
2 and 0.02 wt.% TiC + TiB
2 is reduced by 38.3%, 28.9% and 44.5%, respectively. Using the sandpaper with the abrasive particle size of 14 μm, the volumetric wear rate of H13 steel with 0.02 wt.% TiC, 0.01 wt.% and 0.02 wt.% TiC + TiB
2 is, respectively, reduced by 37.3%, 32.8% and 47.8% compared to the H13 steel. Therefore, the H13 steel with 0.02 wt.%TiC + TiB
2 shows the best wear resistance at different sandpaper size, and the anti-abrasive wear order of different steels is H13 steel with 0.02 wt.% TiC + TiB
2 > H13 steel with 0.02 wt.% TiC > H13 steel with 0.01 wt.% TiC + TiB
2 > H13 steel.
Figure 10a–h shows the abrasive worn surface morphologies of H13 steels without and with different contents and types of nanoparticles at the abrasive particle size of 28 μm and 14 μm. Compared with the abrasive particle size of 14 μm, the surface of the steel at the abrasive particle size of 28 μm has a higher degree of wear and the deeper furrow (
Figure 10a–d).
Figure 10e shows that the surface of H13 steel without nanoparticles has a more severe degree of wear, and a deeper and wider furrow. However, the degree of the surface wear of H13 steel with 0.02 wt.% TiC is reduced, and the depth and width of the furrow are also reduced (
Figure 10f).
Figure 10g shows that the degree of the surface wear of H13 steel with 0.01 wt.% TiC + TiB
2 is slighter than that of the H13 steel but is more severe than that of the H13 steel with 0.02 wt.% TiC. The H13 steel with 0.01 wt.% TiC + TiB
2 shows a poor wear resistance compared with the H13 steel with 0.02 wt.% TiC. In short, the H13 steel with 0.02 wt.% dual-phase TiC + TiB
2 has the best wear resistance.
Figure 10(e
1–h
3) and
Table 6 show the white light interference results of nanoparticles of different types and contents reinforced H13 steel at the load of 25 N after abrasive wear of sandpaper with abrasive particle size of 14 μm. The corresponding results of abrasive wear of sandpaper with abrasive particle size of 28 μm are given in
Figure 8(e
1–h
3). Compared with the surface with abrasive particle size of 28 μm, the number of furrows on the surface with particle sizes of 14 μm is decreased and the degree of abrasive wear is reduced.
Figure 10(e
1–e
3) shows that the surface wear of H13 steel without nanoparticles is serious, accompanied by a very deep furrow, the depth of worn surface ranges from −3250 nm to 2500 nm, and the maximum difference of depth is 5750 nm. Notably, the wear scar on the worn surface reaches 150 μm. As the addition of nanoparticles, the surfaces of different H13 steels become relatively flat, the depth of the furrow is reduced, and the wear resistance is enhanced. The depth of the worn surface of H13 steel with 0.02 wt.% TiC ranges from −2300 nm to 1800 nm, the maximum difference of depth and the maximum width of the wear scar are 4100 nm and 150 μm, respectively (
Figure 10(f
1–f
3)). Furthermore, the depth of the surface furrow of H13 steel with 0.01 wt.% TiC + TiB
2 is slightly higher than that of H13 steel with 0.02 wt.% TiC. However, compared with H13 steel without nanoparticles, the wear resistance is significantly improved. The depth of worn surface ranges from −2600 nm to 2200 nm, the maximum difference of depth and the maximum width of the wear scar is 4800 nm and 115 μm, respectively. Same as above, the H13 steel with 0.02 wt.% dual-phase TiC + TiB
2 has the flattest surface and the best wear resistance. The surface depth ranges from −1950 nm to 2000 nm, and the maximum difference of depth of worn surface is 3950 nm. Compared to the H13 steel, the depth of the surface of the steel manipulated by 0.02 wt.% dual-phase TiC + TiB
2 is reduced by 31.3%. Furthermore, the maximum width of the wear scar is 75 μm, reduced by 50%, and the depth and width of the furrow are significantly reduced. Therefore, it has the best abrasive wear resistance at the room temperature. Smaller sized abrasive particles produce more finer and more uniform furrow marks on all steel surfaces. With the increase of the abrasive size, the mass loss and wear rate of the composite increase, and the depth and width of the grooves increase, which is also confirmed by Tressia et al. [
77]. Due to the irregularity of abrasive particles, larger size abrasive particles are more likely to damage and crush the surface of H13 steel and cause secondary wear. At present, the relevant literature [
78] has reported the effect of abrasive particle size on wear, but there is no unified understanding that can effectively explain its effect.
In the abrasive wear process, a furrow is formed when the matrix is plowed out under the action of friction. The addition of nanoparticles, especially dual-phase nanoparticles, can make the steel matrix resist the indentation of abrasive particles and reduce the indentation depth of abrasive particles, thus leading to the formation of denser microstructure of steel. Compared with single-phase nanoparticles, dual-phase nanoparticles have better resistance to abrasive particle invasion. The denser microstructure of H13 steel manipulated by dual-phase nanoparticles can effectively prevent abrasive particles from being pressed into the steel, thereby protecting the matrix from excessive plowing and effectively reducing the generation of grooves.