The Effects of Different Ultrasonic Composite Surface Modifications on the Properties of H13 Steel for Shield Tunnel Machine Cutter Ring

Abstract

Tunnel boring machines (TBMs) are exposed to the impact of the ground shattering force and the friction of sandstone during excavation work, and are prone to wear and breakage, and other failures. Traditional heat treatment processes cannot simultaneously achieve the required high-energy composite structure of hard external and tough internal properties for cutter rings, leading to inadequate wear resistance and impact toughness under working conditions. This study utilizes H13 steel as the base material, and based on a study of carburizing, nitriding, and ultrasonic impact processes for H13 steel analyzing the effects of different high-energy composite modification processes on the hardness distribution, microstructure, and residual stress of H13 steel, the mechanisms by which high-energy composite modification processes affect the wear resistance and impact resistance of H13 steel are revealed. The results indicate that the wear amount and impact toughness of the sample subjected to carburizing and ultrasonic surface rolling composite strengthening were 1.9 mg and 27.34 J/cm², demonstrating the best wear and impact resistance. This combination of properties allows the H13 steel cutter ring to achieve the optimal overall performance in terms of wear resistance and impact resistance.

1. Introduction

Tunnel boring machines (TBMs) have advantages such as high automation, fast excavation speed, and good safety performance, which can effectively reduce tunnel vibrations, and are widely used in tunnel engineering for infrastructure projects [1,2]. During the excavation process, TBMs operate in extremely harsh environments, where the cutter rings are subjected to friction and impact from rocks, experiencing immense contact stresses [3]. Particularly when encountering granite rock with varying degrees of weathering, the cutter rings are subjected to a complex working environment characterized by either soft upper with hard lower conditions or hard upper with soft lower conditions. This mismatch between the hardness of the outer ring and the toughness of the inner core results in severe wear on the cutter ring [4,5]. The cutter ring used in this research is a disc cutter ring, which is often used in the excavation of marble, sandstone, limestone, granite, and other high-contact-stress environments; the rock-breaking capacity is required to be 30–180 Mpa. Due to the direct contact between the cutter body and the rock, it is subjected to high-intensity contact stress, which is a direct challenge to the cutter ring’s hardness and impact toughness. If the cutter ring does not have high hardness and high impact toughness, it can easily crack, crimp, fracture, and experience other failures, seriously shortening the life of the cutter ring. The cutter ring is a critical component of the TBM; when the cutter ring fails, it requires manual replacement, which typically takes 5 to 7 days each time, thereby increasing the excavation costs of TBMs. According to the engineering data report, the economic loss caused by cutter ring failures accounts for one-fifth of the total tunneling cost, the time for replacing the cutter head accounts for one-third of the total construction time, and wear failure accounts for 87% of the cutter ring failures [6,7]. Therefore, developing a long-lasting and high-performance TBM cutter ring is of great significance for further improving the tunneling level of TBMs and reducing the construction cost [8,9].

The improvement of tool boring life is mainly dependent on the synergistic effect of surface hardness and core toughness of the material. In TBMs, high hardness of the material is essential for cutting through rock. However, if the material experiences impact loads in a short period, insufficient toughness in the core can result in the fracture of the cutting ring [3,10]. The German company Würth [11] has utilized high-hardness cutting tools made from X50CrVMo5 steel, a material known for its excellent hardenability, providing a uniform hardness distribution and good wear resistance, although its toughness is inadequate. Based on research on hot-work mold steels, such as 5CrNiMn and 5CrMnMo, researchers have developed a series of hot-work mold steels (H11, H12, andH13). These H-series mold steels demonstrate excellent comprehensive properties, including high hardness, outstanding wear resistance, high toughness, good heat resistance, and machinability [12]. Among them, H13 steel (AISI H13) is widely used in tasks such as tunneling with tunnel boring machines (TBMs). However, the lifespan of H13 steel cutting rings remains insufficient, necessitating frequent downtime for replacements. Therefore, investigating processes to achieve ultra-hardness and toughening properties in H13 steel is urgent. Currently, improvement strategies for enhancing the surface performance of H13 steel mainly involve heat treatment [13], surface coatings, and cutting ring design [14,15]. Hu et al. [15] prepared Ni3Ta-TaC and Ni-based WC alloy coatings on cutting rings through laser cladding, which significantly improved the surface hardness and wear resistance of the cutting rings. However, the bonding strength between the coatings and the substrate was insufficient; under brief impact loads, the coatings could not meet the impact toughness requirements of the core, leading to damage or even detachment. Jagota V et al. [16] discovered that H13 steel exhibited fine grains and high toughness during austenitization at 1000 °C, but the impact toughness decreased with increasing austenitization temperature. Zhu et al. [17] investigated the effects of tempering temperature and time on the impact toughness of H13 steel, finding that the impact toughness significantly increased after two tempering processes. These studies indicate that a suitable heat treatment process can markedly enhance the impact toughness of the material. There is a challenge in matching the hardness and impact toughness of TBM cutting rings. Therefore, establishing a hardness gradient structure in the surface layer of the material, based on an appropriate heat treatment process, is an effective method to ensure impact toughness while improving wear resistance. This approach is also of significant importance in this research.

Ultrasonic surface rolling peening (USRP) is a finishing process that can improve the surface quality, is a kind of surface modification technology caused by the generation of heavy plastic deformation (SPD) on the surface of the material, relying on the impact head to exert pressure on the surface of the workpiece being processed, so that the surface layer of the processed workpiece produces plastic deformation. After ultrasonic impact, the uneven part of the surface of the workpiece becomes flat and smooth due to plastic flow, and at the same time, under the effect of ultrasonic vibration and impact, the surface grain of the workpiece is refined to improve the comprehensive performance of its surface layer, which has received extensive attention from researchers [18,19,20]. Zou et al. [21] created nanocrystalline fields on the 40CrNiMoA steel surface on the rolling blade material structured by ultrasonic rolling, so that the test had good residual pressure stress and hardness gradient structure, achieving high impact resistance while increasing its wear life. Chen et al. [22] studied 25CrNi2MoV steel under the USRP process by different static loads, and found that both the friction coefficient and the wear volume were reduced after the treatment. USRPs can effectively improve the residual stress and hardness of the material and achieve better wear resistance. In summary, this study is the first to investigate the H13 steel heat treatment process to improve the impact resistance of the material core; on this basis, USRP treatment was used to improve surface quality and thereby improve wear resistance. In previous work, we explored the different heat treatment techniques for H13 materials [23]; this study will compare the wear resistance and impact resistance of H13 steels after different ultrasonic composite modification, which is important for guiding the TBM cutter ring life cycle.

2. Materials and Methods

2.1. Materials and Heat Treatment

2.1.1. Materials Preparation

In this study, to facilitate the investigation of the effects of ultrasonic composite modification processes on the impact resistance and wear resistance of H13 steel used for cutter rings, the processes were applied to standard specimens. As shown in Figure 1a, the impact experimental materials consisted of H13 steel rods, which were cut into straight rods with a diameter of Φ10 mm and a length of 55 mm. As shown in Figure 1b, the wear materials had a diameter of Φ40 mm and a thickness of 15 mm, using wire electrical discharge machining. The chemical composition was analyzed using a SpectroMAXx BT model direct-reading spectrometer from Germany’s Spectro Company, with a discharge frequency of 200 μs for 400 discharges and an energy resolution of 125 mW, and a maximum spark power of 4 kW. The detection results were obtained as the average of five measurements, with the elemental composition (wt%) summarized in

Table 1. Sample element composition.

Element wt%
Si	Mn	C	Cr	V	P	S
1.03	0.4	0.41	5.05	0.89	0.002	0.002

.

2.1.2. Heat Treatment Process

In order to ensure the stability of the quenching process and to harden the H13 steel as a whole by quenching, as shown in

Table 2. Critical transition temperature of H13 steel.

Critical Point	Ac1	Ac3	Ar1	Ar3	Ms	Mf
Temperature (°C)	860	915	775	815	340	215

, an appropriate full austenitizing temperature as well as the quenching program were selected based on the Ac1 (austenite start transformation temperature point), Ac3 (austenite end transformation temperature point), Ms (martensite start transformation temperature point), and Mf (martensite end transformation temperature point) of the H13 steel. To ensure the feasibility of the experiments and enhance their efficiency, the heat treatment process for the materials was selected to match that of prior experiments [23]. The high-energy composite modified structure of the surface layer of the material was constructed by carburizing and nitriding chemical heat treatment on the basis of vacuum air quenching, resulting in a superficial layer composed of high-hardness martensitic microstructure, while the core maintained a tempered martensitic structure. This configuration provides the material with a combination of high surface hardness and toughness in the core [24], simultaneously increasing the fatigue strength of the material [25]. The heat treatment process is shown in Figure 2; Figure 2a shows the real air quenching process—the furnace was first heated to 880 °C uniform heat for 0.5 h and then heated to the quenching temperature of 1020 °C insulation for 0.5 h, and the specimen was subjected to 0.2 MPa gas quenching and then cooled to room temperature. Gas quenching results in a more uniform temperature throughout the product, which improves the consistency of product performance. The carburizing process is depicted in Figure 2b. Initially, the furnace was heated to 880 °C and held at this temperature for 0.5 h for uniform heating. The temperature was then increased to the carburizing temperature of 950 °C, where the sample was subjected to aggressive carburization for 3 h at a carbon potential of 1.2%. Subsequently, the sample was allowed to hold and diffuse at a carbon potential of 0.8% for an additional 3 h. Finally, the temperature was raised to the quenching temperature of 1020 °C for 0.5 h, after which the sample was quenched in gas at 0.2 MPa until it reached room temperature. To stabilize the microstructure after carburizing and quenching, the samples were placed in a vacuum tempering furnace, heated to 520 °C, which was held for 2 h, followed by gas cooling at 0.2 MPa until room temperature was achieved. Carburizing enhances the surface hardness of the material, as well as obtains high-carbon martensite, which improves the mechanical properties of the material surface. The nitriding heat treatment process is illustrated in Figure 2c. This was conducted in a gas atmosphere consisting of N2:H2 = 450 mL/min:150 mL/min under a pressure of 300 Pa, maintained at a temperature of 480 °C for 8 h, and finally cooled along with the furnace. Nitriding enhances the surface hardness and gives good wear resistance. However, a white glossy layer is produced, which may affect the mechanical properties and will be examined in this study.

2.2. USRP Treatment

To investigate the effects of USRP on the impact toughness and friction–wear performance of H13 steel, USRP treatment was performed on the specimens using the HKC30-50 ultrasonic impact modification equipment. A 7 mm diameter carbide ball (SR7 mm) was employed as the impactor, and ultrasonic rolling strengthening was conducted under a static pressure load and ultrasonic vibration load. The quality of the material surface after ultrasonic surface reprocessing (USRP) is influenced by various processing parameters. The process variables included the static load (N), rotational speed (rpm), vibration frequency (kHz), coverage rate, feed rate (mm/r), and number of impacts. Among these, the static load, vibration frequency, feed rate, and rotational speed significantly affect the processing quality. Based on preliminary experimental experience [26], the optimal number of processing cycles was determined to be between 3 and 6 times. The ultrasonic processing parameters are summarized in

Table 3. USRP parameters.

Specimen	Frequency (kHZ)	Force (N)	Attack Speed (mm/r)	Rotate (rpm)	Impact
Wear	23	1500	0.1	200	6
Shock	23	1200	0.1	200	3

. The impact specimen was a cylindrical specimen; when the load was selected and the number of impacts was too large, the cylindrical surface would break, so the load was selected as 1200 N and the number of impacts was 3.

Under the combined action of the ball rolling and ultrasonic vibration, the specimens’ surfaces were processed in a rotational manner. After the machining process, the specimens underwent ultrasonic cleaning and were subsequently sealed with lubricating oil to prevent scratches and oxidation. The specific experimental groups established for this study are as follows: corresponding to the quenched and tempered (QT) specimens, the QT specimen will be used as the base specimen in this study; carburized and USRP-processed (CQTU) specimens; nitrided and USRP processed (QTNU) specimens; carburized and nitrided USRP-processed (CQTNU) specimens. This resulted in four experimental groups: QT, CQTU, CQTNU, and QTNU, as presented in

Table 4. Heat treatment hardening process of H13 steel.

Process	Quench (Q)	Tempering (T)	Carburizing (C)		Nitriding (N)				Modification	Name
Parameter	T/°C	°C	T/°C	t/h	T/°C	t/h	P/kPa	Gas/L·min−1
No.1	1020	510								QT
No.2	1020	510	950	6	—	—	—	—	USRP	CQTU
No.3	1020	510	—	—	480	8	300	N2:H2 = 0.45:0.15	USRP	QTNU
No.4	1020	510	950	6	480	8	300	N2:H2 = 0.45:0.15	USRP	CQTNU

. Finally, performance evaluations were conducted, which included measurements of residual stress, hardness testing, and microstructural observations. Additionally, each group of specimens underwent ball-on-disk friction–wear tests, as well as Charpy impact tests, with the experimental results being recorded and analyzed.

2.3. Surface Performance Testing

The hardness of H13 steel under different processing conditions was measured using a Falcon 500 Vickers microhardness tester. Initially, the specimens were subjected to ultrasonic cleaning in anhydrous ethanol. Subsequently, they were polished in sequence using metallic grinding papers of grits 80#, 150#, 400#, 800#, 1200#, 1500#, and 2000# to ensure that the two faces of the specimens were parallel. Following this, a polishing paste was applied to a polishing machine to smooth the surfaces, and the samples were rinsed with alcohol to maintain a bright and clean finish. Microhardness measurements were then performed along the transverse cross-section of the specimens using a diamond cone indenter with a load of 200 g applied for 10 s. In order to investigate the changes in the microstructural properties of the specimens under different processing conditions, the samples were cut into smaller pieces and polished to a mirror finish using an automatic polishing machine with various grades of sandpaper. The polished samples were then subjected to metallographic etching. The etching solution used was a 5% nitric acid in ethanol mixture, and the etching duration was approximately 10 to 15 s. After etching, the samples were immediately wiped with an alcohol-soaked cotton swab to remove any residual etching solution, followed by drying. The microstructural observations were carried out using a VK-X1000K confocal microscope. Surface morphology was examined with a CrossBCam 550 Zeiss scanning electron microscope. Different fracture morphologies of H13 steel samples under various treatments were observed and analyzed.

Residual stress, an essential indicator affecting material properties, was measured using an HDS-I type X-ray stress-measuring instrument. The specimens, after ultrasonic cleaning with alcohol, were placed on the measuring platform. Chromium (Cr) was selected as the target material, and residual stress measurements were conducted using the sin²Ψ method with Ψ angles set to 0° and 45°. The scanning step was 0.1°. To minimize experimental error, multiple measurements were taken for each specimen, discarding any outliers with significantly high or low values. The average value was calculated, and the residual stress on the surface of the specimens was determined using the average of three measurements.

2.4. Friction and Wear and Impact Toughness

As shown in Figure 3a, the wear resistance of H13 steel specimens was tested by using a UMT-2 multifunctional friction and wear tester and ball-on-disc friction. The diameter of the ZrO2 grinding ball was 0.9525 cm, the hardness was 72 HRC, the rotating radius was 10 mm, the load was 20 N, the rotating speed was 250 r/min, and the wear time was 3 H. Before and after the friction test, the samples were washed and dried with absolute alcohol, and the weight of the samples before and after the wear was measured with an electronic balance. The precision of the balance was 0.01 mg, and the mass loss of the samples before and after wear was calculated, avoiding accidental errors. Finally, a Japan’s VK-X1000k3D confocal electron microscope and Germany’s CrossBCam 550zeiss scanning electron microscope were used to observe and analyze the wear morphology. The principle of friction is shown in Figure 3b. During the process of friction and wear, the protrusions between the friction pairs came into contact first and produced debris particles under the forward load and the interaction force; this was partly due to the delayed formation of softer iron oxide due to heat diffusion and its shedding in continuous friction.

As shown in Figure 3c, the impact toughness of the material can be measured by Charpy impact test. In this paper, in order to reflect the effect of high-energy composite surface modification on the properties of samples, the experiment was carried out with a Φ10 × 55 mm round bar on China’s ZBC230 Charpy Tester. The impact toughness value αk is one of the comprehensive performance indexes reflecting the impact resistance of materials, and its calculation formula [27] is as follows:

$${\mathrm{\alpha }}_{\mathrm{k}}={\mathrm{A}}_{\mathrm{K}}/{\mathrm{S}}_0$$

(1)

In the formula, AK is the impact absorption work (J) and S₀ is the cross-sectional area (CM2)

3. Results

3.1. Microhardness and Residual Stress

The hardness gradient distribution of H13 specimens subjected to different ultrasonic composite modification processes exhibited varying effects. The hardness gradient measurement results for H13 steel specimens after different surface modification treatments are presented in Figure 4. From the figure, it is evident that the hardness distribution of the QT specimen was quite uniform. After undergoing various chemical heat treatments and USRP, the hardness of the specimens showed varying degrees of improvement. Specifically, the hardness curves for the CQTU, QTNU, and CQTNU specimens displayed a hook-shaped pattern, where the hardness initially increased and then decreased. During carburizing, the surface layer had a relatively high carbon content, leading to a significant volume of retained austenite after cooling, which resulted in a lower surface hardness. In the case of nitriding, during the high-temperature austenitization phase, reactive nitrogen atoms tended to recombine into nitrogen gas molecules and escape from the surface, causing a de-nitriding phenomenon that resulted in the formation of voids and cracks. Consequently, the maximum hardness after both carburizing and nitriding occurred in the subsurface layer. For the CQTU specimen, the effective hardened layer was measured to be 0.9 mm, with the highest hardness of 1167.16 HV0.2 detected at a depth of 30 μm from the surface. This hardness gradually decreased with increasing depth, stabilizing after it exceeded 0.8 mm. Conversely, both the QTNU and CQTNU specimens exhibited their maximum hardness at a distance of 10 μm from the surface. As depth increased, the hardness rapidly declined, with the QTNU specimen showing the steepest decline in its hardness curve. The effective hardened depth for the QTNU specimen was approximately 80 μm, beyond which the hardness levels stabilized.

The measurement of residual stress on the surface of H13 steel-impact-equivalent specimens is illustrated in Figure 5, where Figure 5a shows the surface residual stress. It is evident that ultrasonic composite modification induces strain on the specimen surface, enhances the surface hardness, refines the microstructure, and simultaneously enhances the residual compressive stress. The USRP-processed specimens exhibited varying degrees of enhancement in residual compressive stress. For the CQTU specimen, USRP significantly increased the surface hardness while refining the needle-like martensitic structure, resulting in a substantial increase in residual compressive stress to −728 MPa, which represented a 2.02-fold improvement compared with the untreated specimens. In the case of the CQTNU specimen, USRP refined the surface microstructural characteristics and mitigated defects, such as voids and microcracks resulting from nitriding. As a result, this specimen achieved a substantial increase in residual compressive stress, measured at −674 MPa, which corresponded to a 1.87-fold enhancement. Conversely, for the QTNU specimen, the steep gradient of surface hardness led to complications during the USRP process. As the nitrogen diffusion layer was fragmented during the treatment, loosening and detachment of the nitrided layer occurred, resulting in the lowest residual compressive stress measured at −301 MPa. This value is similar to that of the QT specimen, indicating that the modification process in this case did not contribute to an increase in residual compressive stress, as observed in the other specimens. Figure 5b shows the residual stresses along the depth, and it can be seen that the maximum residual stresses in the QT specimen were in the superficial layer, while the maximum residual stresses after USRP treatment were in the subsurface layer. The maximum value of CQTU in ultrasonic composite modification was −983 Mpa, the maximum value of QTNU was −342 Mpa, and the maximum value of CQTU was −780 Mpa, in which CQTU affected the deepest layer, which reached 600 μm, and QTNU affected the shallowest layer, which was only 200 μm, and was comparable to those of the QT specimen.

3.2. Microstructure

Different high-energy composite modification processes have distinct influences on the microstructure of specimens of H13 steel, as shown in Figure 6. In Figure 6a,b, the surface layer of the QT specimen is acicular tempered martensite in the surface layer, and the core contained a small amount of lath martensite. This indicates a uniform structure throughout the specimen, but with limited enhancement in mechanical properties. In Figure 6c,d, the microstructure of the CQTU specimen revealed a more compact structure, characterized by high-density high-carbon acicular tempered martensite. High-carbon martensite typically exhibits ultra-fine twins and a high density of dislocations, which contribute to improved tensile ductility [28]. This observation suggests that the CQTU composite treatment enhanced the mechanical properties of H13 steel significantly.

Figure 6e,f illustrate the QTNU specimen, where a considerable amount of elongated nitrides is observed at the surface. While nitrides can effectively improve the material’s corrosion resistance and surface hardness, their presence can have detrimental effects on the mechanical properties of the nitrided layer. Elongated nitrides tended to precipitate along the original austenite grain boundaries and exhibited a parallel distribution relative to the surface. This configuration can lower the bonding strength at the grain boundaries, becoming potential nucleation sites for cracks under high contact stresses [29]. In Figure 6g,h, the CQTNU specimen showed a mixture of elongated nitrides and tempered martensite. The quantity of elongated nitrides was significantly reduced compared with the QTNU specimen, leading to a refinement of the structure. When an appropriate balance of carbon and nitrogen is achieved, the elongated structure can be composed of both nitrides and carbonitrides [30]. From the microstructural observations, it is evident that carburizing and nitriding processes can form high-density needle-like martensite, nitrogen-containing martensite, and elongated nitrides at the surface layer of the specimens, thereby enhancing the microhardness of the surface. Among these, the microstructure of the CQTU specimen, primarily comprising high-carbon needle-like martensite, effectively improves the mechanical performance of the material. Overall, the combination of treatments contributed to an optimized microstructure that enhanced the wear resistance and mechanical properties of H13 steel.

A large amount of residual austenite can be observed through the surface and core metallographic organization of the QT specimens, and the residual austenite in the surface layers of CQTU, QTNU and CQTNU decreased, while a large amount of aggregated residual austenite was still retained in the core, which indicated that, after the ultrasonic composite modification treatment, the surface layer of residual austenite was transformed to martensite. It is shown that the mechanical properties were enhanced after ultrasonic composite modification.

3.3. Wear

3.3.1. Friction Coefficient, Wear Amount, and Wear Pits

The friction test was carried out at room temperature and dry friction was carried out at 20 N. The dynamic coefficient of friction (COF) curves and average friction coefficients for H13 steel under different ultrasonic composite processing conditions are presented in Figure 7a,b. From Figure 7a, it can be observed that, during the friction process, the trend of the dynamic friction coefficient curve of the H13 steel specimens after ultrasonic composite modification was similar to that of the untreated specimens. At the start of wear, both sets of specimens underwent a running-in phase, during which the COF rapidly increased with time. This phase primarily involved friction occurring at the material’s surface. The rapid increase in the COF could be attributed to the continuous wear of the specimens, which increased the roughness of the friction surface, thereby increasing the dynamic friction coefficient. As the material surface became damaged, wear progressed through the modified layer resulting from the ultrasonic composite treatment. At this stage, the grain refinement of the surface layer enhanced the compressive strength of the material, leading to a stable and relatively low COF, even after ultrasonic composite modification [31]. Approximately at the 300 s mark, the friction coefficients of the H13 steel specimens processed with different ultrasonic composite treatments stabilized, entering the steady-wear phase.

Figure 7b compares the average friction coefficients, revealing that the average friction coefficient of the QT specimen was 0.627. After undergoing composite modification treatments, there was a slight decrease in friction coefficients, with the average values for CQTU, QTNU, and CQTNU being 0.619, 0.617, and 0.613, respectively. The results indicated that all modified specimens exhibited a reduction in friction coefficients, thereby improving their wear resistance. As shown in Figure 7c, the USRP treatment resulted in a significant reduction in the wear of H13 steel, where the QT specimen had a wear amount of 142.4 mg, the CQTU specimen had a wear of 1.9 mg, the QTNU had a wear of 4.2 mg, and the CQTNU had a wear of 4.1 mg. where CQTU had the least wear and the greatest reduction in wear compared with the QT, which was 98.67%. These results confirmed that ultrasonic composite modification refined the martensitic microstructure at the specimen surface, increased hardness, and introduced residual compressive stress. Under dry friction conditions, this modification led to a decreased friction coefficient and reduced wear, thereby enhancing the wear resistance of the steel.

The wear track profiles under different heat treatment and USRP conditions are illustrated in Figure 7d. Among the comparative groups, the QT specimen exhibited the poorest wear resistance, showing the greatest reduction in wear track depth, approximately 25 μm. In contrast, the wear track depth for the CQTU specimen was notably lower, at 5 μm, the depth of wear was the shallowest, indicating a significant enhancement in wear resistance due to the composite treatment For the QTNU specimen, the wear track depth was around 20 μm, while the CQTNU specimen showed a wear track depth of approximately 15 μm. Both values were greater than that of the CQTU specimen. This difference can be attributed to the presence of intermetallic compounds formed during nitriding processes, which, although they contributed to higher surface hardness, also increased brittleness at the surface layer. This brittleness could adversely affect the wear resistance of the samples compared with the CQTU specimen, but the abrasion resistance was still higher than that of the QT specimen. By correlating the wear depth with the wear amount observed previously, it is clear that the wear resistance of the CQTU, QTNU, and CQTNU specimens was superior to that of the QT specimen. Among these, the CQTU treatment provided the most pronounced improvement in wear resistance for H13 steel. This suggests that the synergistic effects of carburization combined with ultrasonic impact processing yielded optimal enhancements in the material’s durability against wear.

3.3.2. Wear Morphology

The surface morphology of H13 steel under different ultrasonic composite surface modification treatments following friction wear testing is illustrated in Figure 8. The wear appearance of the comparative QT specimen is shown in Figure 8a–c. The untreated specimen, characterized by lower hardness and elastic modulus, exhibited significant wear damage. The worn surface was marked by numerous plowing grooves and small abrasive particles adhering to it. This indicates that, during sliding, fragments were shed from the specimen and the friction ball, leading to enhanced surface plowing. The wear scars appeared severe, and the wear resistance was poor, indicating that the primary wear mechanism for this steel was abrasive wear. In Figure 8d–f, the wear appearance of the CQTU specimen is shown. After carburizing and USRP treatment, the surface hardness increased, resulting in a gradient structure. Compared with the QT specimen, the friction scratches and plowing grooves were shallower, reflecting superior wear resistance. However, the worn surface of the CQTU specimen exhibited a large amount of wear debris and adhesion, where the adhesion was produced by tearing of the upper specimen during the wear process, suggesting that the wear mechanism included both adhesive and slight abrasive wear following ultrasonic composite treatment. Figure 8g–i display the wear morphology of the QTNU specimen. It can be found that the surface wear was not uniform, which was caused by surface modification and the uneven distribution of abrasive debris, which is the same as the results of Ref. [32]. From the microscopic morphology, it is observed that metallic iron debris is visibly accumulated on the worn surface. Additionally, the adhesion of more white microparticles to the worn surface was observed, which resulted from the action of grinding forces and inadequate heat dissipation. The gradient structure of the QTNU specimen contained a significant amount of elongated nitrides, which contributed to higher brittleness and lower mechanical properties. As a result, during friction, these characteristics did not provide adequate protection to the material surface; as the friction progressed, the friction ball peeled partially, making the abrasive grains stick to the wearing surface. Furthermore, as the temperature increased, severe thermal effects also developed on the specimen’s surface, leading to significant adhesive wear and transfer damage. The wear mechanisms observed during this process included adhesive wear, oxidative wear, and slight abrasive wear. Figure 7g,h illustrate the wear morphology of the CQTNU specimen. Following ultrasonic composite treatment, this specimen had enhanced surface hardness and presented a gradient structure. The worn surface showed an abundance of adhered micro-particles, with some clustering into larger adhesive masses. Although large accumulations of debris were present, the wear surface quality was improved compared to that of the QTNU specimen. After carburizing treatment, the amount of elongated nitrides at the surface decreased significantly, enhancing the mechanical properties of the material and reducing the extent of adhesive wear. The wear mechanisms exhibited by the CQTNU specimen primarily consisted of a combination of adhesive wear, oxidative wear, and slight abrasive wear. Figure 8j–l illustrate the wear morphology of the CQTNU specimen. Following ultrasonic composite treatment, this specimen had enhanced surface hardness and presented a gradient structure. The worn surface showed an abundance of adhered micro-particles, with some clustering into larger adhesive masses. Although large accumulations of debris were present, the wear surface quality was improved compared with that of the QTNU specimen. After carburizing treatment, the amount of elongated nitrides at the surface decreased significantly, enhancing the mechanical properties of the material and reducing the extent of adhesive wear. The wear mechanisms exhibited by the CQTNU specimen primarily consisted of a combination of adhesive wear, oxidative wear, and slight abrasive wear. Overall, the results indicated that the ultrasonic composite treatments significantly enhanced the wear resistance of H13 steel, and different treatment processes led to variations in the wear mechanisms and surface characteristics.

The wear pits, COF, wear quality, and the above-mentioned wear morphologies were summarized. It can be concluded that the ultrasonic composite-treated samples showed better wear resistance than the QT samples; after ultrasonic composite modification, a plastic deformation layer was introduced on the surface of H13 steel, and the grain refinement caused the material to exhibit plastic resistance, and experience slightly abrasive wear and adhesive wear. In the ultrasonic composite modification, the CQTU sample showed high anti-wear quality and the best wear resistance, while QTNU was affected by surface pulse nitride, showing severe adhesive wear and slight abrasive wear with the worst wear resistance.

3.4. Impact Resistance

3.4.1. Impact Toughness and Surface Hardness

During the tunneling process, TBMs often experience sudden impact loads. The impact toughness of the roller cutter ring is one of the critical factors in extending its service life. The effects of different high-energy composite modification processes on the impact toughness of H13 steel equivalent samples are presented in Figure 9. It is evident that the CQTU treatment exhibited the highest impact toughness, reaching 27.3 J/cm², which is a significant increase of 135% compared with the QT specimen’s impact toughness of 11.6 J/cm², and was accompanied by a surface hardness of 1027 HV_0.2. The CQTNU specimen also showed enhanced impact toughness, achieving 19.9 J/cm², representing a 71% increase over the QT specimen, along with an improved surface hardness of 1101 HV0.2. The carburizing process helped to precipitate a large amount of high-carbon tempered martensite in the surface microstructure during tempering, which featured a high density of dislocations. This resulted in improved surface hardness while maintaining a favorable gradient structure, ensuring that the core of the material retained excellent impact toughness and indicating that the carburizing process contributed to a better distribution of mechanical properties across the material’s structure. Conversely, the QTNU specimen exhibited significantly lower impact toughness, recorded at only 3 J/cm². This reduction was attributed to the presence of numerous elongated nitrides in the nitrided surface layer, which adversely affected the mechanical performance of the material and resulted in a poorly constructed gradient structure. The impact toughness of the QTNU specimen suffered greatly due to this brittleness caused by the nitrogen-enriched regions, which could easily lead to crack formation upon impact.

In summary, the combination of USRP and carburizing processes effectively enhanced the impact toughness and surface hardness of H13 steel while preserving excellent toughness in the core. This resulted in improved resistance to impact loading, thereby providing H13 steel with superior impact performance. Although nitriding enhanced surface hardness, the brittle nature of the elongated nitrides formed in the nitrided layer significantly diminished the impact toughness of the treated samples. Thus, the impact toughness of samples subjected to nitriding and ultrasonic composite treatment dramatically decreased.

3.4.2. Impact Fracture Morphology

The fracture surfaces of H13 steel samples treated with different composite processes were analyzed using scanning electron microscopy (SEM), as shown in Figure 10a,b, depicting the fracture morphology of the QT specimen. The surface and core regions exhibited a mix of brittle fracture and some ductile features, characterized by tearing ridges, numerous cleavage steps, and river-like patterns. The flow direction of the river patterns aligned with the direction of crack propagation, which was typical of quasi-cleavage intergranular fracture morphology. In Figure 10c,d, the fracture morphology of the CQTU specimen is presented. Here, the ductile dimples were less pronounced, with the primary features observed being river-like patterns and cleavage steps. The core region of the CQTU specimen showed deeper dimples compared with the surface, indicating that the plasticity resistance of the core was lower than that of the surface. The elemental analysis table showed that 4.23% of carbon was still present in the core, achieving a reasonable gradient structure. This corresponded to the rational gradient structure developed in the specimen, aligning with the observed increase in impact toughness following carburizing and ultrasonic composite modification. As shown in Figure 10c,d, there were fewer surface dimples on the CQTU samples, which mainly displayed river turbulence and cleavage steps, and also showed quasi-cleavage fracture characteristics. The radiative dimples in the center of the CQTU specimen were deeper than those in the surface layer, which indicated that the plastic resistance of the center of the specimen was lower than that of the surface layer, and a reasonable gradient structure was formed, which was consistent with the increase in the impact toughness of the carburized ultrasonic composite-modified specimen. Figure 10e,f show the fracture of the QTNU specimen, the surface morphology conformed to the fracture along the crystal, which was standard brittle fracture characteristics, while the core still exhibited quasi-decompositional fracture characteristics; furthermore, the nitrogen content of the core was only 0.98%, and the nitrogen ions did not penetrate, indicating that nitriding treatment constructed a gradient structure that was not reasonable, and could not meet the high impact toughness. The fracture morphology of the CQTNU specimen is shown in Figure 10g,h. The surface fracture still exhibits quasi-cleavage characteristics; however, no significant ductile dimples are observed, indicating a high plastic deformation compared to the CQTU specimen. The core region reveals deeper ductile dimples, forming a rational gradient structure, which aligns with the increased impact toughness of the CQTNU specimen.

From the microscopic examination of the impact fractures, it is evident that the H13 steel samples treated with CQTU and CQTNU exhibited ductile fracture characteristics with multiple dimples in the core region. The increased surface hardness resulted in fewer ductile dimples and more minor flow features in the surface fracture, indicating the introduction of a highly plastically deformed layer at the surface and increased residual stress. Additionally, the grain refinement led to higher surface hardness while the core retained a certain level of toughness, thereby forming a rational gradient structure. In conclusion, after the ultrasonic composite treatment, the fracture surfaces showed significant river-like patterns and intergranular fracture characteristics. Among all treatments, the CQTU specimen featured large and deep dimples in the core region, along with many ductile dimples on the surface, demonstrating excellent impact toughness, which is consistent with the high measured impact toughness of the specimen. The carburizing combined with ultrasonic treatment of the CQTU specimen resulted in high surface hardness and good core toughness, achieving the ideal “hard surface and tough core” configuration, which afforded it the best overall impact toughness among the treated specimens.

4. Discussion and Conclusions

This study focuses on H13 steel, a commonly used material for roller cutter rings. It investigates the effects of various high-energy composite processing techniques for carburizing combined with USRP, ion nitriding combined with USRP, and a combination of carburizing and nitriding with USRP, on the microhardness, residual stress, and microstructure of the samples. Through ball-on-disk friction wear tests and Charpy impact tests, the friction coefficient, wear volume, wear depth, and impact toughness values were analyzed. A comparison of the results of different composite modifications is shown in

Table 5. Comparison of different composite modification processes (data in the table are based on the results of the QT experiment, “+” indicates an increase compared with QT, “−” indicates a decrease compared with QT).

Composite Process	Surface Microhardness	Surface Residual Stress	Coefficient of Friction	Wear Amount	Depth of Wear	Impact Toughness
CQTU	+174.56%	+205.07%	−1.28%	−98.67%	−80.00%	+235.08%
QTNU	+162.80%	−15.21%	−1.59%	−97.05%	−20.00%	−74.2%
CQTNU	+180.49%	+189.86%	−2.23%	−97.12%	−40%	+171.10%

.

Connecting these results with the wear surface morphology and fracture surface characteristics, we can summarize the mechanisms by which different high-energy composite treatment processes affect the wear resistance and impact performance of H13 steel. The main conclusions are as follows:

1. The different heat treatment methods combined with ultrasonic impact modification were important for affecting the surface hardness and residual compressive stress of H13 steel samples. For the composite modified specimen, the carburizing composite ultrasonic strengthening was the most obvious, the maximum hardness was 1167 HV_0.2, the hardness gradient changed gently, the gradient structure was the most reasonable, and the impact specimen residual compressive stress was 728 Mpa; the maximum hardness and surface residual compressive stress of the carburizing–nitriding composite ultrasonic impact specimen were 1299 HV_0.2 and 674 Mpa respectively, while the surface residual stress of the nitriding composite ultrasonic strengthening specimen was only 301 MPA and the effective hardening layer was the steepest, which was not consistent with the gradient structure of wear resistance and impact resistance.

2. Appropriate surface modification treatments could significantly improve the wear resistance of H13 steel samples. Carburizing, nitriding, and ultrasonic composite strengthening processes all enhanced the hardness and residual compressive stress of the samples. A greater residual compressive stress and higher hardness resulted in better wear performance. The effect of different phase transformation heat treatments in the USRP composite process on the wear resistance of H13 steel samples varied. Under the same experimental conditions, the wear volume of the samples ranked from largest to smallest as follows: QTNU > CQTNU > CQTU, while the wear scar depth followed the same order: QTNU > CQTNU > CQTU. The CQTU surface composite modification treatment significantly increased the hardness of the samples’ surfaces, refined the acicular martensite structure, and improved the wear resistance, resulting in the best wear performance among all samples.

3. A reasonable composite modification process could refine the surface microstructure, increase the surface hardness and residual compressive stress, and improve the impact toughness of H13 steel. The impact toughnesses of the CQTU and CQTNU samples were 27.34 J/cm² and 19.9 J/cm², respectively, compared with the QT specimen, the improvements in the impact fracture surface of the composite modified specimen were 135% and 71%, respectively, and the impact fracture surface of the composite modified specimen mainly exhibited a cleavage fracture surface, while the core exhibited a ductile fracture surface with multiple dimples, so while the surface hardness is increased, the impact fracture surface of the composite modified specimen mainly exhibited a cleavage fracture surface, and it still had excellent impact toughness. Because of the brittle and thin surface layer, the impact toughness was only 3 J/cm² in the QTNU composite modification, which led to an unreasonable gradient structure and a sharp decrease of the impact toughness; this process was not suitable for composite modification of H13 steel.

In summary, carburizing composite USRP-modified H13 steel had a high surface hardness, the hardness gradient changed more gently, the distribution was reasonable, and the sample as a whole had better impact resistance and showed excellent wear resistance; thus, the carburizing composite ultrasonic impact process is suitable for the production of “surface hardness and toughness” of high-energy composite-modified cutter rings.