3.1. Microstructure
The microstructure of the tested materials under varying pressures was examined, as illustrated in
Figure 2. Given that ZG25MnCrNiMo steel is classified as hypoeutectoid steel, it is characterized by a microstructure that comprises gray pearlite and a white network of ferrite. When no pressure is applied during preparation, the steel exhibits coarse dendritic grains (
Figure 2a). With increasing pressure, the distance between the secondary dendrite arms decreases, resulting in a finer structure. The pressure causes the eutectoid point of the Fe-Fe
3C system to shift toward a lower temperature and lower carbon content, resulting in a diminished α phase region [
21]. Consequently, the carbon content of the prepared steel approaches eutectoid composition, facilitating the formation of a network ferrite that precipitates along the protoaustenite grain boundaries. As illustrated in
Figure 2b–f, the overall uniformity of the solidification structure of the prepared steel under pressure demonstrates notable consistency without discernible directionality. This phenomenon can be attributed to the fact that crystallization under pressure minimizes the diffusion time of solutes in liquid steel [
22], significantly reduces dendrite segregation, and enhances the uniformity of chemical composition. Moreover, given that the elongated dendrites are fused and remelted under the action of pressure, the directivity of the grains is diminished, thereby further promoting the homogenization of the microstructure.
The secondary dendrite arm spacing of prepared steel under varying pressures was quantitatively measured and statistically analyzed, and each sample was photographed taking 15 fields of view; the results are shown in
Figure 3. In comparison to the sample prepared without applied pressure, the secondary dendrite arm spacing of the sample subjected to 30 MPa exhibits a decrease of approximately 32.4%. As the pressure continues to increase, the grain refinement effect becomes increasingly pronounced. And when the pressure is elevated to 150 MPa, the secondary dendrite arm spacing constitutes only 54.6% of that observed in gravity casting (0 MPa). This can be attributed to the applied pressure diminishing the nucleation activation energy [
23], elevating the melting point of the alloy, and enhancing the degree of supercooling, thereby increasing the nucleation rate. In addition, the pressure augments the cooling rate, preventing the grains from growing adequately, thereby facilitating grain refinement [
24].
The ferritic content of prepared steel under different pressures was also quantitatively determined (
Figure 3). As the pressure increases, the ferrite content within the microstructure gradually increases, as reflected in the statistical results presented in
Figure 3. Compared with that of the sample prepared without pressure, the ferrite content of the sample with a pressure of 150 MPa is elevated by 39.1%. The refinement of the original austenite grains results in an increase in grain boundaries, thereby facilitating the nucleation of ferrite in more locations. In addition, when the degree of supercooling is greater, the phase transition kinetics of the austenite–ferrite transformation depend on the rate of interfacial reaction [
25]. The pressure induces thermal deformation of the austenite phase and enhances its chemical potential. The increment in chemical potential, Δ
d, can be expressed as shown in Equation (4) [
26]:
where
VV represents the molar volume of austenite phase;
G and
b denote the shear modulus and Bertrand vector of the austenite phase, respectively; and
d indicates the dislocation density, which is contingent upon the deformation conditions. The work of deformation induced by pressure on the austenite is partially transformed into stored deformation energy. This transformation leads to an increase in dislocation density within the austenite phase and elevates its chemical potential. Consequently, the driving force for dynamic phase transformation is enhanced, which manifests as an increase in the amount of ferrite precipitation within the microstructure.
Figure 4 presents the SEM images of the microstructure of the prepared steels at a higher magnification. The prominent black network structure depicted in
Figure 4 represents ferrite, while the black and white regions correspond to flake pearlite. As shown in
Figure 4a, ferrite nucleates at the grain boundary of the protoaustenite and grows in a “block” form, resulting in a spun crystal morphology. With the increasing pressure, as depicted in
Figure 4b–f, the isometric ferrite located at the grain boundary progressively transforms into a lamellar structure and intragranularly grows along the grain boundaries. Meanwhile, the lamellar spacing gradually diminishes, subsequently transforming into an acicular shape that integrates into the grain. This phenomenon results from a substantial increase in the degree of supercooling induced by the elevated pressure. A small quantity of proeutectoid ferrite is expelled from the austenite and precipitates along the grain boundary [
27], forming network ferrite and acicular ferrite. Upon cooling below Ar1, eutectoid decomposition occurs, and the structure is transformed into a lamellar perlite structure. The integrated structure comprising intragranular acicular ferrite and extremely fine lamellar pearlite is referred to as the Widmannstatten structure [
28]. Considering that the Widmannstatten structure can only be formed within a certain range of cooling rates, it requires a sufficiently high degree of supercooling to provide an adequate driving force for the phase transition, and then the coherent strain energy necessary to create the interface of the pin ferrite semi-common format becomes attainable [
29]. Consequently, the accelerated cooling rate facilitated by pressure promotes the formation of the Widmannstatten structure, and with the increase in supercooling degree, this microstructure is progressively refined.
The XRD analysis of the samples prepared at various pressures is illustrated in
Figure 5. With the increase in applied pressure, the broadening of the X-ray Bragg diffraction peaks of the sample become more pronounced, and the intensity of the diffraction peaks gradually increases, concurrent with the observed shift of the diffraction angle toward a lower value, as illustrated by the magnified α-Fe (110) peaks in the figure. The broadening of Bragg diffraction peaks is primarily influenced by grain refinement [
30]; increased pressure promotes grain size refinement, contributing to the strengthening effect. Additionally, changes in diffraction intensity are influenced by various factors, including crystal structure, grain size, and dislocation density. As illustrated by the localized magnification of the diffraction spectra, the diffraction intensity of the α-Fe (110) grain surface is significantly enhanced, indicating that grain refinement increases the number of grain boundaries, facilitating the scattering of X-rays by atoms in the crystals and thereby enhancing the diffraction intensities.
3.2. Mechanical Properties
The density of the alloy serves as a crucial indicator of the porosity of the casting. The variation in the density and porosity of the prepared steel under different pressures is illustrated in
Figure 6. It is evident that as the pressure increases, the density of the steel rises, while conversely, the porosity of the castings decreases. During the squeeze-casting process, the application of pressure not only increases the density of the casting but also reduces the average distance between atoms, thereby minimizing macro defects in the casting. Under a pressure of 150 MPa, the sample’s density attains a maximum value of 6.8135 g/cm
3, while its porosity achieves a minimum of 1.33%, indicating that the casting exhibits optimal densification. Meanwhile, the density and porosity of the steel exhibit the most significant changes when the pressure is increased from 0 to 30 MPa; the density increases by 1.61%, while the porosity decreases by 49.5%. Furthermore, the rate of change in density and porosity diminishes as the pressure continues to rise.
There are several factors contributing to porosity, primarily including two main reasons: First is the gas dissolved in the alloy. During the melting process of the alloy, the liquid metal contains a higher concentration of dissolved gas, which is correlated with the temperature of the molten metal. As the temperature of the metal rises, the solubility of the gas increases. Additionally, during the pouring process, the gas tends to break down into numerous smaller monomers, which possess lower buoyancy and struggle to escape from the liquid surface, resulting in the formation of pores during the casting process. When the pressure is 0 MPa, the spacing between dendrites poses challenges for filling, making it difficult to eliminate shrinkage. However, when pressure is applied, the flowing distance for the molten metal is reduced. Consequently, less energy is consumed, facilitating easier filling of the liquid. In addition, pressure inhibits the formation of bubbles and increases the solubility of gas in the molten metal, thereby reducing the occurrence of defects such as shrinkage.
Figure 7a illustrates the Brinell hardness of prepared steel at various pressures. All samples exhibit hardness values within the range of HB241 to HB311. As the pressure increases, the hardness of the sample initially decreases before subsequently increasing. The hardness reaches a peak value of HBW286.4 at an applied pressure of 120 MPa, and as the pressure continues to increase, the hardness exhibits a slight decrease.
Figure 7b illustrates the influence of pressure on the low-temperature impact energy of the prepared steels. When the pressure is set at 30 MPa, the low-temperature (−40 °C) impact toughness of the steel reaches its maximum value of 31.79 J, representing an increase of 65.4% compared to the sample that exhibited a value of 19.22 J without pressure applied. The low-temperature impact toughness of the steels exhibits a linear decrease with increasing pressure. Consequently, the empirical relationship illustrated in Equation (5) can be derived through linear regression:
where
Akv represents low-temperature impact absorption energy (J);
P denotes prepared pressure (MPa). The fitting slope of the empirical relationship is −0.147, with a correlation coefficient reaching 96.14%.
When the pressure reaches 60 MPa, the low-temperature impact absorption energy decreases to 27.21 J, approaching the lower limit of 27 J for ZG25MnCrNiMo steel, as stipulated for casting samples or Kiel test blocks in TB/T2942–1999 [
31]. As the exerted pressure continues to rise, the low-temperature impact performance fails to satisfy the necessary requirements for application. In the current study, the maximum pressure for squeeze casting has been raised to 150 MPa, while the impact absorption energy remains merely 14.54 J. It should be noted that the impact toughness of samples is generally much lower than that of cast samples and Kiel test blocks, even when utilizing the same liquid metal [
7]. In summary, it can be concluded that the low-temperature impact toughness of the test steel is significantly influenced by the applied pressure within the range of 0 to 150 MPa, and the impact property is superior under lower pressure. In addition, the impact toughness of the steel without applied pressure is also relatively low, failing to meet the required standards.
The macro-impact fracture of the tested steel at −40 °C was examined, as illustrated in
Figure 8a–f. The fracture is characterized by obvious plastic deformation. The fiber region and shear lip of the fracture are formed following plastic deformation, and the two constitute the primary components responsible for impact energy absorption. In comparison to the schematic diagram of the impact fracture surface [
32], it is evident that when the pressure ranges from 30 to 90 MPa (
Figure 8b–d), the proportion of the fiber zone and shear lip in the impact fracture is larger, indicating relatively good toughness. As depicted in
Figure 8e,f, the fiber area and shear lip of the macro fracture are diminished, resulting in relatively poor impact toughness under conditions of elevated pressure and without pressure applied (
Figure 8a).
Additionally,
Figure 8a’–f’ illustrates the impact fracture morphology at a higher magnification. The impact fracture of the sample prepared without pressure exhibits quasi-cleavage morphology, which is characteristic of a brittle transgranular fracture. Impurity particles located at the bottom of the secondary dendrite arms are distinctly observable, which is attributed to the concentration of solute elements in the liquid phase at the crystallization front, leading to dendrite segregation. At an applied pressure of 30 MPa, as depicted in
Figure 8b’, the low-temperature impact toughness of the steel reaches the optimal value, and the fracture consists of numerous finer dimples, indicative of a typical ductile fracture. The increase in pressure shortens the solidification time, consequently decreasing the diffusion time of solute elements within the liquid phase, minimizing alloy segregation, and promoting the uniform distribution of solute elements. Consequently, the low-temperature impact toughness of the steel prepared under lower pressure significantly exceeds that of the steel prepared by gravity casting (0 MPa). However, as the pressure continues to increase, the dimples progressively become shallower, as shown in
Figure 8c’. In
Figure 8d’,e’, the material exhibits quasi-cleavage fracture characteristics, but notable plastic deformation remains observable at the edge of the tear ridge. And in
Figure 8e’, cleavage steps, tongue-shaped patterns, tear ridges, and small cleavage faces are visible. When the pressure increases to 150 MPa, as depicted in
Figure 8f’, numerous micro-cracks emerge at the fracture. This phenomenon occurs because the increase in pressure elevates the carbide content in the material and alters the carbide morphology, resulting in the quasi-cleavage cracks, which are more likely to originate from fine carbides within the crystal structure [
33], thereby diminishing the toughness of the material.
EDS spectrum analysis was conducted on the particulate matter at the discontinuity in
Figure 8b’, and the results are presented in
Figure 9. The particles at site 1 predominantly consist of sulfide MnS and FeS. Given that the S readily segregates at the grain boundaries [
34], the interfacial energy diminishes, consequently increasing the likelihood of crack propagation along the cleavage plane. The particles at site 2 are alloy cementite (Fe,Mn)
3C with high hardness. Mn possesses a greater affinity for C than Fe and exhibits a higher propensity for segregation, facilitating precipitation along the grain boundary and forming the network carbides such as Mn
3C or (Fe,Mn)
3C. The precipitation of these particles adversely affects the low-temperature impact toughness of the tested steel to some degree, which can be mitigated by precisely controlling the chemical composition during the liquefaction process preceding the furnace.
During the squeeze-casting process, when the pressure is increased from 0 to 30 MPa, it can enhance the solidification rate of metals, increase the degree of supercooling, and facilitate the grain refinement of the steel. Meanwhile, this process reduces the stress concentration, increases the grain boundary ratio, and renders the crack growth path more tortuous and directed, thereby absorbing more impact energy and resulting in fine crystalline strengthening and toughening. Furthermore, the application of pressure induces the thermal deformation of the austenite, shifting the austenite–ferrite transformation toward increased ferrite formation. Consequently, this leads to an increased ferrite content, which enhances the impact toughness of the steel [
35,
36]. The pressure toughening effect is substantiated by the experimental results obtained in this study. When the pressure of squeeze casting is raised to 60 MPa, a minor quantity of the Widmannstatten structure precipitates along the grain boundaries and grows into the intragranular region, creating a partitioning effect on the grains and leading to the formation of cleavage fracture channels. The inherent morphology of the crystalline structure cannot be eliminated, resulting in a substantial reduction in the impact toughness of the prepared steel. The carbides with pointed features are prone to serving as crack initiation sites during impact, resulting in the fracture of samples. In addition, the pressure of squeeze casting intensifies the solid solution strengthening of the alloy, resulting in decreased plasticity and toughness [
37]. Even at a pressure of 150 MPa, micro-cracks may develop, indicating pressure embrittlement. In summary, during the squeeze-casting process under lower pressure, fine crystalline strengthening and toughening predominantly influence the impact process, resulting in pressure toughening of the steel. Conversely, during high-pressure squeeze casting, solid solution strengthening and the appearance of the Widmannstatten structure play a crucial role, intensifying the embrittlement trend of the material and making the material behave according to pressure embrittlement.
3.3. Wear Property
The influence of wear time on the wear loss of the samples prepared under different pressures is plotted in
Figure 10a. The wear resistance of the samples prepared under 0 MPa and 30 MPa is moderate. Excluding the pre-grinding process during the initial 60 min, the samples remain in a stable wear stage within 60 to 220 min, characterized by a low wear rate and a gradual increase in wear amount. After 220 min, the wear rate increases, leading to accelerated wear. When the pressure ranges from 60 to 90 MPa, the wear resistance of the tested steel is poor, characterized by a short stable wear stage and significant total wear. And when the pressure exceeds 120 MPa, the wear resistance of the tested steel is optimal. Even after 300 min of wear, the samples remain in a gentle stable wear stage, with total wear loss remaining below 50 mg.
The wear rate per unit area of the steel corresponding to pressures ranging from 0 to 150 MPa are illustrated in
Figure 10b. The wear rate initially increases before subsequently decreasing as the pressure increases. When the pressure is increased within the range of 0 to 60 MPa, the wear rate increases by 151.6%, reaching its lowest wear resistance at 60 MPa, where the wear rate attains 40.25 mg/(h·cm
2). When the pressure falls within the range of 60 to 150 MPa, the wear rate decreases by 83.2% as the pressure increases, ultimately decreasing to 6.75 mg/(h·cm
2) at the high pressure of 150 MPa, indicating optimal wear resistance.
The worn surface morphology of samples after 300 min of impact abrasive wear under conditions of 2.5 J impact energy and various pressures was examined, as shown in
Figure 11. Under the action of impact, the surface of the material primarily exhibits plastic deformation, and distinct extrusion marks are evident. Since the rotational direction of the lower sample in the experiment remains consistent, the extrusion marks display pronounced directional characteristics. As illustrated in
Figure 11a,b, the worn surface of samples prepared without pressure and under 30 MPa squeeze casting is relatively flat, with wear debris that can push the material to one side, thereby forming a plastic ridge and wedge. A limited number of spalling pits resulting from matrix fractures and detachment due to insufficient strength are observable [
38], and the furrows created by the scratches of the wear debris are shallow, indicating relatively good wear resistance. When the pressure reaches to 60 MPa and 90 MPa, as depicted in
Figure 11c,d, the material experiences damage from stripping under impact, and the spalled material is rolled between contact surfaces, forming abrasive debris that exhibits significant scratching effects, causing deeper grooves, increased inclusions, and reduced wear resistance. When the pressure exceeds 120 MPa, as illustrated in
Figure 11e,f, the wear surface becomes notably flat, with minimal damage caused by abrasive debris. The resulting minor deformation makes it challenging to remove the surface material, thereby leading to reduced wear loss.
The lower sample of the impact wear experiment is a 20Cr ring, which is also a kind of alloy steel material as the tested samples. Under the action of impact, significant local stress at the contact surface between the two materials is generated, resulting in a pronounced tendency for cold welding. Subsequently, the adhesion points fracture due to shear effects during relative sliding, leading to the formation of wear debris that further scratches the surface [
39]. Then, the surface experiences a combined action of normal and tangential forces, resulting in a significant material loss and forming furrows and a limited number of inclusions. The wear mechanisms observed on the worn surface under the condition of 2.5 J impact energy primarily include impact wear, adhesive wear, and minimal abrasive wear, along with notable occurrences of plastic removal, furrows, and spalling.
When the pressure of squeeze casting is lower (38 MPa), the wear resistance of prepared steel does not significantly differ from that of gravity casting (0 MPa). It can be considered that low-pressure squeeze casting has a minimal effect on the wear resistance of ZG25MnCrNiMo steel. As the pressure is increased to 60 MPa, the strengthening and toughening effects of fine grains become apparent, leading to an improvement in material toughness while concurrently causing a significant decrease in wear resistance. Furthermore, the wear resistance of the material continues to improve with increasing prepared pressure, which is primarily attributed to the dominant role of solid solution strengthening on both hardness and wear resistance. Increased pressure enhances the solid solubility of alloying elements, exacerbates lattice distortion of the solid solution, generates an elastic stress field, interacts with dislocations [
37], hinders the dislocation movement on slip planes, and ultimately results in increased hardness and improved wear resistance.