3.1. CCT Curves of Steel
Figure 2a shows the continuous cooling transformation (CCT) curves of the spring steel. The 0 Ni steel curves are displayed in gray, while the curves of 0.5 Ni steel and 1 Ni steel are depicted in red and blue, respectively. The
Bs temperature of spring steel decreased significantly as the nickel content increased from 0 wt.% to 1 wt.%. This is because adding nickel reduces the chemical driving force for the nucleation and growth of the bainite transformation, hindering its kinetics, which enhances steel hardenability [
20]. According to the hardness distribution diagram in
Figure 2b, when the cooling rate reached 20 °C/s, the hardness of all three types of steel exceeded 550 Hv. The critical cooling rates to obtain a complete martensitic microstructure varied between 20 °C/s, 10 °C/s, and 5 °C/s for 0 Ni steel, 0.5 Ni steel, and 1 Ni steel, respectively. The thickness specification of the leaf spring exhibited a positive correlation with its hardenability. Currently, the development of leaf springs is moving towards variable cross-section single leaf springs, which require spring steel with higher hardenability. Adding the element Ni can improve the hardenability of the material, thereby expanding the potential applications of the new generation of high-strength leaf spring steels [
2].
3.2. Microstructural Characterization
Figure 3 shows the typical morphologies of PAG in experimental steel with varying nickel content. The grain size is determined using the linear intercept method followed by the ASTM E 112 standard [
23], which involves counting the intersections of grain boundaries with a test line and calculating the average length of these intercepts. This measurement is used to determine the size of the prior austenite grains by analyzing at least eight random images per sample using six lines. The grain sizes of the steel with different nickel contents exhibited uniformity after austenitization at 900 °C. Moreover, the grain size showed a slight decrease with an increase in the nickel content, and the grain sizes for 0 Ni steel, 0.5 Ni steel, and 1 Ni steel were measured as 7.4 ± 0.7 μm, 7.2 ± 0.5 μm, and 6.9 ± 0.6 μm, respectively.
Figure 4 shows the microstructure of experimental steel with different nickel contents after heat treatment. The heat treatment resulted in a tempered martensitic microstructure, showing an insignificant variation amongst the experimental types of steel that contain different nickel contents.
EBSD boundary maps and inverse pole figure (IPF) maps of 0 Ni, 0.5 Ni, and 1 Ni steel are shown in
Figure 5. The EBSD orientation mapping technique effectively showcases the typical hierarchical structure of martensite. The low-angle grain boundaries (LAGB) are defined by misorientation angles ranging from 2° to 15°, indicated by red lines, while high-angle grain boundaries (HAGBs) are defined by misorientation angles above 15°. To visualize the martensitic substructure, the HAGBs were classified into the following two categories: grain boundaries and packet boundaries. They are denoted by blue lines with misorientation angles less than 55°, and block boundaries are indicated by orange lines with angles greater than 55° [
24].
Figure 5g–i illustrates the misorientation distributions of experimental steel. It is observed that the proportion of HAGBs increases considerably as the nickel content decreases. This study reveals that the percentages of HAGBs in 0 Ni, 0.5 Ni, and 1 Ni steel are approximately 70.3%, 73.6%, and 75.3%, respectively. The addition of nickel refined the PAGS and reduced the
Ms temperature, as shown in
Table 2, refining the martensitic substructure and increasing the proportion of HAGBs in experimental steel.
The packet size (PS) and block size (BS) of experimental steel were analyzed using SEM (
Figure 4) and EBSD (
Figure 5), respectively; the corresponding results are collated in
Table 3. The average values of PS and BS were determined by measuring a total of 300 packets and 300 blocks, respectively. The PAGS, PS, and BS of experimental steel with different nickel contents showed a consistent trend: the increase in the nickel content led to a refinement in both the grain size and martensitic substructure in experimental steel, as shown in
Table 3. In 0 Ni steel, the PS was higher than in the 0.5 Ni and 1 Ni steel. The PS decreased from 2.5 μm in 0 Ni steel to 2.2 μm in 0.5 Ni steel and 2.1 μm in 1 Ni steel due to the decrease in PAGS. The BS of 0.5 Ni and 1 Ni steel were about 0.9 μm and 0.8 μm, indicating similar values, while the BS decreased rapidly from 1.16 μm in 0 Ni steel to 0.84 μm in 1 Ni steel.
The XRD patterns of experimental steel with different nickel contents are shown in
Figure 6 (0 Ni/black; 0.5 Ni/red; 1 Ni/blue). Notably, the diffraction peak of the retained austenite was absent from the observed XRD pattern, suggesting that the presence of retained austenite could be disregarded. The Williamson-Hall (W-H) method has long been used to characterize dislocation density [
25]. The dislocation density of martensitic steel calculated by the W-H method could always be overestimated due to the strong strain anisotropy of martensitic steel [
26]. The modified Williamson-Hall (MWH) method takes into account the effect of strain anisotropy by defining a scaling parameter
, called the average contrast factor of dislocations [
27]. The modified Williamson-Hall (MWH) method was used to calculate the dislocation in medium-carbon martensitic steel [
28].
where Δ
K is the full width at half-maximum (FWHM).
D is the crystallite size, which can be obtained using
D =
kλ/
βcosθ.
M is the dislocations distribution parameter, and
M = 2 [
29].
B is the Burgers vector, and
b = 0.248 nm [
28].
K is the magnitude of the diffraction vector, which can be obtained using
K = 2
sinθ/
λ.
θ and
λ, which are the diffraction angle and the wavelength of the X-rays.
is the average contrast factor of dislocations [
27,
28]. The value of
ρ obtained via XRD results was estimated to be about 4.20 × 10
15 m
−2, 4.71 × 10
15 m
−2, and 4.84 × 10
15 m
−2 for 0 Ni, 0.5 Ni, and 1 Ni steel.
The TEM results reveal that the microstructure of the experimental steel predominantly consisted of lath martensite, with a significant dispersion of VC particles within the lath structure (
Figure 7). The VC carbides in
Figure 7 has been marked with red arrows. According to the energy dispersive spectroscopy (EDS) analysis, as shown in
Figure 7d, the spherical carbides were identified as VC carbides. The average diameter and volume fraction of VC were measured using Image-Pro Plus 6.0 software, and the results are shown in
Table 4, revealing insignificant disparities for the 0 Ni, 0.5 Ni, and 1 Ni steel.