Gray iron casting occupies a pivotal position in various industrial sectors, particularly in automotive manufacturing and machine tool structures, owing to its favorable combination of properties such as good machinability, damping capacity, and cost-effectiveness. The continuous push towards lightweight automotive components and high-precision machinery has escalated the demand for gray iron castings with superior mechanical strength. Conventionally, enhancing the strength of gray iron casting involves optimizing the silicon-to-carbon ratio, introducing alloying elements like niobium or chromium, or employing specific inoculants. While effective, these methods often increase production costs. An alternative and economically attractive approach is the alloying with nitrogen. Nitrogen, an abundant element, has been demonstrated to significantly refine the microstructure of gray iron casting, leading to improved tensile strength. This has led to the development of high-strength nitrogen-alloyed grades, such as the HT350 material under investigation. However, this enhancement in mechanical properties invariably influences its machinability, presenting new challenges in tool wear management during machining operations like milling, which is a critical process for generating flat surfaces, slots, and profiles on cast components.
This study focuses on evaluating the milling performance of a high-strength nitrogen-alloyed gray iron casting and elucidating the associated wear mechanisms of coated carbide tools. We employ a systematic orthogonal experimental design to investigate the effects of key cutting parameters and compare the performance of two prevalent coating technologies: Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD). The goal is to provide foundational knowledge for optimizing the machining processes of this advanced grade of gray iron casting.
1. Experimental Materials and Methodology
1.1 Workpiece Material
The workpiece material used in this investigation was a high-strength nitrogen-alloyed gray iron casting, designated HT350. The key mechanical properties and the primary chemical composition of this grade of gray iron casting are summarized below:
Mechanical Properties: Tensile Strength: 374 MPa; Hardness: 232 HBW.
| Element | Content (wt.%) |
|---|---|
| C | 3.15 – 3.25 |
| Si | 1.5 – 1.6 |
| Mn | 0.15 – 0.25 |
| P | ≤ 0.07 |
| S | 0.06 – 0.08 |
| Cu | 0.35 – 0.45 |
| Cr | 0.25 – 0.30 |
| N | 0.017 |
The microstructure of this nitrogen-alloyed gray iron casting is characterized by type A graphite flakes that are shorter, thicker, and exhibit blunted tips compared to standard grades. This graphite morphology, induced by nitrogen addition, reduces stress concentration at the graphite edges. Furthermore, nitrogen promotes a high pearlite content with a refined interlamellar spacing, which is the primary contributor to the enhanced strength of this gray iron casting.
1.2 Cutting Tools and Experimental Design
Two types of commercially available coated cemented carbide square shoulder milling inserts were selected for the tests:
- PVD-coated tool (Grade YBG102): Features a fine-grained substrate coated with a TiAlN layer via PVD.
- CVD-coated tool (Grade YBD152): Features a wear-resistant substrate coated with a multilayer of TiC and a thick Al2O3 layer via CVD.
Both inserts had identical geometry: an 0.8 mm corner radius, 90° lead angle, and a diameter of 11 mm. They were mounted on the same tool holder.
The milling experiments were conducted under dry cutting conditions on a vertical machining center. An orthogonal array L9(34) was employed to study the influence of three cutting parameters at three levels each. Each milling test had a duration of 15 minutes. The experimental factors and levels are defined in the table below.
| Factor | Level 1 | Level 2 | Level 3 |
|---|---|---|---|
| Cutting Speed, Vc (m/min) | 150 | 200 | 250 |
| Feed per Tooth, fz (mm/rev) | 0.10 | 0.14 | 0.18 |
| Depth of Cut, ap (mm) | 0.4 | 0.8 | 1.2 |
The specific test runs according to the L9 array are shown in the following layout.
| Test No. | Vc (m/min) | fz (mm/rev) | ap (mm) |
|---|---|---|---|
| 1 | 150 | 0.10 | 0.4 |
| 2 | 150 | 0.14 | 0.8 |
| 3 | 150 | 0.18 | 1.2 |
| 4 | 200 | 0.10 | 0.8 |
| 5 | 200 | 0.14 | 1.2 |
| 6 | 200 | 0.18 | 0.4 |
| 7 | 250 | 0.10 | 1.2 |
| 8 | 250 | 0.14 | 0.4 |
| 9 | 250 | 0.18 | 0.8 |
1.3 Measurement and Characterization
Post-machining, tool wear was assessed using a deep-field 3D optical microscope. The primary metrics recorded were the wear morphology on the rake face (crater wear) and the average flank wear land width (VB). For a more detailed analysis of wear mechanisms, selected inserts were examined using a Field Emission Scanning Electron Microscope (FE-SEM) equipped with an Energy Dispersive X-ray Spectroscopy (EDS) system. This allowed for high-resolution imaging of worn surfaces and elemental analysis at specific wear zones.
2. Results and Discussion
2.1 Analysis of Rake Face Wear (Crater Wear)
The observation of the rake face after 15 minutes of milling the high-strength nitrogen-alloyed gray iron casting revealed distinct differences between the two tool types. Under identical cutting conditions, the PVD-coated tools consistently exhibited more severe crater wear compared to their CVD-coated counterparts. The wear scar on the PVD tools was larger in area, and a distinct crescent-shaped crater was often visible, especially at higher cutting speeds. In contrast, the CVD-coated tools showed a more uniform and less pronounced wear pattern on the rake face, with no well-defined crater formation.
The progression of rake face wear with increasing cutting parameters can be qualitatively described. For both tools, when comparing tests at the same feed and depth of cut but increasing speed (e.g., Tests 2, 5, 8), the wear scar area appeared to reduce slightly, but the localized depth or severity of wear within that area increased. This is attributed to the reduced tool-chip contact time at high speed, but a significant increase in cutting temperature leads to more intense thermo-chemical wear mechanisms. Conversely, increasing the depth of cut $a_p$ (e.g., Tests 6, 4, 5 or 8, 9, 7) led to a clear increase in both the wear scar area and its severity for both tools, due to the increased engagement volume and cutting forces. The influence of feed per tooth $f_z$ on rake face wear morphology was less pronounced and not visually distinct across the tested range.
2.2 Statistical Analysis of Flank Wear Land Width
The measured average flank wear land width (VB) for both tools under the nine test conditions is presented in the table below, along with a range analysis (also known as an analysis of means and ranges for orthogonal arrays).
| Test No. | Vc (m/min) | fz (mm/rev) | ap (mm) | VBPVD (µm) | VBCVD (µm) |
|---|---|---|---|---|---|
| 1 | 150 | 0.10 | 0.4 | 18 | 27 |
| 2 | 150 | 0.14 | 0.8 | 27 | 31 |
| 3 | 150 | 0.18 | 1.2 | 35 | 40 |
| 4 | 200 | 0.10 | 0.8 | 46 | 53 |
| 5 | 200 | 0.14 | 1.2 | 55 | 57 |
| 6 | 200 | 0.18 | 0.4 | 44 | 48 |
| 7 | 250 | 0.10 | 1.2 | 68 | 77 |
| 8 | 250 | 0.14 | 0.4 | 60 | 63 |
| 9 | 250 | 0.18 | 0.8 | 65 | 69 |
The range analysis calculates the mean wear (K) for each factor at each level and the overall range (R) for that factor. The results of this analysis are summarized below:
For PVD-coated tool:
Mean Wear (K):
– For Vc: K1=26.7, K2=48.3, K3=64.3 µm
– For fz: K1=44.0, K2=47.3, K3=48.0 µm
– For ap: K1=40.7, K2=46.0, K3=52.7 µm
Range (R): RVc=37.7, Rfz=4.0, Rap=12.0 µm
Primary influence order: Vc > ap > fz
Optimal level for minimum VB: A1B1C1 (Vc=150 m/min, fz=0.10 mm/rev, ap=0.4 mm)
For CVD-coated tool:
Mean Wear (K):
– For Vc: K1=32.7, K2=52.7, K3=69.7 µm
– For fz: K1=52.3, K2=50.3, K3=52.3 µm
– For ap: K1=46.0, K2=51.0, K3=58.0 µm
Range (R): RVc=37.0, Rfz=2.0, Rap=12.0 µm
Primary influence order: Vc > ap > fz
Optimal level for minimum VB: A1B2C1 (Vc=150 m/min, fz=0.14 mm/rev, ap=0.4 mm)
The analysis clearly shows that cutting speed $V_c$ is the most dominant factor affecting flank wear when milling this high-strength nitrogen-alloyed gray iron casting, followed by depth of cut $a_p$. The feed rate $f_z$ has a comparatively minor influence within the tested range. The wear progression can be related to the extended Taylor’s tool life equation, often formulated as:
$$ VT^n f^m a_p^p = C $$
where $V$ is cutting speed, $T$ is tool life, $f$ is feed, $a_p$ is depth of cut, and $n$, $m$, $p$, and $C$ are constants. Our results indicate that the exponent $n$ for $V$ is the largest (most negative), confirming its paramount influence on wear rate.
2.3 Investigation of Tool Wear Mechanisms
To understand the underlying reasons for the observed wear patterns and the superior rake face performance of the CVD tool, a detailed SEM and EDS study was conducted. The analysis revealed the activation of different dominant wear mechanisms depending on the cutting conditions and the coating type.
2.3.1 Abrasive Wear
At lower cutting speeds (e.g., Vc = 150 m/min), abrasive wear was a primary mechanism. The high-strength nitrogen-alloyed gray iron casting contains hard phases such as carbides and oxides originating from alloying elements like Si, Cr, and the pearlitic matrix itself. These hard particles, or fragments of built-up edge, plow grooves on the tool surface. SEM images showed significantly more pronounced and deeper scratching (plowing marks) on the rake face of the CVD-coated tool compared to the PVD-coated tool under identical low-speed conditions. This suggests that the initial interaction with the abrasive constituents of the gray iron casting is more damaging to the CVD coating’s top layer, likely the Al2O3.
2.3.2 Adhesive Wear
Adhesive wear, characterized by material transfer from the workpiece to the tool, was also evident, particularly at lower to medium cutting speeds. EDS analysis of material adhered to the rake face showed high concentrations of Fe, C, and O. On the CVD tool, the distributions of Fe and O were closely correlated, suggesting the adhesion of iron oxides (e.g., FeO) formed during cutting. This oxide layer can sometimes act as a protective film, potentially explaining the less severe crater formation on the CVD tool at lower speeds. On the PVD tool, the adhered material showed less oxygen, indicating a different adhesion mechanism, possibly involving more direct Fe-to-coating bonding, which could contribute to more pronounced crater wear.
3.3.3 Diffusion and Oxidative Wear
At higher cutting speeds (Vc ≥ 200 m/min), thermally activated mechanisms became dominant. EDS analysis within the wear scars on tools used at high speeds revealed the presence of elements like Mn, Si, and S from the gray iron casting within the tool’s surface layer, indicating interdiffusion between the tool coating and the workpiece material. This diffusion wear was more pronounced for the PVD-coated tool, as its elemental composition showed greater deviation from the original state compared to the CVD tool.
The most critical finding relates to oxidative wear. The EDS analysis showed a substantial increase in oxygen content on the worn rake face of both tools compared to a new tool. However, the relative increase was far greater for the PVD tool. The new PVD coating (TiAlN) contained minimal oxygen, while the worn surface showed oxygen atomic percentages exceeding 30%. For the CVD tool, which originally has a thick Al2O3 layer, the oxygen increase was smaller.
This points to a significant oxidative degradation mechanism for the PVD coating during the milling of high-strength gray iron casting. The TiAlN coating reacts with atmospheric oxygen at elevated cutting temperatures to form aluminum oxide:
$$ 2TiAlN_{(s)} + \frac{3}{2}O_{2(g)} \rightarrow Al_2O_{3(s)} + 2TiO_{2(s)} + N_{2(g)} $$
While Al2O3 is a stable and hard oxide, the key difference lies in its adherence and the dynamics of its formation. On the CVD tool, the Al2O3 is a dense, well-bonded, pre-existing layer that offers inherent oxidation resistance. On the PVD tool, the Al2O3 forms in situ on the surface. The hard abrasive particles present in the nitrogen-alloyed gray iron casting continuously abrade this newly formed, less adherent oxide film. This cyclic process—formation of Al2O3 followed by its immediate removal by abrasion—continuously exposes fresh TiAlN to oxidation. This self-perpetuating cycle leads to accelerated consumption of the PVD coating, resulting in more severe crater wear and flank wear at higher speeds, despite TiAlN’s higher intrinsic hardness. This explains the paradox where the theoretically more wear-resistant PVD coating suffered worse rake face damage when machining this particular grade of gray iron casting.
3. Conclusions
Based on the orthogonal milling tests and subsequent analysis on high-strength nitrogen-alloyed gray iron casting HT350, the following conclusions can be drawn:
- The cutting speed $V_c$ is the most significant parameter influencing flank wear land width (VB), followed by depth of cut $a_p$, while feed per tooth $f_z$ has a minimal effect within the tested range. For the PVD tool, the optimal combination for minimum VB was Vc=150 m/min, fz=0.10 mm/rev, ap=0.4 mm. For the CVD tool, it was Vc=150 m/min, fz=0.14 mm/rev, ap=0.4 mm.
- Under identical cutting conditions, PVD-coated tools exhibited more severe rake face (crater) wear compared to CVD-coated tools when machining this grade of gray iron casting.
- The dominant wear mechanisms are dependent on the cutting regime. At lower cutting speeds (100-200 m/min), abrasive wear and adhesive wear are predominant. At higher cutting speeds (200-250 m/min), diffusion wear and, most critically, oxidative wear become dominant.
- A key wear mechanism for PVD (TiAlN) coated tools is a cyclic process of oxidative wear exacerbated by abrasion. The TiAlN oxidizes to form Al2O3, which is then rapidly removed by the abrasive components in the high-strength nitrogen-alloyed gray iron casting. This continuous exposure and degradation lead to accelerated coating loss. The pre-existing, stable Al2O3 layer on the CVD tool provides better protection against this cyclic oxidative-abrasive mechanism, resulting in better rake face integrity despite potentially higher initial abrasive grooving.
This study underscores that the improved mechanical properties of nitrogen-alloyed gray iron casting, while beneficial for component performance, introduce specific challenges in machining. Tool selection must consider not just hardness but also the coating’s chemical stability and resistance to the synergistic oxidative-abrasive environment generated when cutting this advanced material. The findings provide essential guidance for selecting and applying coated carbide tools in the machining of high-strength nitrogen-alloyed gray iron casting components.