Why Stainless Steel Demands Specialized Turning Grades
Stainless steel, classified under ISO M in the international machining standard, is one of the most demanding workpiece materials for turning operations. Its combination of high work-hardening tendency, low thermal conductivity, and high strain rate sensitivity creates a perfect storm of cutting-edge stress. Unlike plain carbon steels, stainless steels—including the ubiquitous AISI 304 and 316 austenitic grades—can harden at the cut surface to 35–45 HRC during machining, rapidly degrading insert edges that are not engineered for the task.
Two of the most respected names in metalcutting, Iscar and Sandvik Coromant, have developed dedicated grade portfolios for the ISO M group. This article compares their stainless steel turning technologies head-to-head, examining coating architecture, substrate design, recommended cutting parameters, and insert geometry to help machinists make an informed selection.
Iscar Stainless Steel Turning Grade Portfolio
Iscar’s stainless steel turning strategy is built on a dual-pillar system: PVD-coated grades for finishing and medium operations, and CVD-coated grades for heavy roughing. This division reflects the fundamental trade-off between edge sharpness and wear resistance.
IC805 — PVD TiAlN for Finishing and Medium Cutting
The IC805 grade is Iscar’s flagship PVD offering for stainless steel. It features a TiAlN (titanium aluminum nitride) coating applied via advanced arc evaporation PVD process at a thickness of approximately 2–3 μm. The thin coating preserves a sharp cutting edge—critical for generating clean surfaces on gummy austenitic stainless without built-up edge (BUE).
The substrate beneath IC805 is a fine-grained cemented carbide (grain size ~0.8 μm) with a cobalt content optimized for toughness. This combination allows IC805 to handle the interrupted cuts and fluctuating cutting forces common in stainless turning.
IC908 — CVD MT-CVD for Roughing
For heavy stock removal, Iscar deploys the IC908 grade, utilizing moderate-temperature CVD (MT-CVD) coating technology. The multilayer architecture typically consists of:
- Base layer: TiC/TiCN (3–4 μm) for adhesion and crater-wear resistance
- Middle layer: Al₂O₃ (alumina, 2–3 μm) for thermal barrier protection
- Top layer: TiN (golden, ~1 μm) for visual wear indication and friction reduction
Total coating thickness reaches 7–10 μm, providing the thermal insulation needed for high material removal rates where cutting temperatures at the edge can exceed 900°C.
IC20 — PVD General Purpose
The IC20 grade serves as Iscar’s workhorse PVD carbide for a broad range of ISO P and M applications. With a TiAlN coating at ~2–3 μm on a moderately fine substrate, it offers a balance of versatility across stainless steel types, particularly effective on free-machining stainless grades like AISI 303.
Sandvik Coromant Stainless Steel Turning Grade Portfolio
Sandvik Coromant takes a slightly different strategic approach, organizing its stainless steel grades around a CVD-optimized platform for roughing and a dedicated PVD platform for finishing, with a broader range of geometry-specific substrates.
GC2025 — CVD for Roughing and Medium Operations
The GC2025 grade is Sandvik’s primary recommendation for stainless steel roughing. It uses a thick CVD coating with a multilayer structure:
- TiCN base (~4 μm) providing mechanical wear resistance and adhesion
- Al₂O₃ (alpha-alumina) middle layer (~3 μm) acting as a thermal barrier
- Thin TiN top coat for chip flow and wear visibility
Sandvik’s substrate for GC2025 uses a gradient sintering process, creating a cobalt-enriched zone near the cutting edge (5–10 μm depth) that improves edge toughness without sacrificing bulk hardness. This gradient technology is a key differentiator—reducing the risk of edge chipping that is endemic in stainless steel roughing.
GC2015 — CVD for Medium to Finishing
The GC2015 grade bridges the gap between roughing and finishing. It shares the CVD coating philosophy of GC2025 but with a thinner overall coating (~6 μm) and a sharper edge geometry. This makes GC2015 ideal for medium-depth cuts on stainless components where surface finish matters but stock removal is still significant.
GC1115 — PVD for Finishing
The GC1115 is Sandvik’s dedicated PVD grade for stainless steel finishing. Its TiSiN-based nanocomposite coating (~2 μm) offers higher hardness (approaching 35–40 GPa) and superior oxidation resistance (stable up to ~1100°C) compared to conventional TiAlN. This allows higher cutting speeds in finishing passes without triggering adhesive wear.
Coating Technology Head-to-Head Comparison
| Parameter | Iscar IC805 (PVD) | Sandvik GC1115 (PVD) | Iscar IC908 (CVD) | Sandvik GC2025 (CVD) |
|---|---|---|---|---|
| Coating Type | PVD TiAlN | PVD TiSiN nanocomposite | CVD (MT-CVD multilayer) | CVD multilayer |
| Coating Thickness | 2–3 μm | ~2 μm | 7–10 μm | ~8 μm |
| Coating Hardness | ~28–30 GPa | ~35–40 GPa | ~22–25 GPa | ~22–25 GPa |
| Max Service Temp | ~850°C | ~1100°C | ~1000°C | ~1000°C |
| Edge Sharpness | High (sharp) | High (sharp) | Moderate (rounded) | Moderate (rounded) |
| Best Application | Finishing, medium | Finishing, high-speed | Roughing | Roughing, medium |
| Substrate Feature | Fine grain (~0.8 μm) | Gradient cobalt zone | Medium-fine grain | Gradient cobalt zone |
Cutting Parameters for Stainless Steel Turning
The following parameters are recommended for austenitic stainless steels (AISI 304/316) and represent starting values for stable machining conditions. Actual values should be adjusted based on machine rigidity, workpiece fixturing, and coolant availability.
| Operation | Grade | Vc (m/min) | fn (mm/rev) | ap (mm) | Coolant |
|---|---|---|---|---|---|
| Roughing | Iscar IC908 | 120–180 | 0.3–0.6 | 2.0–4.0 | Flood / dry |
| Roughing | Sandvik GC2025 | 130–200 | 0.3–0.7 | 2.0–6.0 | Flood / dry |
| Medium | Sandvik GC2015 | 150–220 | 0.2–0.4 | 1.0–3.0 | Flood |
| Finishing | Iscar IC805 | 180–280 | 0.08–0.25 | 0.5–1.5 | Flood |
| Finishing | Sandvik GC1115 | 200–320 | 0.05–0.2 | 0.3–1.2 | Flood / MQL |
| General | Iscar IC20 | 100–160 | 0.1–0.3 | 0.5–2.5 | Flood |
Key parameter notes:
- Cutting speed (Vc) should be reduced by 20–30% for duplex stainless steels (e.g., 2205) versus austenitic grades due to higher strength and work-hardening rate.
- Feed (fn) must always exceed 0.05 mm/rev to avoid rub-hardening—cutting below the minimum chip thickness accelerates work hardening and edge failure.
- Depth of cut (ap) should be kept above the work-hardened layer from the previous pass (typically >0.2 mm) to prevent progressive edge degradation.
- Coolant is strongly recommended for PVD finishing grades; CVD roughing grades can run dry or with flood coolant, but avoid high-pressure interruption that causes thermal shock.
Insert Geometry and Chip Control
Both brands offer insert geometries specifically designed for stainless steel. The geometry choice is as critical as the grade—stainless steel’s tendency to form long, stringy chips demands aggressive chip-breaking geometries.
Iscar Geometry Options
Iscar’s stainless-focused geometries include the ISSIR and ISSM chip breaker families, characterized by:
- Positive rake angle (8–12°) to reduce cutting forces and work hardening
- Positive land with a small hone radius (20–40 μm) to balance edge sharpness with chip-breaking ability
- Curved chip breaker profile designed to curl chips tightly for evacuation in bore and facing operations
Sandvik Geometry Options
Sandvik’s geometry portfolio for stainless steel centers on the -SM (stainless medium) and -SF (stainless fine) suffix designations:
- SM geometry: Positive rake with a T-land (0.1–0.15 mm) and hone for roughing—designed for stability at higher depths of cut
- SF geometry: Sharp positive edge without T-land for finishing—produces surface roughness values down to Ra 0.4–0.8 μm
- Wiper geometries available in both PVD grades for doubled feed rates at equivalent surface finish
Wear Mechanisms and Failure Mode Analysis
Understanding how each grade system responds to stainless steel wear mechanisms is essential for optimizing tool life:
| Wear Mechanism | Iscar Best Defense | Sandvik Best Defense |
|---|---|---|
| Crater wear (chemical dissolution) | IC908 Al₂O₃ layer | GC2025 Al₂O₃ layer |
| Adhesive wear / BUE | IC805 sharp PVD edge | GC1115 TiSiN low friction |
| Thermal fatigue cracking | IC908 thick CVD | GC2025 gradient substrate |
| Plastic deformation (edge softening) | IC908 + coolant | GC2025 + coolant |
| Notch wear (depth-of-cut line) | Reduce ap, increase fn | Reduce ap, increase fn |
Application Scenario Recommendations
Scenario 1: High-Volume 304/316 Bar Stock Roughing
For maximum material removal rate on standard austenitic stainless bar stock, Sandvik GC2025 holds a slight edge due to its gradient substrate technology and marginally higher recommended cutting speed range. The cobalt-enriched edge zone better resists the chipping that initiates premature failure in high-feed roughing. Pair with an SM geometry insert and maintain flood coolant.
Scenario 2: Precision Finishing on 316L Components
For fine-finishing operations where surface finish and dimensional tolerance are critical, Sandvik GC1115 offers superior oxidation resistance and higher achievable cutting speeds. However, Iscar IC805 remains highly competitive and may be preferred in shops already standardized on Iscar tooling systems. Both grades should use a sharp positive geometry with minimal land.
Scenario 3: Mixed Material / Job Shop Flexibility
For shops handling a mix of 303 free-machining stainless, 304, and occasional 410 martensitic parts, Iscar IC20 provides the broadest operational window without grade changes. Its balanced TiAlN PVD coating and versatile substrate handle the full stainless spectrum at moderate parameters.
Scenario 4: Duplex and Super-Duplex (2205, 2507)
Both brands recommend their CVD roughing grades (IC908 / GC2025) for duplex stainless, with cutting speeds reduced to 80–120 m/min. The thick alumina barrier is essential for managing the extreme heat generated by duplex’s high strength and low thermal conductivity. PVD grades are generally not recommended for duplex roughing.
Tool Life and Cost-Per-Edge Considerations
While this article avoids pricing, the technical factors influencing cost-per-edge are relevant:
- Sandvik GC2025 typically achieves 15–25% longer tool life in continuous roughing due to gradient substrate toughness
- Iscar IC805 provides more predictable wear in interrupted cuts, potentially yielding more parts per edge in challenging fixturing
- Insert indexing strategy matters more than brand: both offer 4–6 usable edges per square or round insert, and consistent indexing at 70–80% of expected life is the single largest cost lever
- Coating integrity at the nose radius is the life-limiting factor in finishing; reduce fn when approaching end of life to preserve edge integrity for the last pass
Conclusion: Which Grade System Wins?
The Iscar vs Sandvik comparison for stainless steel turning reveals two technically mature, well-matched systems with distinct philosophical emphases:
- Sandvik Coromant leads slightly in roughing productivity through its gradient substrate technology and optimized CVD platform (GC2025). Its PVD GC1115 also pushes the speed envelope in finishing.
- Iscar matches Sandvik in finishing versatility with the IC805 PVD grade and offers broader general-purpose flexibility with IC20 for mixed-material job shops.
- Both brands’ CVD grades are essentially equivalent for duplex and super-duplex applications—selection here should be driven by geometry availability and shop tooling standardization.
The decisive factor in most real-world scenarios is not the grade itself but the insert geometry, cutting parameter optimization, and process stability. A well-applied Iscar grade will outperform a poorly-applied Sandvik grade and vice versa. For shops standardized on either brand, the respective stainless steel grade portfolios provide complete coverage across all ISO M subgroups.
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