Introduction
Stainless steel machining remains one of the most demanding operations in modern manufacturing. With work-hardening tendencies, low thermal conductivity, and built-up edge (BUE) formation, ISO M-category materials (austenitic, martensitic, and duplex stainless steels) push cutting inserts to their limits. Selecting the right turning insert — in terms of geometry, coating, and grade — directly affects tool life, surface finish, and dimensional accuracy.
In this guide, we compare two leading Japanese/Korean insert manufacturers — Tungaloy and Korloy — across their stainless steel turning portfolios. We analyze their grade systems, chipbreaker geometries, and recommended cutting parameters to help you make an informed selection for turning operations on 304, 316, 17-4PH, and duplex grades.
Why Stainless Steel Turning Demands Specialized Inserts
Before diving into brand comparisons, it is essential to understand why generic ISO P (steel) grades often fail on stainless steels:
- Work hardening: Austenitic grades (304, 316) harden by 30–50% during machining, accelerating flank wear on subsequent passes.
- Low thermal conductivity: At ~16 W/m·K for 316L versus ~50 W/m·K for carbon steel, heat concentrates in the cutting zone, degrading coatings faster.
- High ductility and toughness: Leads to chip control challenges, BUE formation, and poor surface finishes if chipbreakers are mismatched.
- Chromium oxide layer: Abrasive oxide inclusions accelerate crater wear and notch wear at the depth-of-cut line.
These factors necessitate inserts with optimized M-class substrates, PVD coatings (typically TiAlN or AlCrN based), and sharp-edged geometries with dedicated chip control.
Tungaloy Stainless Steel Turning Grades
Tungaloy’s stainless steel turning lineup centers around the AH725, AH740, and T9125 grades within their TUNGALOY brand portfolio.
Tungaloy AH725 — Versatile M-Class Workhorse
AH725 is a multi-layer PVD-coated (TiAlN + Al2O3) micro-grain carbide grade targeting ISO M and P materials. Its key attributes:
- Substrate hardness: ~1,650 HV3 with 10% Co binder
- Coating thickness: ~3–4 μm total (multi-layer architecture)
- Edge preparation: honed edge with 15–25 μm hone radius for balance between edge strength and sharpness
- Best suited for: general-purpose stainless steel turning at moderate speeds (Vc = 120–200 m/min)
Tungaloy AH740 — High-Speed Finish Turning
AH740 employs an advanced AlCrN-based PVD coating with superior oxidation resistance. Designed for finishing applications:
- Coating microstructure: nano-structured AlCrN with Si3N4 interlayers
- Max operating temperature: 1,100°C without coating degradation
- Surface finish capability: Ra 0.4–0.8 μm achievable with fz = 0.08–0.12 mm/rev
- Optimal for: continuous finishing passes on 304L, 316L at Vc = 200–300 m/min
Tungaloy T9125 — CVD-Coated Heavy Roughing
For aggressive roughing with large depths of cut (ap > 3 mm), T9125 offers a thick MT-CVD TiCN + Al2O3 coating on a tough substrate:
- Coating thickness: ~8–10 μm for maximum wear resistance
- Substrate: gradient cobalt-enriched surface zone for added edge toughness
- Negative-rake geometries available for maximum cutting forces
- Ideal for: heavy roughing of duplex and super-duplex stainless steels
Korloy Stainless Steel Turning Grades
Korloy’s approach to stainless steel turning emphasizes their KS3035, KS3050, and KC5310 grades.
Korloy KS3035 — Multi-Purpose M/P Turning
KS3035 is Korloy’s flagship PVD-coated grade for stainless steel, utilizing a proprietary multi-layer TiAlSiN coating:
- Substrate: ultra-fine grain WC-Co with ~6% binder content
- Coating: TiAlSiN multilayer (~3 μm) with high aluminum content for oxidation resistance
- Edge: precision-ground with controlled hone (20 μm typical)
- Best for: medium finishing to semi-roughing on austenitic and martensitic stainless steels
Korloy KS3050 — Toughness-Oriented Roughing
KS3050 is optimized for interrupted cuts and roughing on difficult-to-machine stainless grades:
- Substrate: 8–10% Co with coarse-grain structure for maximum fracture toughness
- Coating: thick TiCN/TiAlN PVD bilayer (~5 μm)
- Chipbreaker: aggressive curl geometries (MR3, MR5) for effective chip breaking at high feed rates
- Optimal for: roughing 17-4PH, duplex stainless, and operations with interrupted cuts
Korloy KC5310 — Precision Finishing Grade
KC5310 targets high-quality surface finish requirements in stainless steel finishing:
- Coating: ultra-thin AlCrN PVD (~2 μm) preserving cutting edge sharpness
- Edge preparation: minimal hone (8–12 μm) for near-mirror finishes
- Surface finish: Ra 0.2–0.5 μm achievable at fz = 0.05–0.10 mm/rev
- Optimal for: finishing pass on medical-grade and food-grade stainless components
Head-to-Head Grade Comparison
The following table compares the two brands across key technical specifications for stainless steel turning:
| Parameter | Tungaloy AH725 | Tungaloy AH740 | Korloy KS3035 | Korloy KC5310 |
|---|---|---|---|---|
| Application | General M/P | Finishing | General M/P | Precision Finishing |
| Coating Type | PVD TiAlN + Al2O3 | PVD AlCrN + Si3N4 | PVD TiAlSiN | PVD AlCrN |
| Coating Thickness | 3–4 μm | 2–3 μm | ~3 μm | ~2 μm |
| Edge Hone | 15–25 μm | 10–15 μm | ~20 μm | 8–12 μm |
| Max Temp (°C) | 900 | 1,100 | 950 | 1,050 |
| Vc Range (m/min) | 120–200 | 200–300 | 130–220 | 180–280 |
| Achievable Ra (μm) | 0.8–1.6 | 0.4–0.8 | 0.6–1.2 | 0.2–0.5 |
Chipbreaker Geometry Comparison
Chip control is arguably the most critical factor in stainless steel turning. Both brands offer dedicated M-class chipbreaker designs:
| Feature | Tungaloy (NS/NU Series) | Korloy (MR/MP Series) |
|---|---|---|
| Finishing Geometry | NS style — narrow land, positive rake (15–18°) | MP style — wide land, moderate rake (12–16°) |
| Semi-Finish Geometry | NM style — medium width, balanced curl | MR3 — multi-stage curl with ramp |
| Roughing Geometry | NM-M style — wide seat, aggressive curl | MR5 — high curl angle, deep groove |
| ap Range (Finish) | 0.2–1.5 mm | 0.2–2.0 mm |
| ap Range (Rough) | 1.5–6.0 mm | 2.0–7.0 mm |
| fz Range (Finish) | 0.08–0.20 mm/rev | 0.05–0.18 mm/rev |
| fz Range (Rough) | 0.20–0.50 mm/rev | 0.20–0.45 mm/rev |
Tungaloy’s NS finishing geometries tend to produce tighter 6-9 form (C-type) chips at lower feeds, which is advantageous for Swiss-type lathes and small-diameter work. Korloy’s MR geometries, with their multi-stage curl design, excel at wider ap ranges and handle the transition between light and heavy cuts without chipbreaker change.
Recommended Cutting Parameters by Stainless Steel Grade
Turning AISI 304 / 304L (Austenitic)
| Operation | Brand/Grade | Vc (m/min) | fz (mm/rev) | ap (mm) | Chipbreaker |
|---|---|---|---|---|---|
| Heavy Roughing | Tungaloy T9125 | 130–160 | 0.30–0.50 | 3.0–6.0 | NM-M |
| Heavy Roughing | Korloy KS3050 | 120–150 | 0.25–0.45 | 2.5–6.0 | MR5 |
| Semi-Finish | Tungaloy AH725 | 160–210 | 0.15–0.25 | 1.0–2.5 | NM |
| Semi-Finish | Korloy KS3035 | 150–200 | 0.15–0.25 | 1.0–3.0 | MR3 |
| Finishing | Tungaloy AH740 | 220–280 | 0.08–0.12 | 0.2–0.8 | NS |
| Finishing | Korloy KC5310 | 200–260 | 0.05–0.10 | 0.1–0.5 | MP |
Turning AISI 316 / 316L (Higher Alloy Austenitic)
| Operation | Brand/Grade | Vc (m/min) | fz (mm/rev) | ap (mm) | Chipbreaker |
|---|---|---|---|---|---|
| Roughing | Tungaloy AH725 | 120–170 | 0.25–0.40 | 2.0–5.0 | NM-M |
| Roughing | Korloy KS3050 | 110–150 | 0.20–0.40 | 2.0–5.0 | MR5 |
| Finishing | Tungaloy AH740 | 180–240 | 0.08–0.12 | 0.2–0.6 | NS |
| Finishing | Korloy KC5310 | 170–230 | 0.05–0.10 | 0.1–0.4 | MP |
Note: 316/316L requires approximately 10–15% lower cutting speeds than 304 due to higher molybdenum content increasing work-hardening severity.
Turning 17-4 PH (Precipitation Hardening)
| Condition | Brand/Grade | Vc (m/min) | fz (mm/rev) | ap (mm) | Notes |
|---|---|---|---|---|---|
| Solution Treated | Tungaloy AH725 | 100–150 | 0.15–0.30 | 1.0–3.0 | Relatively machinable state |
| Solution Treated | Korloy KS3035 | 110–160 | 0.15–0.28 | 1.0–3.0 | Good chip control |
| Aged (H900) | Tungaloy T9125 | 60–100 | 0.10–0.20 | 0.5–2.0 | Hardened state — use CVD grade |
| Aged (H900) | Korloy KS3050 | 65–110 | 0.10–0.20 | 0.5–2.0 | Toughness critical in aged condition |
Turning Duplex Stainless (2205 / 2507)
| Operation | Brand/Grade | Vc (m/min) | fz (mm/rev) | ap (mm) | Notes |
|---|---|---|---|---|---|
| Roughing | Tungaloy T9125 | 90–140 | 0.20–0.35 | 2.0–4.0 | CVD grade for thermal shock resistance |
| Roughing | Korloy KS3050 | 85–130 | 0.18–0.35 | 2.0–4.0 | Tough substrate prevents edge chipping |
| Finishing | Tungaloy AH740 | 140–200 | 0.08–0.12 | 0.2–0.5 | AlCrN coating resists ferrite abrasion |
| Finishing | Korloy KC5310 | 130–190 | 0.06–0.10 | 0.1–0.4 | Sharp edge minimizes work hardening |
Insert Shape Selection Guide
Both Tungaloy and Korloy offer their stainless steel grades in standard ISO insert shapes. Here is a practical selection guide based on application priorities:
| Insert Shape | Strength | Best Application | Recommended ap Range |
|---|---|---|---|
| CNMG (80° Diamond) | Good balance | General turning, ID/OD | 0.5–4.0 mm |
| DNMG (55° Diamond) | Lower cutting forces | Finishing, thin walls, low-rigidity | 0.2–2.5 mm |
| WNMG (60° Triangle) | Stronger tip | Roughing, heavier cuts | 1.0–5.0 mm |
| SNMG (Square 90°) | 4 usable edges | Facing, shoulder turning | 1.0–6.0 mm |
| RNGS (Round) | Highest edge strength | Heavy interrupted cuts, profiling | 0.5–6.0 mm |
| VCGT (35° Diamond) | Sharpest edge | Finish turning, low force | 0.1–1.0 mm |
Brand Selection Recommendations
Choose Tungaloy When:
- High-speed finishing is the priority — AH740’s AlCrN + Si3N4 nano-coating outperforms at Vc > 250 m/min
- Heavy roughing duplex or super-duplex — T9125’s thick CVD coating handles thermal cycling better
- Swiss-type or multi-axis lathes — NS chipbreakers produce consistent short chips at lower ap/fz
- High-temperature alloys overlap — AH740 performs well on ISO S materials (Inconel, Hastelloy)
Choose Korloy When:
- Interrupted cuts and unstable conditions — KS3050’s tough coarse-grain substrate resists chipping
- Wide ap range flexibility — MR3/MR5 geometries handle semi-finishing and roughing without tool changes
- Precision finishing requirements — KC5310 delivers the best surface finish (Ra 0.2 μm) due to its minimal hone
- Cost-effective general-purpose turning — KS3035 offers competitive performance across M/P materials
Practical Tip: Running Both Brands
Many shops successfully run both brands in complementary roles. A common strategy:
- Roughing pass: Korloy KS3050 (MR5 chipbreaker) for aggressive material removal with maximum edge toughness
- Semi-finish pass: Tungaloy AH725 (NM geometry) for balanced wear resistance and chip control
- Finish pass: Tungaloy AH740 (NS geometry) or Korloy KC5310 (MP geometry) depending on required Ra value
Coolant and Tool Life Optimization
Both brands perform best with high-pressure coolant (70+ bar) directed at the chip-tool interface. For stainless steel turning:
- Minimum 8% concentration of semi-synthetic coolant with good lubricity
- High-pressure through-spindle coolant (≥70 bar) can increase tool life by 40–60% on 316L by breaking chips and cooling the shear zone
- Avoid flooding at low pressure — this can cause thermal shock cracks in CVD-coated grades like T9125
- For dry cutting with AH740 or KC5310, increase Vc by 15–20% and reduce fz by 10% compared to wet parameters
Conclusion
Both Tungaloy and Korloy offer technically mature insert grades for stainless steel turning. Tungaloy excels in high-speed finishing (AH740) and heavy CVD-coated roughing (T9125), while Korloy provides superior toughness for interrupted cuts (KS3050) and ultra-precise finishing (KC5310). The optimal choice depends on your specific workpiece material, machine rigidity, coolant system, and surface finish requirements.
For the best results, consider the combination strategy — using Korloy’s tough grades for aggressive roughing and Tungaloy’s high-tech PVD grades for finishing passes. This approach maximizes material removal rates while maintaining consistent surface quality across production batches.
Always validate cutting parameters with test cuts on your specific machine-tool-workpiece combination, and monitor flank wear (VBmax ≤ 0.3 mm is a standard replacement criterion for stainless steel turning).
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