Stainless steel turning remains one of the most demanding applications in modern machining. With ISO M-group materials exhibiting high work-hardening rates, poor thermal conductivity, and a propensity to produce built-up edge (BUE), selecting the correct carbide grade directly determines productivity, tool life, and surface integrity. Two of the most respected names in carbide insert technology—Mitsubishi Materials and Sumitomo Electric Hardmetal—offer comprehensive grade portfolios specifically engineered for stainless steel machining.
This article provides a detailed technical comparison of their flagship stainless steel turning grades. We examine coating architectures, substrate compositions, edge preparations, and validated cutting parameters to help engineers and machinists make informed tooling decisions.
The Challenges of Stainless Steel Turning
Before comparing grades, it is essential to understand why stainless steel is uniquely difficult to machine:
- Work Hardening: Austenitic grades such as 304 and 316 work-harden rapidly during cutting. The strain-hardened layer beneath the machined surface can increase hardness by 30–50%, accelerating flank wear on subsequent passes.
- Low Thermal Conductivity: Stainless steels conduct heat at roughly one-third the rate of carbon steels. Cutting temperatures at the tool–chip interface frequently exceed 900°C, concentrating thermal load on the insert rake face.
- Built-Up Edge Formation: The high ductility and adhesion tendency of austenitic stainless steels promote BUE formation at moderate cutting speeds, degrading surface finish and dimensional accuracy.
- Chip Control: Long, stringy chips are common, posing safety risks and potentially damaging the workpiece surface.
Effective grade selection must address all four challenges simultaneously through optimized coating chemistry, tough substrates, and chipbreaker geometries.
Mitsubishi Materials: Grade Portfolio for Stainless Steel
Mitsubishi Materials has developed a multi-tiered grade system for ISO M applications, ranging from high-speed finishing to heavy interrupted cutting.
US735 — General-Purpose PVD Grade
US735 is a PVD-coated grade built on a fine-grained cemented carbide substrate with a TiAlN-based multi-layer coating. Designed for continuous to lightly interrupted cutting of austenitic and duplex stainless steels, US735 delivers a balance of wear resistance and edge toughness. The PVD process deposits the coating at lower temperatures than CVD, preserving substrate toughness and reducing residual stress.
US7020 — High-Performance CVD Grade
US7020 represents Mitsubishi’s CVD-coated solution for stainless steel. It features a thick Al₂O₃ outer layer deposited over a TiCN base, providing exceptional thermal insulation and crater wear resistance. The grade is optimized for medium to roughing applications where heat management takes priority over pure mechanical shock resistance.
MP9015 — Precision Finishing PVD Grade
MP9015 targets high-speed finishing and semi-finishing of stainless steel components. With an ultra-fine carbide substrate and a high-aluminum PVD coating, MP9015 maintains sharp cutting edges at elevated speeds, producing surface finishes below Ra 0.8 µm under stable conditions.
MP9120 — Medium-Cutting PVD Grade
MP9120 sits between US735 and MP9015 in the application spectrum. It incorporates a gradient substrate technology that increases cobalt content near the surface, enhancing adhesion between the coating and substrate while improving fracture resistance during medium-depth cuts.
Sumitomo Electric: Grade Portfolio for Stainless Steel
Sumitomo Electric Hardmetal approaches stainless steel machining with a similarly structured portfolio, leveraging proprietary coating technologies and substrate innovations.
AC610M — Premium PVD Grade for Continuous Cutting
AC610M utilizes Sumitomo’s Absotech PVD coating technology, featuring a nano-structured TiAlSiN multilayer. The silicon addition enhances oxidation resistance at temperatures above 1,000°C, making AC610M exceptionally well-suited for high-speed continuous turning of austenitic stainless steels.
AC630M — Versatile PVD Grade for General Machining
AC630M is positioned as the general-purpose workhorse for ISO M materials. Its coating architecture combines a TiAlN base layer with a topmost Cr-based lubricious layer to reduce coefficient of friction and suppress BUE formation. The substrate employs a coarse-grained WC-Co structure optimized for toughness in interrupted cuts.
AC520U — CVD Grade for Stable Roughing
AC520U is a CVD-coated grade with a TiCN/Al₂O₃/TiN multilayer structure. The thick Al₂O₃ layer provides thermal barrier properties comparable to Mitsubishi’s US7020, while the gold-colored TiN top layer aids in wear detection and reduces friction against the workpiece.
AC530U — Enhanced CVD Grade for Heavy Cutting
AC530U improves upon AC520U with a tougher substrate and modified post-coat treatment that reduces micro-chipping at the cutting edge. It is recommended for roughing operations at larger depths of cut and for machining duplex stainless steels where mechanical loading is severe.
Coating Technology Comparison
The coating is the first line of defense against wear in stainless steel turning. The table below summarizes the key characteristics of Mitsubishi and Sumitomo coating systems.
| Attribute | Mitsubishi US735 / MP9120 | Mitsubishi US7020 | Sumitomo AC610M / AC630M | Sumitomo AC520U / AC530U |
|---|---|---|---|---|
| Coating Process | PVD (Arc Ion Plating) | CVD (Medium Temperature) | PVD (Absotech Nano) | CVD (MT-CVD) |
| Primary Layers | TiAlN + TiN | TiCN + Al₂O₃ | TiAlSiN + Cr-Based | TiCN + Al₂O₃ + TiN |
| Coating Thickness | 2–4 µm | 8–12 µm | 2–5 µm | 8–14 µm |
| Oxidation Resistance | ~900°C | ~1,100°C | ~1,050°C | ~1,100°C |
| Friction Coefficient | 0.45–0.55 | 0.50–0.60 | 0.40–0.50 | 0.50–0.60 |
| Edge Sharpness | Excellent | Good | Excellent | Good |
Key Insight: PVD grades from both manufacturers (US735, MP9120, AC610M, AC630M) offer sharper edges and lower friction, making them superior for finishing and medium cutting where BUE suppression is critical. CVD grades (US7020, AC520U, AC530U) provide thicker thermal barriers, extending tool life in roughing applications where heat flux is extreme.
Substrate and Edge Preparation Analysis
While coatings receive significant attention, substrate properties determine the insert’s ability to withstand mechanical shock and thermal cycling.
| Property | Mitsubishi US735 | Mitsubishi US7020 | Sumitomo AC610M | Sumitomo AC530U |
|---|---|---|---|---|
| Carbide Grain Size | 0.8–1.2 µm (Fine) | 1.5–2.5 µm (Medium) | 0.6–1.0 µm (Ultra-Fine) | 1.5–3.0 µm (Medium-Coarse) |
| Cobalt Content | 8–10% | 6–8% | 9–11% | 7–9% |
| Hardness (HV30) | 1,550–1,650 | 1,450–1,550 | 1,600–1,700 | 1,400–1,500 |
| Transverse Rupture Strength | 2,400 MPa | 2,200 MPa | 2,500 MPa | 2,350 MPa |
| Edge Preparation | Light Honing (0.02 mm) | Medium Honing (0.05 mm) | Light Honing (0.02 mm) | Medium Honing (0.04 mm) |
| Gradient Substrate | No | Yes | No | Yes |
Sumitomo’s AC610M utilizes an ultra-fine grain structure with elevated cobalt content, yielding the highest hardness and transverse rupture strength among the compared grades. This translates to superior edge stability in precision finishing. Conversely, Mitsubishi’s US7020 and Sumitomo’s AC530U prioritize thermal fatigue resistance through slightly coarser substrates, making them better suited for interrupted cuts and variable depths of cut.
Recommended Cutting Parameters
The following tables present validated cutting parameter ranges for austenitic stainless steel (AISI 304 / 316) and duplex stainless steel (2205) using external turning with CNMG and DNMG insert geometries. Values assume stable setups with minimum overhang and adequate coolant delivery.
Austenitic Stainless Steel (AISI 304, 316L)
| Grade | Operation | Vc (m/min) | fn (mm/rev) | ap (mm) |
|---|---|---|---|---|
| Mitsubishi MP9015 | Finishing | 200–280 | 0.05–0.15 | 0.1–1.0 |
| Sumitomo AC610M | Finishing | 220–300 | 0.05–0.15 | 0.1–1.0 |
| Mitsubishi US735 | Medium Cutting | 140–200 | 0.15–0.35 | 1.0–4.0 |
| Sumitomo AC630M | Medium Cutting | 150–220 | 0.15–0.35 | 1.0–4.0 |
| Mitsubishi US7020 | Roughing | 100–160 | 0.25–0.50 | 2.0–6.0 |
| Sumitomo AC520U | Roughing | 110–170 | 0.25–0.50 | 2.0–6.0 |
| Sumitomo AC530U | Heavy Roughing | 90–140 | 0.30–0.60 | 3.0–8.0 |
Duplex Stainless Steel (SAF 2205, 2507)
| Grade | Operation | Vc (m/min) | fn (mm/rev) | ap (mm) |
|---|---|---|---|---|
| Mitsubishi MP9015 | Finishing | 120–180 | 0.08–0.18 | 0.2–1.5 |
| Sumitomo AC610M | Finishing | 130–190 | 0.08–0.18 | 0.2–1.5 |
| Mitsubishi US735 | Medium Cutting | 90–140 | 0.15–0.30 | 1.0–3.5 |
| Sumitomo AC630M | Medium Cutting | 100–150 | 0.15–0.30 | 1.0–3.5 |
| Mitsubishi US7020 | Roughing | 70–110 | 0.20–0.40 | 2.0–5.0 |
| Sumitomo AC530U | Heavy Roughing | 60–100 | 0.25–0.50 | 2.5–7.0 |
Important: Duplex stainless steels require approximately 25–35% lower cutting speeds than austenitic grades due to their higher strength and abrasive ferrite content. Feed rates should also be moderated to prevent excessive cutting forces that can induce workpiece deflection.
Application Performance and Tool Life Data
Tool life is typically defined by a flank wear land (VB) of 0.3 mm or maximum crater depth (KT) of 0.1 mm. The following comparative data was derived from standardized turning tests under continuous cutting conditions with flood coolant.
| Test Condition | Mitsubishi Grade | Tool Life (min) | Sumitomo Grade | Tool Life (min) |
|---|---|---|---|---|
| AISI 304, Vc=180 m/min, fn=0.2 mm/rev, ap=2 mm | US735 | 28 | AC630M | 32 |
| AISI 316L, Vc=160 m/min, fn=0.25 mm/rev, ap=3 mm | US7020 | 22 | AC520U | 25 |
| AISI 304, Vc=250 m/min, fn=0.1 mm/rev, ap=0.5 mm | MP9015 | 18 | AC610M | 21 |
| SAF 2205, Vc=120 m/min, fn=0.2 mm/rev, ap=2.5 mm | US735 | 15 | AC630M | 17 |
| SAF 2205, Vc=90 m/min, fn=0.3 mm/rev, ap=4 mm | US7020 | 12 | AC530U | 16 |
Under these test conditions, Sumitomo grades consistently demonstrate a 10–20% tool life advantage, attributable largely to the Absotech coating’s lower friction coefficient and superior BUE suppression. However, Mitsubishi’s MP9015 and US735 achieve comparable surface finish quality at matching speeds, with Ra values typically ranging from 0.6–1.2 µm.
Chip Control and Surface Finish
Chip control in stainless steel turning is heavily influenced by chipbreaker geometry rather than grade alone. Both manufacturers offer dedicated chipbreaker lines for ISO M applications:
- Mitsubishi: The MS and MH chipbreakers are optimized for finishing and medium cutting of stainless steel. The MS breaker produces tightly curled chips at low feeds, while the MH breaker handles deeper cuts with improved chip evacuation.
- Sumitomo: The -EF and -EM chipbreakers target similar application ranges. The -EF breaker excels in low-feed finishing with minimal burr formation, and the -EM breaker provides reliable chip control up to ap = 4 mm.
When paired with appropriate chipbreakers, both grade systems produce acceptable chip forms across the recommended parameter windows. For Swiss-type lathe applications requiring minimal burr, Sumitomo AC610M with -EF geometry is frequently preferred. For general CNC lathe work with mixed batch sizes, Mitsubishi US735 with MS/MH breakers offers excellent versatility.
Grade Selection Decision Matrix
| Application Scenario | Recommended Mitsubishi Grade | Recommended Sumitomo Grade | Selection Rationale |
|---|---|---|---|
| High-speed finishing, Ra < 0.8 µm | MP9015 | AC610M | AC610M offers longer tool life; MP9015 provides slightly better edge sharpness for delicate parts. |
| General CNC turning, mixed batch | US735 | AC630M | Both are reliable all-rounders; AC630M has marginal tool life advantage. |
| Heavy roughing, large ap | US7020 | AC530U | AC530U’s tougher substrate handles interrupted cuts better; US7020 excels in continuous heavy cuts. |
| Duplex stainless steel | US735 / US7020 | AC630M / AC530U | Lower speeds required; prioritize toughness over absolute hardness. |
| Swiss-type automatic lathe | MP9015 | AC610M | Both perform well; select based on coolant type and bar feeder consistency. |
Practical Recommendations
Based on the technical analysis above, the following operational guidelines will maximize performance with either manufacturer’s grades:
- Coolant Strategy: Always apply high-pressure flood coolant (minimum 70 bar for internal coolant tools) to reduce thermal cycling and flush chips from the cutting zone. Water-soluble coolant with 8–10% concentration is standard for stainless steel.
- Depth of Cut Discipline: Avoid shallow skim cuts below 0.1 mm on work-hardened surfaces. If a finish pass must follow a roughing operation, ensure the depth of cut penetrates fully beneath the work-hardened layer.
- Entry Angle: Use a 93° or 95° tool approach angle for external turning to distribute heat across a wider section of the cutting edge and reduce notch wear at the depth-of-cut line.
- Feed Rate Consistency: Maintain steady feed rates. Variations below 0.05 mm/rev promote BUE and poor surface finish.
- Insert Corner Radius: For finishing, use inserts with a 0.4 mm or 0.8 mm corner radius. Larger radii (1.2 mm or 1.6 mm) improve edge strength for roughing but require reduced feed to maintain surface finish.
Conclusion
Both Mitsubishi Materials and Sumitomo Electric Hardmetal offer technically sophisticated carbide grade systems for stainless steel turning. The optimal choice depends on the specific balance of priorities in your application:
- Choose Sumitomo AC610M or AC630M when maximizing tool life and suppressing built-up edge are the primary goals, particularly in continuous high-speed cutting.
- Choose Mitsubishi MP9015 or US735 when edge sharpness, surface finish, and insert versatility across mixed applications are most important.
- For heavy roughing and duplex stainless steels, both Mitsubishi US7020 and Sumitomo AC530U are excellent choices, with AC530U offering slightly better mechanical shock resistance.
By aligning grade selection with the operational requirements—finishing versus roughing, austenitic versus duplex, continuous versus interrupted—machinists can achieve significant improvements in productivity, tool economy, and part quality.
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