Machining MP35N: The Toughest Cobalt-Nickel Alloy

Machining MP35N: The Toughest Cobalt-Nickel Alloy in Production

MP35N (UNS R30035) is a multiphase cobalt-nickel alloy that earns its reputation as one of the most difficult metals to machine in the aerospace and medical device industries. With a nominal composition of 35% Co, 35% Ni, 20% Cr, and 10% Mo, MP35N delivers an extraordinary combination of tensile strength (up to 2600 MPa), corrosion resistance (superior to 316L in chloride environments), and non-magnetic behavior. Its work-hardening rate is among the highest of any engineering alloy, making every machining decision critical to tool life and part quality.

Material Conditions

Annealed condition: 250-300 HB, 800-950 MPa UTS, 55-65% elongation. This is the recommended condition for bulk machining.

Cold-worked and aged: 45-52 HRC (450-530 HB), 2000-2600 MPa UTS, 10-20% elongation. Machining in this condition is extremely challenging and generally limited to final sizing operations.

Machinability: Approximately 10-15% of B1112 in the annealed state, dropping to 3-5% in the cold-worked and aged condition. This places MP35N below even Inconel 718 and Rene 95 in machinability rankings.

Why MP35N Is So Difficult to Machine

Three metallurgical factors combine to make MP35N exceptionally tough on cutting tools:

1. Extreme work hardening: The alloy transforms from FCC (austenite) to HCP (epsilon martensite) under cutting deformation. Surface hardness can increase from 300 HB to 500+ HB within 0.1mm of the machined surface in a single pass. Each subsequent pass cuts into this hardened layer.
2. Low thermal conductivity: At 11 W/m-K (compared to 16 for Inconel 718 and 50 for carbon steel), heat concentrates at the cutting edge rather than flowing into the chip or workpiece.
3. High cobalt content: Cobalt in the workpiece chemically attacks the cobalt binder in carbide inserts, causing accelerated crater wear and substrate dissolution.

Insert Grade Recommendations

Roughing – Annealed MP35N:

• Sandvik GC1115 with Inveio CVD coating. The alumina layer resists crater wear from cobalt chemical attack. CNMG 120412 with -6 deg negative rake and 0.03mm T-land edge prep.
• Kennametal KC5525 (PVD TiAlN). Offers better edge toughness for interrupted cuts on splined or keyed components.
• Seco CP2501 (CVD Al2O3/TiCN). Excellent crater wear resistance for continuous turning.

Finishing – Annealed:

• Sandvik GC1105 (PVD fine-grain). DNMG 150404 with polished rake face to reduce built-up edge from the ductile matrix.
• Mitsubishi VP15TF. Provides good surface finish at moderate speeds.

Cold-Worked and Aged (45-52 HRC):

• PCBN is mandatory. Sandvik CB7020 (high CBN content, ceramic binder) or Kennametal KD250. CNMX 120408 with 25 deg x 0.04mm T-land.
• Ceramic: Kyocera KS6000 (SiAlON) for high-speed roughing at Vc 200-300 m/min with light cuts (ap below 1.0mm).
• Carbide inserts will fail within 1-3 minutes in aged MP35N. Do not attempt production machining with carbide above 42 HRC.

Cutting Parameters

Turning – Annealed MP35N:

• Vc: 18-30 m/min (CVD carbide), 25-40 m/min (PVD carbide)
• fn: 0.12-0.22 mm/rev roughing, 0.06-0.12 mm/rev finishing
• ap: 2.0-4.0 mm roughing, 0.3-0.8 mm finishing
• Coolant: High-pressure flood at 100-150 bar (higher than most superalloys due to extreme heat generation)
• Tool life: 5-12 minutes per edge at 25 m/min (significantly shorter than Inconel 718)

Turning – Cold-Worked and Aged MP35N (50 HRC):

• Vc: 50-90 m/min (PCBN), 150-250 m/min (ceramic)
• fn: 0.06-0.14 mm/rev (PCBN), 0.04-0.10 mm/rev (ceramic)
• ap: 0.3-1.5 mm (PCBN), 0.2-0.8 mm (ceramic)
• Tool life: 8-20 minutes (PCBN at 70 m/min), 3-8 minutes (ceramic at 200 m/min)

Critical Process Strategies

Never take light cuts: MP35N’s extreme work hardening means shallow passes (ap below 0.3mm) cut into the hardened layer from the previous pass. This destroys tool life. Always maintain depth of cut above the work-hardened zone depth, typically 0.5mm minimum for roughing.

Constant engagement: Avoid dwell, pause, or retract at the workpiece surface. Even brief contact without cutting creates a hardened dwell mark that damages the next engagement.

Rigid setups: Any vibration or chatter accelerates the phase transformation and creates localized hardened spots. Use heavy-duty tool holders (Capto C6 or larger), minimize overhang, and employ steady rests for shaft turning.

Chip control: MP35N produces long, tough, work-hardened chips that can wrap around the workpiece and tool holder. Use inserts with aggressive chipbreaker geometries (Sandvik -MH or Kennametal -HP). If chip control is impossible, program pauses for manual chip removal between passes.

Milling MP35N

Milling is significantly more challenging than turning due to the interrupted cutting action and thermal cycling:

• Use 4-flute solid carbide end mills with AlTiN-Si (HiPIMS) coating, 10-16mm diameter
• Vc: 12-25 m/min
• fz: 0.02-0.05 mm/tooth (slotting), 0.04-0.07 mm/tooth (side milling)
• Radial engagement: 3-5% of diameter (trochoidal), 30-50% (peripheral)
• Axial depth: 0.3-0.7 x diameter
• Tool life: Expect 15-40 minutes of cutting time for a 12mm end mill at 18 m/min

Indexable milling cutters should use round inserts (RCMT 10T3 or 1204) with tough CVD grades. Avoid square-shoulder cutters with sharp corners, which chip rapidly in MP35N.

Drilling

Drilling MP35N requires patience and rigid peck cycles:

• Solid carbide drills, 135-140 deg point angle, TiAlN coating, through-tool coolant
• Vc: 10-20 m/min (among the lowest drilling speeds for any engineering alloy)
• fn: 0.02-0.05 mm/rev for 6-10mm drills
• Peck depth: 0.5-1.0 x diameter with full retract
• Coolant: Through-tool at 50-80 bar
• Expected life: 5-15 holes per drill (8mm x 20mm deep)

For high-volume production, consider gun drilling or BTA (boring and trepanning association) drilling with carbide-tipped tools at dedicated deep-hole drilling stations.

Medical Device Considerations

MP35N is widely used in orthopedic cables, guidewires, and fasteners for implantable devices. Medical machining adds requirements:

• Surface finish: Ra 0.4 micrometers or better on bearing surfaces
• No embedded iron contamination (dedicated tooling that has never cut ferrous materials)
• Full traceability of insert edge counts per workpiece lot
• Clean-room compatible cutting fluids (vegetable-oil based MQL preferred over mineral oil emulsions)

Cost and Planning Summary

MP35N machining costs run 6-10x higher than 316L stainless steel. Key planning factors:

• Allow 4-6x tooling budget versus standard austenitic stainless programs
• Plan for 3-5 minute tool life per edge in roughing; stock extra inserts for each setup
• Machine in the annealed condition and cold-work/age after final machining where possible
• Factor 15-25% scrap rate during initial process development
• MP35N bar stock costs $80-120/kg; minimize roughing stock allowance through near-net forging