Inconel 718 vs Inconel 625: Different Alloys, Different Machining Strategies
Inconel 718 and Inconel 625 are the two most widely machined nickel-based superalloys in aerospace, oil and gas, and power generation. While they share a common nickel-chromium base, their precipitation-hardening mechanisms and mechanical properties create distinctly different machining behaviors. Treating them as interchangeable is a costly mistake that leads to premature tool failure, poor surface integrity, and scrapped components. This guide details the metallurgical differences and provides material-specific cutting parameters.
Material Properties Comparison
| Property | Inconel 718 (Precipitation Hardened) | Inconel 625 (Solution Strengthened) |
|---|---|---|
| Hardness | 38-44 HRC (aged condition) | 20-28 HRC (annealed) |
| Tensile Strength | 1,240-1,380 MPa | 830-965 MPa |
| Yield Strength (0.2%) | 1,035-1,170 MPa | 415-550 MPa |
| Thermal Conductivity | 11.4 W/m·K (at 100°C) | 9.8 W/m·K (at 100°C) |
| Elongation at Break | 12-20% | 30-45% |
| Strengthening Mechanism | Gamma double-prime (γ”) precipitates | Solid solution (Mo, Nb) |
| Work Hardening Rate | High | Very high |
Inconel 718’s gamma double-prime precipitates create a material that is harder but somewhat more predictable in cutting behavior. Inconel 625, despite being softer, has an extreme work-hardening rate and high ductility that makes chip formation unpredictable and promotes notch wear on inserts.
Insert Grade Selection for Inconel 718
Inconel 718 demands a tough, heat-resistant insert with a sharp cutting edge. PVD-coated grades with AlTiN or TiAlN coatings perform best because they maintain edge sharpness under the extreme cutting temperatures.
| Manufacturer | Grade | Type | Application |
|---|---|---|---|
| Sandvik Coromant | S1125 / 1105 | PVD TiAlN | Finishing / medium turning |
| Iscar | IC907 / IC5400 | PVD TiAlN / SiAlON ceramic | General turning / high-speed finishing |
| Korloy | PC3625 / NC6110 | PVD / CVD | Finishing and medium cuts |
| Kennametal | KC5525 / KCS10B | PVD / SiAlON ceramic | Turning / high-speed machining |
| Tungaloy | AH725 / NS9530 | PVD | Heat-resistant alloy turning |
Insert Grade Selection for Inconel 625
Inconel 625 requires even tougher grades because of its higher ductility and work-hardening tendency. The insert must resist notch wear and built-up edge formation.
| Manufacturer | Grade | Type | Application |
|---|---|---|---|
| Sandvik Coromant | 1125 / 2135 | PVD TiAlN / CVD | General purpose / roughing |
| Iscar | IC807 / IC907 | PVD TiAlN | General turning and grooving |
| Korloy | PC3625 / PC215K | PVD | Finishing to medium cutting |
| Kennametal | KC5025 | PVD TiAlN | Nickel-based alloy turning |
Cutting Parameters: Turning Inconel 718 (Aged, 38-44 HRC)
| Operation | Cutting Speed (m/min) | Feed (mm/rev) | Depth of Cut (mm) | Coolant |
|---|---|---|---|---|
| Rough Turning | 25-40 | 0.15-0.25 | 1.5-3.0 | Flood emulsion 10% or HP coolant |
| Semi-Finish | 35-55 | 0.12-0.20 | 0.8-1.5 | Flood emulsion or HP coolant |
| Finishing | 45-70 | 0.08-0.15 | 0.2-0.8 | HP coolant preferred |
| Grooving | 20-35 | 0.06-0.12 | Groove width | HP coolant (70+ bar) |
Cutting Parameters: Turning Inconel 625 (Annealed, 20-28 HRC)
| Operation | Cutting Speed (m/min) | Feed (mm/rev) | Depth of Cut (mm) | Coolant |
|---|---|---|---|---|
| Rough Turning | 30-50 | 0.18-0.30 | 1.5-3.5 | Flood emulsion 10% or HP coolant |
| Semi-Finish | 40-60 | 0.15-0.25 | 0.8-1.5 | Flood emulsion |
| Finishing | 50-80 | 0.10-0.18 | 0.3-0.8 | HP coolant preferred |
| Grooving | 25-40 | 0.08-0.15 | Groove width | HP coolant |
Milling Parameters
| Parameter | Inconel 718 | Inconel 625 |
|---|---|---|
| Cutting Speed (roughing) | 25-45 m/min | 30-50 m/min |
| Cutting Speed (finishing) | 40-65 m/min | 45-75 m/min |
| Feed per Tooth | 0.05-0.10 mm | 0.06-0.12 mm |
| Radial Depth (ae) | 50-70% of cutter Ø | 50-70% of cutter Ø |
| Axial Depth (ap) | 0.5-1.0 × Ø | 0.5-1.5 × Ø |
| Recommended Cutter | Solid carbide AlTiN 5-flute | Solid carbide AlTiN 5-7 flute |
Critical Machining Strategies for Both Alloys
1. High-Pressure Coolant is Not Optional. Both Inconel alloys benefit enormously from high-pressure coolant (70-300 bar). HP coolant breaks chips, reduces notch wear, and lowers cutting zone temperatures. Shops that switch from flood to HP coolant routinely report 40-80% improvement in tool life.
2. Maintain Constant Engagement. Inconel work-hardens rapidly under the cutting edge. Every time the tool dwells or reverses direction, a hardened zone forms. Use trochoidal or adaptive tool paths in milling to maintain constant tool engagement and avoid dwell points.
3. Depth of Cut Must Exceed the Work-Hardened Layer. After each pass, the surface beneath the cut is work-hardened to a depth of approximately 0.1-0.3 mm. Your next pass must cut deeper than this layer, or the tool will rub against hardened material instead of cutting fresh stock.
4. Lead-Angle or Approach Angle Matters. For turning, a 45° lead angle (approach angle) distributes cutting forces more evenly and reduces notch wear at the depth-of-cut line. Standard 90° tools concentrate force at a single point.
5. Ceramic Inserts for High-Speed Finishing. SiAlON ceramic grades like Iscar IC5400 or Kennametal KCS10B can run at 200-400 m/min on Inconel 718 in light finishing passes (DOC 0.2-0.5 mm). This is 5-8× faster than carbide but requires a rigid machine and dry cutting.
Drilling Inconel
Solid carbide drills with through-coolant are mandatory. Use a cutting speed of 15-30 m/min and feed of 0.05-0.12 mm/rev depending on diameter. Peck drilling (G83) is essential — retract every 1-2× diameter to clear chips. Without pecking, chips will pack in the flutes and break the drill.
Summary
Inconel 718 and 625 are not interchangeable. The aged 718 requires lower speeds but can tolerate slightly higher feeds thanks to its precipitation-hardened structure. The annealed 625 allows somewhat higher speeds but demands aggressive feeds and HP coolant to manage its extreme work-hardening behavior. In both cases, PVD-coated tough grades like Iscar IC907, Sandvik 1125, and Korloy PC3625 are the foundation of successful machining.
hooguu.com supplies a comprehensive range of carbide inserts, end mills, and coolant-through drills optimized for Inconel 718, Inconel 625, and other nickel-based superalloys. Explore our heat-resistant alloy tooling section to find the right grades for your aerospace and energy applications.
Shop Related Products at HOOGUU
