Machining Maraging Steel 250 and 300: Aging Effects on Tool Life

Machining Maraging Steel 250 and 300: How Aging Condition Drives Tool Life and Insert Selection

Maraging steels occupy a unique position in high-performance machining. Grades 250 (18Ni-250) and 300 (18Ni-300) combine ultra-high strength (1700-2100 MPa after aging) with exceptional dimensional stability. Unlike conventional high-strength steels, maraging grades are machined in the soft, solution-annealed condition and then age hardened after machining. However, some applications require machining in the aged state for final sizing. This guide covers both scenarios with specific insert grades and cutting parameters.

Material Properties by Condition

Maraging 250 (AMS 6514):

• Solution annealed: 28-32 HRC, 900-1000 MPa UTS, excellent machinability (~50-60% of B1112)
• Age hardened (480 deg C, 3 hours): 48-52 HRC, 1750-1850 MPa UTS, machinability drops to 8-12% of B1112

Maraging 300 (AMS 6517):

• Solution annealed: 30-34 HRC, 1000-1100 MPa UTS, machinability ~40-55% of B1112
• Age hardened (480 deg C, 3 hours): 52-56 HRC, 2000-2100 MPa UTS, machinability 5-10% of B1112

The key alloying difference is higher cobalt (12% vs 8%) and titanium (1.4% vs 0.6%) in Grade 300, which produces more intermetallic precipitates and greater hardness after aging.

Insert Grade Selection

Solution-Annealed Condition (Preferred for Machining):

• Roughing: Sandvik GC4325 (CVD TiCN/Al2O3/TiN triple layer) or Kennametal KCP25B. CNMG 120412 with wiper geometry. These medium-to-tough grades resist flank wear in the relatively soft but gummy martensitic matrix.
• Semi-finishing: Mitsubishi MC6025 or Seco TP2501. CNMG 120408 geometry with 0.4mm nose radius.
• Finishing: Sandvik GC4315 or Tungaloy T9115. DNMG 150404 with polished rake face for chip control.

Age-Hardened Condition (Final Sizing Only):

• PCBN (primary choice): Sandvik CB7015 (50% CBN content, ceramic binder) or Kennametal KD050 (high-purity CBN). ISO geometry CNMX 120408 with 25 deg x 0.03mm T-land edge prep.
• Ceramic (high-speed finishing): Sandvik CC6050 (SiAlON) or Kyocera K620 (Al2O3-based). These allow Vc up to 200-350 m/min for light finishing passes with ap below 0.5mm.
• Fine-grain carbide (budget alternative): Sumitomo AC6105U or Mitsubishi VP15TF at reduced speeds. Expect 5-8 minutes per edge.

Cutting Parameters

Maraging 250 – Solution Annealed (30 HRC):

• Vc: 120-180 m/min for CVD carbide
• fn: 0.20-0.35 mm/rev roughing, 0.10-0.20 mm/rev finishing
• ap: 2.0-5.0 mm roughing, 0.3-1.0 mm finishing
• Tool life: 40-80 minutes per edge at 150 m/min
• Coolant: Standard flood emulsion at 8% concentration

Maraging 250 – Age Hardened (50 HRC):

• Vc: 80-150 m/min for PCBN, 40-60 m/min for fine-grain carbide
• fn: 0.08-0.18 mm/rev for PCBN, 0.06-0.12 mm/rev for carbide
• ap: 0.3-1.5 mm for PCBN, 0.2-0.8 mm for carbide
• Tool life: 20-40 minutes per edge (PCBN at 100 m/min), 5-10 minutes (carbide at 50 m/min)
• Coolant: High-pressure through-tool at 100-150 bar. Dry machining acceptable for PCBN finishing.

Maraging 300 – Solution Annealed (32 HRC):

• Vc: 100-160 m/min for CVD carbide
• fn: 0.18-0.30 mm/rev roughing
• ap: 2.0-4.0 mm
• Tool life: 30-60 minutes per edge

Maraging 300 – Age Hardened (54 HRC):

• Vc: 60-120 m/min for PCBN, 30-50 m/min for carbide
• fn: 0.06-0.15 mm/rev for PCBN
• ap: 0.2-1.2 mm
• Tool life: 15-30 minutes (PCBN), 3-7 minutes (carbide)

Aging Effects on Tool Wear Mechanisms

Understanding how the aging process changes wear behavior is critical for insert selection:

1. Before aging: The soft martensitic matrix produces long, stringy chips and primarily adhesive (built-up edge) wear. Polished rake faces and positive rake geometries reduce BUE formation. TiN-coated inserts perform well due to low chemical affinity with iron-nickel.
2. After aging: Intermetallic precipitates (Ni3Mo, Ni3Ti, Fe2Mo) act as hard abrasives. Dominant wear shifts to abrasive flank wear and crater wear. Al2O3-containing coatings resist crater wear. PCBN outperforms all carbide grades due to superior hot hardness.
3. Over-aged condition: If parts are over-aged (exceeding 500 deg C or extended times), coarsened precipitates reduce abrasiveness slightly but lower hardness. Tool life may paradoxically increase 20-30% while part strength drops below specification.

Milling and Drilling Guidelines

Milling (solution annealed):

• Use 5-6 flute carbide end mills, 12-20mm diameter, with TiAlN or AlCrN coating
• Vc: 80-120 m/min
• fz: 0.05-0.10 mm/tooth
• Radial engagement: 50-70% of diameter
• Axial depth: 1.0-1.5 x diameter

Drilling (solution annealed):

• Solid carbide drills with 135 deg split point, TiAlN coating
• Vc: 50-80 m/min
• fn: 0.08-0.15 mm/rev for 6-12mm drills
• Peck depth: 2-3 x diameter with full retract

Drilling (age hardened):

• Reduce speeds by 50-60% and feed by 30-40%
• Use shorter peck cycles (1 x diameter)
• Consider EDM drilling for holes under 3mm in aged material

Process Recommendations

1. Always perform bulk machining (roughing, semi-finishing, most drilling) in the solution-annealed condition. This maximizes material removal rate and tool life.
2. Leave 0.3-0.5mm stock on all surfaces for final finishing after age hardening. Account for dimensional growth during aging (approximately 0.03-0.05% linear expansion for both grades).
3. Age hardening at 480 deg C for 3 hours is standard. Air cooling is acceptable; furnace cooling minimizes distortion on thin-walled components.
4. After aging, finish only with PCBN or ceramic inserts. Carbide is technically possible but uneconomical for production volumes exceeding 10 parts.
5. Monitor for grinding burn if post-machining grinding is specified. Maraging steels are susceptible to temper colors above 300 deg C local temperatures, which indicate surface softening.