How to Increase the Hardness of CNC Parts?

Detailed Analysis of Hardening CNC Parts

1. Core Hardening Technologies

• Heat Treatment for Bulk Hardness:
  • Quenching & Tempering: For carbon steels (e.g., 1045, 4140), heating to 800–900°C (austenitizing) dissolves carbon, followed by rapid cooling (water/oil quenching) to form martensite (hard but brittle). Tempering at 200–600°C reduces brittleness while retaining hardness (30–50 HRC for 4140, ideal for shafts/gears). Critical parameters: heating rate (5–10°C/min), hold time (30–60 mins per 25mm thickness), and cooling rate (water quenching achieves ~50 HRC vs. oil’s 45 HRC).
  • Solution Annealing & Aging (Aluminum/Titanium): Aluminum alloys (6061, 7075) are heated to 460–500°C (solution annealing) to dissolve precipitates, then quenched and aged (120–180°C) to form fine precipitates, increasing hardness from 100–150 HV to 150–200 HV (7075 reaches 150–180 HRC-equivalent). Titanium alloys (Ti-6Al-4V) use β-annealing (800–900°C) followed by aging to boost hardness to 35–40 HRC.
• Surface Hardening (Case Hardening):
  • Carburizing: Low-carbon steels (1018) are heated in a carbon-rich atmosphere (900–950°C) to diffuse carbon into the surface (0.5–2mm depth), then quenched. Creates a hard surface (58–62 HRC) with a tough core (30–35 HRC), ideal for gears or shafts needing wear resistance and impact tolerance.
  • Nitriding: Parts (alloy steels, stainless steels) are heated to 500–550°C in ammonia gas, forming nitride compounds (Fe₄N) on the surface. Achieves 60–70 HRC with minimal distortion—suitable for precision parts like CNC tool holders or aerospace fasteners, as it avoids high-temperature quenching.
  • Carbonitriding: Combines carbon and nitrogen diffusion (820–880°C) for a 0.1–0.5mm hard layer (55–60 HRC). Used for small parts (e.g., bolts, pins) where thin, uniform hardness and low distortion are critical.

2. Hardening Process Workflow

1. Pre-Treatment:
   • Cleaning: Remove oils/coolants from CNC parts (ultrasonic cleaning with alkaline solutions) to prevent surface contamination during heating, which can cause uneven hardening.
   • Stress Relief: For complex parts (e.g., gears with thin webs), pre-heat to 600–650°C for 1–2 hours to release machining stresses, reducing post-hardening warpage by 40–60%.
2. Hardening Operation:
   • Quenching & Tempering (Steel):
     • Heat 1045 steel to 840°C (austenitizing), hold 30 mins (for 25mm thickness), quench in water (cooling rate >50°C/sec) to form martensite (60 HRC).
     • Temper at 350°C for 1 hour to reduce brittleness, achieving 35–40 HRC with balanced toughness—ideal for axles.
   • Nitriding (Stainless Steel):
     • Heat 420 stainless steel to 520°C, expose to ammonia gas for 20–40 hours. Nitrogen diffuses into the surface, forming chromium nitrides (65–70 HRC) with a 0.1–0.3mm case depth. No quenching avoids distortion, critical for precision molds.
3. Post-Treatment:
   • Grinding/Polishing: Remove oxide scales from hardened surfaces (e.g., using 120-grit grinding wheels) to restore dimensional accuracy. For tight-tolerance parts (±0.01mm), post-hardening grinding corrects minor distortion.
   • Inspection: Verify hardness with a Rockwell tester (HRC scale for steels, HV for aluminum). Check for cracks via dye penetrant testing—critical for high-stress parts like tooling inserts.

3. Material-Specific Hardening Strategies

• Carbon Steels (1018, 1045):
  • Low-carbon steels (1018, <0.2% C) rely on carburizing to achieve surface hardness (55–60 HRC) while keeping cores ductile (20–25 HRC)—used for gears, where surface wear resistance and core toughness are needed.
  • Medium-carbon steels (1045, 0.45% C) use quenching + tempering for bulk hardness (30–45 HRC), suitable for shafts or structural parts.
• Alloy Steels (4140, 4340):
  • Chromium-molybdenum steels (4140) harden to 50–55 HRC via quenching (840°C, oil-cooled) + tempering (200°C). Added alloys (chromium, molybdenum) improve hardenability, ensuring uniform hardness in thick sections (50+mm)—ideal for aerospace landing gear components.
• Tool Steels (D2, A2):
  • High-carbon, high-chromium D2 steel is air-quenched (1010°C) to 60–62 HRC, with low-temperature tempering (150°C) for abrasion resistance. Used for CNC cutting tools, dies, and molds requiring long service life.
• Non-Ferrous Metals:
  • Aluminum Alloys (7075): Solution heat-treated at 475°C, water-quenched, then aged at 120°C for 24 hours. Precipitates (magnesium-zinc phases) boost hardness to 150–180 HV (vs. 80 HV in annealed state)—used for aerospace brackets.
  • Titanium Alloys (Ti-6Al-4V): β-heat treatment (920°C, water-quenched) + aging (500°C) forms α+β phases, increasing hardness to 35–40 HRC—critical for high-strength, lightweight aerospace parts.

4. Hardness Requirements for Key Products

• Cutting Tools (End Mills, Drills): Require high bulk hardness (55–65 HRC) for wear resistance. D2 tool steel, hardened via air quenching + low-temperature tempering, ensures edges stay sharp during CNC machining of steel.
• Gears & Bearings: Need surface hardness (58–62 HRC) for wear and core toughness (30–35 HRC) for impact resistance. Carburized 1020 steel meets this via a 1mm hard case and ductile core.
• Aerospace Fasteners: Titanium (Ti-6Al-4V) parts use β-aging to reach 35 HRC, balancing strength (1,100 MPa) and weight—critical for fuel-efficient aircraft.
• Molds & Dies: Stainless steel molds (420) use nitriding to achieve 65 HRC surfaces, resisting plastic wear while maintaining precision (±0.005mm) for injection-molded parts.

5. Industry Applications & Hardening Needs

• Automotive: Transmission gears (1020 steel) undergo carburizing to 58 HRC surface hardness, with core toughness to withstand torque. Axles (4140 steel) use quenching + tempering for 35 HRC, resisting bending fatigue.
• Aerospace: Turbine blades (Inconel 718) use precipitation hardening (720°C aging) to reach 35 HRC, withstanding high temperatures (600°C+) and centrifugal forces.
• Tooling & Manufacturing: CNC machine tool guides (4340 steel) are induction-hardened to 55 HRC, reducing wear from sliding contact with workpieces.
• Medical Devices: Surgical instrument blades (440C stainless) are hardened to 55–58 HRC via quenching, ensuring sharpness during repeated use and sterilization.

6. Challenges & Mitigations

• Distortion: High-temperature quenching can warp thin parts (e.g., 1mm steel sheets). Use low-temperature processes like nitriding (500°C) or induction hardening (localized heating) to limit thermal stress.
• Brittleness: Over-hardening (e.g., untempered martensite in steel) causes cracking under impact. Control tempering temperature (e.g., 200–300°C for 4140) to balance hardness and toughness.
• Complex Geometries: Hard-to-reach areas (e.g., blind holes in gears) may harden unevenly. Use gas nitriding (uniform nitrogen diffusion) or localized induction heating for selective hardening.
• Non-Ferrous Limitations: Aluminum’s low melting point (660°C) prevents high-temperature quenching. Instead, use T6 aging (solution + artificial aging) to maximize hardness without melting.