Thermal Cracking in Inserts: Coolant Prevention

Thermal Cracking: The Silent Insert Killer

Thermal cracking (also called comb cracking or heat checking) is a failure mechanism where a network of fine cracks forms on the cutting edge of a carbide insert, perpendicular to the cutting edge. These cracks propagate with each thermal cycle until they connect, causing pieces of the cutting edge to spall off. Thermal cracking is particularly insidious because it can destroy an insert in minutes and is often misdiagnosed as mechanical chipping or a defective insert.

The Physics of Thermal Cracking

Thermal cracking occurs when the cutting edge experiences rapid, repeated temperature cycling:

• Hot phase: During cutting, the insert surface reaches 600-900 C. The carbide expands thermally (coefficient of thermal expansion: 5.0-6.5 x 10^-6 /C for WC-Co).
• Cold phase: When coolant hits the hot surface, or when the insert exits the cut (in milling), the surface cools rapidly to 100-200 C. The surface contracts while the interior remains hot.
• Stress generation: The temperature gradient creates tensile stress on the surface (up to 800-1,200 MPa) that exceeds the tensile strength of the carbide (600-900 MPa for standard grades).
• Crack initiation: Micro-cracks form perpendicular to the cutting edge at intervals of 0.1-0.5 mm. These are the characteristic “comb” pattern visible under 10-20x magnification.
• Crack propagation: With each thermal cycle, cracks grow deeper into the substrate. After 1,000-5,000 cycles (which may represent only 2-10 minutes of cutting in milling), the cracks connect and material spalls from the cutting edge.

Conditions That Promote Thermal Cracking

Milling with Intermittent Coolant

Milling is the most crack-prone operation because each insert tooth alternately engages and exits the workpiece. When flood coolant is applied, the tooth is quenched on every exit:

• Temperature swing: A tooth in a face mill cutting steel at 180 m/min may reach 750 C during engagement and cool to 150 C during exit, a 600 C swing per revolution.
• Frequency: At 500 RPM with 8 inserts, each tooth cycles 500 times per minute. Cracks can appear within 3-5 minutes of cutting.

Turning with Intermittent Coolant Application

In turning, thermal cracking occurs when coolant is applied and removed cyclically:

• Operator practice: Some operators turn coolant on and off during the cut, or use a coolant nozzle that only covers part of the cut length. This creates temperature cycling on the insert.
• Cross-hole turning: When turning a shaft with cross-holes, the insert exits and re-enters the cut at each hole, creating natural thermal cycling even without coolant variation.

Interrupted Turning

Turning parts with keyways, splines, or flats creates periodic engagement/disengagement:

• Impact + thermal cycling: The combination of mechanical shock at each entry and thermal shock from coolant creates the most severe conditions for insert failure.
• Speed dependency: At low speeds (below 80 m/min), the temperature swing is smaller and thermal cracking is less likely. At high speeds (above 150 m/min), the combination of high temperature and rapid quenching is destructive.

Distinguishing Thermal Cracking from Other Failures

Thermal cracking has distinct characteristics that differentiate it from mechanical chipping or normal wear:

• Crack pattern: Multiple parallel cracks perpendicular to the cutting edge, evenly spaced at 0.1-0.5 mm intervals. Mechanical chipping produces irregular, random fractures.
• Location: Cracks appear on both the rake face and clearance face, near the cutting edge. Flank wear produces a smooth wear land, not cracks.
• Material loss pattern: When cracks connect, they create small, flat spalls (0.2-1.0 mm wide) rather than the larger, irregular chunks typical of mechanical fracture.
• Time to failure: Thermal cracking appears suddenly after a period of seemingly normal operation, whereas flank wear progresses gradually and predictably.

Prevention Strategies

1. Eliminate or Modify Coolant Application

The most effective prevention is to eliminate the thermal cycle entirely:

• Dry cutting: For milling of steel and cast iron, dry cutting eliminates the quench cycle entirely. The insert runs at a stable, elevated temperature (700-800 C) without thermal cycling. Modern PVD TiAlSiN coatings maintain hardness at these temperatures. Metal removal rates are maintained; cycle times are unchanged or slightly improved.
• Air blast: Compressed air (6-8 bar) directed at the cutting zone removes chips without creating a thermal cycle. Surface temperatures remain 50-100 C lower than dry cutting due to convective cooling, but without the quench effect of liquid coolant.
• MQL (Minimum Quantity Lubrication): Oil aerosol at 5-30 ml/hour provides lubrication without significant cooling. The small volume of oil evaporates instantly at the cutting zone, providing boundary lubrication without thermal shock.

2. If Coolant Is Required: Use Continuous, High-Pressure Delivery

If the application requires coolant (e.g., deep cavity milling, aluminum machining, or thread milling), ensure it is applied continuously:

• Through-tool coolant: Coolant delivered through the spindle and tool body reaches every tooth consistently, eliminating the dry-wet cycling that occurs with external nozzles.
• Flood volume: Minimum 20-40 liters per minute for face milling operations to ensure complete coverage of all cutting teeth throughout the rotation.
• Coolant temperature: Maintain coolant at 20-25 C (not chilled below ambient, which increases the thermal gradient).

3. Select Crack-Resistant Insert Grades

Some carbide grades are inherently more resistant to thermal cracking:

• Tough substrate: Grades with higher cobalt content (10-12% Co vs standard 6-8% Co) have higher fracture toughness and can accommodate more thermal stress before cracking. ISO P30-P40 and M25-M35 grades typically have higher cobalt content.
• Fine-grain carbide: Sub-micron grain carbide (grain size 0.4-0.8 um) has higher transverse rupture strength (TRS) than coarse-grain carbide, resisting crack initiation.
• PVD coatings: PVD-coated inserts (TiAlN, AlTiSiN) are more resistant to thermal cracking than CVD-coated inserts because PVD coatings are thinner (2-4 um vs 10-20 um) and introduce less residual stress in the coating-substrate system.
• Avoid CVD for interrupted cuts: CVD coatings have inherent tensile residual stress from the coating process (applied at 900-1,050 C). This residual stress combines with thermal cycling stress to accelerate crack formation. For milling and interrupted turning, PVD is the preferred coating type.

4. Optimize Cutting Parameters

• Reduce speed: Lowering the cutting speed by 20-30% reduces the peak temperature and therefore the temperature swing per cycle. This is the most straightforward parameter change.
• Increase feed: A higher feed rate moves the heat source away from the cutting edge (thicker chip carries more heat away), reducing the surface temperature of the insert.
• Reduce radial engagement: In milling, reducing radial engagement from 75% to 30-50% of cutter diameter gives each tooth more air time between engagements, reducing peak temperature.

Case Study: Face Milling 4140 Steel

A manufacturer milling AISI 4140 steel plates (280-320 HB) with an 80 mm face mill (6 inserts, SEKT geometry) experienced thermal cracking at 12-15 minutes per index using CVD TiCN/Al2O3-coated inserts at 180 m/min with flood coolant:

• Solution: Switched to PVD AlTiSiN-coated inserts, eliminated flood coolant (dry cutting with air blast for chip removal), and reduced speed to 150 m/min.
• Result: Tool life increased to 45-55 minutes per index (3-4x improvement). Surface finish improved from Ra 2.5 um to Ra 1.8 um due to elimination of spalled-edge marks.

Conclusion

Thermal cracking is a preventable failure mode caused by rapid temperature cycling at the cutting edge. The root cause is almost always the interaction between coolant application and intermittent cutting (milling or interrupted turning). The most effective solutions are eliminating coolant entirely (dry cutting with air blast or MQL), using continuous through-tool coolant delivery, or selecting PVD-coated, tough-substrate insert grades designed for interrupted cutting. In most cases, eliminating coolant also reduces cost and environmental impact, making the fix a double benefit.