Selection of Coolants in CNC Machining of Hastelloy Impellers

Hastelloy is a high-temperature corrosion-resistant and mechanical nickel-based alloy widely used in aerospace, chemical, marine, energy, and other harsh (working condition) sectors. Due to its high strength, low heat conductivity, and work-hardening characteristics, Hastelloy tends to develop issues such as high cutting temperature, aggravated tool wear, and poor quality of the machining surface during CNC machining. Therefore, the selection of suitable coolants is necessary for improved machining quality, tool life, and stability in machining.

What is a Coolant?

A coolant is an industrial fluid medium used in machining, primarily to cool and lubricate the cutting zone and prevent overheating or corrosion of workpieces and tools. Coolants consist of water, oil, and functional additives, primarily divided into three types:

• Water-soluble Coolants: Based on water as the main base, with additions of rust inhibitors, lubricants, and preservatives, they have good cooling properties and moderate lubrication qualities, being the most common industrial coolant group.
• Oily Coolants: Made based on mineral oil or synthetic oil, they have excellent lubrication qualities, which are best suited for heavy-load cutting and accurate machining but with relatively poor cooling qualities.
• Synthetic Coolants: Oil-free and composed mainly of some chemically synthesized compounds, they have high cleaning ability and biological stability, suitable for machining conditions requiring high cleanliness.

Cooling Challenges in Hastelloy Machining

Hastelloy creates a number of cooling difficulties while machining, i.e.：

• Low Thermal Conductivity: Hastelloy has low thermal conductivity, with heat during machining diffusing at low velocities, leading to the accumulation of a very high temperature in the cutting zone, further aggravating tool wear and workpiece deformation.
• Strong Work-Hardening: At elevated temperatures, Hastelloy’s surface layer hardens rapidly, increasing cutting force, decreasing machining efficiency, and even tool edge chipping.
• Strong Tool Adhesion Tendency: Under elevated cutting temperatures, diffusion reactions easily occur between tool materials and Hastelloy, causing alloy materials adhering to the tool surface, increasing friction and promoting wear.
• Difficult Chip Removal: Due to its high viscosity and strength, Hastelloy chips adhere very easily to the tool, causing chip build-up and affecting the surface quality of the machined surfaces.

To eliminate these issues, coolants having high cooling, lubrication, and high-temperature stability are chosen to be the remedy.

Analysis of Coolant Types and Applicability

Depending upon the cutting behavior of Hastelloy, different coolants can be employed for different machining operations. Main types of coolants and characteristics are as follows:

• Water-soluble Synthetic Coolants: Based on water and synthetic additives, they provide effective cooling and average lubricity. Suitable for high-speed light-load machining, but their lubricity may be insufficient for machining hard alloys, which may affect machining stability.
  • Advantages: Effective cooling, easy to clean, economical, and environmentally friendly.
  • Disadvantages: Insufficient lubrication, not preferred for high-hardness or heavy-load cutting.
• Emulsified Coolants (Emulsified Oils): Mixed by oil and water in proportion, having both cooling and lubrication. Suitable for medium-load cutting operations, e.g., Hastelloy cutting at medium speed.
  • Advantages: Well-balanced holistic performance, suitable for various cutting conditions.
  • Disadvantages: Unstable emulsification, prone to bacterial contamination over a period of time.
• Pure Oil Coolants (Cutting Oils): Ideally used in low-speed cutting operations involving high loading like drilling and tapping. Lubricating in nature but poor coolants.
  • Advantages: Good lubrication, finish capabilities, and difficult-to-machine positions.
  • Disadvantages: No cooling, tends to smoke and char easily, puts considerable pressure on the environment.
• Minimum Quantity Lubrication (MQL): This cooling system provides lubrication by a small amount of high-pressure oil mist but with low cooling effect, not suitable for rough machining of Hastelloy.
  • Advantages: Environmentally friendly, reducing the consumption of coolant.
  • Disadvantages: Insufficient cooling, not suitable for conditions of excessive heat accumulation.

Coolant Performance Requirements

In machining Hastelloy impeller, coolants must possess the following indicators:

Coolant Application Methods and Control Strategies

Rational application of coolants plays a critical role in achieving thermal management and chip removal during impeller machining and has a direct impact on machining efficiency and tool life. Major application methods are:

High-Pressure Cooling System (HPC)

By employing a high-pressure pump (20–70 bar pressure), coolants are precisely sprayed on the tool-workpiece contact area, effectively and rapidly transferring cutting heat away while also removing chips. Such a system is particularly well suited to hard-to-cut high-speed machining of materials like Hastelloy, significantly reducing tool temperature and wear.

Through-Coolant Tool Channels

Internal vacancies are offered in the tool, and coolants are discharged right from within the tool in order to enhance cooling in compound shape and deep cavity regions so that effectively machining deformation and heat accumulation caused damage to the tool is avoided.

Minimum Quantity Lubrication (MQL)

By misting a little lubricant in the form of a gas-mixture, it achieves energy saving, environmental protection, and reduced consumption of lubricants. It is appropriate for high-environmental-demand and fine machining conditions but doesn’t impact much on rough machining of high-strength materials like Hastelloy.

Recommended Coolant Formulations and Application Suggestions

Conclusion

CNC machining of Hastelloy impellers places rigorous requirements on the choice of coolant. Coolants not only affect tool life and quality of machining but also directly relate to machining efficiency as well as system stability. By logically selecting coolant types and satisfying requirements of different machining processes, machining quality and Hastelloy’s machining performance can be remarkably improved. For machining Hastelloy impellers, water-based emulsified coolants or synthetic coolants with high performance, with suitable additives (preservatives, extreme pressure agents, lubricants), are recommended for use personally to satisfy the demands of corrosion protection, lubrication, and cooling in machining and ensure machining quality and tool life.