Top Tips for Milling and Drilling High-Hardened Steels

Machining high-hardened steel presents unique challenges that can quickly wear down even the most experienced machinists.

These ultra-tough alloys with a Rockwell hardness of 45 HRC and above are incredibly abrasive, putting immense stress on cutting tools.

However, with the right techniques and tooling, you can successfully mill and drill these demanding materials with excellent surface finishes and tight tolerances.

Choose the right cutting tools.

The first and most crucial step is selecting cutting tools for high-hardened steels. Standard tooling won't provide the toughness and wear resistance needed to withstand the abrasive nature of these materials. Look for solid carbide end mills, drills, and other tooling with specialized coatings like titanium aluminum nitride (TiAlN) or diamond. These coatings significantly improve lubricity, heat resistance, and tool life.

Another option to consider is polycrystalline diamond (PCD) tooling, which offers unmatched hardness and abrasion resistance. While more expensive upfront, PCD can pay dividends through reduced tool changes and longer machining runs on challenging alloys. 

Optimize speeds and feeds. 

While it may seem counterintuitive, higher speeds and lighter chip loads tend to perform better on hardened steels than soft materials. The increased speeds help maintain higher cutting temperatures, allowing the workpiece material to soften slightly and improve machinability.

Additionally, use conservative feed rates to minimize stress on the cutting edges. Aim for feeds around 50–75% of your mild steel use. This prevents excessive rubbing and premature tool failure from edge chipping or fracturing.

Mind your cutting fluid.

Flood coolant is an absolute must when machining hardened steels. Not only does it lubricate the cutting zone and extend tool life, but it also prevents thermal shock that can crack or shatter carbide inserts. Use generous flow rates of a premium synthetic or semi-synthetic coolant designed for extreme-pressure machining.

If flood coolant isn't an option, consider minimum quantity lubrication (MQL) with a high-quality oil-based aerosol spray. MQL has proven quite effective, particularly for shallow passes and drilling operations.

Stabilize and support the workpiece.

Hardened steels readily transmit vibrations, which can quickly lead to chatter, poor surface finishes, and shortened tool life. Take extra care to support the workpiece properly and minimize deflection or vibration.

Use sturdy fixturing, maximize clamping forces, and leave as little unsupported overhang as possible. Adding supplementary support structures like adjustable jack screws can dramatically improve stability on long workpieces or deep cavities.

Furthermore, tuned mass dampers or damping devices near the cutting zone can help absorb vibrations before they amplify into harmful chatter. Actively damped tool holders are another excellent investment for milling high-nickel alloys and other vibration-prone materials.

Climb or conventional milling?

Conventional (up) milling is the go-to method for many machinists, providing a stronger cutting action and better accommodating entry into the workpiece. However, some studies have shown that climb (down) milling may be more favorable on ultra-hard materials above 55 HRC.

In climb milling, the cutting edges engage gradually with a lighter load, reducing the initial cutting pressure compared to plunging straight in. This can result in longer tool life, better finishes, and less risk of chipping or fracturing the cutting edges on that initial contact.

Tapping high-hardened materials 

Tapping is one of the most demanding operations when working with ultra-hard steels. Extremely rigid machine setups are required to prevent tap breakage or seizing in the hole.

For blind holes, consider using the "hole first" technique. In this technique, you drill a through hole slightly larger than the final tapped size. Then, a bottom-forming tap creates the thread depth you need. This minimizes side loading and tap breakage risks.

For through holes, spiral-point pipe taps offer an excellent solution. Their specialized geometry allows them to start in existing holes easily and re-start mid-thread without binding. When combined with a suitable rigid tapping head, pipe taps are far less prone to breakage in hardened materials.

Post-processing for hardened parts

Once the machining is complete, consider additional post-processing to enhance component performance and longevity. Options include low-temperature stress-relieving, cryogenic treatment, or controlled shot peening.

Stress relief helps remove damaging internal stresses induced by the machining process. Cryogenic treatment precipitates eta-carbides for improved wear resistance, and shot peening hardens the surface to boost fatigue life.

Conclusion

Machining ultra-hard, abrasive alloys will always be more challenging than mild carbon steels. However, you can achieve excellent results by employing the proper tooling, techniques, and strategies specifically optimized for these demanding materials.

Be patient, start conservatively, and thoroughly test different variables until you find the ideal combination for your particular job. With the right experience and persistence, even the hardest steels can be machined productively and profitably.