Cryogenics, or the behavior of materials at very low temperatures, has proven to have extremely useful applications across multiple disciplines. From cryonics to cryosurgery, cryotronics, and cryopreservation, low-temperature conditions enable scientists to carry out tasks otherwise impossible at higher temperatures.
Understanding the Demand
In manufacturing, high temperatures are the bane of machine operators whose cutting tools are softened and deformed by heat caused by friction and energy loss. Bob Warfield writes that at room temperature, “carbide tools have a hardness of 1600-1800 kg/mm^2. By 900C that’s dropped to about 1/3. [As] the tool softens, it rapidly loses its cutting edge and is soon rendered useless.”
Liquid coolant reduces friction, slows tool wear, and speeds up cutting, but usually involves harmful chemicals and oily residue; this can contribute to slip-and-fall accidents).
Cryogenic machining refers to the use of liquid nitrogen as an alternative coolant for tools. Apart from the need to handle the -321°F (-196°C) liquid with gloves and aprons, cryogenic machining has a number of key advantages including:
- Speed improvements between 40% and 400%
- Better cycle times when machining harder materials, including steel and iron harder than 45 HRC, heat-treated powdered metals, hard-metal matrix composites, medical alloys such as cobalt-chromium (where it’s important to avoid contamination from metal cutting fluids), aerospace superalloys, and polymers and plastics that machine better when frozen
- Improved environmental sustainability than chemically-complex coolants. LN2 evaporates into nitrogen (nontoxic and largely inert) and eliminates the need for post-machining coolant clean-up
- A smoother finish, better surface and subsurface hardness, improved wear resistance, and higher compressive residual stresses.
Different Types of Machining
Cooling through cryogenic machining can take two forms:
- Convective cooling: heat removal due to the temperature difference between the coolant temperature and the cutting zone, usually by jetting LN2 onto the cutter
- Conductive cooling: a “heat sink” effect due to the temperature difference between the hot cutting zone and the (cool) back end of the tool. Cooling the bulk tool temperature with LN2 can create a significant heat difference and substantial heat-sink effect.
CNC manufacturer Okuma aims to have no more than a 12-month ROI on its cryogenic machine kit, compared to an industry average for capital equipment purchases of 2–3 years. Nearly any machine, regardless of brand or type, can be retrofitted with "cryo” capabilities.