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Cryogenic Processing Cryogenics is invaluable for all cutting tools-carbide, high speed steel, coated or uncoated tools. The percentage of life expectancy increase is dependent on several factors. First and foremost is the heat-treatment process itself. Inexpensive tools purchased from industrial suppliers (known in the industry as bargain brands) are likely to have the worst content of retained austenite. In fact, that is a great way to experiment to see what these tools will do before and after cryogenic treatment. It's possible these tools may outlast better grades of tools and reduce the initial out-of-pocket purchase cost. A good example of this was an aircraft engine manufacturer that was using T15, TiN coated premium drill bits to drill a very tight tolerance hole in an aircraft alloy. They could get only 15 holes before losing tolerance. A very inexpensive, off the shelf, M3 drill bit was tried and it wouldn't produce one single good hole. But, after cryogenic treatment and resharpening to remove the thin film, this drill bit produced an unprecedented 200+ holes before it had to be resharpened. The savings was estimated at over $100,000 per year for just this one application.
Other factors that affect the results are the properly ground cutting angles. Different steels can require vastly different cutting angles, rake angles, clearances, etc. In addition, if abusive grinding or poor control of the grinding practices is performed, the tool cannot be expected to do its job with or without cryogenics. Speeds and feeds need to be interpreted correctly then and religiously used. Proper lubrication, machine tightness, tight work hold down, and all of the normal machining practices need to be observed. Once these basic factors are under control, then cryogenics can really be successful.
Not only do tools benefit, but often the product produced from the tool action will demonstrate improvement. Aluminum die castings, forgings, cast parts, jigs, fIXtures, etc., cryogenically processed to remove stresses will show reduced defor- mation or distortion during and after machining. This is a great machining aid when producing tight tolerance parts or parts that hold tight flatness specifications. It is not a good idea to process zinc or tin in a cryogenic process. Carbide inserts and carbide tools really respond well to the cryogenic process. Now, there is no martensite or austenite in carbide, so what makes the difference? The major difference is in the binder material that holds the carbide grains in place. Most of the problem associated with wear on carbides comes from the loss of carbide grains on the cutting edge caused by the binder not grasping the carbide as tightly as it could. The cryogenic process improves that holding strength of the binder material to keep the micro-grained carbide on the cutting edge in place. Cryogenics also acts as a stress relief from the high pressure sintering process under which carbide tools are created. Typically carbides tend to double in life once cryogenically processed. Would you like to cut your carbide cost in half?
Stamping tools and dies, punches, and cutting blades are among the prime items that show great results from cryogenic processing. The best way to process a new tool is to cryogenically process and finish grind after processing. Cryogenics doesn't make the components any harder or easier to grind. It has no effect on grinding. However, there will be a slight reintroduction of stress from the grinding process caused by the intense heat created at the nip point where the grinding wheel meets the part being ground. Many toolmakers are unaware that 3000°F (704°C) or more is introduced to the surface of a metal being ground. This freshly created stress layer can be relieved by a followup cryogenic process if you feel it's necessary. The only reason to perform this extra process is in the event the die has a section that is long and very slender, or a strange cutout configuration. Be mindful that there can and should be some growth after processing if there are large amounts of retained austenite. Even good heat treatment with small amounts of retained austenite will exhibit some growth (but it will be very minor). On tools that are already made and in service, the same above-mentioned things apply, but run the tool until resharpening is required, then cryogenically process and sharpen. Watch the growth. In the book Heat Treatment, Selection and Application of Tool Steels, there is a helpful hint in Chapter 20 that deals with extending tool life by retempering a tool after it has been in use. Specifically, information is included for stamping dies and plastic injection molds using a heat tempering to relieve stresses. It explains that the heat tempering process performs a "mild stress relief to the fatigue that builds up in a tool." However, cryogenics performs a nearly full stress relief and becomes a very valuable method to relieve later stress buildups. Using this information, a stress relief on active or critical
tools can be performed on a regular time table to reduce compressive stresses. This action can dramatically extend tooling life from the constant compressive forces of a stamping die or plastic mold, and is ideal for long run parts.
Another real benefit from cryogenically treating dies is the reduction of galling. Gall is caused by the buildup of scraped off metal by the jagged peaks of a surface. The valleys pick up the material and, like magnets, any added scraped material bonds to itself creating gall. Because cryogenically treated surfaces are smoother, the amount of galling is reduced. It still requires good die break in and stoning to reduce the peaks, but cryogenics really helps. Die casting dies also benefit from cryogenic processing, and the same precautions need to be taken as stated for plastic molds |
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