Cryogenic treatments of tool materials, metals, and alloys have been applied to various industries to improve wear resistance of the materials since the mid-1960s. The cryogenic treatment subjects the material generally below -280F and frequently below -300F for 20-60 hours.
The dry cryogenic process is precision controlled and the materials to be treated are not directly exposed to any cryogenic liquids such as nitrogen. Overall, all the treated materials retain their size and shape. Cryogenically treated materials with some occasional heat treatment generally improve hardness, toughness, stability, corrosion resistance and reduce friction. Some materials have shown little or no improvement after a cryogenic treatment. However high speed steels (HSS) and tungsten carbide have shown improved tool life in many industries and somewhat in the woodworking industry after a cryogenic treatment.
High-temperature corrosion/oxidation has been shown to be a major contributor to the wear of tungsten carbide when machining medium density fiberboard (MDF). However, a deep cryogenic treatment showed promise for increasing tool life. Cryogenic treatment has been successfully applied to die and high speed steel (HSS), ferrous alloys and may enhance tungsten carbide.
For all its advantages, however, deep cryogenic tempering is no panacea. In some cases it has produced sterling results. One manufacturer of titanium alloy parts reports that, after treating the M-42 twist drills it uses, the company needed 63 percent fewer of the tools to do the same work. In another instance, a 400 percent improvement in tool durability was achieved using cryogenically treated C-2 carbide inserts to mill 4340 stainless steel. However, other metals, such as T-2 tungsten HSS, have been left with little or no change after treatment. Even where deep cryogenic treatment has been shown to be effective, results have not been consistent.
Slowly cooling a tool steel to cryogenic temperatures and soaking it for several hours changes the material's microstructure. Almost all of the austenite (a soft form of iron) retained in the steel after heat treating is transformed into a harder form of martensite by deep cryogenic tempering.
A second result of a deep cryogenic "soak" is the formation of fine carbide particles, called binders, to complement the larger carbide particles present before cryogenic treatment.
One study by Randall Barren, Louisiana Polytechnic Institute, looked at how the changes brought about by cryogenic treatment affected steel's ability to resist abrasive wear.
The Barren study found that the martensite and fine carbide formed by deep cryogenic treatment work together to reduce wear. The fine carbide particles support the martensite matrix. These effects reduce abrasive wear but also form a solid state solution of carbides and phase changes which are less chemically reactive or refractory.
The cryogenic process enhances the conversion from one phase (austenite) to another phase (martensite), which is a common change in ferrous metals as a result of heat treating and now cryogenic treating. Tungsten carbide tool materials generally have a cobalt binder. Cobalt is next to iron in the periodic table as part of the VIII B group, has the same valences, and forms similar phases in crystalline structures. The tungsten carbide is a fairly stable and constant crystal structure under many conditions. Consequently, a cryogenic treatment of tungsten carbide tool material may have an effect upon the cobalt binder to enhance tool life. Since the cryogenic treatment hardens and toughens a HSS, the material is probably more chemically inert at high temperatures.
Previous results have shown that tool wear can be reduced by selecting tool materials, coatings, or treatments that are chemically refractory or less reactive. Cryogenically tested tungsten carbide was tested in a wood machining turning test with MDF to demonstrate that a cryogenic treatment may enhance tool life.
Turning tests were conducted on 3/4-inch-thick MDF with C2 tungsten carbide (WC -6 percent Co) tool material at 0.005 ipr and 550 rpm with a 15-degree rake angle and a 10-degree clearance angle. Three pieces of C2 were untreated and three pieces were cryogenically treated to -306F. Both samples were replicated three times. The turning test turned 20 disks at approximately 18,000 inches per disk for a total length cut of 360,000 inches.
The results indicate cryogenic-treated tungsten carbide wears less and slower than the untreated tungsten carbides. Some transformation or changes probably resulted from the cryogenic treatment. Also, the tungsten carbide (WC) crystal is a stable structure; therefore, the change probably occurred in the 6 percent cobalt (Co) binder.
Examination of the wear scar or zone was wider for the untreated carbide and showed more cobalt binder retention among the tungsten grains for the cryogenic-treated carbide. These observations tend to substantiate that some phase or structure transformation occurred in the 6 percent cobalt binder of the tool material. The transformation produced a more refractory structure of the binder.
Previous results have shown that the chemical degradation of WC -6 percent tool material was at least a two-stage process when machining MDF. Subsequent research has shown additional wear mechanisms occur. These mechanisms include sulfidation, halogenation, and oxidation. One dominant reaction is the chemical degradation of the cobalt binder at lower temperatures. At higher temperatures, oxidation of both the WC grains and cobalt matrix also occurs. The cryogenic treatment apparently reduces the chemical degradation of the cobalt matrix at the lower temperatures and perhaps also the WC at higher temperatures. Although the WC-cobalt/MDF system was the objective of this investigation, the results show the importance of chemical degradation of tool materials when machining dry wood as well as reconstituted wood products and perhaps many other materials. These results indicate cryogenic treating of tungsten carbide with a cobalt binder may enhance tool life. Additional CNC machining test results comparing cryogenically treated and untreated solid carbide router bits reinforce that cryogenic treating of tungsten carbide tools with a cobalt binder may enhance tool life. These tests were conducted at Purdue University and will appear in a subsequent article.
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