"COATING" THE STRUCTURE OF METALS AT DEEP CRYOGENIC TEMPERATURES

Durability is the most important criterion when defining the quality of a tool steel. Cryogenic tempering of metals is becoming acknowledged as an effective method for increasing durability, or "wear life", and decreasing residual stress in tool steels. The process creates many benefits for steel, including an exceptional increase in durability and wear resistivity, generally exceeding 300% as the greatest benefit. Tensile strength, an increase in toughness and greater stability through the release of internal stresses are also created.

Deep cryogenics (below -300°F­) is creating new applications in science. High temperature superconductors, the super-conducting super-collider, cryobiology, magnetohydrodynamic drive systems for ships and low-temperature physics have all developed recently. The deep cryogenic tempering process for metals is now cost effective thanks to developments by 300 ­Below, Inc., a Decatur, IL company. The process is a one-time permanent treatment affecting the entire part, not just the surface. New and used tools alike benefit from the treatment. Steel surfaces receiving wear, such as drill bits, end mills, surgical scissors, bearings, racing engines, slicers and granulator knives, all benefit from the treatment. Completing the Heat Treating Process: Martensitic transformationn

A research metallurgist at the National Bureau of Standards states, "When carbon precipitates form, the internal stress in the martensite is reduced, which minimizes the susceptibility to micro cracking. The wide distribution of very hard, fine carbides from deep cryogenic treatment, also increases wear resistance." The study concludes, "...fine carbon carbides and resultant tight lattice structures are precipitated from cryogenic treatment. These particles are responsible for the exceptional wear characteristics imparted by the process due to a denser molecular structure, the resulting larger surface contact area, reducing friction, heat and wear."

New Findings

Metallurgists have been skeptical of the cryogenic process for some time because it imparts no apparent visible changes to the metal. They often stated that proper heat treating would change 85% of the retained austenite to martensite. The deep cryogenic process only transformed an additional 8 - 15%. Therefore, deep cryogenic treatment was an inefficient process.

Those are true statements, with an inaccurate conclusion. The subjected metals also develop a more uniform, refined microstructure with greater density. These particles were known but never quantified scientifically until recently. "Carbide fillers" are precipitated as a result of the deep cryogenic processing. In the study below, the carbides tripled in the structure. The carbides fill the open spaces, or micro-voids, resulting in a much denser, coherent structure of the tool steel. The end result is increased wear resistance. These particles are identified and counted in the study using a scanning electron microscope with field particle quantification (an automatic particle counter). It is now believed that these particles are largely responsible for the great gains in wear resistivity. Unlike coatings, the change created is uniform throughout and will last the life of the tool regardless of any subsequent finishing operations or regrinds. It is a permanent, irreversible molecular change.

The cryogenic cycle is an extension of standard heat-treatment and creates an increase in durability. For example, a major aircraft manufacturer testing deep cryogenics found that, with only six different tools subjected, the savings in tool purchases could exceed $5 million. Also, an Arizona State University study used C-2 deburring tools and achieved a 400% improvement based on weight, after five cuts of 0.003" (0.007 cm) on Inconel alloy 718 from deep treating treatment of an 8% cobalt end mill showed improvement in two ways. The number of milling cuts was increased from three cuts before deep cryogenic processing to 78 cuts after processing (26 times the wear life). Also, resharpening the end mills after deep cryogenic treatment required only one-third the amount of stock removal to restore the tool geometry. In another example, Rockwell, a major aircraft manufacturer, is running C-2 carbide inserts to mill epoxy graphite. They doubled their output after deep cryogenic treatment. In a second test milling 4340 stainless steel, they achieved a 400% improvement.

Process Developments

The deep cryogenic process has had an Achilles heel. It has been inconsistent. In the past, improvements to cutting tools would vary from little improvement to over a 1000% increase in useful life. The trick is in the processing. If a cutting tool is dropped in liquid nitrogen, the tool could shatter. Temperature changes must be controlled exactly for consistent results. Metals require an optimal cooling curve. Process with the wrong curve and the treatment will be ineffective. The computer solves the problem by duplicating the optimal cooling curve exactly, time after time.

The older cryogenic tanks did not have adequate control. Using them was like baking a cake in a wood-fired stove. New systems like the deep cryogenic tempering system Model 925 shown in the photo (penguin optional) are necessary to achieve consistent results. This enables the heat treater to improve the profit margin, the manufacturer of tooling to improve his product and the industrial consumer of perishable tooling to save on tool expense. Deep cryogenic tempering equipment is coming out of the laboratory and into the marketplace. Over 100 deep cryogenic processors are now used nationwide with consistent results.

Deep Cryogenic Tempering Machine with Liquid Nitrogen: The Cryogenic Tempering Process

The new machines operate with controlled dry thermal treatment. "Controlled" simply means that the process is performed according to a precise prescribed timetable. A computer acts as a process controller to operate the descent, soak and ascent modes. The material is cooled slowly to -317­°F, held for 20-60 hours then raised to +300­°F, and slowly returned to room temperature. The "dry" process prevents the metals from being subjected to liquid nitrogen and eliminates the risk of thermal shock. The inexpensive manufactured system is as simple as placing an item in the freezer and pushing a button.

The Cryogenic Industry

The process has been used in the U.S. in one form or another for some time. A small Massachusetts firm has been processing items for 12 years. The strings on a piano which was previously tuned every six months were treated. The piano has not been re-tuned for five years. Musicians who play guitar and violin swear the strings are brighter in sound Oscilloscopes confirm a shift after treatment. A firm in Michigan has been quietly processing with the method for 27 years. They also specialize in stress relief of the plastic material used in contact lenses, among other items. A cryogenic treating company in Phoenix treats many aerospace parts. Another processor in Ohio treats many carbide tools. The treatment is gaining acceptance nationwide.

While deep cryogenic processing is not a "magic-wand" which will extend the life of everything, most parts, such as reamers, taps, dies, broaches, drills, endmills, slicers and cutting knives, do respond consistently to the process, saving extra tool expense dollars for end-use customers. The process is also effective on parts with TiN coatings.