Cryogenic Heat Treatment
Published on Feb 15, 2021
Cryogenic temperatures are defined by the Cryogenic Society of America as being temperatures below 120 0 K (-244 0 F, -153 0 C).
Durability is the most important criterion used to define the quality of a tool steel. Cryogenic treatment and tempering of metals has been ac- knowledge for almost thirty years as an effective method for increasing durability, or "wear life" and decreasing residual stress in tool steels.
Deep cryogenics (below -300°F) is creating many new applications in science. High temperature superconductors, the super-conducting super collider, cryo-biology, magneto-hydrodynamic drive systems for ships, and low temperature physics have all developed recently.
The deep cryogenic treatment and tempering process for metals is economical. It is a one time permanent treatment, affecting the entire part, not just the surface. The treatment may be applied to new or used tools, sharp or dull, and reshaping will not destroy the imparted properties. Benefits achieved from subjecting tools to this treatment include: increases in tensile strength, toughness, and stability through the release of internal stresses.
Cryogenic Treatment for Improved Properties
A research metallurgist at the National Bureau of Standards in Boulder Colorado, 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 and resulting larger surface area of contact, reducing friction, heat and wear." There have been skeptics of the cryogenic process for some time, because it imparts no apparent visible changes to the metal. Since proper heat treating can transform 85% of the retained austenite to martensite and the deep cryogenic process only transforms an additional 8 to 15%, the deep cryogenic treatment has been considered an inefficient process.
While these percentages are correct, the conclusion drawn from them is inaccurate. In addition to the trans- formation to martensite, the subjected metals also develop a more uniform, refined microstructure with greater density. Although known to exist, this type of microstructure was only recently quantified scientifically. Particles known as "binders" are coupled with the precipitation of the additional micro fine carbide "fillers". The fillers take up the remaining space in the micro-voids, resulting in a much denser, coherent structure of the tool steel.
These particles are identified and counted in the above study cited, using a scanning electron microscope with field particle quanti- fiction (an automatic particle counter). It is now believed that these particles are largely responsible for the great gains in wear resistivity. The permanent irreversible molecular change created is uniform throughout the tool, unlike coatings, and will last the life of the tool, regardless of any subsequent finishing operations or regrinds.
Deep Cryogenic Treatment Potential
The cryogenic cycle is an extension of standard heat-treatment, and creates many outstanding increases in durability. Some examples are as follows. A major aircraft manufacturer testing deep cryogenic treatment found that with only six different tools treated, the savings in tool purchases could exceed $5 million. An Arizona State study conducted by Laurel Hunt, used deep treated C-2 debarring tools on INCONEL alloy 718, achieving a 400% improvement based on weight, after five cats of .003 in. (.007 cm) on this alloy. This deep cryogenic treatment of an 8% cobalt end mill has made dramatic improvements in two important ways.
The number of milling cats was increased from three before deep cryogenic processing, to 78 cats after processing (26 times the wear life). Resharpening the end mills after deep cryogenic treatment required only 1/3 the amount of stock removal to restore the tool geometry. Rockwell, a major aircraft manufacturer, using C-2 carbide inserts to mill epoxy graphite, doubles their output after deep cryogenic treatment of the inserts.
In a second test, a 400% improvement was achieved upon milling 4340 stainless steel with cryogenic treated tool. Other applications include: Leading national stock car drivers who previously raced only 4-8 races between equipment teardowns, drove in 40+ races before teardown after cryogenically treating block, crank, cam, pistons and heads.
Process Advancement through New Equipment and Computerization
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 1000% increased in useful life. The key to effective improvements consistently is proper processing. If a cutting tool is dropped in liquid nitrogen, without tem- perature control, the tool could shatter. Metals require specific cooling rates; temperature changes must be controlled exactly to obtain the optimal cooling curve. The computer processor solves the problem, since it al- lows exact duplication of the optimal cooling curve, repeatedly.
The older cryogenic tanks did not have adequate controls. A relatively new cryogenic system (model 2953, lead illustration) achieves consistent results. The new cryogenic machines operate with controlled dry thermal treatment. "Controlled" simply means that the process is performed according to a precise prescribed time table. A process controller (Yokogawa UP 25) operates the descent; soak and ascent modes (see Fig. 5). Generally, the material is cooled slowly to -317°F, held for 20-60 hr then raised to +300°F, and slowly returned to room temperature.
The machine switches over to the electrical resistance mode for the tempering operation. The "dry" process prevents the metals from being subjected to liquid nitrogen, and eliminates the placing of an item in the freezer and pushing a button. A breakthrough in system insulation has been achieved as a result of the space program. The system is de- signed to accomplish thermal transfer, and the more efficient the better. It is essential to transfer themes from the liquid nitrogen to the metal parts being treated, without losing the therms to the outside.
Thus, the vacuum chamber is designed for three- level insulation. Walls of the chamber are 3 in. stainless steel. On the inside wall are 125 wrapped layers of aluminized polyester film. Inside the chamber it self is 2 in. polyisocyanurate high density foam which is coated with Ceramaseal amorphous vapor barrier comprised of micro spherical ceramic globes. A 93% increase in processing ability is accomplished with the "space shuttle insulation" in the vacuum walls, providing considerable savings in processing and making the treatment economical for a variety of items in addition to tooling.
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