Cryo-Rifles: Deep Cryogenic Stress Relief

(Editor's introduction: In our December issue, page 24, Geza Nagy reported on a remarkable barrel improvement... a new match grade barrel from a top-flight maker just had not been shooting well at all... but once the barrel was "deep frozen" things changed dramatically for the better. The article elicited considerable reader response. The company president, Peter Paulin, was persuaded to author a more technical description for our readers.)

Residual stress is an important criterion when defining the accuracy of a rifle barrel. Deep Cryogenic tempering of metals is becoming acknowledged as an effective method for decreasing residual stress and also increasing durability, or "wear life" of steels. The process creates many benefits for steels, including an exceptional increase in durability and wear resistivity, generally exceeding 300%. Tensile strength, an increase in toughness, as well as greater stability through the release of internal hoop, compressive and tensile stresses are all benefits of cryogenic processing.

Residual stresses are inevitable in any given piece of steel. They are uneven, and located variously throughout the structure. Steel expands as it is heated, and stress impedes expansion. If the stress in a steel barrel is uneven, the steel will warp off axis as it is heated or cooled. Additional uneven stresses are induced from machining the barrel, and will further cause the barrel to expand in another direction as it heats or cools. Twisting the barrel on fabrication helps to equalize the stress to all quadrants, but does not eliminate it. Residual stress will therefore cause a barrel to progressively "warp an arc" as it heats from firing. A barrel placed in a vise and fired repeatedly will generally show an arc of round placement. The effect has a memory and will continue group after group of shots fired. The "loose cannon effect" is measured in microns at the barrel end, but the multiple of distance to the target amplifies the problem causing wide groups or patterns. The problem is not a crooked barrel, but one which moves on repeated firing.

Deep Cryogenics (below -300F) has created many new applications in science. High temperature superconductors, the super-conducting supercollider, Cryo-biology, magneto-hydrodynamic drive systems for ships, and low temperature physics have all developed through cryogenic research. How is Deep Cryogenic Processing effective relieving stress?

Stresses in steels are created by mechanical methods such as machining, boring and forming. Residual stresses are also created in castings or forgings as a result of differential cooling. Thermal stresses are created in steels after heat treating through the quench hardening process. An ice cube when dropped into a cup of hot coffee illustrates this effect. The heating creates expansive stress on the exterior of the ice cube while the core is still frozen. The result is stress shear or cracking due to the differing rates of thermal "growth" caused by the coefficient of expansion. Dropping a rifle barrel into liquid nitrogen would have the same effect, creating stress. Stress relief (the opposite effect) takes place when the entire mass is at an EQUAL temperature (core and surface) and cycled through a wide temperature range. If the rate of temperature change is slow enough, thermal compression and expansion take place equally from the core to the surface, releasing internal stresses. The result is a homogenously stabilized material. Understandably this process takes a long period of time, generally more than a day. Taking a mass to extremely low temperatures creates a very dense molecular state. Technically, absolute zero (-475°F) is the zero motion molecular state of mass.

Process Developments

The deep cryogenic tempering process for metals is now inexpensive and very cost effective due to recent developments. The process is a onetime permanent treatment, affecting the entire part, not just the surface. New and used rifles benefit from the treatment. Steel surfaces receiving wear such as: drill bits, end mills, surgical scissors, bearings, racing engines, rifle barrels, slicers and granulator knives, also benefit from the inexpensive treatment. New applications are being discovered regularly. Even saxophones and trumpets benefit with easier play and truer notes. Some music professionals are hailing the process as the greatest advancement for musical instruments in the last fifty years.

The Deep Cryogenic Processing previously had an Achilles heel. It was inconsistent. Improvements to steels would vary from little improvement to over a 1000% increase in useful life. The trick is utilizing strict controls in the processing. Temperature changes must be controlled exactly and the part subjected must be run with thermal stability, for homogeneous temperature changes throughout the metal. ie: the core temperature must be equal to the surface temperature for consistent results. Metals require an optimal cooling curve. Process with the wrong curve and the treatment will be ineffective or create stress. The computer processor solves the problem. The com

puter can be programmed to duplicate the optimal cooling curve exactly within 1

10 of one degree Fahrenheit, so the mass descends at an even, slow rate. The computer control is repeatable, time after time. A contractor for NASA in 1994 hailed the 300 Below, Inc. equipment as the most accurate thermal system they had ever seen. The same system was used to stress relieve an optical bench for a planetary probe. It works.

The older cryogenic tempering systems 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 (pictured below) achieve consistent results. Deep cryogenic tempering equipment is now coming out of the laboratory and into the marketplace. Over 50 Deep Cryogenic Processors are now used worldwide.

Deep Cryogenic Tempering Machine with Liquid Nitrogen.

The Cryogenic Tempering Process The new machines operate with controlled dry thermal treatment, "Controlled" simply states that the process is performed according to a precise prescribed time table. A 486 computer acts as a process controller to operate the descent, soak and ascent modes. The material is cooled slowly to -310F, held for 20-60 hours then raised to +310°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 now as simple as placing an item in the freezer and pushing a button.

Since Liquid Nitrogen is the largest processing cost, a cascade compressor system using electricity cools the chamber to -110F, where it seamlessly transitions to LN2 to continue the descent to -310F. Thermal transfer is what the half, making it economical for all kinds of items, not just rifles.

Completing the Heat Treating Process; Martensitic transformationn

Metals are affected by heat treating. These changes are called Phase Transformations and are named variously: Bainite, Austenite, and Martensite. Austenite is a softer form than Martensite. Cryogenic Processing will complete martensitic transformationn from austenite. 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 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 structure and resulting larger surface area of contact, reducing friction, heat and wear." New Findings

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 a university study from Jasy Romania, the carbides were shown to have tripled in the structure after Cryogenic Processing. The carbides fill the open spaces, or micro-voids, resulting in a much denser, coherent structure

of the steel. The end result is increased wear resistance. These particles were identified and counted 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. The change created is uniform throughout the steel unlike coatings, and will last the life of the steel, regardless of any subsequent finishing operations or regrinds. It is a permanent irreversable molecular change.

The following two 1000X Micrographs represent samples from an S-7 (tool steel) bar stock. The first is untreated S7. The second was deep cryogenically treated. The martensitic transformationn is readily apparent.

The Cryogenic Industry

The process has been used in Europe and Australia under the tradename CryoTouch, a BOC treatment. 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. Feature articles about Cryogenic Processing have been written in Popular Science, Omni, Wall Street Journal, Modern Applications News, Cutting Tool Engineering, Heat Treating and Circle Track Magazine.

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 company in Phoenix treats many aerospace parts. Another processor in Ohio treats many carbide tools. The treatment is gaining acceptance nationwide.

Set your sights, Hit your Target

Wind, temperature, vibration and personal skill are enough variables to keep you busy. A moving barrel should not be part of the equation. Among the properties which define the attributes of a high quality rifle, accuracy is the highest importance. Cryogenic Processing increases accuracy.