Basics of Cryogenic Metallurgy

Table of Contents
Introduction
Why Cryogenic Tempering is Important
An Overview of How Cryogenics Works
Alterations in Metal Structure
The Ideal Role of Cryogenic Tempering
Conclusions
Bibliography

Introduction

The thermal treatment of metals must certainly be regarded as one of the most important developments of the industrial age. After more than a century, research continues into making metallic components stronger and more wear-resistant. One of the more modern processes being used to treat metals (as well as other materials) is cryogenic tempering. While the science of heat treatment is well known and widely understood, the principles of cryogenic tempering remain a mystery to most people in industry. Information regarding this process is full of contradictions and unanswered questions. Until recently, cryogenic tempering was viewed as having little value, due to the often brittle nature of the finished product. It is only since the development of computer modeled cooling and reheat curves that the true benefits of cryogenically treated materials have become available to industry and the general public.

The purpose of this work is not to break new ground in cryogenic science, nor will it answer all of the questions surrounding this process. Rather, this is a condensation of much of the information available concerning the effects cryogenic treatment has on metal stucture, as well as an overview of the actual process involved in treating parts. Also included are theories and conclusions regarding the optimum use of cryogenic tempering on steels, costs and feasability notwithstanding. All information is as up to date as possible, having been gathered from various scientific and industrial databases (no books were harmed during the preparation of this treatise).

An Overview of How Cryogenics Works

Cryogenic tempering may be oversimplified into a process of chilling a part down to relatively near absolute zero and maintaining that condition until the material has cold-soaked. The temperature is then allowed to rise until ambient equilibrium is reached. The part may then be subjected to a normal tempering reheat, although this step is not always included in the process. The complexity of the process involves determining and achieving the proper duration for the cooling, soaking, and warming cycles. It is here that developments in computer modeling and controls have placed cryogenic tempering on the cutting edge of metal treatment. Scientists in provinces of the former Soviet Union typically disagree with western methods of cryogenic treatment, as tests there have revolved around unceremoniusly dumping parts into a flask of liquid nitrogen, removing them, and allowing the material to cool uncontrolled in ambient air. Predictably, reports of extended tool life have not been as favorable as those achieved using more tightly controlled processes (History).

Alterations in Metal Structures

An explanation of the effect of deep cooling on metallic structure requires a connection be drawn to the more standard elevated temperature treatment processes. When a metal (high carbon steel, for example) is heated, the increase in energy expands the molecules. A ferrite (iron) molecule, like everything else, is mostly empty space between tiny atoms. As the iron atoms separate, atoms of carbon present in the steel "fall" into the larger empty spaces, creating a condition known as interstitial solid solution. This hot, carbon enriched iron is known as austenite. Hardened steel is simply austenite which has been rapid quenched to trap the carbon atoms in solution. This hardening process is the first step in any thermal treatment of steel. Because the grain structure of austenite is normally unstable at ambient temperatures, simply quenching results in steel which is brittle and of little value to industry. If the steel is partially quenched and held at a certain elevated temperature (dependent on actual carbon content) austenite will re-form into a much more stable structure known as martensite. After the steel has reached thermal equilibrium at the martensite start temperature (Ms), it is allowed to cool slowly to promote martensitic transformation.

It is at this point that cryogenic tempering becomes important, especially in the hypereutectoid steels (above 0.83% carbon content). Martensite structures do not form at a constant temperature, rather the austenite is converted to martensite as the steel cools to ambient temperature. The temperature range for martensite formation is determined by the particular carbon content of the material. Figure 1 is a graph of martensite formation temperatures as they relate to carbon content. The ranges are not exact, they are merely intended to show the concept of martensite growth with cooling. As the graph shows, the martensite growth completion line (Mf) drops below 0C at approximately 0.7% carbon content. As most steel producing plants are considerably warmer than this, it is easy to see that the higher carbon steels cannot undergo a complete austenite conversion without artificial refrigeration.

Figure 1

The Mf line is extrapolated below 0C, but it serves to point out the major reduction in temperature necessary to completely transform austenite in the higher carbon steels. Obviously, the cryogenic process is key to continuing the transformation of retained austenite into martensite in hypereutectiod steels. A study conducted by the Polytechnic Institute of Jassy, Romania showed that cryogenic tempering of 0.83% carbon steel reduced the percentage of retained austenite from 42.6% to 0.9% when compared to normal tempering (Frozen Gears). It is this completion of martensitic transformation which gives cryogenically treated steel good hardness characteristics while still maintaining reasonable ductility (fracture resistance).

Electron micrographs of cryogenically treated steel reveal another phenomenon which is less easily understood. During the martensite transformation process (hot or cold), a certain amount of free carbon atoms will precipitate out of the interstitial solid solution. These atoms are grouped together by pressures exerted during martensite crystal growth. These tiny pockets of carbon are known collectively as carbides. Under the microscope, carbides appear as tiny lumps of coal wedged into the martensite grain boundaries. These carbides upset the uniform structure of the martensite crystals, and are a significant factor in the brittleness of hardened and tempered steel. Cryogenic treatment appears to have the effect of significantly reducing the size of these carbides. While roughly the same amount of free carbon is present, the cryogenic process seems to slow the development of these "lumps of coal", distributing the carbon atoms more evenlyand allowing a tighter overall grain structure with less voids (Fuerst). One hypothesis is that the very low temperatures inhibit the formation of covalent atomic bonds in the free carbon, preventing the larger carbide structures from forming. Figure 2 shows microscopic views of martensitic steel before and after cryogenic tempering. Note the more even carbon distribution and larger grain structure visible even at relatively low magnification (100X).

Figure 2

The Ideal Role of Cryogenic Tempering

As previously discussed, the transformation of austenite into useful martensite is dependent on two factors, carbon content and temperature. The steel must first be heated to a temperature sufficient to allow carbon atoms to enter into solution. This temperature must then be maintained until enough time has passed for a complete austenitic reaction to take place. This is known as soak time, and is dependent on the mass of the part being treated. Next, the part must be partially quenched down to the martensite start temperature for the particular carbon content of the steel. The part is then held at this temperature until thermal equilibrium is reached (the entire thickness of the part is the same temperature). Under ideal circumstances, the part would immediately be placed into the cryogenic chamber and cooled to the relevant Mf temperature for carbon content and maintained at that temperature until thermal equilibrium was once again reached at Mf. It is at this point that complete martensite transformation will have occured and the part may be allowed to return to ambient temperature at a rate which will minimize internal stresses.

Cryogenic tempering of room temperature steels is effective at transforming retained austenite, however, the resulting crystalline structure is not as uniform as martensite which has all formed at the same time, as in the above example. Integral cryogenic tempering is not cost effective for volume production parts, as the logistics of having a large cryo chamber in close proximity to a steel furnace would be interesting, to say the least. Interrupted tempering or tempering of room temperature steels are much more practical ways of obtaining a majority of the benefits of cryogenic treatment.

Conclusions

It is apparent that cryogenic tempering offers many benefits where ductility and wear resistance are desirable in hardened steels. These benefits extend to cast iron, aluminum, stainless steels, and other materials. While this paper discussed only the effect on high carbon steel, the concept of continuing alloy grain structure formation through temperature reduction applies to these other materials in the same fashion. While various experts dispute the benefits of time-at-temperature control, available research, along with a correlation with standard heat treating processes indicates that this control is the key to maximizing the potential of cryogenic tempering. As is the case with many scientific dicoveries, the cost factor limits the usefulness of this process in the production phase of the materials industry.

Bibliography

Fuerst, J.D. Cryotreatment, Panacea or Black Magic. 30 may 1996. On-line. Available from Netscape @ http://www-csa.fnal.gov/csa_bin/csa_spring96

Thornton, Peter A., and Vito J. Colangelo. Fundamentals of Engineering Materials. Englewood Cliffs: Prentice-Hall. 1985.

Frozen Gears. 2 Jan 1998. On-line. Available from Netscape @ http://www.duro-chrome.com/articles

History of Cryogenic Tempering. 3 Mar 1998. On-line. Available from Netscape @ http://www.pm300.com/history

Meng, Fanju, kohsuke Tagashira, Ryo Azuma, and Hideaki Sohma. Role of Eta-carbide Precipitations in the Wear Resistance Improvements of Fe-12Cr-Mo-V-1.4C Tool Steel by Cryogenic Treatment. ISIJ International. Vol. 34 , No.2 (1994). On-line. Available from Netscape @ http://www.duro-chrome.com/articles