Cryogenic Treatment of Steels and Gun Barrel Accuracy
Steel is a complex substance. It is not an element, like iron or copper, nor is is a compound, like water or salt. Steel is a "solid solution" of various elements and crystaline structures, frozen in place when it is cooled. There are two major types of steels, plain carbon steels and alloy steels. Plain carbon steels consist almost entirely of iron and carbon, with the carbon ranging from 0.1 to 2.3% by weight, while alloy steels have additional elements such as manganese, chromium, molybdenum, silicon, cobalt, tungsten, etc. Since each alloying element causes different changes to the structure of the steel, and since different combinations may act in unexpected ways, we'll concentrate on plain carbon steel.
When you heat a bar of steel to about 1400F, a transition occurs. The carbon and iron molecules form a face centered cubic (fcc) structure known as austenite. Austenite is non magnetic, and very soft and ductile. If you want to forge steel, you generally do it above this temperature where the steel is easy to work. When the austenite cools, the carbon diffuses out to form cementite particles, leaving behind layers of pure iron in a body centered cubic (bcc) structure known as ferrite. This layered structure of ferrite and cementite is known as pearlite (the layres make it appear iridescent at high magnification). Annealed steel is in the pearlite state, and it is soft and very tough, since the layers of cementite and ferrite slide over each other easily.
If the austenite is cooled very quickly, so that the carbon has no time to diffuse and form layers of pearlite, then we end up with a new structure. The removal of the carbon forms martensite, which is a tetragonal crystal, and free carbon. The carbon distorts the crystal lattice of the martensite, placing it under considerable stress, and this stress is what makes martensite hard and brittle. Martensite is an unstable structure, and aging the steel will allow the carbon to diffuse and form pearlite, although this can take thousands of years at room temperature. The diffusion is sped up considerably with increased temperatures; at a 1000F it happens almost instantly, and you're left with nearly pure pearlite. Plop a good, high carbon steel file (nearly pure martensite) into an oven at 600F, and you'll get a softer, tougher steel that makes a good knife blade, since a certain portion of the martensite will convert to ferrite and cementite.
The reason martensite is so hard is the residual stress from the rapid cooling. The tetragonal structure of the martensitic crystals is distorted by the free carbon, and the resulting stresses in the matrix make the structure resistant to deformation. For example, the method used in the making of Japanese katana blades: wrap the back edge of the blade blank in clay, heat it to austenizing temperatures, and quench in water; the exposed edge cools very quickly, resulting a file-hard edge of nearly pure martensite, while the back edge cools slowly, resulting in a tough, largely ferrite-cementite structure. The curve of the blade is a result of the cooling: martensite is a less dense form of the steel, so the contraction of the back edge of the blade is what gives it the curve. I've austenized a file and quenched one edge in water; the difference in length of the martensitic and ferrite-cementite edges warped the 6 inch long file about 1/8 of an inch. Heat the file back up, and the martensitic side contracts, the stress is relieved, and the blade straightens.
Cryogenic treatment claims to "improve" steels by converting any retained austenite to martensite. This is done by cooling the steel from the initial quenching temperature (room temperature) down to a very low temperatore (usually 77K, the temp of liquid nitrogen). The normal method is a slow cooling over a period of a couple of hours to the desired temperature, leaving it there for 16 to 20 hours, then bringing it slowly up over a period of several hours to 400K, and letting it cool to room temperature.
This statement goes against the previous description of the formation of martensite, which says that the transformation from austenite must be rapid to form the "solid solution" of martensite; if it is done slowly, then the carbon will have time to diffuse and form pearlite. The heating to 400K is a tempering stage, raising the steel above it's "quenching" temperature of 77K, and converting martensite to ferrite and cementite, which does toughen the steel but must also reduce the hardness (you can't have it both ways).
Link to a table showing quantitative results of 77K and 189K cryogenic treatments of various steels with respect to wear resistance. Note: This chart only concerns wear resistance, it says nothing about the hardness or toughness of the steel, which are far more important to a gun barrel.
The tool steels, which generally have high alloy content, exhibited a significant increase in wear resistance after the 77K treatment and a lesser increase after 189K treatment. Stainless steels showed a small improvement with cryotreatment, but the difference between the 77K and 189K was less than 10%. The plain carbon steel and the cast iron showed virtually no change in wear resistance with treatment. This is consistent with the theory that the cryogenic treatment deals with austenite, since the alloy steels tend to retain large amounts of austenite, and they form martensite when cooled at a much slower rate. For example, the A series of tool steels can be cooled down from the critical temperature over a period of hours and still form the hard structure; to soften an A series steel, it must be cooled over a period of days. The fact that carbon steel did not react to the cryo treatment and the stainless steels did implies that the cryo treatment relies on some factor of the alloy hardening process.
Rifle barrels are either stainless steel or a low alloy, medium carbon steel (Shilen uses 4140 chrome-moly and 416 stainless), and therefore would show minimal if any change in wear resistance.
Here's what Lilja, maker or world record holding rifle barrels, has to say about cyrogenic treatment:
"The cryogenic treating of barrels at a temperature of -300 degrees below zero [77K] has been a hot topic of discussion lately. Our short answer is that it will not harm your barrel but we are not completely convinced of all of the benefits claimed by some. The only benefits that we feel are likely to result from the treatment are possibly a longer barrel life and a slight increase in machinability."
Claims for increased accuracy through stress relief are not founded in our opinion. When barrels are button rifled no material is removed, it is just displaced. This causes stresses to be formed in the steel. If these stresses are not removed problems will result. These negative conditions include warping of the barrel during other machining operations, an increase in the bore diameter towards the muzzle end of the barrel during the contouring phase, and in the extreme, lengthwise splitting of the barrel. Also, if there are stresses remaining in the barrel they can be slowly released as a barrel warms up during firing. This causes the barrel to actually move during the course of shooting, causing inaccuracy."
In our testing we have found that the only effective means to completely remove the types of stresses introduced during rifling are with conventional heat treating using elevated temperatures. The -300 degree treatment alone will not remove these stresses. We have been told by a knowledgeable metallurgist that the deep cold treatment will, at best, remove up to 6% of the remaining stresses in the type of steel used for rifle barrels. The key words here are remaining stresses. In other words if the barrel was not stress relieved conventionally, then only 6% of the original stress will be removed. If the barrel has been treated conventionally with heat and then brought through the -300 degree cycle, up to 6% of any remaining stresses could be removed by the cold treatment. We do know through our testing that the cold treatment alone will not remove any significant amount of stress and that the problems outlined above concerning stress will remain in the barrel."
So, because of the very limited amount of stress that could be removed with the cold treatment (if the barrel has been properly stress relieved with heat as our barrels are) we do not believe that there can be much if any accuracy benefit to the -300 degree treatment of our barrels. It is for these reasons that we feel the cold process has very little potential for increasing the accuracy of our barrels. In our opinion, other than the removal of these stresses, there are no other mechanical factors involved that could benefit accuracy in a rifle barrel, resulting from a heat treating operation, either hot or cold."
For reasons not completely understood however there may be an increase in the wear resistance of the steel. This type of wear however does not contribute greatly to barrel erosion. We invite you to read our comments on this type of barrel wear in the question regarding the use of moly coated bullets."
Another possible side benefit to the freezing process is a slight increase in its machinability."
Post Script: Since I originally wrote this an excellent article by Kevin Thomas of Sierra Bullets was printed in the September, 1998 issue of Precision Shooting magazine. Mr. Thomas found, in a controlled test, that there was little benefit to deep freezing match grade barrels. He could see no difference in accuracy but probably a slight increase in useful life. I would encourage anyone interested in this subject to take a look at this article."
And from the section on moly coated bullets refferred to above:
"The key to this type of erosion is that it is caused by hot powder gases under high pressures and not by friction between the bullet and the barrel. We have read a report from a military test that examined this type of barrel wear. It was found that over the course of tens of thousands of rounds the actual groove diameter of the barrel was only increased by a few ten thousandths of an inch. It is this type of wear that moly might prevent or slow down. But in this test the throat area grew progressively longer and larger in diameter from gas erosion, not friction between the bullet and barrel."
And from Shilen, another world class rifle barrel maker:
"If you have heard that the cryogenic treatment stress relieves steel, this is false. We have measured the residual stress in 4140 and 416 steel with a process called x-ray diffraction. After much R&D, we have not been able to measure any changes in molecular stress after cryo treatment. For this reason we do not endorse the cryogenic process, but we can safely say that it is not detrimental to the barrel either."
Keven Thomas did a comparison of three identical rifle barrels, two of which were cryogenically treated and one of which was not, with comparisons of groups before and after treatment. His test, refferred to by Lilja, showed no change in accuracy in the cryogenically treated barrels.