Forgive me folks but how could I resist posting the first topic on the “Metallurgy: Heat Treating” forum of the ABS website. But I also wanted it to be as useful as possible to others so I hope this is helpful.

Quenching is one of the most common anxiety producing operations we bladesmiths do. Every other part of the process we have control over, but all of us hold our breath as our hard work goes into the fickle hands of fate in the quench, and pray it comes out at least as good as it went in. But due to its short duration many of us do not realize how much goes on in those incredibly complicated seconds when the blade is born. Careful study of this process can not only increase our understanding, but also alleviate some worry by giving us a little more control. Many years ago ASM produced a fantastic little book called “Quenching and Martempering”, that is the single most valuable tool I have encountered in taking control of the quenching operation. Obviously the greatest factor in our control is the quench medium we use, and here are just a few important words from that handy little book:

Agitation: Every industrial quenching process, most involving shapes much more complex than knives, uses some form of agitation in order to obtain even hardness and keep distortion to a minimum. Most makers fear distortion from moving the blade, but proper movement helps minimize this problem, just remember tip to tang or edge to spine, never flat to flat. Movement helps defeat the vapor jacket and greatly aids in heat extraction; think of it as artificial convection. Also by limiting the time any volume of oil is exposed to the hot steel oxidation is reduced, prolonging the life of the oil.

Clean blades: Ever had to remove a pesky black skin or ugly mottled stain from your quenched blades? Repeated exposure to heat and aging will lead to oxidation and breakdown of most oils, producing sludge and other negative effects. The oxidation can lead directly to staining of the surfaces of quenched steel, and the sludge will leave carbonaceous deposits.

Clean oil: Contaminants are big trouble for consistent, quality quenching. Scale, ash or dirt carried by the blades, soot or sludge from burning the oil or dust, and moisture from the environment, all drastically affect the abilities of a quenchant. Suspended particles interfere with the liquids natural convective and conductive properties. Continual alterations of chemistry will make consistent results all but impossible. Moisture is fine if you wish to quench with water but its presence in an oil turns normally minimal vapor jackets into a major problem, slowing the oils cooling ability. Always keep your quenchant covered when not in use.

Conduction: How quickly can heat move though the quenchant? To visualize the difference between this and convection, think of clamping the hot blade between two large blocks of metal, the heat moves out into the blocks despite the fact there is no physical movement of the materials. Does the quenching liquid move heat energy quite well?

Consistent results: Predictability is the hallmark of quality control, and the ability to produce the same results again and again are invaluable to heat treating since unseen variables are the most vicious gremlins in any bladesmiths shop. A well formulated and properly maintained quenchant is capable of producing the exact same results time after time, but if your quenchant is prone to chemical changes each quench can be a whole new adventure in trouble shooting. Years ago I developed a witches brew of a quenchant that for one month gave me the exact hamon activities I wanted in 1095, but it quickly went south on me and I was never able to get the mix exactly the same. Then I turned the formulation over to people who have labs and know-how to make it exactly the same every time, and adjusted my techniques to match a product that will also last much longer than a month without any change in behavior.

Convection: Localized heating of liquids results in movement from changes in density. The ability to move overheated liquid away from the blade to be replaced with fresh cool quenchant can be critical and is often connected to other factors such as viscosity, vapor points, preheat temperatures and agitation rates.

Cost: This is not about the cost of the oil but instead the total cost of producing the knife. Tools that cost even thousands of dollars are capable of reducing overall production costs enough to easily pay for themselves. Steel prices are on the rise and certain quenchants can limit your steel selection options. I quench primarily into low temperature salts, this limits my steels to more expensive alloys that has me feeling he financial pinch more than those with the freedom to work with simpler steels. Also, how much time or labor can be saved by blades which come from the quench as clean as they went in? The cost savings of eliminating warped and cracked blades, or redoing a blade that didn’t fully harden, is undeniable. Making a living from blades is tough enough without losing any in heat treating, careful selection of a quenchant to meet your needs can be a great boon to that pursuit.

Flash Points: We are thrusting red hot steel into flammable liquids after all. Every oil has a flash point, but some have been designed for our purpose with a much higher temperature at which its vapor will ignite. Bladesmithing is hazardous enough without dealing with vats of flaming oil in our face. Anytime an oil flashes its usefulness as a quenchant is drastically changed. This is particularly true of a quenching oil with a chemistry fine tuned to provide repeatable top performance, catching it on fire is a wasted investment. Really good quench oils use formulation to suppress volatile vapors and control this issue. My favorite fast quench oil technically has a very low flash point but, due to virtually no vapor formation, in all my years using it I have never had it flame in a proper quenching operation

Resitance to oxidation: All oils will break down and oxidize when exposed to the temperatures involved in quenching steel. This oxidation will result in sludge formation and unwanted changes in the quenching characteristics. A good quenchant will be designed to handle these conditions to provide a long life of consistent quality quenching.

Steel Compatibility: Do you work with a shallow hardening, water quenching steel or a deep hardening, oil quenching steel? Almost any steel in cross sections of less than ¼” will harden in a thoughfully matched oil, avoiding the risks associated with water. There are medium speed quench oils and there are fast quenching oils and your steel of choice will tell you what you need. O-1, L6, 52100 or 5160 are examples of deep hardening steels which would do best in a medium speed oil that will provide enough cooling to thoroughly harden them without unnecessary stress from excessive quench speeds. 1075, 1080, 1084, 1095, W2 and W1 are a few of the shallow hardening steels that need the quick cooling power of a fast quenching oil.

Vapor Formation: A good quenchant will resist forming heavy vapor jackets when exposed to hot steel, thus allowing quicker and more even cooling. Also the more vapor the quenching operation produces the more you have to breath it in your shop air. The liquid will not burn, you need a proper fuel/air mixture for combustion, and thus the quenchants vapor forming tendencies are critical in limiting flash fires.

Viscosity: Sluggish liquids will suffer from reduced convection and lower thermal extraction rates. Lower viscosity oils make agitation easier and will not cling to the blade, limiting waste due to drag out as well as a mess in the shop and on the blade. Properly preheating an oil can be important in increasing its effectiveness by reducing its viscosity. Most oils respond best from 80 F to 150 F. but be certain to find out the recommended working temperature for your oil since over 150 F. little more is gained in lowering viscosity and working above the oils range will significantly shorten its life.

For more on the topic see “Quenching and Martempering” by ASM (often on used book sites) and also the front of the single greatest book ever for a heat treater “The Heat Treater’s Guide” by ASM, contains a chapter that distills the topics down quite well.