What is Heat Treating?
Custom Heat Treating
Hardening Steel, Tempering Knife Steel
Annealing Knife Steels. Does It Really Matter?
What is heat treating steel all about? How does it happen? WHY does it happen? Why should it happen? We'll explore these and other questions in this post as I try to give you a lay person's understanding of some common heat treating terms and the various phases of steel. So dive in and let's get going!
Annealing is the opposite of hardening, as it softens steel, making it easier to grind, drill, saw, or perform other operations required to make a knife. Annealing also performs the function of stress relief, relaxing internal stresses created at the mill or in grinding/shaping your knife.
Annealing happens by heating the steel to slightly above its critical temperature and holding it there for a period of time. After this step a SLOW cooling is required, in the dozens of hours for some steels. All this time/temp is dictated by the alloying elements in the steel, and recommendations are provided by the manufacturer for each.
Choosing proper times and temperatures can also improve the grain structure, carbide distribution and size, and help set up the steel for the best hardening response possible. Most knife steels are delivered in some sort of annealed state, although some manufacturers do a better job that others at maximizing the properties of the steel. It is always in the best interest of a knife maker to run "test coupons" of each new batch of steel to determine the appropriate recipe for that batch.
There are other recipes to anneal steel that serve different purposes based on the alloy of steel in question, and the mechanical properties desired. The DET Anneal (Divorced Eutectoid Transformation) is great for setting up lower alloy carbon steels as a finely spheroidized carbide structure for a great heat treat response. The Temper Anneal helps refine grain size in higher alloy steels. The Subcritical Anneal heats the steel to BELOW its critical temp, and then is removed from the furnace to cool in still air.
Normalizing most often happens after forging or heavy grinding/machining operations on simpler CARBON steels. Air hardening steels like stainlesses and other high alloy steels do not typically benefit from this step. This happens at higher temperatures than annealing, and is used primarily to break up coarsely spheroidized carbides enlarged in the forging process or by poor annealing processes at the foundry. After soaking at this higher temperature for an appropriate time, the steel is removed from the furnace and allowed to cool in still air.
With simple carbon steels a grain refinement cycle is a good idea after normalizing, where that elevated temperature of can cause grain growth, requiring this cycle. This refinement happens when you heat the steel into the lower range of its hardening window for a short soak of 15-20 minutes or so, and then cool the steel in still air. Some folks believe several of these cycles at progressively lower temps improves the microscopic structure of the steel, others believe repeated refinement cycles to be unnecessary. This process leaves the steel in a microstructure called "pearlite".
After grain refinement the DET anneal can further refine the microstructure of the steel and set it up for the best possible heat treatment response. This anneal require a controlled furnace, and this process is called a "Divorced Eutectoid Transformation" anneal. It involves heating the steel to the lower range of its hardening window for a period of time, then cooling in the furnace at 665F or less per hour down to 1200-1250F. This will place the steel in a very fine "spheroidized" state and further assures very fine grain in the final product. This is thought by many to be the best state to harden knives from.
The austenitizing, or "hardening" temperature of a steel is determined by the alloying and carbon content of the steel type, and can range from as low as 1425-1450F for some low alloy steels, to over 2100F for high alloy high speed tool steels. This temperature along with time changes the steel into a phase called austenite, and helps dissolve some of the carbides, providing carbon in solution to harden the steel matrix. Austenite is nonmagnetic and has a different crystalline structure than room temp steel. Carbides give up some carbon into the austenite to increase hardness upon quenching. High alloying percentages of chromium, tungsten, cobalt, vanadium, or molybdenum will raise the hardening temperature (austenitization temperature) and soak time at temp, to give these complex carbides time to dissolve and give up some carbon. Too high a temperature and you risk grain growth, reduced toughness, and lowered stain resistance. Too low a temp and the steel won't fully harden.
After the steel has reached the proper temp and soak time in the oven, it is time to "quench". Quenching is a rapid cooling to room temperature from the high austenitization temperature. Depending on the steel this can be as harsh and severe as plunging into warm water or brine, oil quenching in an engineered quench oil, clamping between aluminum or steel plates, or for air hardening steels, simply sitting or hanging in still air is sufficient. The object for each steel is to quench the steel at the appropriate cooling rate to lock in the hard martensite phase, without stressing the steel so much that it cracks, so the proper quenching media is very important. This again transforms the steel from the soft, ductile austenite phase to the harder, stronger martensite phase. This is what we've been working for all along!
After the quench, some steels do not fully convert to martensite at room temperature. Again we can blame alloying elements in this case. Some alloys lower what is called the Mf, or martensite finish temp. With enough of these elements in the composition, Mf falls to below room temperature or even below zero F. This is where liquid nitrogen comes in. A dip in LN2 for a few minutes to a few hours lowers our steel to -320F and guarantees a near full conversion of austenite to martensite in any alloy. The steel is now harder and stronger, sacrificing a small reduction in toughness for greater strength and hardness. Find out more about liquid nitrogen treatments HERE!
After cryo processing, the steel is allowed to warm back to room temp, then goes through the tempering process at a temperature just high enough to reduce internal stresses and soften the martensite a little to make it more ductile and tough. The tempering process is nearly always under 400F for the steels I use, always at least two temper cycles minimum, and always for two hours or so. This ever so slightly softens the steel to its final working hardness by allowing a small amount of carbon to precipitate out of the steel matrix as fine "precipitation" carbides. Tempering also reduces the extreme stresses created in the steel from the hardening and quenching processes. If all these steps are done at precise temperatures, for precise hold times, the steel will be the best it can possibly be, for a knife. The tempered steel will exhibit the maximum toughness and edge retention for the given alloy, have adequate hardness and other desired properties for its intended purpose, and make someone a fine knife!
There are MANY other ways to heat treat steels: surface hardening, case hardening, nitriding, and more. Absolutely none of these apply to knife making.
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Thanks for reading,