Chart for 1080 carbon steel
One of the most interesting areas of bladesmithing is the heat treatment. I only use carbon steels and do not have experience with more highly alloyed steels, I will share what I know, but encourage you to do your own homework. I would also suggest that you learn one steel and how to get the most from it before you add a new steel to your inventory. Whenever I add a new steel and whenever I change my process, I make test blades that are tested to destruction. Destruction testing is the only way you will be sure you know what your blades are capable of doing, it is educational and a lot of fun. I am self taught and am continually learning, if there are any glaring errors or omissions in this please let me know.
I like to forge iron and was a blacksmith for a very short time, but I put that aside when I moved up to steel. It is the hidden quality in steel that makes it so exceptional and fascinating. That quality is its ability to change it's hardness and physical characteristics by controlling the heating and cooling of the steel.
For this discussion, I will take you through the hardening process that I use on a high carbon steel blade, but first a few asides. When you place the steel in the fire it begins to gain heat. The steel will begin to give off visible color just above 900F it will continue to pick up color until it reaches a point where it seems to hang. It is still gaining heat, but it is undergoing an internal transformation from its cold structure into a metastable condition called austenite. This point at which it seems to hang is called decalescence and it represents the bottom of the critical temperature. It usually begins around 1335F in carbon steel depending on the carbon content.
Once it passes through this point, the crystal structure of the steel changes as the ferrite reacts with some of the carbide and begins to pool into austenite. As the temperature increases more of the austenite will begin to form in other places and continue until it reaches a point 10 or 15 degrees above the critical temperature where all of the ferrite should be consumed. At this point the steel should consist of austenite and undissolved carbides. The austenite grains start from a small nucleus and continue to grow until they impinge on other growing grains. The initial grain size is established at this point and if the excess carbide is in large quantities it will maintain this size with little increase, pinned by the carbide.
You can see this transformation if you watch the steel carefully and bring the steel up slowly. The Japanese talked about watching the shadows on the blade and quenching when the shadows turned to liquid. If you take the blade out of the fire at this point and watch the colors drop, you will notice a point where the steel will brighten even as it is cooling. On a tapered cross section like a knife blade it will appear to travel up from the edge to the spine of the blade. This is call recalescence and represents the transformation from austenite back to pearlite. After I am done forging a blade, I cycle the blade just above critical and down to dark heat at least three times. I watch for these two points to establish critical in my mind and to set up a very fine grain pearlite structure in the steel.
After reaching critical temperature, the steel should be fully austenized, but the carbides will continue to dissolve. It may be necessary to soak at temperature to fully dissolve all the carbides. In some steels it may be necessary to continue to raise the temperature for this to be accomplished especially in the presence of alloying elements that retard the transformation.
Once the steel is above critical and austenite, it may be quenched and hardened. The structure of the steel can be established by carefully controlling the time it takes the steel drop from critical through the various temperature sensitive points.
The structure and hardness of the steel is established by the rate of cooling from the austenitic condition. If brought down slowly the steel will be annealed and soft. The structure will be mostly ferrite and cementite, carbides. This can be done in a temperature controlled furnace by dropping the temperature through a known rate over a set period of time dependent on the type of steel. Another method is to preheat a heavy bar of low carbon to the same temperature as critical for the steel and bury both of them together in vermiculite. The vermiculite, obtained in bags from garden supply, is made from chipped mica and is an excellent insulating material. It will slow the cooling rate down so that the blade will still be hot to the touch the next day. For most of the carbon steels this will be enough to anneal the piece.
If allowed to air cool it will be normalized, a tougher condition comprised of fine pearlite and carbides. Blades can be ground and prepared for heat treatment in either normalized or annealed states. Another treatment that is particularly effective for workability and for dimensional stability is called sphereodizing. With the steel in a normalized condition you reheat, usually in salt to inhibit oxidization, to a temperature just below lower critical, 1300F and hold for at least an hour. What occurs is that the carbides will begin to aglomulate or pool into larger more evenly spaced particles in a ferrite matrix. It makes handfinishing much easier.
It is important to precondition your blades not only because it helps workability, but also to stress relieve the steel after forging. This will reduce chances of cracking and warping in the quench. It is helpful to think of the forging stage as the beginning of the heat treatment and to pay careful attention to the heats especially in the final forging. My last heats are always at critical. When the blade is finally shaped, I cycle the blade just above critical and down to almost black heat at least three times, cooling between by moving it back and forth in the air gently.
You have a lot of options when it comes to hardening carbon steel. Even the slightest change in alloy content can make a remarkable difference in the hardening characteristics of the steel, so I would again encourage you to study the steels you will be using.
The transformation temperatures and times are described using a chart that shows the Ae1 line, the temperature at which austenite begins to form and the Ms line, the temperature at which martensite starts to form from austenite.
Chart for 1080 carbon steel
The time line at the bottom of the chart is in seconds and side bars give temperature. This is called an "S" curve chart and it is very useful in determining the quench speeds for each steel. The top curve of the "S" is known as the nose of the curve. When quenching from critical, the temperature of the steel must drop below the nose of the curve within a precise amount of time in order for the steel to harden to martensite. In this case, it must get below 900F in under five seconds to form martensite.
If the steel is quenched to below the Ms, martensite will be the predominate structure, however if the blade is quenched to a point slightly above the Ms point, say around 500F and held until it has stabilized at that temperature, the steel has the promise to form martensite, but will not set up until it drops below Ms. This is called marquenching and is commonly used because it is less stressful particularly in difficult cross sections like we encounter in knife blades. When the blade is removed from the quench it is still above the Ms point and has very unusual properties. It can be easily bent or straightened and is still quite soft. As it cools however, it begins to setup martensite and will harden at room temperature. Again, you need to look at the chart for each steel you will be using because the Mf, or martensite finish point can be well below room temperature on some highly alloyed steels. These steels benefit from sub zero quenching because the colder temperatures are necessary to complete the austenite transformation and to reach the martensite finish. Care must be taken that the blade is not chilled by placing on a cold surface or even by being placed in a breeze or draft. The safest method is to allow it to cool in still air. The blade should be tempered after it has cooled to the point where it can be handled with bare hands.
If the steel is quenched from Ae3, critical, to a point between the Ms and the nose of the curve, say 600F and held at temperature for a long time, the austenite will convert to banite. Banite is a much tougher structure than martensite and will maintain the hardness of the steel as tempered to that temperature. This process requires a salt bath and good controls, but makes an really tough spring and is being used by some makers on steels like 52100.
The method of controlling the speed of cooling is the quenchant. The quench rate is determined by how quickly the quenchant can remove the heat from the steel. When a piece of hot steel enters the quenchant the area surrounding the blade absorbs heat from the blade until it is heated itself.
Water is a common quench especially in steels with low hardenability. It is fast and clean, but it is also severe especially with odd cross sectioned pieces like knife blades. If we look at what happens when steel is quenched in water it will give us an idea of how this process works. As the hot steel enters the water, the water immediately takes heat until it reaches boiling temperature. As the water boils, it forms a vapor jacket around the steel and unless it is circulated or flushed rapidly from the surface, this jacket will inhibit cooling. In thin cross sections this is not as much of a problem as it would be in thicker sections, but if not taken into consideration results can be spotty. I like to use rainwater for my water quenches because the mineral content of municipal or well water can affect the results of the quench.
A faster quench than water is brine. In some steels that have a low hardenability it may be necessary to go to a brine quench. Brine solution is made by adding salt, sodium chloride, to water. The backyard rule of thumb is to add enough salt to float an egg and can be varied from between 5% and 12% sodium chloride to water. The effect of brine on the quench is to make the water more efficient by precipitating on the steel and then blowing off very rapidly creating rapid agitation and disrupting the vapor jacket.
Polymer quenchants use a polymer additive to water. There are many kinds of polymer, POG, PAG etc, and each have particular attributes. The effect of the polymer is that you fairly precisely control the speed of the quench by varying the amount of concentration. Polymer quenchants are used in industry where the temperature of the quench and concentrations are tightly controlled and are most effective when used with induction or atmosphere controlled furnaces.
Oil quenches are slower than water based quenches, but in thin cross section will harden most knife steels. It is not as drastic a quench as water or brine and lessens the chance for cracking. Talk to any bladesmith and you will get a new formula for quenching oil. The safest approach is buy a known quenching oil. I like Tough Quench from Brownell's. It is fast, has high flash point and is reliable.
For marquenching and austempering you can use low temperature salts as a quenchant. These salts have effective ranges that make them suitable for this application and are not toxic like lead nor will they flash like hot oil.
There are a number of ways to bring the steel to temperature for heat treating. Expensive industrial or commercial heat treating equipment will not be discussed, but rather we will cover what is within the means of a small shop.
Perhaps the oldest method for heating and heat treating metals has been the charcoal fire. Charcoal is an excellent fuel because you can make an even heat over a long bed, it is a non oxidizing environment and is easily controlled. Charcoal cut to the right size chunks makes a soft bed and is not likely to bend or deform the blade as it is move in the fire. On the negative side however, charcoal is a fire hazard. It throws sparks, gives off intense radiant heat and is sometimes hard to put out, reigniting itself long after it has been doused. It is also a nuisance to make, expensive to buy and very messy.
Coal has been used for most of this century and I feel it is absolutely the worst fuel for steel that could be imagined. I do not recommend it for anything especially heat treating.
You can heat treat in an open environment propane forge. The atmosphere should be adjusted so that it is neutral or slightly rich. The heat will be radiant heat from the walls of the forge and invariably there will be hot and cool spots in the forge. Once you recognize the limitations, you can successfully heat treat in a propane forge. I have set up a digital controller on my long hardening furnace to insure that I do not over shoot the temperature, but still must rely on eye to judge the temperature of the steel.
For small blades it is possible and efficient to heat them with a torch. You need a large tip, adjusted to a slightly rich flame. This will show as a blue inner cone off the white hot point in the flame. Moving the flame back and forth over the blade will give an even heat. Be careful to shoot the flame at an angle to the blade so that no spot gets hotter than another. Another use for a torch is flame hardening of the edge. By moving the flame just along the edge at a consistent and fast pace, the edge will rapidly austenize and air harden. It requires practice and a good hand, but can yield consistent results and can be useful in some applications.
High temperature salts can be melted using electric or propane furnaces. The salts are usually held in a stainless pipe, but black iron will work also though are not as durable. Salts can be purchased in various temperature ranges and melting points and should be selected for the steels you will be working. You can purchase salts in small quantities from Jeff Carlisle. The advantage to working with salt bath is that it provides a uniform heat through out the container due to the natural convection of the liquid, it is non oxiding and no scaling will occur, and it provides a rapid transfer of heat to the steel. The temperature can be accurately controlled and a simple furnace can be easily built. It is the ideal solution for the small shop. I recommend buying some carbon stirring rods available through any casting or jewelry supply. If you stir your bath regularly it will help prevent decarb on your blades, a tip from Al Pendray.
There are several electric heat treating furnaces available for heat treating. These can be fitted with programmable digital controllers and are very accurate tools. Paragon makes furnaces designed for the custom knifemaker and even will custom build furnaces for specific applications. These are atmospheric, but can be adapted with by introducing inert gases to create a neutral atmosphere. These are a good out of the box solution and provide excellent control.
After the steel has been quenched and hardened to martensite, it will be hard, but also quite brittle. By reheating the steel toughness is added and also the hardness can be controlled. Tempering is a function of time and temperature and is greatly affected by the various alloys in the steel. A polished piece of steel will oxidize as it is reheated and begin to show colors, the higher the temperature the thicker the oxide layer and the darker the color. These have been used to judge the "temper" of the steel, but the color is also affected by the surface polish, the alloy elements in the steel and the duration of the tempering cycle and are not reliable indicators of the tempered condition of within the structure of the steel.
Careful control of the temperature and the time of the tempering cycle will give more reliable results. Each steel responds differently to tempering and you should make sample test pieces and test them thoroughly to find the best combination for the steel and job it is expected to perform. The purpose of tempering is to relieve the stresses built up during the transformation from austeninte to martensite and to control the hardness of the steel to suit the use of the tool.
Tempering can also trip retained austenite to make fresh untempered martensite. A common and effective practice is to double temper steel. The first tempering heat is for at least a one hour cycle and then a second heat is used to temper any fresh martensite that has been transformed.
A two to three hour drawing cycle is more effective than a shorter duration higher temperature draw. Care should be taken not to over heat the piece while tempering since it will soften the steel and diminish its potential.
Tempering may be done in almost any heat source that can be accurately controlled. The size of the piece to be tempered will often be the determining factor. For long pieces, low temperature salt is useful.
Iron Carbon Equilibrium Phase Diagram