Simple Heat Treating for a High Carbon Steel Knife Blade – Part 1, by Steve A.

Modern civilization owes its existence in part to the early discovery that iron containing small amounts of carbon could be made much harder than other iron compounds. This substance, iron with between about 0.6 and 1.7 percent carbon and no other alloying elements, was the first predictably hardenable steel and can be referred to as “high carbon plain steel”. Such steels can be made as hard as a file and form the most basic group of tool steels, where tool steel is defined as steel that is able to cut softer steel. The range of these steels include the simple mixtures of iron and carbon we will discuss here as well as steels that exhibit very different behaviors from their older kin due to the addition of alloying elements such as chromium, cobalt, molybdenum, nickel, niobium, titanium, tungsten, vanadium, zirconium, and others that significantly change the properties of the steel.

Many manufactured products such as files, automotive leaf springs, and wood cutting saw blades are made from various alloys of high carbon steel. If you elect to use such scrap to make knives, chisels, gouges, or other cutting tools, first test harden a piece of the scrap steel to insure that it has enough carbon content to harden file hard when quenched. This testing will prevent an unwelcome surprise when you later unsuccessfully try to harden that particular steel.

Many hardenable steels can be shaped and taken to the desired degree of hardness by using relatively simple processes. Cutting tools, springs, and high strength machinings can all be made from appropriate new or used steels using these processes to anneal, shape, harden, and temper such items. Fully hardened steel can be tempered using different times and temperatures to have the properties needed for fasteners, tools, springs, knives, and myriad other items. We will concentrate on applying this information to the making of a simple but very serviceable knife.

Implicit in this work is a source of high temperature heat such as an oxy-acetylene torch or a coal/charcoal forge. Indeed, blacksmiths were well known for recycling high carbon steel scrap into tools for their own and other’s use. These craftsmen had a practical understanding of the processes used to work high carbon steel. The need for high temperatures immediately mandates cautions such as proper clothing, fire protection, and ventilation before starting. All temperatures will be given in degrees Fahrenheit.

A little theory will help us understand how simple high carbon steel can be changed by the actions of heating and cooling it. Imagine a cube. On each of the eight corners of the cube is an iron atom. In addition, there is an iron atom in the center of the cube for a total of nine. This “unit cell” is the smallest grouping of iron atoms that forms the basis for the crystalline structure of iron. It is referred to as “body-centered cubic” structure, a cube with an iron atom in the center of its body.

Each unit cell shares its corner atoms with the unit cells that surround it. Body-centered cubic is the form iron has at temperatures ranging from below freezing up to approximately 1400 degrees F. It also is the form associated with the magnetic properties of iron. When steel is heated to the” critical temperature” of around 1400°, the heat makes the iron atoms more active. The iron atoms reorganize as the metal expands. The central atoms leave their positions and new locations are established at the center of all faces of each unit cell in the red-hot metal. This reorganization means each unit cell and all the surrounding unit cells now effectively have fourteen iron atoms each, one at each corner of the cube and one in the center of each face. These atoms are, of course, shared with all the adjoining cubic cells. The number of atoms has not changed, but the organization of the crystalline structure has. This form of iron is called “face-centered cubic” and is not magnetic.

In simple tool steels, there is carbon present. This takes the form of iron carbide, one carbon atom and three iron atoms. As the body-centered cubic iron is heated to the critical temperature, the carbon slowly dissolves in the iron. When the iron shifts from body-centered cubic to face-centered cubic, the spaces between the atoms enlarge. The carbon atoms move to these spaces, called interstitial spaces. This form of red-hot iron and carbon is called “Austenite.” Austenite does not have the magnetic properties of body-centered cubic iron; hence, a loss of attraction between a magnet and a piece of heated steel signals that the “critical temperature” has been reached where the unit cell of iron has become face-centered cubic, and carbon atoms can enter the now larger spaces between the iron atoms. This diffusion of carbon atoms among the iron atoms takes time, so the steel must be held at the critical temperature or slightly above for a few minutes before the carbon can migrate uniformly. If the iron is slowly cooled from the austenitic state, the carbon atoms are pushed out of the austenite, and the iron returns to a body-centered cubic state. The resulting mixture of iron carbide and soft iron is called “pearlite.” This is a very soft, readily machinable state.

If a previously hardened carbon tool steel is heated to the critical temperature and then cooled slowly, it will likewise become soft, and is said to have been “annealed.” If, however, the austenite is cooled down from the critical temperature rapidly enough by being quenched in a suitable liquid, the interstitial carbon atoms are trapped in between the iron atoms as the iron atoms quickly move closer together. This sets up a high degree of stress and hardness in the steel. This solid solution of carbon and iron is now in a body-centered tetragonal structure called “martensite.” Under a microscope, the martensite appears as a mass of fine needle-like structures. The neat rows of cubes are no more, and the steel is a very hard and brittle Rockwell 65 or higher.

Before the hardened steel cools much under 200°, it is again heated, but to much lower temperatures of between 200° and 800°. This process, called “tempering,” breaks down some of the martensite, relieving stress and reducing the hardness. This important step adjusts the steel hardness to match its intended use. As the tempering temperature increases, the steel hardness decreases while toughness and flexibility increase.

There is a predictable relationship between hardness and flexibility. Each tempering point is a different trade-off between the two. A cutting tool, for example, would have less of its stress and hardness removed than would a spring. While both an axe and a knife are cutting tools the tempering of each is different due to the intended application.

If the tempering temperature gets hot enough, the steel will soften completely, just as in annealing. The tempering temperature for various uses can be determined by published charts for that particular alloy of steel. The temperature required can be delivered by thermostatically controlled furnaces, heating until a temperature crayon melts, or by observing the oxide colors heat forms on the surface of clean steel.

Let us use a representative high carbon simple steel composed of iron and roughly one percent carbon, such as readily available commercial 1095, to make a knife blade. This steel is delivered in the annealed state, which is very good thing. Many, but certainly not all, older machinist files are made from a similar steel. The first step is shaping the work while the steel is soft. Machining, forging, grinding, and filing can all be used to shape the knife blade.

The prepared part is then heated to the critical temperature. Take care to heat the work uniformly, or distortion will certainly occur. The critical temperature can be found quite accurately by using a magnet. When the heated work is no longer attracted to the magnet the critical temperature has been reached. If viewed in dim light the steel would be glowing with a bright cherry red color, bordering on orange in dim light. Hold this temperature uniformly for a couple of minutes to allow the carbon atoms to travel and mix uniformly into the iron. Don’t let the metal get much hotter than this, since grain growth that weakens the steel structure will occur.

The red-hot steel must now he quenched. A light oil is used for this steel. Surprisingly, inexpensive Canola oil works very well. The oil should be heated to around 130 degrees F. before the hot blade is quenched.

Most thin sections of high carbon steels will harden adequately in 10 weight automotive oil or Canola oil and are less likely to warp due to the slower cooling rate of the oil as compared to older quench mediums such as water or brine. This can be important for thin knife blades. An oil quench is flammable, so use proper procedures when quenching with oil and have a fire extinguisher at hand. Take precautions before you start in case the hot oil should for some reason spill out of the holding tank.

The work should be quickly and completely submerged in the quenching solution. Have the quench tank close to the heat source so the time from heat source to quench is very close to one second. Move the work up and down in the quench. This keeps the hot steel from boiling the oil away from its surface, which would slow the cooling rate and possibly reduce the initial hardness. The time to leave the blade in the quench can vary due to the size of the blade and the volume of the quenching medium, but one minute should be fine for most knife blades.

Remove the blade from the quench and test the hardness of the work with a fine file. The file should skip over the work without biting into it. This indicates full hardness was reached during the quench. Occasionally, the work will warp from being heated unevenly before the quench, or from uneven cooling during the quench. This cannot be entirely avoided, only minimized by proceeding carefully.

It is for this reason that in industry, many items are ground after they are hardened. The warpage occurred, but was ground out. Minor warpage in a knife blade will not affect its use. You will notice is that the steel has discolored or even developed surface scaling from being in contact with the atmosphere while at its elevated temperature. This surface coating can be removed after tempering by using fine sandpaper or other abrasives.

Atter the blade is removed from the quench but before it cools below about 200°, it should be tempered. The knife blade will be tempered at 450° F. This should give a Rockwell hardness of between 57 and 61. The blade will be hard and hold a good edge. Some of the brittleness and hardness has been traded off for a certain amount of flexibility. The blade will, however, snap if bent very far.

The initial heating and quenching of 1095 is pretty straightforward. The tempering step is more difficult. The specific hardness related to each use is very important. The actual hardness of the work is dependent upon the maximum tempering heat reached and the time at that temperature. The problem is it’s very hard to accurately judge the wide range of temperatures in a small shop. Any errors in tempering make the work harder or softer than desired and less suitable for its intended use.

The kitchen oven, when used in conjunction with an oven thermometer, makes for a pretty reliable tempering oven up to its normal top temperature of around 500°. Be aware that most home ovens overshoot the selected temperature and then cool down below that temperature before overshooting it again. A setting of 375° may well reach 400° at some time during the cycle. The highest temperature reached will determine the hardness of the work. You can use two independent oven thermometers inside the oven to get a better idea of the oven’s actual temperature. When using an oven, a two-hour tempering time gives good results.

At the risk of some repetition, let’s apply the theory to making a knife from a file in a practical, step by step manner. Say that the only potential blade steel you have to work with is an old high carbon steel machinist’s file that is big enough to make a usable size of knife such as a general-purpose utility knife blade.

Finalize the knife design before you start making the knife. If possible incorporate one edge of the file into the design to save some work. A sketch is very useful in this regard, as is a wooden mockup. What makes this example different is that, unlike the first example, our steel is already fully hard and must be softened to be used. In addition, we will not fully harden the entire blade but only the cutting edge.

The file in its present state is very hard and not easily worked. It is so hard and brittle that it will fracture if bent or struck sharply. This hardness can be removed and the file annealed by heating it until it reaches the critical temperature and slowly cooling it.

The critical temperature can be determined by a bright cherry red glow the steel gives off as it is heated to this temperature. This color should be judged in dim indoor light and not in sunlight. Do not overheat the steel. Use the magnet test to double check the temperature. The red-hot steel must then slowly cool until it is below 800°F, somewhat cooler than the point at which the steel becomes Incandescence and begins to give off a glow.

(To be concluded tomorrow, in Part 2.)