By Matthew ParkinsonDownload Tutorial
Dealing with Medium and High carbon steels
When making a knife or axe, a high or medium carbon steel is needed. This kind of alloy is also called “spring steel” or “tool steel”. When working with these steels, the higher the carbon content and the higher the alloying content, the more sensitive the steel will be to working in the correct temperature ranges. Some of these alloys can be red hard (a temperature range that the steel to hard to work) or red short (a temperature range that the steel is prone to cracking or crumbling), generally these problems are more common in high alloy steels. Simple high carbon steels tend less to these problems, but will develop large grain size if over heated or are held at high temperature. Large grain size weakens the steel, and is detrimental to the cutting ability of the finished knife or axe.
The way to avoid damaging the steel you are working with is to know what alloy you are working with. Look that alloy up on line or in one of the many reference books. Find out what that alloy is prone to, (if it is red short or red hard) and what the hardening and temper ranges are (you will need this info later). With any of these alloys there are a few things that should be done. First Do not soak the steel in the forge, second do not heat the steel to a higher temperature than is necessary to work it, and third as you forge closer to finished shape work at progressively cooler temperature. Finally normalize the steel before finishing the knife or axe (filing grinding etc) to normalize heat the steel to critical temp, this temp can be found by using a magnet to find the curire point,(the point that heated steel turns nonmagnetic) critical temp is a few hundred deg. Higher than the curie point. Heat to critical and let cool in still air to about 400 deg F., do this three times ( or cycles) this will reduce the grain size break down any carbides that might have formed, and soften the steel making the grinding/filing easier.
In the USA Steel alloys are graded and sold using two main systems. The first is a numeric based system (SAE, AISI) in this system there are 4 or 5 digits that determine the alloy, the first two determine the alloy content and the last two or three the carbon content, theses are called points, 100 points equals 1 percent by weight of carbon so 1050 steel would be a simple carbon steel (10=simple carbon steel) with .50% carbon content. The minimum carbon content to make a good knife is about 40 points (.40%) and the maximum is around 1%.
The second grading system is the letter number system of tool steels, theses are specialty alloys that were developed for a purpose so with in one set of steels (O series for example) there can be a total change of alloys with similar fished properties. Some of the more common steels in this system are O1,W1,W2 ,L6,S7 and D2. Most of these steels can make vary good knives and axes but can also be very difficult to work.
The slit and drift method
The slit and drift method is a more modern development in the making of axes. It has the advantages or being entirely formed of hardenable material so the whole axe can be heat treated (not just the edge) this can be an advantage in some thin light war axes. It also removes the risks involved in a poor weld as there are no welds in this method.
Begin with a 3- 4” long piece of 1”SQ of some hardenable steel .(1050, 4140, W1, 1080 etc) lay out and drill two 3/16” holes 1” on center 3/8” from the end then drill a series of holes in between to remove more of the material to be broached. For a pipe hawk or a spike hawk, use a slightly longer piece of material and set the first hole in by an 1” or so to leave material to forum the pipe or spike. using a slot punch 3/16”/1” hot brooch the eye. Use a high forging heat. Line the punch up with the holes and strike it hard. Drive the punch down half way through the part. Remove the punch and cool it off every now and then and between heats. Flip the part and drive the punch through the other half of the slot. Clear the punched slug from the slot.
Drifting the eye
Use a drift to open the eye by hammering around the sides of the eye to expand it and then drive the drift in, this will place less strain on the drift and on the material around the eye. Be sure to work all sides of the eye evenly so that the wall thickness will stay even. The width of the eye can be adjusted by drawing material directionally using a cross peen. Do not drive the drift all the way, wait until the body has been forged out leaveing room to adjust the eye to center with the edge after the body is forged out.
Forging to shape
Forge down the thickness of the bit, beginning at the edge. Greater width to the edge can be obtained by using a crass peen to direct the material movement. Work the sides over the horn to form the shape of the head. The goal is a cross section that is 3/8”-1/2” thick at the eye thinning to 3/16”-1/4” near the edge. Insert the drift and check to be sure that the edge and body of the axe is in line with the eye/ handle. If not, correct this now. Forge in a edge bevel or take the edge down to 1/16” -1/8” thick
Flattening and straightening
Using a progressively cooler heats refine the shape of the axe using a lighter hammer (1-2LB ) use a firm blow and pay extra attention that the edge and body of the axe are both flat, straight, inline, and centered to the eye .
Mark the drift for the desired eye size. (this is determined by the handle size if a premade handle is to be used) work the edges and drive the drift down working the sides of the eye until the mark is even with the top of the eye or just a little shy. Be sure that the eye stays centered to the body and edge of the axe. Adjusting the eye for center as needed
Grinding and finishing
grind the profile to shape , be sure that all the grinder marks are running parallel to the edge. Once the profile is established, as much or a little finishing as you like can be done to the body and eye of the axe. The only other area or the blade that needs to be ground is the edge bevels. This area should be ground and polished , the thickness at the edge can vary with the intended use of the axe. A heavy working axe can have and edge a full 1/8” thick before sharpening. A lighter war axe can have an edge thickness before sharpening of as little as 1/16”. The remainder of the axe can be left as a forged finish or ground and polished .
Drifting the eye
Use a drift to open the eye, work the edges and shape the out side of the eye begin with the corners and then thin out the rest of the eye. Langets can be formed by using a cross peen to direct the flow of the material working over the horn or a bick don’t try to use the drift to back up the wall of the eye as one side of the axe will form the langet nicely, the other side will be too thin to form it from being worked by the anvil. Pull the material down with the peen and then adjust the shape using the horn.
Forging the shape of the axe is much the same as with the wrap and weld method. The main difference is in forming the area around the eye. This should be done hot with the drift in place.
Fitting a handle
Drive the blade onto the handle. If necessary carve, sand, or file the handle or eye for the best fit. When a good fit has been achieved, remove handle and hand sand up to 220 grit. Use linseed oil, Danish oil, wax etc, to seal and finish the handle. After heat treating and refinishing the head , Reassemble and drive the handle home, or Insert wedges and trim the end flush depending on handle design being used.
Basic Metallurgy and heat treating
Understanding what is happening to the steel during heat treating allows the bladesmith to know when it is safe “to get away with something” and when it isn’t. It also allows the bladesmith to find solutions to the problems that crop up from time to time when working with new steel. Steel is defined as iron alloyed with carbon. All modern steels have alloys other than carbon, but all steels must have carbon present to be steel.
Definition of terms:
- Hardness is a measure of a resistance of a material to deformation. For steels, this is measured on the Rockwell C Scale.
- Hardenability is a measure of the steel’s ability to reach hardness, both absolute hardness (at surface) and in depth of hardening (hardness at center).
- Toughness is a measure of the steel’s ability to withstand stress (resistance to shock, flexibility, deformation, etc)
Each different alloying metal will change the properties of the steel. What each alloy and what different alloys together can do is a lifetime of study. As such, I will not go into further detail other than to say that most alloys are present to change the qualities of the steel (i.e. finer grain, higher hardenability, etc.).
Steel is a crystalline material and can form several distinct structures within the crystalline matrix. The first structure is ferrite, which is pure iron crystals in the steel with cementite (iron carbide) binding up the vast majority of the carbon. Ferrite is a body-centered cube of 9 atoms (8 iron atoms at the corners and one iron atom in the center) in which metallic alloys such as nickel can replace one or more of the iron atoms. When steel is heated above its “critical” temperature, a structure called austenite is formed. This is a face-centered cube of 14 iron atoms (again, metallic alloys can replace iron atoms in the structure), which can hold up to 2% carbon by weight between the iron atoms. For the most part, austenite is only present at temperatures above the austenizing temperature (beginning at 1375˚F). When quenched, austenite becomes martensite, which is hardened steel. Martensite is formed when austenite is “frozen” in the quench and is structured as a body centered tetragonal.
The goal of heat treating for bladesmiths is to free up the carbon from carbides and take it to solution with the iron (austenite) then quench to freeze the carbon into solution. In practice this is 3 main steps; normalizing, hardening, and tempering. The purpose of normalizing is to break up carbides, reduce grain size, and allow the ready formation of austenite. This will allow for a shorter soak time at temperature during hardening and finer grain martensite after quenching. Normalizing is defined as heating to the upper transformation point (about 1400-1500˚F) and slow cooling to the lower transformation point about (about 900˚F). Multiple cycles of normalizing can have greater benefits.( this is also called thermal cycling.)
The hardening step consists of heating to above the upper transformation point and cooling within a prescribed rate of time (quench). The length of time between heating and cooling is determined by the alloy (speed of quench). This rate can be found in a TTT (Time-temperature Transformation) chart. When mapped on a TTT chart the hardening curve will look like a nose. So long as the steel is cooled below the tip of the nose within the allowed time, it will harden. The TTT chart also shows the exact upper and lower transformation points, as well as the austenizing points, and the Curie point (the point at which steel becomes non-magnetic). After hardening the steel will mostly be martensite with residual carbides, and in the case of the higher-alloy steels there is often some retained austenite as well. Once quenched, the steel is in a highly stressed state. It is very hard, but also very brittle. By tempering (heating between 250-1100˚F) much of the stress is relieved, a portion of any retained austenite is converted to martensite and the overall hardness is lessened. As the hardness is lessened, the brittleness is lessened, and toughness is increased. A second cycle will temper both the original and newly formed martensite and convert more of the retained austenite to martensite. If the temper cycle is repeated 3 times 90% or more of the retained austenite will be converted to tempered martensite. For the average blade steel this isn’t really necessary since low-alloy steels have almost zero retained austenite after quenching. For blades made from high-alloy steels it can be worth the extra effort, and in some cases is actually necessary.
My method is to begin tempering 50 degrees below the finishing temper (i.e. a temper of 375˚F would be started at a temper of 325˚F). Soak at the lower temperature for 1 hour, remove and let cool. Then re-set the oven for 25 degrees higher, temper for 1 hour, remove and let cool. Then complete a final temper at 25 degrees higher, temper for 1 hour, remove the blade and let cool.
Steels come in three classes: hypo-eutectoid (less carbon than eutectoid), eutectoid, and hyper-eutectoid (more carbon than eutectoid). The eutectoid point (roughly 0.75% carbon by weight) in steel is the point at which the amount of carbon present has “saturated” the low temp material but is not yet sufficient for the formation of “free” carbides. In un-hardened steels all of the material should be pearlite, which is a mixture of ferrite (pure iron) and cementite (iron carbide). Below the eutectoid point the material will be a mixture of ferrite and pearlite and above the eutectoid point the material will be a mixture of pearlite and free carbides.
Hypo-eutectoid steels contain between 0.01% to 0.75% carbon by weight. Those steels above 0.4% carbon will harden and tend to be rather tough, though not especially hard. Addition of other alloys can improve hardness and hardenability. The hypo-eutectoid steels are generally easy to forge, grind, and heat treat.
Eutectoid steel is the range right around 0.75% carbon by weight. These steels will harden well and tend to be forgiving when working with them, but do not have the added toughness of hypo-eutectoid steels without added alloys. These are the best steels for beginning bladesmiths due to their forgiving nature and relatively high performance.
Hyper-eutectoid steel is between 0.75% to 1.25% carbon by weight. These steels can yield the highest performance because the excess carbon can form various carbides. They are almost always found with high alloy content, especially such carbide-formers as chromium, vanadium, and tungsten. When treated properly these steels have the best edge-holding and wear-resistance properties, but they are temperamental to work with and react poorly to overheating. Good knowledge of metallurgy and proper control of forging and heat treating temperatures are a must before delving into this group
Basic heat treating
Basic heat treating for knife or Axe making is a three step process, it is the heat treating that is the most important part of making a knife. It is heat treating that turns a Knife shaped object into a knife. The steps are step one normalizing, step two hardening, step three tempering.
normalizing heat the blade to a orange heat and let cool to still air down to a black heat, do this three times. This will remove any stresses built up by grinding, reduce the grain size, and leave the steel in the best condition to be hardened.
Hardening is heating the blade to critical temp.(the temp. at with all carbon is in solution with the iron) and quenching it (in most cases in oil.) this will force the steel into it’s hardest state. Critical temp varies from alloy to alloy (usually between 1450-1550 DEG F) to find critical, heat the steel and check it with a magnet, the temp at which it looses magnetism is called the curie point, about 100deg above this point is critical. In practice quenching from the point that the steel looses magnetism is close enough. judging the temp by color is affected by ambient light so even if when using a steel you are familiar with it is a good Idea to check the temp using a magnet. Heat the blade to this point and quench the blade in oil, quench the blade, edge down or tip first in oil, do not angle the blade when entering the quench or the blade will warp. For most steels vegetable or peanut oil works fine and is non toxic, motor oil can also be used,(fresh not used) as can transmission fluid. For a more consistent quench and when working with faster hardening steels a commercial quenchant like Parks-50 should be used. Quench the blade until all color is gone from the blade then let cool to room temperature. Check the edge using a file to be sure the blade hardened, if the file “skates “ then proceed to tempering. If the file “bites” the blade didn’t harden, reheat to a slightly higher temp and requench then check again. If the blade still isn’t hardening the edge may have decarburized, lightly grind the blade and check again if it is still not hard the steel you are using may not have enough carbon to harden.
Step three Tempering.
Tempering is heating the steel to 150-1000 deg F. This will take away the brittleness along with some of the hardness in the steel. The tempering temps will vary depending on the alloy used , size and type of knife being made. For the most part a temper of 300-450 Deg F for an hour is common. Hardness in steel is measured using the Rockwell C scale (RC) this scale ranges from RC30 (unhardened steel) to about RC70 for a med sized knife (6-8” blade) a hardness of around RC58-60 is about right a smaller knife can be harder (RC58-62) and a larger knife should be a bit softer.(RC52-58)
For a bit greater performance three cycles of temper can be done, the first 50 deg below the finished temper. For a temper of 350 start with one hour at 300, let cool then one hour at 325 let cool and a final temper of 350 for an hour.
Temper ranges for common blade steels
Steel AS Hard 300 Deg 400Deg 500deg
1050 RC59 RC55 RC52 RC48
1075 RC64 RC62 RC59 RC58
1084 RC66 RC64 RC60 RC55
5160 RC62 RC59 RC56 RC54
O1 RC64 RC62 RC60 RC58
W1 Rc65 RC63 Rc61 RC59
(Temper ranges found online from various manufactures website)
The first edge on a newly finished blade should be cut in with the grinder to establish a secondary bevel. Once this is done, the edge can be re-sharpened or further dressed with stones.
On all knives with a secondary bevel (basically anything other than Japanese style work and razors), there are three types of edge: flat ground, convex, and concave (hollow ground edge), Most production knives have a flat ground edge of 15o to 25o. A flat ground edge can be easily re-sharpened and cuts well, Most custom knife makers use a convex edge of the same basic angle of 15-25. This type of edge is just as sharp as a flat edge, but it is stronger, and able to hold an edge longer. It is how ever slightly more difficult to re-sharpen with hand stones. The concave edge is a style that works well for some knives, such as meat cutting knives, that a steel will be used to sharpen but is of limited utility for an everyday knife as it is a relatively weak edge, will dull quickly and is impossible to re-sharpen with hand stones.
As said most custom knife makers use a convex edge, to set this type of edge the blade is ground on the slack belt of the grinder. ( I generally cut the edge with a 120 grit belt making sure to cool the edge frequently. ) Hold the knife edge down at a 10o angle, (as measured from the center line of the blade to the grinder) starting at the base of the blade press in slightly, and take one continuous pass along the whole edge. Cool the blade down and repeat these steps on the opposite side. Continue this process alternating sides until a burr (wire edge) develops all along the edge.
At this point move to a finer belt (220 grit) and continue alternating sides. Then, use 400 grit belt to re-polish the edge. Afterwards strop the edge on the buffer to remove the bur. The blade should be sharp. If the edge is still not sharp enough after buffing, re-cut the edge at a slightly steeper angle and go through the steps again. Unlike knives, axes should have an edge angle of between 25-30 deg. This thicker edge angle is stronger and better able to resist damage when used for heavy chopping.
Forge welding depends on three things to be successful. First, a perfectly clean joint (no scale or other contaminates). Second is a totally inert atmosphere. Third is complete contact of the mating surfaces. These three things are put in place via the flux and heat. At welding temps the flux will strip away the oxides on the surface and reveal a clean surface below. The flux will also seal off the joint creating an inert atmosphere. When struck, the metal will push the flux out of the way and bring the two surfaces into perfect contact. This kind of welding is also call solid state welding.
To forge weld a scarf must first be forged on the two sides of the joint. The scarf ensures that the flux will be forced out of the joint and not be trapped in the weld. Next a heat is taken and the joint is wire brushed until cool (this removes much of the scale giving the flux a helping hand). Then the joint is reheated to a dull red and flux is applied to the joint. In a CLEAN fire the joint is heated to a bright yellow heat and quickly taken to the anvil. Strike the center of the joint with a good blow to set the weld and then work the joint with firm blows until it is to shape or has cooled down to a red heat. If the weld took the whole joint should cool at the same rate. If there are cool spots, these are areas that the weld did not take. Wire brush and reflux any areas that didn’t take. Take another welding heat then reweld any areas that didn’t take. Once the joint is welded finish forging the area to shape. Common problems
Weld looks good but fails when forging to shape- this is due to flux trapped in the center of the joint. Use a slightly higher heat and reshape the scarf to allow the flux to escape.
Weld will not take- this can be caused by many factors. Most common is a dirty fire and too much air getting to the joint. Other common causes are not enough heat, too much scale in the joint for the flux to deal with (reflux and try again), not enough heat/ heat did not reach center of joint.
The wrap and weld method.
This is a very common method for the making of tomahawks. There are several variations of this method. The first and most common is the cored body, this is when the core of the bit is lined with a high carbon bar, running from the edge to the eye. The main problem with this method is it is very difficult to get a complete weld, invariably the weld near the eye will not take. The next method is the welding of the eye and then the addition of a high carbon bit, ( inset, over layed etc) other similar methods utilize a larger block and the eye is slit from the end and wrapped over then welded to form the eye. (placing the weld at the back of the eye. Other methods involve pre shaping the piece to be wrapped before welding to form features such as langets or deeply bearded edges.
To begin cut a 6-7 long piece of 3/8-1/2” thick 1-1 ¼” wide mild steel.(depending on the size axe being made) Mark the center with a chisel or center punch, and set down a 4-5” long section(before drawing out) to ¼” thick.
Use the edge of the anvil to set down a section approximately 2” from the center of the bar. Set down again 2” in the other direction from center and then draw and flatten the material in between the two to ¼”/1”. A flattener can be used to square up the shoulders and dress the set down section flat.
Trim the ends even approximately 2” from the shoulders. Bevel the tips of the ends so that one side is flat and the other has both the shoulders and the bevels. Bevel for a length of approximately ¾” and down to a thickness of 3/16” -1/4” thick. Alternately this step can be skipped and later a slot for the high carbon bit can be cut with the hardie or chisel.
Now grind the flats and the bevels clean of scale. Carefully heat the set down section and bend so that the two flats are lined up at the shoulders. Heat, flux and weld the joint. (see section on welding) When the weld is solid, clean up the V formed by the bevels with a file. Use a chisel to raise burs on the inside of the V. Fit a piece of 1095 to the V by forging / grinding to shape. Cut and set the piece of 1095 in place hot. (The burs will hold the steel in place) Flux and weld the joint. Alternately the High carbon bit can be chiseled to raise burs to hold it in place for welding . Once the bit is fit, flux and weld the bit in place.