Dealing with Medium and High Carbon Steels

High or medium carbon steel is needed when making a knife. 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 correct temperature ranges. Some of these alloys can be red hard (a temperature range in which the steel is too hard to work) or red short (a temperature range in which the steel is prone to cracking or crumbling). Generally these problems are more common in high alloy steels. Simple high carbon steels tend to have these problems less but will develop a large grain size if held at high temperature. Large grain size weakens the steel, and is detrimental to the cutting ability of the finished knife.

The way to avoid damaging the steel you are working with is to know what alloy you are using. 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 temperatures. Finally, normalize the steel before finishing the knife (filing grinding etc). To normalize, heat the steel to critical temp. This temp can be found by using a magnet to find the curie point (the point that heated steel turns nonmagnetic). Critical temp is a few hundred degrees higher than the curie point. Heat to critical temperature 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 by 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. These are called points. 100 points equals 1 percent by weight of carbon. 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. These are specialty alloys that were developed for a purpose. Within 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 very good knives but some can also be very difficult to work.
Basic Metallurgy

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 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 knife steel this isn’t really necessary because 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 because they 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.
Forging the Blade Profile to Shape

Begin by forging a point on the end of the bar. Draw this point out into taper approximately 1/3 shorter than the desired finished length of the blade. If beginning with round or square stock, forge a 4 sided taper first, then flatten. Remember that the shape and length of taper will be reflected in the shape and point of the finished blade but the finished blade will be longer and wider after beveling. Once the profile is forged to shape, the tang can be set in using a spring fuller. To better control the length of the finished blade the tang can be set in after forging in the bevels. The tang should remain ½ to 2/3 the width of the blade at this point. If making a self handled blade, forge in the transition to the grip and cut off excess material. If making a hilted blade, after forging in the transition cut off leaving 1-2 inches of stock after the step forged in from the spring fuller. Then forge out a tang.

Tips

Leave point of taper at least 3/16 square to ease in beveling.
Use the end of each heat to flatten the blade before re-heating.
Only work the steel down to a bright red heat. DO NOT soak the steel at a high temperature in the forge.

Forging in the Bevels

Begin forging in the bevels at the point of the tang. To do this, angle the cutting edge side of the blade on the anvil and strike at this same angle. Strike as near to the edge as possible and once a bevel is established shallow out the angles and begin working higher up on the blade to shift the bevel up the side of the blade until it reaches the spine.

Because the edge is expanding as it is forged thinner, the blade will curve away from the edge. Correct this at the end of each heat when the blade is still in the reds by setting the spine of the knife on the anvil and firmly hammering the edge back to straight. As the bevels are forged in be sure to work both sides evenly. Flip the blade over and work the other side keeping the same angle used on the first. It is best to work both sides in the same heat, but if this proves difficult a workable option is to alternate side to side from one heat to the next. As the bevels are forged in concentrate on keeping the edge centered, the bevels even and of an even thickness. Forge the bevels to a thickness about that of a dime at the edge.
Beveling Double Edged Blades

Forging the bevels on a double edged blade is much the same as on a single edged blade just done on both sides. All four facets of the diamond must be worked evenly. You can work from the tang to the point or from the point to the tang. Use the same angle in all 4 facets. The blade will not curve if the bevels are forged in evenly each heat. This is a good indicator that you are forging evenly. Be sure to match hammer angle to angle the bar is held to anvil on ALL FOUR facets. Carefully forge the bevels up to the ricasso (if present) or into the tang if no ricasso is present. Work the edge down to 1/16” or so and forge the bevels until the spine to the edge is one plane of a flattened diamond.

Tips for beveling

Flatten and straighten at the end of each heat. Work only the flats of the bevels to straighten.
Cork screw is caused by changing angles from facet to facet. To correct set the height of the twist down and forge high up on the bevel increasing angles.  Work both opposing facets.

Grinding

Using a worn coarse grit belt, grind the edge on the flat platen or contact wheel of the grinder. Clean up the profile of the blade, re-shaping the tip as necessary, until the blade is even and centered. Next, run the blade edge vertically on the grinder so all grind marks run vertically on the edge. Next, use a small wheel attachment on the grinder to even out tang junction or grip area of knife. If a small wheel attachment is unavailable, use files to refine shape of the tang or grip. The joint area between the tang and blade should be at least a 1/8 radius (1/4 inch circle) ideally with no tool marks crossing the edge. This will prevent stress risers from forming and make for a stronger blade.

Once profiling is complete, begin grinding the bevels using a worn coarse grit belt on the flat platen. After the scale is stripped off use a fresh course grit belt to grind the bevels flat with the edge up. When grinding everyone has a weak side and a strong side. Begin grinding with your weak side and match your stronger side to it as this provides more control. Grind the bevels down until the spine is centered and even, and the edge is of even thickness and about the thickness of a dime. If the design has no hard plunge cuts, move to an 8- inch contact wheel with a 120 grit belt and grind the bevels vertically until all coarse grit marks are removed. If the design calls for a hard plunge, re-grind on the flat platen. First grind with 120 grit, and then grind with 220 grit. At this point the blade is ready for heat treating.
Fitting of Guard or Bolster

Begin by laying out a center line on the guard with a scribe. Next, using calipers measure the width and thickness of the tang. Mark the width on the guard centered where the tang will pass through. Then find the center and scribe a line there. Off the center line mark lay out the slot. Use a drill one fractional size under the thickness of the tang. Mark and punch the outside holes ½ the thickness of the drill bit from the outside lines. Center punch and mark the width of the drill bit from the last center punch mark. Drill out the center punched marks. Use a needle file to cut away the web of material between the holes. Now file the slot to size using the scribed lines as a reference. Once the tang passes through the slot, file a radius to allow the shoulder of the tang to seat fully. A rotary tool or graver can be used to a cut a pocket to seat the blade into for a cleaner transition. Mark this after the blade is seated by tracing the blade on the guard with the scribe. Then cut away inside the lines. Check the fit often and only remove enough material so that the blade just seats. Once fit, the guard can be shaped as desired.
Fitting the Grip

Draw the tang on the handle material. Mark a center line on the side 90Deg. from this. Use a square to continue these lines to the top of the material. Using these lines as guides, clamp the material in a drill press and use a slightly undersized bit, drill out the slot for the tang. Clamp the grip in a vice and carefully heat the tang of the knife to around 900 DEG F. Then use the tang to burn out the slot for a perfect fit. If the blade has been heat treated be sure not to heat the tang past the junction to the guard as this will result in having to re-heat treat the blade. If this is a concern, a bar of steel may be shaped to match the tang and is used to burn out the handle material. Once the tang is fit, draw in the profile on the marked side of the handle. Using a band saw or belt grinder, rough in the shape to these marks (at this point if a pin is included in the design fit that then continue). Repeat for the other 2 facets. Then, round and shape the grip as desired. Check the fit between the guard and handle. Adjust as necessary. Hand sand the handle/grip to about 400 grit for the best finish.

When hand sanding wood always sand with the grain.
Lightly wetting the wood between grits  will raise the grain and  achieve a finer finish

Adding Pins to the Grip

With the handle profiled to shape and well fit up to the guard, mark where the pins will be placed. Using the proper sized drill bit drill pin holes in the grip (pin holes should be drilled on size IE 1/16” thick pin use a 1/16” drill bit). Now assemble the knife and use a long clamp to keep it together. Use the holes in the handle as a guide to drill the holes in the tang to match. Cut a piece of pin stock 1 inch longer than the width of the grip. Rough up the pin with coarse paper and taper one end. Bend the last ¼-1/2 of the un-tapered end over 90Deg. Insert the pin in the hole and check the fit of assembly.

If the fit is off, carefully file away excess material or use a spacer to restore the fit-up. Leather works well as it will compress and take up any inconstancies when the pin is driven in. Once glued up the leather will be stable and sealed from moisture.
Basic Heat Treating

Basic heat treating for knife 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. Step one is normalizing, step two hardening, and step three tempering.

Step one: normalizing. Heat the blade to an orange heat and let cool in 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.

Step two: hardening. Heat the blade to critical temperature (the temp. at with all carbon is in solution with the iron) and quench it (in most cases in oil). This will force the steel into its hardest state. Critical temp varies from alloy to alloy (usually between 1450-1550 DEG F). To find critical temp, 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. 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 re-quench, 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, and the 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 medium 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).

Temper ranges for some common blade steels

Steel AS Hard300 Deg 400Deg 500deg

1050 RC59 RC55 RC52 RC48

1075 RC64 RC62 RC59 RC58

5160 RC62 RC59 RC56 RC54

O1 RC64 RC62 RC60 RC58

W1 Rc65 RC63 Rc61 RC59