Hardening Carbon Steels

This is the fourth Instructable in my series on heat treating carbon steels.

If you read the preceding Instructables: An Introduction To Heat Treating Carbon Steels, Annealing Carbon Steels, and Normalizing Carbon Steels, then you learned that heat treating is the manipulation of a metal's molecular structure via exposure to specific temperatures. Carbon steel's molecular structure is crystalline and has a grainy appearance. Exposure to heat changes the shape of these crystals. Each crystalline shape exhibits a different set of properties, which can be beneficial in different ways depending on the material's application.

The best way to think about heat treating is to consider the shifting of the crystalline structure from one phase to another. In the case of Hardening, martensite is the phase that the steel has transformed into.

When speaking of Hardening, we focus on two terms: martensite and the prevention of slip. Martensite is the hardest state the crystalline structure can have. It is created when the steel is rapidly cooled from its critical temp to below 617 degrees F. Slip is the word used to describe the movement within the crystalline structure along tiny gaps between the grains of the metal. This is essentially the flexibility that metals have in their softer states. Hardened metal has little to no slip.

Metalurgist agree on several common factors in the creation of martensite, but cannot pinpoint one over another as to the primary cause of it. The factors are: Distortion of the crystalline lattice causing internal stress, the dramatic reduction of grain size, the creation of submicroscopic particles of cementite, the shift of carbon atoms into an intralattice structure, and suppression of expansion from lattice distortion.

We could get very deep in the weeds about each of these and so for the sake of brevity, we should just be satisfied with a general understanding that a combination of these things happen during the rapid cooling of carbon steels which causes the creation of martensite, prevents slip, and makes the steel hard.

Steel has been an integral part of the formation of our society and continues to play a major role in our industrial development. The two major reasons for this is because of its abundance and versatility.

Steel's versatility comes from its ability to be shaped into different forms, which makes it an excellent material for making tools. Most tools either strike or cut, and since steel is harder than most other materials, it works well for these operations. What happens though, when steel tools are used to cut and strike other pieces of steel?

If I were to rub my finger across my arm, my arm would not slice open. My finger and arm are made from the same material and so there is minimal effect when the two come in contact with one another. So how then does a steel saw blade cut a steel bar?

The answer lies in the material's hardness. The saw blade is much harder than the bar, thus it is able to be sharpened and maintain that sharpness as it comes into contact with the softer bar. Likewise, if the bar was harder than the saw, then it would be damaged as it tried to cut the bar.

Hardness in steels is directly related to carbon content. Steel is an alloy of iron and carbon. As mentioned in the first Instructable of this series, medium to high carbon steels have a carbon content typically in the 0.30–1.70% carbon by weight range. Low carbon steels do not have enough carbon to be heat treated, and are not relevant to us in this context.

Basically, the more carbon that is present in a steel, the harder it is. Hardness is also related to sharpness and brittleness. As the hardness of a steel increases, so does its ability to get and stay sharp. Unfortunately, the harder a steel gets, the more susceptible is will be to cracking and even shattering.

For this reason selecting a carbon steel that is appropriate for your application is critical. Using steels that are not hard enough will result in marring of the tool. Using steels that are too hard will crack and break. Both can be dangerous, not to mention a waste of your time.

Steel hardens when it is is rapidly cooled from its critical temperate to below 617 degrees F. This may seem simple, but there are several things to consider in order to make this happen correctly: The quench medium, duration of the cooling, the position of the part when quenching, the quality and thoroughness of the heat, and the shape of the part being hardened.

Lets break these down one by one:

Quench Medium -

The only thing more important than the quench medium is the heat itself. The medium in which a part is quenched is of the utmost importance as it will dictate the rate of cooling. Many mediums exits but they basically break down to three categories: oil, water and brine (salt water).

Water is a very useful quenching medium because it is abundant (if you don't live in California), easily attainable and inexpensive. It is also desirable because it can be disposed of without any environmental or pollution issues. Plain water offers close to the near maximum rate of cooling available in a liquid and works well to pop scale off steel as it is being quenched.

One drawback is that the rate of cooling is so dramatic, that it can cause distortion and even cracking. Because of this, water is typically reserved for quenching simple, symmetrical parts of lower hardening grades of carbon steel.

Another disadvantage of plain water quenching is associated with what is called the vapor blanket. This is a matrix of vapor bubbles that surround the part as it is cooling. The trapped bubbles can insulate the piece, causing uneven cooling and irregular distribution of stress in the part. Agitating the piece in the quenching medium can break up the vapor blanket and expose the part to "new" cool water.

Keeping the water clean is also important as contaminants such as algae, soaps, and oils can trap steam and prevent cool water from making contact with the part.

Oil is another widely used quenching medium. It is popular because it slows the cooling rate slightly. There are two general types of quenching oils: "conventional", having no additives, and "fast" which are blended with other ingredients to reduce viscosity.

Oil is desirable because it provides a fast quench speed though the vapor blanket and then tapers the cooling speed off as the piece enters the lower cooling range. This rate of cooling seems to be ideal for most carbon steels.

Air / Gas hardening is not widely used, but can be effective for some alloys that require a slow rate of cooling to achieve hardness. Note that hardening an alloy that specifically requires an air quench, is exactly the same thing as normalizing an alloy that does not require an air quench. Confusing? Yep!

Duration of Cooling -

We know that martensite forms when steel is cooled rapidly from critical temp to below 617 F. This is called the steel's critical speed. If cooled to fast, some austenite may be present; if cooled too slowly martensite may partly break down into softer constituents. The key is to find the correct info on the material data sheet and / or perform tests before quenching important parts.

Position of the Part -

This is just as important in the heating process as it is for quenching. Lets say there is a part with a thin edge on one side. Care must be taken as to not overheat the thin part and melt it. Likewise, that same part must be positioned in the fire so that it reaches critical temp evenly (this may require rotating and flipping). Finally, these same considerations must be made when quenching lest any disproportion in the thickness of the part lead to uneven cooling, distortion, and cracking.

Quality of the Heat -

As mentioned above, the heat must be even throughout the part to be quenched. The effect of heat on steel is to expand it, thus uneven thickness or asymetric shapes may cause a part to heat unevenly. When this happens expansion of the part is also uneven. This is where distortion will take place and remain permanent.

Shape of the Part -

The shape of the part is an important factor to consider when quenching. A long part quenched vertically (long-wise) will cool first at the dipped end before the whole part is submerged. This will create radical unevenness and will likely result in distortion or breakage. The same part turned 90 degrees will submerge much quicker and cool more evenly. Asymetric parts should have their thickest end quenched first. e.g. knife blades and cutting tools.

1. Know your material and choose proper quenching medium,

2. Heat material slowly and evenly,

3. Soak for proper time based on material thickness,

4. Remove material from forge and quench,

5. Agitate material to break up the vapor blanket,

6. Leave in quenching medium until material is able to be handled,

7. Test hardness with a file*.

* Hardened carbon steel will typically become harder than a file. Clean the surface with a wire wheel and gently run a file over the materials surface. If it was hardened properly, then the file should glide over the material without making any cuts in the surface. If the file does cut the material, hardness was not achieved. There is a wide variety of carbon steels, each with a slightly different set of properties. It's these differences that give the materials sensitivity to varying temperatures. One steel might harden at 1475°F, while another hardens at 1500°F. There is not much of a difference there, but if the proper heat is not reached, then hardening is at best, partial, and at worst, non existent. Likewise, quenching at temperatures that are too high can cause enough stress to crack, and in extreme cases, tear the material apart.

When starting a project, knowing the type of steel is ideal. If the material is new, then the manufacturer can supply you with all the material data that you require for a successful heat treat. If the steel has been reclaimed from the scrap yard, then making an attempt to identify it is strongly recommended.

For example, auto manufactures used 5160 alloy steel to make truck leaf springs for many years. Chances are an old spring from the salvage yard will be of good quality. However, you can be sure by checking the make model of the vehicle, looking for any numbers stamped into the parts and plugging all of that into google. There isn't a more tragic ending to a project than getting all the way to the end and loosing it because of mistakes in the heat treating process.

Do your homework.