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Heat treating is a *huge* subject, and depends on the metal, and intended use. Most of the time, this question is asked regarding steel, so we'll give a brief description of that, based on an article in Home Shop Machinist (Sept/Oct 1991, "Heat Treating Basics" by Steve Acker).
[Also thanks to Steve Gaudio (?) for his post of 18-Sep-1992, and clarification by Tim Eisele]
1
Iron will, at common temperatures, organize itself into an atomic structure that is called "body centered cubic." This consists of overlapping cubes with an atom at each corner, and one more in the center of the cube. But above roughly 1400 degrees F there is a change in structure to "face centered cubic" and the central atoms migrate to the faces of the cubes. This latter form is not magnetic.
Steel is basically iron with some carbon mixed in, though modern alloys have various other metals and substances as well. When steel is heated to the critical temperature (about 1400 degrees F), the iron will change to face centered, and the carbon atoms will migrate into the central position formerly occupied by an iron atom. This form of red-hot steel is called austentite. Since it is not magnetic, a magnet may be used to determine when the critical temperature has been reached (though the magnetism may be lost before the transition, so this is only approximate). Complete migration of the carbon atoms may take a minute or two.
If you let this cool slowly, the iron atoms migrate back into the cube and force the carbon back out, resulting in soft steel called pearlite. If the sample was formerly hard, this softening process is called annealing.
If you cool (quench) the sample suddenly by immersing it in oil or water, the carbon atoms are trapped, and the result is a very hard, brittle steel. Too brittle for most uses. The structure is now a body centered tetragonal form called martensite.
So, the next step is to heat it back up, to between 200 and 800 degrees F or so, depending on the desired end hardness. This allows some of the hardness to relieved and is called tempering. The amount of tempering that is desirable depends on the final use. Cutting tools are very hard, knife blades less so because they must flex under use rather than break. Tempering is a trade-off between hardness and flexibility.
Accurately measuring the tempering temperature is important. A nice, expensive thermostatically-controlled oven is great. Or, some special compounds can be applied that melt or change color at the right temp, such as Tempilstik and Tempilaq. If the steel is clean to start with, then you may notice that it goes through certain color changes as it heats up, with understandably vague descriptions such as "light straw" indicating about 440 degrees F, and purple=520. These colors are not incandescence colors, but are viewed in normal room light. The colors are due to types of surface oxidation that are temperature dependent.
When quenching, it is often very important to avoid stirring a part because this will cool one side much more quickly than the other, and might cause warping. For knife blades, as an example, move it strictly up and down during the quench.
Case hardening is a bit trickier, and involves heating the object in some sort of agent that promotes hardening at the surface. Liquid cyanide works well but should be out of the question for the home machinist. Luckily there are substitutes available from suppliers, one being called Kasenit, for example. Note that hardness is often measured using a "Rockwell C" scale, with 63 being very hard and 35 being fairly soft.
A type of steel called "drill rod" is especially useful for home/hobby use. As its name implies, it is the type of steel used for drills, and is available is round or square form (square drills?). Drill rod is also very useful around the shop because it is usually made to very accurate dimensions. Some types of drill rod are formulated for hardening via heating then quenching in oil, while others are quenched in water. The difference is that water will cool more quickly because it's a good conductor (though it may also form a steam "jacket" that moderates this effect), while oil will cool more slowly. Since rapid cooling may warp a part, this could make a difference in the final product.
There is also an "air hardening" steel, though it seems to be quite a bit more expensive than other steels.
It has been reported, by way of example, that you can make springs out of hacksaw blades by annealing, bending, hardening, then tempering by heating to a "metallic blue" and quenching in oil. I suspect lots of experimenting may be in order before you get things just right. Remember the steel must be clean (no paint etc.) to see the colors.
Quenching in oil may be a fire hazard. Take proper precautions, such as removing flammable materials from the area, wear proper clothing, and have an extinguisher handy. Even quenching in water presents the risk of scalding from steam or splattered water.
As one newsgroup reader pointed out, not only are there a gerbillion alloys, but zillions of treatments to choose from, and this is just for steels. Other metals, like brass, can be hardened by "working" the metal, by bending, hammering, peening, etc. Brass is usually annealed with a quench, which is the opposite of steel.
It's best to carefully research your particular project first, especially if it's something that is valuable.
A book, "Simplified Tool Steel Heat Treatment and Selection Guide" by Bill Bryson, may be of some help. $31.95 from Bill Bryson, 336 Governors Road, Milton, NH 03851. I purchased this, and was surprised to spend $32 on 100 pages of loose-leaf pages. The information is geared more towards the small commercial shop than the home shop, and thus deals with issues such as atmospheric control (using stainless-steel foil) and using accurate temperatures. In the home shop, we usually read about methods like "hit it with a torch then drop it in a bucket of oil." Bryson goes beyond this, discussing accurate methods that might be out of reach for some of us, but just barely. He also has a chapter on cryogenic treatment, that can also be used in the home shop via dry ice. http://www.worldpath.net/~hisaim for more info and ordering info. Note the title has been changed to "Heat Treatment, Selection, and Application of Tool Steels."
Power Model Supply recently (December 1992) listed two small heat treating ovens in an HSM ad. 2000 degrees F, 4x4x4 inch $330, 6x6x6 $435. Write them for more info or see the ad.[/SIZE]
[Also thanks to Steve Gaudio (?) for his post of 18-Sep-1992, and clarification by Tim Eisele]
1
Iron will, at common temperatures, organize itself into an atomic structure that is called "body centered cubic." This consists of overlapping cubes with an atom at each corner, and one more in the center of the cube. But above roughly 1400 degrees F there is a change in structure to "face centered cubic" and the central atoms migrate to the faces of the cubes. This latter form is not magnetic.
Steel is basically iron with some carbon mixed in, though modern alloys have various other metals and substances as well. When steel is heated to the critical temperature (about 1400 degrees F), the iron will change to face centered, and the carbon atoms will migrate into the central position formerly occupied by an iron atom. This form of red-hot steel is called austentite. Since it is not magnetic, a magnet may be used to determine when the critical temperature has been reached (though the magnetism may be lost before the transition, so this is only approximate). Complete migration of the carbon atoms may take a minute or two.
If you let this cool slowly, the iron atoms migrate back into the cube and force the carbon back out, resulting in soft steel called pearlite. If the sample was formerly hard, this softening process is called annealing.
If you cool (quench) the sample suddenly by immersing it in oil or water, the carbon atoms are trapped, and the result is a very hard, brittle steel. Too brittle for most uses. The structure is now a body centered tetragonal form called martensite.
So, the next step is to heat it back up, to between 200 and 800 degrees F or so, depending on the desired end hardness. This allows some of the hardness to relieved and is called tempering. The amount of tempering that is desirable depends on the final use. Cutting tools are very hard, knife blades less so because they must flex under use rather than break. Tempering is a trade-off between hardness and flexibility.
Accurately measuring the tempering temperature is important. A nice, expensive thermostatically-controlled oven is great. Or, some special compounds can be applied that melt or change color at the right temp, such as Tempilstik and Tempilaq. If the steel is clean to start with, then you may notice that it goes through certain color changes as it heats up, with understandably vague descriptions such as "light straw" indicating about 440 degrees F, and purple=520. These colors are not incandescence colors, but are viewed in normal room light. The colors are due to types of surface oxidation that are temperature dependent.
When quenching, it is often very important to avoid stirring a part because this will cool one side much more quickly than the other, and might cause warping. For knife blades, as an example, move it strictly up and down during the quench.
Case hardening is a bit trickier, and involves heating the object in some sort of agent that promotes hardening at the surface. Liquid cyanide works well but should be out of the question for the home machinist. Luckily there are substitutes available from suppliers, one being called Kasenit, for example. Note that hardness is often measured using a "Rockwell C" scale, with 63 being very hard and 35 being fairly soft.
A type of steel called "drill rod" is especially useful for home/hobby use. As its name implies, it is the type of steel used for drills, and is available is round or square form (square drills?). Drill rod is also very useful around the shop because it is usually made to very accurate dimensions. Some types of drill rod are formulated for hardening via heating then quenching in oil, while others are quenched in water. The difference is that water will cool more quickly because it's a good conductor (though it may also form a steam "jacket" that moderates this effect), while oil will cool more slowly. Since rapid cooling may warp a part, this could make a difference in the final product.
There is also an "air hardening" steel, though it seems to be quite a bit more expensive than other steels.
It has been reported, by way of example, that you can make springs out of hacksaw blades by annealing, bending, hardening, then tempering by heating to a "metallic blue" and quenching in oil. I suspect lots of experimenting may be in order before you get things just right. Remember the steel must be clean (no paint etc.) to see the colors.
Quenching in oil may be a fire hazard. Take proper precautions, such as removing flammable materials from the area, wear proper clothing, and have an extinguisher handy. Even quenching in water presents the risk of scalding from steam or splattered water.
As one newsgroup reader pointed out, not only are there a gerbillion alloys, but zillions of treatments to choose from, and this is just for steels. Other metals, like brass, can be hardened by "working" the metal, by bending, hammering, peening, etc. Brass is usually annealed with a quench, which is the opposite of steel.
It's best to carefully research your particular project first, especially if it's something that is valuable.
A book, "Simplified Tool Steel Heat Treatment and Selection Guide" by Bill Bryson, may be of some help. $31.95 from Bill Bryson, 336 Governors Road, Milton, NH 03851. I purchased this, and was surprised to spend $32 on 100 pages of loose-leaf pages. The information is geared more towards the small commercial shop than the home shop, and thus deals with issues such as atmospheric control (using stainless-steel foil) and using accurate temperatures. In the home shop, we usually read about methods like "hit it with a torch then drop it in a bucket of oil." Bryson goes beyond this, discussing accurate methods that might be out of reach for some of us, but just barely. He also has a chapter on cryogenic treatment, that can also be used in the home shop via dry ice. http://www.worldpath.net/~hisaim for more info and ordering info. Note the title has been changed to "Heat Treatment, Selection, and Application of Tool Steels."
Power Model Supply recently (December 1992) listed two small heat treating ovens in an HSM ad. 2000 degrees F, 4x4x4 inch $330, 6x6x6 $435. Write them for more info or see the ad.[/SIZE]
