Stacy E. Apelt - Bladesmith
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I get an annealing question about once or twice a week. I though I would make a metallurgical post on annealing. Those with metallurgical experience or significant personal experience feel free to add your thoughts. The rest of the newer folks and you-tube trained smiths might do best to just read and post questions here. I personally feel that the biggest barrier to good annealing and HT processes is the abundance of internet posts and You Tube videos giving inaccurate advice. The usual proof of validity is, "I get good results with XYZ." ... Good compared to what?
Metallurgical gurus - check this over and post any needed changes, as I did this off the top of my head.
Annealing - The process of softening and refining the grain of steel and iron:
There are several basic types of annealing carbon steels ( stainless and high alloy steels are far more complex). Lets break them down.
Process Annealing:
Process Annealing is done by heating to 1300°F and holding for 30 minutes to an hour. The steel is air cooled to ambient. This softens the steel somewhat, but is insufficient for anything but basic machining. Most steel comes from the mill Process Annealed. It is used in foundries and factories where parts are die-struck or mechanically worked.
Sub-critical Annealing:
Sub-critical annealing is a process annealing that works well for most knifemaking tasks. The steel is heated to 1250-1300°F and held for a period of 30 minutes to four hours. It is then air cooled to ambient. This is fairly easy with a HT oven and will allow drilling and grinding sufficiently for most all knife making tasks.
Stress Relief Annealing:
Stress Relief is similar to Sub-critical Annealing. The steel is heated to 1200°F and held for several hours, then air cooled to ambient. This is used to remove stresses from hammering, forging, multiple heat cycles, and severe machining. It is often done at the end of a days work to prevent cracking of machined parts and forgings.
Normalization:
Normalization is done by heating the steel to about 1500°F and holding long enough to become fully austenitic, approximately 5-10 minutes. It is then air cooled to ambient. This makes a moderately soft structure, with patches of uneven hardness. It works fairly well for tasks such as bending and simple working, but not always sufficient for drilling or grinding. The resultant structure is fine pearlite. This process is often repeated at descending temperatures as part of a HT regime. This is done to refine the grain and place all the alloying elements where they are best situated for the final hardening.
Full Annealing - Most of the slow annealing done by smiths in the past, and on You-Tube is some form of an attempt at full annealing. The basic principle is heating to about critical and holding there for a period of time. This is followed by some form of slow cooling. Most of the back yard annealing with a forge and then ash and vermiculite is far too uncontrolled to get good results.
Metallurgically, the process is:
Heat to approx. 1475°F, and hold long enough for everything to go fully into solution. Normally it is held for about 30 minutes to several hours, depending on the alloy. It is then cooled at a rate of about 40°F per hour until it reaches 900°F. This takes about eight hours. Once at 900°F it is cooled to ambient in the air, or quenched in water to rapid cool. This forms a very evenly distributed coarse pearlite, which is soft and easily worked.
If the cooling rate is too slow or prolonged, and continues at the slow cooling rate below 900°F, fine pearlite can form. This is much harder to drill and grind. This is why many back yard annealing in vermiculite doesn't seem to work ... it works too good and goes past the desired structure. That is why rapid cooling below 900°F is often mentioned as part of the process.
Spheroidization ( spheroidal annealing):
Spheroidization make a structure where the carbides are balled up in rods and spheres of cementite in a matrix of ferrite or pearlite ( depending on carbon content). If the steel is to be machined, drilled cut, or ground, spheroidal annealing is what you want.
The process is - heat the steel to 1300°F and hold for a short while. Raise to about 1400°F and hold for a short while, cool to 1300°F, and repeat the cycle several times. This nucleates very fine grains. Then, raise to 1450°F and hold for 10 to 30 hours. Cool in air to ambient.
Note: steel that has been fully spheroidized will need a preliminary HT step before final hardening to dissolve the spheres of carbides and put them into even solution. Otherwise the steel may not harden. Heat the steel to 1650°F and hold for 10-15 minutes. Air cool to black. Heat the 1500°F and either air cool to black or quench in oil. Heat to the target temperature and quench as per the HT regime.
Steel from Aldo and other knife suppliers is often Spheroidal annealed.
Kevin Cashen and many other smiths use a hybrid annealing which is a combination of processes. This is the basic annealing I use:
Normalize at 1600
Normalize at 1475
Heat to 1475°F, quench (very important)
Place blade in 1275° kiln for one hour, air cool.
Note: In most of these processes where the final cooling is done in still air, once the steel reached 900°F ( black heat) the steel can be water quenched to speed up cooling time, instead of slower air cooling to room temperature. There is little difference in the final product either way.
Metallurgical gurus - check this over and post any needed changes, as I did this off the top of my head.
Annealing - The process of softening and refining the grain of steel and iron:
There are several basic types of annealing carbon steels ( stainless and high alloy steels are far more complex). Lets break them down.
Process Annealing:
Process Annealing is done by heating to 1300°F and holding for 30 minutes to an hour. The steel is air cooled to ambient. This softens the steel somewhat, but is insufficient for anything but basic machining. Most steel comes from the mill Process Annealed. It is used in foundries and factories where parts are die-struck or mechanically worked.
Sub-critical Annealing:
Sub-critical annealing is a process annealing that works well for most knifemaking tasks. The steel is heated to 1250-1300°F and held for a period of 30 minutes to four hours. It is then air cooled to ambient. This is fairly easy with a HT oven and will allow drilling and grinding sufficiently for most all knife making tasks.
Stress Relief Annealing:
Stress Relief is similar to Sub-critical Annealing. The steel is heated to 1200°F and held for several hours, then air cooled to ambient. This is used to remove stresses from hammering, forging, multiple heat cycles, and severe machining. It is often done at the end of a days work to prevent cracking of machined parts and forgings.
Normalization:
Normalization is done by heating the steel to about 1500°F and holding long enough to become fully austenitic, approximately 5-10 minutes. It is then air cooled to ambient. This makes a moderately soft structure, with patches of uneven hardness. It works fairly well for tasks such as bending and simple working, but not always sufficient for drilling or grinding. The resultant structure is fine pearlite. This process is often repeated at descending temperatures as part of a HT regime. This is done to refine the grain and place all the alloying elements where they are best situated for the final hardening.
Full Annealing - Most of the slow annealing done by smiths in the past, and on You-Tube is some form of an attempt at full annealing. The basic principle is heating to about critical and holding there for a period of time. This is followed by some form of slow cooling. Most of the back yard annealing with a forge and then ash and vermiculite is far too uncontrolled to get good results.
Metallurgically, the process is:
Heat to approx. 1475°F, and hold long enough for everything to go fully into solution. Normally it is held for about 30 minutes to several hours, depending on the alloy. It is then cooled at a rate of about 40°F per hour until it reaches 900°F. This takes about eight hours. Once at 900°F it is cooled to ambient in the air, or quenched in water to rapid cool. This forms a very evenly distributed coarse pearlite, which is soft and easily worked.
If the cooling rate is too slow or prolonged, and continues at the slow cooling rate below 900°F, fine pearlite can form. This is much harder to drill and grind. This is why many back yard annealing in vermiculite doesn't seem to work ... it works too good and goes past the desired structure. That is why rapid cooling below 900°F is often mentioned as part of the process.
Spheroidization ( spheroidal annealing):
Spheroidization make a structure where the carbides are balled up in rods and spheres of cementite in a matrix of ferrite or pearlite ( depending on carbon content). If the steel is to be machined, drilled cut, or ground, spheroidal annealing is what you want.
The process is - heat the steel to 1300°F and hold for a short while. Raise to about 1400°F and hold for a short while, cool to 1300°F, and repeat the cycle several times. This nucleates very fine grains. Then, raise to 1450°F and hold for 10 to 30 hours. Cool in air to ambient.
Note: steel that has been fully spheroidized will need a preliminary HT step before final hardening to dissolve the spheres of carbides and put them into even solution. Otherwise the steel may not harden. Heat the steel to 1650°F and hold for 10-15 minutes. Air cool to black. Heat the 1500°F and either air cool to black or quench in oil. Heat to the target temperature and quench as per the HT regime.
Steel from Aldo and other knife suppliers is often Spheroidal annealed.
Kevin Cashen and many other smiths use a hybrid annealing which is a combination of processes. This is the basic annealing I use:
Normalize at 1600
Normalize at 1475
Heat to 1475°F, quench (very important)
Place blade in 1275° kiln for one hour, air cool.
Note: In most of these processes where the final cooling is done in still air, once the steel reached 900°F ( black heat) the steel can be water quenched to speed up cooling time, instead of slower air cooling to room temperature. There is little difference in the final product either way.
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