A good heat treatment (HT) can make an ordinary steel sing yet a poor HT can ruin a great knife steel. Some aspects of heat treatment were discussed in a recent thread and piqued my interest (again) in this important yet mysterious element of knife making. Rather than derailing that thread, I thought it would be better if I started a new one.
I'll kick off with my understanding of some important concepts of HT. While I find the whole process fascinating, I'm not a knife maker or a metallurgist. I'd be grateful for the input of those knowledgable in metallurgy and knife making. Please feel free to tell me if and where I'm wrong (or right) or have missed something (as long as you know what you are talking about). Please be as specific as you feel comfortable without giving away anything that you consider to be a trade secret.
My impression of the aims of HT are:
1) Making a blade that is hard enough and tough enough for its intended use.
2) Creating a fine grained blade (simple carbon steels) OR optimising the competing problems of grain growth and carbide growth (alloyed steels) to facilitate sharpenability and aid toughness.
My understanding of the phases of steel are:
Very high temp: Molten.
High temp: Austentite (also known as gamma). Almost a semi-solid. The iron atoms are in a cubic lattice with a place for carbon in the middle of each square (face centred cubic lattice).
Room temp: All are solids.
Ferrite (aka alpha): The iron atoms are in a tighter lattice, with only enough room for 1 carbon in the centre of each cube (Body centred cubic lattice). Contains a maximum 0.02% carbon by weight.
Cemetite: Iron Carbide (Fe 2.5: C). Can form in grain boundaries in steels over 0.77% C or in spheres within the grains (depending on the specific heat treatment).
Pearlite: Microscopic plates of ferrite sandwiched between plates of cementite. Forms when austenite is slowly cooled. Pearlite is soft and (I think) brittle.
Bainite: Filaments of cementite in a ferrite matrix. Forms when austenite is cooled at faster rate. Tougher than martensite and almost as hard.
Martensite: This is the phase that we want in our knives. A tetragonal lattice of iron with a space for carbon in the centre of each cube (body centred tetragonal lattice). The more carbon in the lattice, the more the Fe-Fe bonds are stretched, so the more tension they have, hence the harder (and more brittle) the steel. Forms when austentite is cooled rapidly. Fresh martensite is quite brittle and needs to be tempered.
My understanding of the processes involved in HT are (with thanks to Kippington for the order of the list):
1) Forge or cut to shape (you can normalize at any stage during forging): The (unhardened) steel is shaped into a knife, for example, by hitting it with a hammer.
2) Normalization: The blade is heated well above austenisation temperature and "soaked" at this temperature (perhaps an hour?) to distribute carbon and alloying elements evenly throughout the blade, resulting in smaller carbides. This can be partially undone during the quench, especially if the quench is low temp (e.g.: honyakii), which can result in segregation banding. This process can result in grain size growth so there is sometimes a tradeoff between small grain size and small carbide size.
3) Grain refinement: The steel is forged (to fracture the grain boundaries) and/or temperature cycled (from austenisation temp to room temp) several times to reduce grain size. This works because when the steel is austentised, small new austentite grains form in the boundaries of the old ferrite/ pearlite grains.
4) Anneal/stress relieving: I don't really understand how this works. Please enlighten me.
5) Quench: The steel is heated to austenite. It is then cooled quickly (for example by placing in water, oil, molten salt, liquid nitrogen or air, depending on the steel) so that the Austenite is "frozen" into martensite (hard) before it has a chance to form pearlite (soft) or bainite (somewhat soft). For high carbon steels (over about 0.8%), a cryo quench is required to obtain full martensitastion (and prevent retained austenite, which reduces hardness).
6) Tempering: The knife is heated to below the austenisation temperature and cooled to room temperature. This causes a change in the martensite which makes it a lot less brittle but a little less hard.
7) Grinding, sharpening and blade finishing.
I look forward to your insights and refining my knowledge of this process. Thanks.
I'll kick off with my understanding of some important concepts of HT. While I find the whole process fascinating, I'm not a knife maker or a metallurgist. I'd be grateful for the input of those knowledgable in metallurgy and knife making. Please feel free to tell me if and where I'm wrong (or right) or have missed something (as long as you know what you are talking about). Please be as specific as you feel comfortable without giving away anything that you consider to be a trade secret.
My impression of the aims of HT are:
1) Making a blade that is hard enough and tough enough for its intended use.
2) Creating a fine grained blade (simple carbon steels) OR optimising the competing problems of grain growth and carbide growth (alloyed steels) to facilitate sharpenability and aid toughness.
My understanding of the phases of steel are:
Very high temp: Molten.
High temp: Austentite (also known as gamma). Almost a semi-solid. The iron atoms are in a cubic lattice with a place for carbon in the middle of each square (face centred cubic lattice).
Room temp: All are solids.
Ferrite (aka alpha): The iron atoms are in a tighter lattice, with only enough room for 1 carbon in the centre of each cube (Body centred cubic lattice). Contains a maximum 0.02% carbon by weight.
Cemetite: Iron Carbide (Fe 2.5: C). Can form in grain boundaries in steels over 0.77% C or in spheres within the grains (depending on the specific heat treatment).
Pearlite: Microscopic plates of ferrite sandwiched between plates of cementite. Forms when austenite is slowly cooled. Pearlite is soft and (I think) brittle.
Bainite: Filaments of cementite in a ferrite matrix. Forms when austenite is cooled at faster rate. Tougher than martensite and almost as hard.
Martensite: This is the phase that we want in our knives. A tetragonal lattice of iron with a space for carbon in the centre of each cube (body centred tetragonal lattice). The more carbon in the lattice, the more the Fe-Fe bonds are stretched, so the more tension they have, hence the harder (and more brittle) the steel. Forms when austentite is cooled rapidly. Fresh martensite is quite brittle and needs to be tempered.
My understanding of the processes involved in HT are (with thanks to Kippington for the order of the list):
1) Forge or cut to shape (you can normalize at any stage during forging): The (unhardened) steel is shaped into a knife, for example, by hitting it with a hammer.
2) Normalization: The blade is heated well above austenisation temperature and "soaked" at this temperature (perhaps an hour?) to distribute carbon and alloying elements evenly throughout the blade, resulting in smaller carbides. This can be partially undone during the quench, especially if the quench is low temp (e.g.: honyakii), which can result in segregation banding. This process can result in grain size growth so there is sometimes a tradeoff between small grain size and small carbide size.
3) Grain refinement: The steel is forged (to fracture the grain boundaries) and/or temperature cycled (from austenisation temp to room temp) several times to reduce grain size. This works because when the steel is austentised, small new austentite grains form in the boundaries of the old ferrite/ pearlite grains.
4) Anneal/stress relieving: I don't really understand how this works. Please enlighten me.
5) Quench: The steel is heated to austenite. It is then cooled quickly (for example by placing in water, oil, molten salt, liquid nitrogen or air, depending on the steel) so that the Austenite is "frozen" into martensite (hard) before it has a chance to form pearlite (soft) or bainite (somewhat soft). For high carbon steels (over about 0.8%), a cryo quench is required to obtain full martensitastion (and prevent retained austenite, which reduces hardness).
6) Tempering: The knife is heated to below the austenisation temperature and cooled to room temperature. This causes a change in the martensite which makes it a lot less brittle but a little less hard.
7) Grinding, sharpening and blade finishing.
I look forward to your insights and refining my knowledge of this process. Thanks.
