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- Jan 11, 2010
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There is a huge amount of information available on heat treat and steel phases, but it's rather like taking a sip from a fire hose. I'm left staring at the ceiling wondering what hit me.
I do the newsletter for a knife makers club (5160 Club) and want to include a "hallway water fountain" sip of basics in the newsletter I'm writing this week. Some of our folks know this front-to-back but there are newbies and a lot of folks in-between.
I'm hoping that those knowledgeable in metallurgy <cough><cough>Kevin Cashen?<cough><cough> could spare a few minutes to look this over and make sure I'm not propagating any errors:
For the newsletter:
At the OKCA show in April one of my table-mates was wondering why a freshly hardened blade might crack if just left sitting and what exactly happens during tempering. What follows is based on what I've learned from books and talks by Wayne Goddard, the ABS instructors in Old Washington, and others, the postings of Kevin Cashen, and the PDF document by John Verhoeven. If I'm made aware of any errors due to my limited understanding of metallurgy I'll publish a correction.
A lot of this is generalized for simple carbon steels. Alloying elements change the temp and speed at which a steel will change phase, so every steel is a little different and has it's own charts.
Here's my brief & basic on steel phases & heat treat:
I bet you know that simple steel is iron (Fe) with less than 1.5% carbon (C) and trace amounts of impurities. If you get more than 2% C you are getting into the realm of cast iron. The eutectoid point for C in Fe is 0.77%C. This is the amount of C that fully occupies the austenite phase of steel. More C in the steel is referred to as hypereutectoid, less than 0.77% is referred to as hypoeutectoid. In numbered steels the last digits refer to the 100th of a percent of C. The reference is not always exact so 5160 (0.56-0.64% C) is hypoeutectoid... and 1084 (0.80-0.93% C) is hypereutectoid.
Ferrite is the phase steel wants to be in at room temperature. It is like a cubic crystal lattice with Fe atoms at the cube corners which can hold one C atom in the center of each cube called Body Centered Cubic (BCC) or alpha iron. Only 0.02% C can be held in ferrite.
Austenite is phase of steel created by heating that also has a cubic lattice structure but in this phase C atoms can settle into the lattice at each face of the cube called Face Centered Cubic (FCC) or gamma iron. This allows the steel to suck much more C into the lattice. 0.77%C can be held in austenite, and when you include other metallurgical magic, I've read that austenite can hold 80 times the C that ferrite can hold.
Cementite and other carbides are where that extra C wants to live when it can't get into the Fe lattice. Cementite (Iron Carbide Fe3C) is harder than the regular steel lattice but softer than other carbides we find in alloyed steels.
If austenite is cooled slowly, the C atoms have time to migrate out of the lattice and bind into carbides. In plain steels this will be cementite which forms tiny pure cementite plates between layers of pure ferrite. This combination is called pearlite and is about as soft as you will get a steel at room temp.
If austenite is cooled very rapidly the C atoms do not have time to get out of the lattice and this causes a stressed lattice structure called martensite. The lattice is literally stretched into a Body Centered Tetragonal form (BCT) to accommodate the C that could not diffuse out in time. When quenching for martensite, each steel has a Martensite Start (Ms) temp and a Martensite Finish (Mf) temp. Martensite starts forming at Ms, but you need to reach Mf within the time given in the steel's Time Temperature Transformation (TTT) diagram in order to get full martensite without retained austenite. Martensite is both hard and brittle due to this stress.
Like martensite, bainite is formed by initially cooling austenite very fast to miss the nose of the steel's TTT diagram, but stopping and holding the temp well above Ms for long enough to go through the TTT diagram's lines at a constant temp before finishing the quench. I gather that this generally requires use of a molten salt bath to achieve the fast initial quench and then hold at that a temp below the nose of the diagram and above Ms. Bainite is similar to pearlite in that it is a mix of ferrite and cementite but in bainite, cementite forms in smaller filaments and loose particles. Actually there are two forms of bainite upper and lower with lower bainite having finer cementite structures. Lower bainite can be almost as hard as martensite and can be tougher than tempered martensite with the same Rockwell (Rc).
Retained austenite refers to austenite that does not transform (to martensite or pearlite or bainite) on quenching. In my comments on martensite (above) I noted that if you quenched to a temp between Ms and Mf that you would only transform a portion of the austenite. The rest is retained austenite. Retained austenite is unstable in the long run at room temp and is living on borrowed time. When it does transform it tends to form untempered martensite. The unrelieved stresses gradually accumulating as retained austenite transforms into more untempered martensite might explain stories of an untempered blade just cracking for no reason if left too long on the workbench.
Tempering is primarily to transform untempered martensite into tempered martensite. When martensite is heated to a few hundred degrees (exact temp varies depending on the steel and the desired effect), some of the C atoms trapped in the martensite will migrate out of the lattice, forming carbides and leaving the martensite in a less stressed state. This adds toughness to the blade, making it less brittle. For simple steels, tempering up to about 375°F causes very little loss of hardness.
All phases of steel (ferrite, austenite, pearlite, martensite, bainite) are formed of grains. Within a grain the crystal-like Fe lattice is oriented in one direction. In neighboring grains the Fe lattice will be oriented in other random directions. When steel changes from one phase to another (ferrite to austenite, austenite to pearlite, etc.) seed grains for the new phase generally start scattered along existing grain boundaries and grow from there. This is why thermal cycling between steel phases tends to reduce grain size. This occurs during phase change both on heating and on cooling. If the steel is not overheated then these new grains tend to remain smaller than the old grains. Overheating causes some grains to consume their neighbors, forming larger grain sizes.
The temperature at which steel becomes non-magnetic (Curie temp) is 1414°F regardless of the %C in the steel and has something arcane to do with Fe electron's angular momentum and spin.
The theoretical transformation of ferrite to austenite is marked on phase diagrams by the A3 line for hypoeutectoid steels and the Acm line for hypereutectoid steels. This theoretical austenizing temp is lowest at the eutectoid point (1340°F at 0.77%C). The theoretical austenizing temp varies according to %C. Above and below 0.77%C the austenizing temp increases as shown in the A3 and Acm lines. The recommended real-life austenizing temps are higher than the A3 and Acm lines I'm assuming this is to fully transform ferrite into austenite and to allow for existing carbides to dissolve properly. For instance, a recommended temp for austenizing 5160 is 1525°F even though the diagram's A3 line at 0.60%C is about 1380°F.
And there you have the 10 cent tour of my current understanding of heat treatment and steel phases. Let me know if any of this looks wrong or misleading.
Thanks!
I do the newsletter for a knife makers club (5160 Club) and want to include a "hallway water fountain" sip of basics in the newsletter I'm writing this week. Some of our folks know this front-to-back but there are newbies and a lot of folks in-between.
I'm hoping that those knowledgeable in metallurgy <cough><cough>Kevin Cashen?<cough><cough> could spare a few minutes to look this over and make sure I'm not propagating any errors:
For the newsletter:
At the OKCA show in April one of my table-mates was wondering why a freshly hardened blade might crack if just left sitting and what exactly happens during tempering. What follows is based on what I've learned from books and talks by Wayne Goddard, the ABS instructors in Old Washington, and others, the postings of Kevin Cashen, and the PDF document by John Verhoeven. If I'm made aware of any errors due to my limited understanding of metallurgy I'll publish a correction.
A lot of this is generalized for simple carbon steels. Alloying elements change the temp and speed at which a steel will change phase, so every steel is a little different and has it's own charts.
Here's my brief & basic on steel phases & heat treat:
I bet you know that simple steel is iron (Fe) with less than 1.5% carbon (C) and trace amounts of impurities. If you get more than 2% C you are getting into the realm of cast iron. The eutectoid point for C in Fe is 0.77%C. This is the amount of C that fully occupies the austenite phase of steel. More C in the steel is referred to as hypereutectoid, less than 0.77% is referred to as hypoeutectoid. In numbered steels the last digits refer to the 100th of a percent of C. The reference is not always exact so 5160 (0.56-0.64% C) is hypoeutectoid... and 1084 (0.80-0.93% C) is hypereutectoid.
Ferrite is the phase steel wants to be in at room temperature. It is like a cubic crystal lattice with Fe atoms at the cube corners which can hold one C atom in the center of each cube called Body Centered Cubic (BCC) or alpha iron. Only 0.02% C can be held in ferrite.
Austenite is phase of steel created by heating that also has a cubic lattice structure but in this phase C atoms can settle into the lattice at each face of the cube called Face Centered Cubic (FCC) or gamma iron. This allows the steel to suck much more C into the lattice. 0.77%C can be held in austenite, and when you include other metallurgical magic, I've read that austenite can hold 80 times the C that ferrite can hold.
Cementite and other carbides are where that extra C wants to live when it can't get into the Fe lattice. Cementite (Iron Carbide Fe3C) is harder than the regular steel lattice but softer than other carbides we find in alloyed steels.
If austenite is cooled slowly, the C atoms have time to migrate out of the lattice and bind into carbides. In plain steels this will be cementite which forms tiny pure cementite plates between layers of pure ferrite. This combination is called pearlite and is about as soft as you will get a steel at room temp.
If austenite is cooled very rapidly the C atoms do not have time to get out of the lattice and this causes a stressed lattice structure called martensite. The lattice is literally stretched into a Body Centered Tetragonal form (BCT) to accommodate the C that could not diffuse out in time. When quenching for martensite, each steel has a Martensite Start (Ms) temp and a Martensite Finish (Mf) temp. Martensite starts forming at Ms, but you need to reach Mf within the time given in the steel's Time Temperature Transformation (TTT) diagram in order to get full martensite without retained austenite. Martensite is both hard and brittle due to this stress.
Like martensite, bainite is formed by initially cooling austenite very fast to miss the nose of the steel's TTT diagram, but stopping and holding the temp well above Ms for long enough to go through the TTT diagram's lines at a constant temp before finishing the quench. I gather that this generally requires use of a molten salt bath to achieve the fast initial quench and then hold at that a temp below the nose of the diagram and above Ms. Bainite is similar to pearlite in that it is a mix of ferrite and cementite but in bainite, cementite forms in smaller filaments and loose particles. Actually there are two forms of bainite upper and lower with lower bainite having finer cementite structures. Lower bainite can be almost as hard as martensite and can be tougher than tempered martensite with the same Rockwell (Rc).
Retained austenite refers to austenite that does not transform (to martensite or pearlite or bainite) on quenching. In my comments on martensite (above) I noted that if you quenched to a temp between Ms and Mf that you would only transform a portion of the austenite. The rest is retained austenite. Retained austenite is unstable in the long run at room temp and is living on borrowed time. When it does transform it tends to form untempered martensite. The unrelieved stresses gradually accumulating as retained austenite transforms into more untempered martensite might explain stories of an untempered blade just cracking for no reason if left too long on the workbench.
Tempering is primarily to transform untempered martensite into tempered martensite. When martensite is heated to a few hundred degrees (exact temp varies depending on the steel and the desired effect), some of the C atoms trapped in the martensite will migrate out of the lattice, forming carbides and leaving the martensite in a less stressed state. This adds toughness to the blade, making it less brittle. For simple steels, tempering up to about 375°F causes very little loss of hardness.
All phases of steel (ferrite, austenite, pearlite, martensite, bainite) are formed of grains. Within a grain the crystal-like Fe lattice is oriented in one direction. In neighboring grains the Fe lattice will be oriented in other random directions. When steel changes from one phase to another (ferrite to austenite, austenite to pearlite, etc.) seed grains for the new phase generally start scattered along existing grain boundaries and grow from there. This is why thermal cycling between steel phases tends to reduce grain size. This occurs during phase change both on heating and on cooling. If the steel is not overheated then these new grains tend to remain smaller than the old grains. Overheating causes some grains to consume their neighbors, forming larger grain sizes.
The temperature at which steel becomes non-magnetic (Curie temp) is 1414°F regardless of the %C in the steel and has something arcane to do with Fe electron's angular momentum and spin.
The theoretical transformation of ferrite to austenite is marked on phase diagrams by the A3 line for hypoeutectoid steels and the Acm line for hypereutectoid steels. This theoretical austenizing temp is lowest at the eutectoid point (1340°F at 0.77%C). The theoretical austenizing temp varies according to %C. Above and below 0.77%C the austenizing temp increases as shown in the A3 and Acm lines. The recommended real-life austenizing temps are higher than the A3 and Acm lines I'm assuming this is to fully transform ferrite into austenite and to allow for existing carbides to dissolve properly. For instance, a recommended temp for austenizing 5160 is 1525°F even though the diagram's A3 line at 0.60%C is about 1380°F.
And there you have the 10 cent tour of my current understanding of heat treatment and steel phases. Let me know if any of this looks wrong or misleading.
Thanks!
