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- Sep 9, 2003
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I think one topic that is very confusing to smiths is tempering, mostly because smith see it as such a simple follow up operation that we dont bother to look much deeper into the subject. From calling the entire hardening operation tempering (that one drives me insane!!
), to almost dismissing it as a second thought, tempering is too often glossed over in many otherwise well written texts. Due to the number of posts with questions regarding this topic, I feel another voluminous explanation coming on so get comfortable
Forgive me but I will need to cover some of the mechanisms of hardening and quenching in order to emphasis the importance of what happens in tempering.
As-quenched steel of sufficient carbon content will have undergone a radical transformation beginning when the quenching process reaches between 500F and 400F, at this point the fcc atomic arrangement of austenite will find the urge to transform into something a little more stable overwhelming.
Iron prefers to be bcc at room temperature and would have quickly done this at a much higher temperature if you hadnt outpaced its ability to do so by quenching it, but now it has a problem.
In order for the iron atoms to drop back into the bcc configuration it cant have things in the way, but you trapped these carbon atoms in between them and then cooled things to the point that they would loose hands down in a race with an arthritic snail (at higher temperatures, due to their size, the carbon atoms would leave Jesse Owens in their dust but they need heat to do this). So now the show is beginning and you have iron atoms wanting to take their seats but cant get in their theater chairs because the isle and rows are littered with all these narcoleptic carbon atoms. By the way the movie playing is Somethings Got to Give*.
What happens next is what differentiates martensite from most of the other phases you normally deal with in which the carbon atoms just get out of the way and sit in their own little sections reserved for them. As strain builds from cooling the austenite (comprised of the iron and carbon all mixed together) a shift from fcc is inevitable but it will not be gentle diffusion, it will require a radical deformation of the matrix, whole planes of atoms within the matrix will tilt on angle to initiate a shearing action at the interface of the old fcc and the new phase that will be distorted in bct (body centered tetragonal), not bcc, in order to accommodate the trapped carbon.
Unlike diffusive changes this reaction doesnt give a hoot in hell about time, it wants continuous drop in temperature, stop the cooling at any point to hold and there will be no more tilting and shearing until the cooling continues, hold it for an hour or hold it for a year, the shearing will not continue until it gets cooler. What could happen however is the austenite could stabilize and decide not to give even when the cooling starts again. This is also how alloying affects retained austenite, it strengthens the matrix so that it can better resist the tilting and shearing, thus you need extra freezing to totally convert the reinforced austenite.
I hope the previous paragraphs explained why it is important not to mess with the continuous cooling until Mf is reached, however it is worth mentioning that many alloys have Mf above 120F, but some have a lower one, you need to learn about your steel. I never said that tempering warm steel is not a good thing asl long as Mf has been reached. Just it is very important to emphasize certain things to people in order to avoid misinterpretation, I have just seen many people taking the hand warm thing too far and rushing to temper steel that has not reached Mf. Some hands can handle more heat than others and many of the steels we work with could have around 10% left to go at around 200F, so get right to the temper but please be aware of Mf. I also hope that the previous paragraphs will establish some important points for the following paragraphs.
When Mf is reached (Mf by the way stands for martensite finish and designates the point at which the austenite to martensite transformation should be complete) you will have a blade with a make up at the atomic stacking level that is very distorted and full of unimaginable amounts of stored energy, if this is not dealt with as soon as possible, once again something will have to give! And now we get to tempering
* the original unfinished film with Marilyn, not the recent Nicholson nonsense
), to almost dismissing it as a second thought, tempering is too often glossed over in many otherwise well written texts. Due to the number of posts with questions regarding this topic, I feel another voluminous explanation coming on so get comfortable
Forgive me but I will need to cover some of the mechanisms of hardening and quenching in order to emphasis the importance of what happens in tempering.
As-quenched steel of sufficient carbon content will have undergone a radical transformation beginning when the quenching process reaches between 500F and 400F, at this point the fcc atomic arrangement of austenite will find the urge to transform into something a little more stable overwhelming.
Iron prefers to be bcc at room temperature and would have quickly done this at a much higher temperature if you hadnt outpaced its ability to do so by quenching it, but now it has a problem.
What happens next is what differentiates martensite from most of the other phases you normally deal with in which the carbon atoms just get out of the way and sit in their own little sections reserved for them. As strain builds from cooling the austenite (comprised of the iron and carbon all mixed together) a shift from fcc is inevitable but it will not be gentle diffusion, it will require a radical deformation of the matrix, whole planes of atoms within the matrix will tilt on angle to initiate a shearing action at the interface of the old fcc and the new phase that will be distorted in bct (body centered tetragonal), not bcc, in order to accommodate the trapped carbon.
Unlike diffusive changes this reaction doesnt give a hoot in hell about time, it wants continuous drop in temperature, stop the cooling at any point to hold and there will be no more tilting and shearing until the cooling continues, hold it for an hour or hold it for a year, the shearing will not continue until it gets cooler. What could happen however is the austenite could stabilize and decide not to give even when the cooling starts again. This is also how alloying affects retained austenite, it strengthens the matrix so that it can better resist the tilting and shearing, thus you need extra freezing to totally convert the reinforced austenite.
I hope the previous paragraphs explained why it is important not to mess with the continuous cooling until Mf is reached, however it is worth mentioning that many alloys have Mf above 120F, but some have a lower one, you need to learn about your steel. I never said that tempering warm steel is not a good thing asl long as Mf has been reached. Just it is very important to emphasize certain things to people in order to avoid misinterpretation, I have just seen many people taking the hand warm thing too far and rushing to temper steel that has not reached Mf. Some hands can handle more heat than others and many of the steels we work with could have around 10% left to go at around 200F, so get right to the temper but please be aware of Mf. I also hope that the previous paragraphs will establish some important points for the following paragraphs.
When Mf is reached (Mf by the way stands for martensite finish and designates the point at which the austenite to martensite transformation should be complete) you will have a blade with a make up at the atomic stacking level that is very distorted and full of unimaginable amounts of stored energy, if this is not dealt with as soon as possible, once again something will have to give! And now we get to tempering
* the original unfinished film with Marilyn, not the recent Nicholson nonsense
Now every piece of steel I pick up I will be seeing a packed theater instead and wondering where the carbon atoms are sitting!
How do we determine what to fret over
?