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- Sep 9, 2003
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The issue and background basics:
I often get questions regarding expected hardness levels for a given steel, what is maximum, how does their hardness level measure up and what they can do to increase it. It is one most common question topics I get and I just got another e-mail from a friend who is working with O1 (no surprise a very popular steel) and 1095 (he likes higher carbon levels), who wanted to know if cold treatments may help bump up his Rockwell number which he wishes were a bit higher after the quench. I promised I wouldnt go into specifics about his situation, name, actual numbers etc but I asked if I could provide a public explanation to his questions to help other with such a common and misunderstood issue.
At temperatures below 1335F steels iron atoms are in a stacking configuration that will only allow a maximum of .025% of carbon in solution, making it soft since the majority of that strengthening carbon is separated out into iron carbide groups. When we heat the steel above the critical temperature the iron atoms shift to a stacking that allows up to 100 times more carbon into solution with a maximum of 2% before moving on to cast irons.
This solution phase is called austenite. Think of these carbon atoms like wheel chocks stuck between the iron atoms all stacked up like oranges on a fruit stand. The carbon atom distorts the stacking and keeps the iron atoms from easily sliding past each other, this is what gives us hardness and strength at room temperature and even a little stiffness under that hammer it forging heat.
The idea is to cool things fast enough to keep the carbon solution above .025% at temperatures where it was never meant to be. For this discussion I will assume that the cooling is more than sufficient to avoid any other soft phases on the way down so we will have no need to worry about pearlite. If this is done to a temperature below 700F the chances of that carbon staying on solution become very good. At the point where steel begins to harden by making martensite the austenite will need to shift to a new stacking but it cant because of the trapped carbon, so instead it will have to heavily distort in order to accommodate a change. This distortion will be accomplished by a tilting of entire plains of atoms in sequence which is facilitated by a shearing action at the interfaces of these plains. Now that all sounds very technical, and it is, so all we really need to remember is that there is a whole lot of deformation and strain going on at the atomic level in order for steel to harden, and the driving force is the cooling.
Fortunately simple austenite is pretty pliable stuff and will easily accommodate all that distortion unless it is reinforced by something
I often get questions regarding expected hardness levels for a given steel, what is maximum, how does their hardness level measure up and what they can do to increase it. It is one most common question topics I get and I just got another e-mail from a friend who is working with O1 (no surprise a very popular steel) and 1095 (he likes higher carbon levels), who wanted to know if cold treatments may help bump up his Rockwell number which he wishes were a bit higher after the quench. I promised I wouldnt go into specifics about his situation, name, actual numbers etc but I asked if I could provide a public explanation to his questions to help other with such a common and misunderstood issue.
At temperatures below 1335F steels iron atoms are in a stacking configuration that will only allow a maximum of .025% of carbon in solution, making it soft since the majority of that strengthening carbon is separated out into iron carbide groups. When we heat the steel above the critical temperature the iron atoms shift to a stacking that allows up to 100 times more carbon into solution with a maximum of 2% before moving on to cast irons.
This solution phase is called austenite. Think of these carbon atoms like wheel chocks stuck between the iron atoms all stacked up like oranges on a fruit stand. The carbon atom distorts the stacking and keeps the iron atoms from easily sliding past each other, this is what gives us hardness and strength at room temperature and even a little stiffness under that hammer it forging heat.
The idea is to cool things fast enough to keep the carbon solution above .025% at temperatures where it was never meant to be. For this discussion I will assume that the cooling is more than sufficient to avoid any other soft phases on the way down so we will have no need to worry about pearlite. If this is done to a temperature below 700F the chances of that carbon staying on solution become very good. At the point where steel begins to harden by making martensite the austenite will need to shift to a new stacking but it cant because of the trapped carbon, so instead it will have to heavily distort in order to accommodate a change. This distortion will be accomplished by a tilting of entire plains of atoms in sequence which is facilitated by a shearing action at the interfaces of these plains. Now that all sounds very technical, and it is, so all we really need to remember is that there is a whole lot of deformation and strain going on at the atomic level in order for steel to harden, and the driving force is the cooling.
Fortunately simple austenite is pretty pliable stuff and will easily accommodate all that distortion unless it is reinforced by something
