Tá fáilte romhat, Mr Cashen!
You speak Gaelic, Kevin? Many years ago, I signed up three years running for a Gaelic course at the community college but they never had enough interested folk to run the class. Sure would have enjoyed that.
I am sure no more than you, Fitzo. But let's try some technical speak, which is Greek to too many.
Now…
The TTT curve is for determining phases formed at certain temperatures while cooling, and each alloy has its own unique curve, making them much more applicable to heat treating uses. The Fe/Fe3C (iron/carbon) equilibrium diagram is a more general phase diagram that covers the concept of simple iron/carbon alloys, making it less useful in direct application to heat treating, but quite invaluable in understanding the basic concepts of phases when heating iron carbide systems. In short the graph below is applicable to a pure iron carbon system and any alloying or other elements will move some of the lines in significant ways:
On this chart the left hand side has a column of numbers that represent the temperature, along the bottom is a row of numbers that show the carbon content in the mix. You will notice that there is an entire wide area to the right of 2% carbon with many more features to it. I will not delve into this because the only area of the chart that pertains to knifemaking steels rests to the left, or less than, 2%; the area in excess of this deals with cast irons more than steel.
In the middle if the steel range you will see .8% with a dotted line shooting straight up from it, this is a very significant line and it is called the eutectoid. Note- “toid”, not “tic” please do not make the mistake of calling it the “eutectic” there is a eutectic on the chart but it rests far in the cast iron range, don’t feel bad about the confusion I have seen folks with some pretty impressive letters beside their name make this error. The eutectoid is the point where upon cooling the iron and carbon ration is at just the right mix that pure pearlite will be made with no leftover carbon or iron (cementite or ferrite). But also upon heating there will be no leftovers to dissolve so it will be the easiest to convert to austenite.
Steels to the left of this line all called hypoeuctoid and have extra ferrite (iron), while steels to the right of this line have leftover cementite (carbon) and are called hypereutectoid. The leftovers are described as proeutectoid, so if you anneal a piece of 1095 it will be mostly pearlite with proeutectoid cementite mixed in.
Now for the heat, the chart assumes that you have a pearlitic (annealed) steel, it is after all simple carbon steel, on heating everything is pearlite until you reach the first line at around 1335F. This is called A1 (if you are heating it is called Ac1, if cooling it is called Ar1), it is at this temperature that the shift from BCC atomic stacking to FCC stacking begins to occur. Since FCC has more spaces between the iron atoms it facilitates the carbon atoms movement much better and allows the iron to dissolve the carbon to form austenite. So A1 is the line that separates the pearlite field from the zone consisting of iron or carbon mixed with varying degrees of austenite.
The next line is A2, it designates the Currie point for steel (1414F), this gets a bit confusing but it is the point at which steel loses magnetism and, contrary to what virtually 95% of all bladesmiths believe, has nothing to do with determining when a steel has reached proper solution. Not being as keen on physics as I am on metallurgy, the best I gather is that the Currie point is the result of the change in the spin of the electrons in the iron atom shell resulting from the allotropic (bcc to fcc) shift. When enough of the stacking has shifted to bring about this differing electron spin ferromagnetism will begin to fade and thus the Currie point. So it is not heavily affected by alloying or carbon content, and thus is not at all fool proof in determining the proper temperature of most alloys beyond simple iron carbon. It would be great for a steel that had at hardening temperature of 1400F but lousy for one that needs 1525F (like 5160).
I am getting large here so I will cover more in another post.
Once you do a full interrupted quench, do you "soft draw" the back of the blade, riccaso and tang or just temper and rock on?
.
Few do, though they'd tell you otherwise.
