To expand upon Nathan's comments:
The eutectoid point in steel is .83% carbon, so 1084 is eutectic steel.The critical point for eutectoid steel is the lowest on the curve. If you raise the carbon content ,or lower it ,the transformation point goes up. For most situations in HT of a blade, about 50-100F above this critical point is a good temperature to austenitize the steel. Most blade steels have a soak temperature from 1475F to 1525F.
Now, to complicate things, the carbon has a real square dance party when it is above the critical point. In the case of hypo-eutectoid steel it has to run from one partner to the next , because of all the available iron. In hyper-eutectoid steels, it has to play musical chairs, and if it can't find a chair ( too much carbon -too little iron) it just hangs out with the other carbon atoms over at the punch bowl. At the end of the dance, as the austenite cools below the transformation point the carbon gets to go home with the iron atoms it was dancing with. These lucky foursomes become Fe3C ( iron carbides) called cementite. The wallflower iron in the hypo eutectoid steel becomes ferrite, which can only be truly happy Massachusetts and California. The carbon in hyper- eutectoid steel are all a bunch of swingers.The dancers go home in groups, and depending on how fast they leave (cool down at quench) the ones who mosey out to their cars become pearlite, the ones that stay and smoking cigarettes and chatting for hours become bainite, and the really hard core folks rush out and jump in their cars to beat the traffic and become martensite. So the steel usually becomes a mix of structures.
Back to real metallurgy:
To allow the carbon to completely diffuse in hypo-eutectoid steel, and for the carbides to dissolve in hyper-eutectoid steel, you have to hold (soak) the steel at the austenitizing temperature for a little while. About 5-10 minutes for simple carbon steel is enough. The temperature is lowered a bit as you move away from 1084, not because the transformation point is lower (it is higher), but because you will be giving it a little more time to get the carbon situated. Since grain growth occurs as a result of time and temperature, making the time longer necessitates lowering the temperature. So 1050 and 1095 are held for 5 minutes at 1475F and 1084 is held for one minute at 1525F.
Stacy
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