where is the eutectiod point for steel?

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mitch8

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Oct 11, 2010
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i have heard .9, .83 and .77% C listed as eutectiod points for steel but each source makes it sound so cut and dry. how do i know which one is right?
right column on pg 11 of this one
http://books.google.com/books?id=br...l Metallurgy for the Non-Metallurgist&f=false
not necessarily a scientific source but still a competing number
http://dictionary.reference.com/browse/eutectoid+steel
and where i saw it first, in the graph on the top of this page. (this one would explain why people say forge 1084 if your a beginner, it would have the lowest austenitic temp.)
http://info.lu.farmingdale.edu/depts/met/met205/fe3cdiagram.html
 
I've seen .9% listed in older sources so I think you can eliminate that one. Perhaps the amount of Mn, S, P, and etc. can cause a shift in the eutectoid point. If that is the case then you are aiming at a moving target. My best guess is that any simple steel between 1075 and 1084 can be considered eutectic.
 
.83%.

It is often reduced to just .8% or .85% for simplicity. The .9% is often found in searches, but that is hypereutectoid.
To complicate things, it is often referred to as being "Between .7 and .9% carbon". That is a big range in steel.

All these variations come from the days when carbon content in steel was variable, and quality control was less. The equipment was also less technical than today's.

1084 is the eutectoid steel for knife making concerns.

If you look at a phase diagram chart (TTT), you will see the bottom of the trough at .83%.
This is for a pure carbon-iron mix. The actual temperature in steel used to make knives is a bit higher than on the theoretical chart, because there are alloys present. That is part of why we heat the steel 50-100° above the transformation point to austenitize it.
 
I have several references and they all list it between 0.77% and 0.83%. It is indeed a moving target, as most alloying elements will push the eutectoid composition down. Even something like L-6 or 8670M can technically be considered hypereutectoid. I feel pretty certain they have enough alloying to push the eutectoid carbon % below 0.70.
 
Understand the limits of the TTT diagram . It's an equilibrium diagram of just iron and carbon .We don't see that in the real world .
 
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