[TheForge] Heat Treating 5160 was Welding 5160

[theforge@mailman.qth.net]

To All,
Oy!
I just read through the string and I am not sure if I can be clear in a brief 
e-mail.
Lets forget about ferrite and pearlite.  Think ferrite and carbide.  The 
amount of time that it takes to transform a ferrite-carbide structure to austenite 
IS dependent on the alloy content. The method most quoted for the hardening 
of plain carbon tool steel is to place the work in a preheated furnace and when 
the work is the same color as the thermocouple, you quench it.  For tool 
steels with increasing alloy content, the instructions are to let it reach the 
temperature of the thermocouple and then hold for so many minutes per inch of 
thickness.  This is to make you hold it long enough to dissolve all of the alloy 
carbides.  If you do not dissolve all of the carbides, independent of their 
composition, you will not get the full benefit of the hardening process.   For 
very highly alloyed tool steels like T-1 or M-2, the austenitization is carried 
out in two stages.  Metals are generally such good conductors of heat that 
the surface and center of a work piece are no more that 50*F apart almost 
independent of size if the heat input is not intense like the flame of a torch.
The formation of martensite is a temperature dependent process.  You add 
alloy to make it easier, i.e., take longer to cool the work.  Alloy pushes the TTT 
curve to the right so you have more time to clear the "pearlite" nose of the 
curve.  Plain carbon steels have virtually not clear path to martensite so you 
have to use the most drastic quench available.  Yes, you can air or oil 
quench (really) thin sections of W-1 to full hardness all the way to the center.  
You use air on big blocks of A-2 because you can.
Alloy increases hardenability, i.e., the ability to deeply harden a piece 
with less dramatic quenches.  
Carbon is the only element that adds substantial hardness.  You can harden 
W-1 to higher levels than say D-2 because more of the carbon in the W-1 is in 
the martensite than the D-2.
D-2 has a lot of chromium in it which is a more potent carbide former than 
iron.  The chrome carbides impart wear resistance because they are harder than 
iron carbide or the martensite matrix.
The martensitic hardness actually peaks at about 60 points carbon in 
unalloyed steel.  Increasing the carbon content beyond that adds relatively little 
hardness and most the increase is the result of an increasing iron carbide 
content.

Regards,

Hochewa
BS MetE, VPI
PhD MetE, UHK


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