Subject: Hardness vs. hardenability

From: [email-masked] (MR WILLIAM A HOCHELLA)
Date: Sun, 8 Sep 1996 19:24:05, -0500

To All,
Hardness is the ability to resist penetration. 
Hardenability is a measure of the depth to which the work is hardened.

Low hardenability steels, i.e., plain carbon, can be hardened only as 
hard as the carbon content allows. These steels show a very shallow 
case when hardened properly. They require very drastic quenches like 
brine or Gunter's goo.
As the alloy content increases so does the hardenability. Oil 
hardening steels have more alloy than the water hardening grades and 
air hardening grades have more alloy than the oil grades.
The resulting hardness is a function of the carbon remaining in the 
martensite. You can harden a 1% carbon steel to a higher value than a 
1% carbon oil or air hardening grade. The alloying elements that 
increase hardenability also tend to promote carbide formation which 
effectively pulls carbon out of the martensite. The highly alloyed 
grades may be more wear resistant than lower alloy grades but they 
are not harder. The wear resistance comes from hard carbide particles 
in a reasonably hard matrix.
Inreasing the carbon content of a plain carbon steel increases the 
hardness up to a point and it increases the depth of the hardened 
layer up to a point. 
The maximum hardness that can be obtained is about Rc 70 at about .6/.
7% carbon. Most air hardening tool steels might flirt with Rc 62/64 
even at 1.5/2.0% carbon.
The alloying elements like Chromium, Nickel, Molybedenum, Vanadium 
and Tungsten Make the alloy more hardenable, more resistant ot wear, 
more resistant to tempering, harder to heat treat, more susceptable 
to decarburization, more expensive, etc. Nothing is free.

Hochewa