Hypereutectoid steels tempered to lower hardness

R

RX-79G

Joined
Jun 23, 2006
Messages
2,383
Hyperetectoid steels get both abbrasion resistance and measured hardness from the carbides they form. If you properly heat treat a carbide forming steel so it comes out of the quench at 63+ Hrc, how does it behave when you temper down to something like 52-55 Hrc?

I'm imagining that the carbides stay the same - little nuggets of high hardness, and the steel matrix that surrounds it gets soft - like wetting the dust in a compacted gravel driveway.

How does this behave compared to a eutectoid steel at the same hardness? Can you even legitamately compare hardnesses, since the net Rockwell reading is going to be a kind of average of the carbides and the surrounding steel, where the surrounding steel is going to have to have a fairly low hardness to balance the carbides high hardness? In other words, if you tempered the steel matrix itself of hypereutectoid steel to 53, would the actual reading be 56 Hrc because of the carbides?

And if that's the case, are a hypereutectoid steel at 53 really comparable to a eutectoid steel at 53, or does the eutectoid steel have an "advantage" in that there are no carbides to confuse the Rockwell test?

I just keep thinking that using Rockwell to compare steels really starts to break down at lower hardnesses, because the tester would register a solid block of oak as being the same hardness as foam rubber embedded with rocks, even though their relative strengths would be radically different.


I hope I've explained my thinking well enough to have a conversation comparing carbide to non-carbide steels from the perspective of hardness. Thanks.
 
The carbides don't confuse the Rockwell tester, you are getting a true measurement of the steel. You would have to have an unrealistically high carbide content before that would be an issue. It is a common misconception that when carbide precipitation leads to higher hardness such as in high temperature tempering that you are getting soft steel with lots of carbides in it. That's not true. In that case the carbides are so small they actually increase the strength of the steel itself. This is a strengthening mechanism called precipitation hardening. However in the case you are referring to where you temper a steel down to 52 Rc there is no funny business going on, it is just 52 Rc steel with a higher carbide volume than the low carbon steel.
 
I wasn't thinking that the steel in between the carbides was soft. I was thinking that the as-quenched hardness of carbide forming steels is higher than eutectoid steels, and this was due to the presence of the carbides. So while a full quench will get you maybe 62 Hrc with 1080, the presence of iron carbides will take up to 65 Hrc. Given the 3 Hrc difference in as quenched, if you want to temper down fairly low, is going to the same Hrc really comparable? Or should a 55 Hrc 1095 sample really be compared to a 52 Hrc 1080 sample for the most apples and apples?
 
It is not the carbides in hypereutectoid steels leading to higher hardness, but higher carbon content of the martensite.
 
To back up what Larin is saying:
The carbides are not what makes the Rockwell reading higher or lower. It is the hardness of the martensite, which gets harder as the carbon rises ( up to about 1.4%).

The carbides formed by high alloy steels are not the same. The Cr-C, W-C, Mo-C, V-C, etc don't make the steel harder as-quenched. This can easily be observed by looking at the as-quenched of high carbide stainless steels ... which is lower than a simple carbon steel with far less ( or no) carbides. At a certain point, the hardness is affected by hugh carbide percentages, but this is not a factor in knife blades.
 
You must log in or register to reply here.
Back
Top