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And is this a good thing I assume?
When comparing 8620 to Carpenter 158, another common Bolt material, you will notice that 158 has a much lower carbon content, and higher Chromium and Nickel content than 8620.
158 is truly an alloy metal, while 8620 is a low alloy metal.
-- in the world of steels that does not necessarily make it better but different.
Nickel added to steels produces a austenite microstructure which adds strength. Higher carbon contents of steels and (using Molybdenum and Mangenese as alloying agents ,) will generally form a martensite microstrure, and/or carbides upon heat treat which is also very strong.
But remember that Strength, Toughness, Ductility, Elasticity, and Hardness are properties of metals that rise and fall in relation to each other.
Some specs pulled from the internet:
Alloy TechZone - Typical Properties
Note the amount of nickel, chromium, and carbon in 158.
http://www.suppliersonline.com/propertypages/8620.asp
Note the low content of nickel, chromium, and higher carbon content of 8620.

All in all, this only demonstrates the differences in chemistry but the mechanical properties are the true test of a materials application which would have to be tested in a controlled environment (yield, tensile, elasticity, etc.)
-- my personal opinion is that Carpenter 158 is superior to 8620 in that 158 gains its strength from its high alloy, where as 8620 gains its strength from high carbon and low content Mn, Mo, Cr and ability to take a carburized ( Carbon rich ) heat treat.

I haven't seen Carpenter 158 bolts in quite a while, I LIKE the Steels that the Carpenter Co, produces
8620 is a common steel used in machinery parts like gears
 

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And is this a good thing I assume?
They are adequate if well treated. What I meant to say is that carriers sometimes are cheaper deals because they have 8620 bolts.
This doesn't mean a good deal cannot be a decent carrier but re-sellers sometimes do not even know what they are selling.
Carpenter 158 and other alloys are harder but they cost more. Something for folks to take into consideration depending on what the
purpose for the upper and what they are looking for.
That's all.
 
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