Bolt Steel 158 Carpenters or 8620 steel

[bcammons]

A local company in my area has 8620 bolts that are made in their shop for a pretty good price.
They are a very reputable company in my area and have some good quality parts.
Their bolts are made from 8620 steel instead of 158 Carpenters. Any benefits to a 8620 bolt over a 158?
They are MP tested but not HP. Thinking about getting a spare bolt, most likely will be a BCM, but just wanted to know thoughts on the difference in materials used.

[Stickman]

There is a link somewhere that gives ratings for bolts and carriers, and shows what is considered acceptable, I'll see if I can find it, hopefully someone else has it bookmarked.

[bcammons]

Yes, Ive seen the comparison before. Just dont remember seeing anything about which steel is best for a bolt, 158 or 8620

[jmart]

158 is considered better.

Probably depends upon your usage. You can get a BCM bolt made of 158 for not a whole lot, $55-60 IIRC. Not sure how much cheaper a 8620 is going to save you.

[rob_s]

MP has significantly less value without HPT as I understand it.

[Doc Bob]

First post gentlemen. I hover over at Arfcom and thought I might join the party here. I have a moderate background in metals and find the discussions on materials interesting, especially when the discussion turns to mechanical properties and their applications to AR15s. I bought a Carpenter steel bolt years ago from Fulton Amory more out of convenience than choice (back ~1998 or so.) When comparing 8620 to Carpenter 158 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. However 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:
http://www.cartech.com/techcenter.aspx?id=1730
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.) However, 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 heat treat.

Thanks for letting me rattle on... and I look forward to what "The List" has to say.



By the way Carpenter 158 is a proprietary name, tomorrow I will look up an AISI or ANSI equal.