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View Full Version : What makes a grade 8 bolt a grade 8 bolt?



clutch
07-07-2008, 07:37 PM
IIRC a grade 8 is about 175ksi tensile. What are they made out of and what kind of HT profile do they undergo?

Anyone care to share?

Clutch

Chipslinger
07-07-2008, 08:12 PM
According to the Chinese 6 little lines on the head and that is it.

J. R. Williams
07-07-2008, 08:34 PM
One item--- Money
JRW

kvom
07-07-2008, 08:35 PM
one reference I have states:

"medium carbon alloy steel, quenched and tempered, minimum tensile strength 150,000 psi."

lazlo
07-07-2008, 09:33 PM
Like KVom says, the SAE/ASTM bolt standards specify a tensile strength, heat treatment, and rough carbon content, but not the specific alloy:

Grade 0-1-2: 74,000 psi Low Carbon Steel
Grade 3: 100,000 psi Medium Carbon Steel
Grade 5: 120,000 psi Medium Carbon Steel Heat Treated
Grade 6: 133,000 psi Medium Carbon Steel Tempered
Grade 7: 133,000 psi Medium Carbon Alloy Steel
Grade 8: 150,000 psi Medium Carbon Alloy Steel

Grade 8's machine a lot like 4140 Pre-Hard, and have similar Rockwell Hardness and Tensile Strength, so the "medium carbon alloy steel" is probably very similar.

Evan
07-07-2008, 10:01 PM
Note the difference between carbon steel and alloy steel. Carbon steel has only iron and carbon as the significant constituents unless otherwise specified (ie. leaded steel). Alloy steel is alloyed with other metals to achieve the desired properties and may contain very significant quantities of several different metals other than iron.

lazlo
07-07-2008, 10:32 PM
Grade 8: 150,000 psi Medium Carbon Alloy Steel

Grade 8's machine a lot like 4140 Pre-Hard, and have similar Rockwell Hardness and Tensile Strength, so the "medium carbon alloy steel" is probably very similar.

4140 = Chromoly. It's very likely that your Grade 8 bolts are Chromoly, since it's one one of the most common alloys that can meet the 150 K Psi tensile strength requirement.

SmoggyTurnip
07-08-2008, 11:44 AM
Note the difference between carbon steel and alloy steel.


So is "carbon alloy steel" "carbon steel" or "alloy steel" or something else ?

TGTool
07-08-2008, 12:04 PM
So is "carbon alloy steel" "carbon steel" or "alloy steel" or something else ?

Well, steel can be alloyed with constituents, not including a high carbon content. Stainless steel, for instance could be a low carbon composition.

Roughly speaking, the "carbon" parts suggests to you that it can be hardened, and the "alloy" suggests additional characteristics such as heat strength, work hardenability, corrosion resistance or other things.

Peter N
07-08-2008, 12:10 PM
So is "carbon alloy steel" "carbon steel" or "alloy steel" or something else ?

A medium carbon low-alloy steel will usually have around 0.4-0.45% carbon (taking it into the 'medium carbon' range as it is > 0.25%) but will also usually have less then 8% of the alloying element (making it 'low-alloy').

For example, our EN19 grade (equivalent to your 4140) is a 'medium carbon low-alloy steel', having a composition of:

C. 0.40% / Si.0.25% / Mn. 0.70% / Cr. 1.20% / Mo. 0.30%

where the Chromium and Molybdenum are the main alloying elements.

Plain medium carbon steel (our EN8/your 1045) would have the same carbon/silicon/manganese content (Si. & Mn. are essentially process aids for steelmaking), but no other alloying elements.

Does this help or is it still as clear as mud? :D

Peter

lazlo
07-08-2008, 12:30 PM
So is "carbon alloy steel" "carbon steel" or "alloy steel" or something else ?

Carbon steel is steel with varying amount of carbon. Alloy steel has an alloying agent, usually Chromium and Molybdenum.

So like I said: 4140 = Chromoly = carbon alloy steel. So the Grade 8 bolts are most likely Chromoly.

The bolt maker is free to use any alloying agent they want to meet the Grade 8 spec (and in my experience, ultra-premium bolts like Unbrakos are far stronger than "normal" Grade 8 bolts), but Chromoly is the most cost effective alloy, and therefore the most likely.

Swarf&Sparks
07-08-2008, 12:57 PM
If you need bolts to spec, buy em from an industrial supplier that sells to spec. (and get a receipt, ISO 90XX, etc)

If not, buy from the local hardware store to suit you needs.

lazlo
07-08-2008, 01:00 PM
Lin, I think he's trying to figure out what kind of stock a Grade 8 bolt would make.

Grade 8's machine a little worse than 4140 Pre Hard.

Swarf&Sparks
07-08-2008, 01:07 PM
OK, fair call Rob.
But why bother with "grade XX" fasteners?
Chuck it up and see how it cuts.

Unless you have to follow an audit trail, or a moral trail ("would I hang my harness on this?")

Frankly I've often wondered about this "grade xx" bolt/screw I've seen here from US members.
Can you enlighten me?

lazlo
07-08-2008, 01:18 PM
Frankly I've often wondered about this "grade xx" bolt/screw I've seen here from US members.
Can you enlighten me?

People are looking for cheap turning stock.

Swarf&Sparks
07-08-2008, 01:58 PM
Only time I looked for "cheap turning stock", the client paid up happily.

Needed >1" dia by 2" L to fix his rudder stock, 316.
Near $40 for the bolt, near $200 for 200 mm of 32mm 316 bar.

He decided that the bolt shank was "just fine, and how soon can I have my rudder back?"

But, again, that was traceable.

macona
07-08-2008, 05:54 PM
Shoot, you can get a pack of 3' lengths of 4130 from 3/16" to 1" for something like $40 or $50 from Online Metals. Bolts are expensive turning stock with an unknown pedigree.

.RC.
07-08-2008, 06:30 PM
Carbon steel is steel with varying amount of carbon.

Well there is no such thing as a non carbon steel anymore...Wrought iron which had minuscule amounts of carbon in it is no longer made..

lazlo
07-08-2008, 06:49 PM
Well there is no such thing as a non carbon steel anymore...

It's the other way around:they're distinguishing steel with only carbon in it from steel with alloying elements like chromium and molydenum. So 1018 and 1045 are low and medium carbon steels, and 4140, 4150, 4340 are alloys steels, according to SAE's definition.