Rockwell Hardness as a predictor of toughness/brittleness?

Okay, this is yet another SAE Grade 5 vs Grade 8 fastener question - are Grade 8s more brittle than Grade 5 and does it matter for practical purposes.

My main question in this post is "I thought that generally harder materials were more brittle rather than tough?"

I've been going around in circles trying to definitively answer this and just get conflicting answers. Yes it is. No it is not. Yes it is but it doesn't matter because the Grade 5 will have already failed. It is also my understanding that SAE does not specify notch toughness or ductile/brittleness for Grade 5/8.

About the best explaination I've seen is at

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is an automotive enthusiast site.

However, I disagree with what the author wrote in the second paragraph quoted below.

I'm fully prepared to be shown wrong! :-) It's been over 20 years since I've studied the stuff. :-(



****from roughly the middle of the weblink above ****

Again, you can see that the grade 8 will support over 1000 lbs more or a 1/2-ton more. But there?s something more important to note. The grade 5 fastener has already reached its ultimate load and FAILED BEFORE the grade 8 starts to yield or stretch. Therefore, the argument that you should not use grade 8?s because they are more brittle than grade 5?s is not a true statement in most applications.

Toughness is an important feature of a fastener. It is the opposite of brittleness and gives you an idea of how it will handle abuse without being damaged and eventually weakening the fastener or can cause fatigue to appear much earlier than normal. One way to ?measure? toughness is by looking at the hardness rating of a fastener. The higher the number (Brinell, Rockwell ?) the harder the material is and the tougher it is to damage. According to Marks? Standard Handbook for Mechanical Engineers, Grade 5?s typically have a core Rockwell hardness of C25-C34 whereas a grade 8 typically has a core Rockwell hardness of C33-C39. Based on this, grade 8?s are tougher than grade



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In general higher strength metals have less toughness.

Toughness is related to energy. Fracture toughness and impact toughness are higher in more ductile materials. So I think the usage of toughness is incorrect above.

Reply to
Jeff Finlayson

JJ said the following on 5/24/2004 6:31 AM:

When presented with a choice between using Grade 5 or 8, I've always assumed that my bolted joints will be overstressed sometime in the future. I then ask myself "How do I want my joint to fail? Does a brittle or ductile failure mode affect the safety of people who may be standing next to my bolted joint?"

In almost all my applications - pressure vessels or pipelines - I want my overstressed bolted flange joints to open up a bit and start leaking. The leak will serve as a last ditch pressure relief or at least warn people that there's a problem and maybe they should get the hell away from there. This leads me to select Grade 5 almost all the time.


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Stronger bolts tend to lose ductility. There is a general inverse correlation there. So a tension bolt (=no bending) is stronger as grade 8. A bolt which must hold on through a crash etc, when bent would specify grade 5 profitably.

Brian Whatcott Altus OK

Reply to
Brian Whatcott

It's been my experience that the stronger grade 8 screws are superior for tension-only loads, where it's necessary to keep things held tightly together. When shear forces are possible, then it almost always makes sense to include some other kind of device to absorb that load, so the screws don't have to.

One common technique I use in machine designs involves situations where a machine might be crashed, and shear forces might be exerted on an assembly that's screwed together. I typically use tough, soft dowels or taper pins, along with grade 8 screws. The clearance holes for the screws are oversize, and allow a bit of lateral movement between the assembled parts under extreme conditions. When a crash occurs, the screws provide maximum holding power against tension loads that would pull the assembly apart. Any sliding, shearing motion that occurs will act first on the dowels or taper pins, which are sacrificed while absorbing the energy of the crash. The screws don't get shear-tested until the assembled parts have moved enough to use up that extra clearance in the holes. By that time, the energy is mostly gone, and the screws can usually survive.

The result is that a crash will wreck the sacrificial dowels, and maybe beat up the screws a little bit; but the assembly won't just come apart and send bits and pieces of itself flying around to hurt someone, or to cause further damage to the machine. Similar methods sometimes use soft keys, or deformable plates on aligned surfaces of assembled parts.

Screws are tension devices. Other kinds of loads require other kinds of hardware.


JJ wrote:

Reply to
Kirk Gordon

My understanding is that Gr. 8 bolts are for all intents & purposes the same as A 325s. Assuming this to be true, the Gr. 8 bolts are much stronger that Gr. 5s both in shear & in tension. In common construction usage of A 325 bolts of 5/8" to 1-1/4" dia. (typically) used w/ common construction grade steel of ~50 kips the bolts are so much stronger than the steel that there is no question which is going to fail first in shear. It would take an extremely deficient bolt to fail first. (Although over the years counterfeit A 325s have been found, but that's not the point here). The other more interesting way A 325s are used is in what are called "friction connections" These connections are torqued to 70% (?) of yield. This puts so much pressure on the annular area between the nut & bolt head and the 2 plys of steel, that the failure is calculated to be @ the exterior of the annular area in the base metal. The shear strength of the bolt is not considered. The point is that toughness does not come into play. I would think that w/ base metals which are as strong as the bolt, which could deliver a load large enough to impact stress the bolt, then it's toughness could come into question. Much better explanations & data can be found in the AISC Manual of Steel Construction, which references another small book specifically about A-325s & A 490s & the ASME section that covers materials. By the way, toughness does not relate directly to any other property. That's why when toughness is a design concern; the material (whatever it might be; base metal; welding electrodes; bolts) is given a Charpy (misspelled?) impact test.

Hope this helps, John

Kirk Gordon wrote in message news:...

Reply to
John McGraw

JJ: You are right.

Rockwell (or Brinell or Vickers) hardness of non-austenitic steels is a very good estimator of tensile strength. As John and others also correctly stated, hardness does not have much to do with toughness. Tensile strength increases as hardness increases. Toughness decreases as hardness increases.

Hardness is widely used as a proxy for tensile strength both for quality control and failure analysis purposes. In most testing labs there is a wall chart of hardness conversions with a column of estimated tensile strengths. The standard source for this correlation is ASTM A370.

John McGraw understands wrong. Grade 8 bolts (SAE J429) are very similar to ASTM A490 structural bolts. ASTM A325 bolts are very similar to SAE Grade 5. For a quick comparison you can look at the physical properties chart at

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it is best to actually look at the relevant specifications before ordering or making engineering decisions.

Given that bolts are inevitably going to get torqued to their yield strength (FTY), the right question should be whether a Grade 8 fastener has enough fracture toughness (KIC) to tolerate the same flaw size (A) as a Grade 5 fastener, using KIC = FTY* sqrt (A). Since the minimum yield strength for Grade 5 is 92 ksi, and the minimum for Grade 8 is

130 ksi, you would need 1.41 times the fracture toughness to have Grade 8 be better than Grade 5. I doubt that you can get it from these alloy steels.

Another problem with specifying Grade 8 bolts is long term durability in outdoor environments where corrosion may occur. The upper hardness limit for Grade 8 and similar structural bolts like ASTM A490 is literally where things begin to fall apart via stress corrosion cracking. For a fairly recent discussion see Hendrix:

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Grade 8 bolts also are hard enough (above 35 HRC) to be susceptible to hydrogen embrittlement from hydrogen introduced during improper finishing operations such as electroplating. It's a real downer to have the heads pop off a few days after being torqued. However, it does provide business for failure analysts who can write that same report once again. The bottom line is that Grade 5 bolts are preferable, as Lance and Brian have already stated, unless there is a very compelling reason for using Grade 8. Use Grade 8 with caution and understand that it is buying you new problems you would not have with Grade 5.

Pittsburgh Pete

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Pittsburgh Pete

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