Rockwell Hardness of suspension bolts.

[Greg L.]

Does anyone know typicaly what the hardness rating is of these fasteners? I guess grade 8 is probably the highest strength bolt that you would see in this application so is there any way to tell what the hardness of these bolts are?

[Dick W.]

Re: Rockwell Hardness of suspension bolts.

Three lines = grade 5
Five lines = grade 8

I do not think that you will find anything less than grade 5 on suspension fasteners, grade 8 on critical fasteners

[Terry M.]

Re: Rockwell Hardness of suspension bolts.

Hey Dick. Take out your other hand -- you need more fingers.

3 lines = grade 5
6 lines = grade 8

I have never seen a hardness specification for fasteners. They are generally tested for yield strength in a machine that pulls them until they break -- and the specifications I have seen give the yield strength in PSI. There must be some Rockwell spec someplace though because the Grade 8 bolts are generally heat-treated and they must test that treatment somehow, although the yield test would do that also.

[Chuck S.]

Re: Rockwell Hardness of suspension bolts.

Three lines = grade 5
Five lines = grade 8

I do not think that you will find anything less than grade 5 on suspension fasteners, grade 8 on critical fasteners

Reading between the lines here, I'm thinking the Gregster is still worrying about hydrogen embrittlement from not stress relieving fasteners after replating.

Forget it, Gregster; if it's strong enough to have six lines on it, any slight variation in the range of Rockwell hardness suitable for that strength is probably not enough to keep those replated fasteners from stress cracking.

[Greg L.]

Re: Rockwell Hardness of suspension bolts.

Well ya sure hit that nail on the head Chuck!

I figure that since grade 8 is the strongest bolts that we have on our suspension that if I can find a rockwell spec for it that I will have an idea for how long I should bake my bolts. I bought what I could of repop fasteners and what I could buy I bought nice used pieces. I have the used pieces all cleaned up and ready for plating but before I go there I want to get a bake procedure in place and I think a rockwell spec will give me this.

[Chuck S.]

Re: Rockwell Hardness of suspension bolts.

Google SAE J429 Grade 8...You'll find more information than you need. There are some very interesting spec tables on the web, actually.

I believe you're looking at about Rockwell C33-39 for Grade 8, and Rockwell C25-34 for Grade 5. This was taken from Grainger's online catalog.

Let me know what you decide on this stress relieving procedure; I'm interested.

[Rod K.]

Re: Rockwell Hardness of suspension bolts.

Greg, I don't think a Rockwell spec will give you the info you're looking for. It is simply a surface hardness test which may be translatable to a tensil spec, depending on the material.

As Terry mentioned, the grade designation is a tensil strength spec and would be (destructively) confirmed by pulling the bolt until it yields. The Rockwell test is done by pushing a very sharply pointed probe into the surface with a given force and measuring the depth of the indentation made, which gives you the surface hardness, but not the core hardness which is critical to overall strength. (Case hardened parts are very hard at the surface, soft in the middle, unlike the bolts you're interested in, but the surface hardness may be about equal in both cases.)

I'm going from memory, and it's very faded, but hyd relief probably would be baking at about 400 F for 2-3 hrs within 4 hours of plating. I'd caution against plating these critical fasteners yourself, if that's your intent, and find a competent plater to do the work, then ask questions about the post plating hydrogen embrittlement process until you're satisfied he knows what he's doing. I think we'd all hate to hear of your project upside down from a failed bolt.

[Chuck S.]

Re: Rockwell Hardness of suspension bolts.

...The Rockwell test is done by pushing a very sharply pointed probe into the surface with a given force and measuring the depth of the indentation made, which gives you the surface hardness, but not the core hardness which is critical to overall strength. (Case hardened parts are very hard at the surface, soft in the middle, unlike the bolts you're interested in, but the surface hardness may be about equal in both cases.)...

That's true, Rod, but if you're working with a known entity like heat treated fasteners of a given material, rather than a material specimen of unknown heat treatment, the surface hardness implies the strength of the fastener.

If the fastener doesn't have that hardness, then it is inherently not of the expected strength. You don't need to know the hardness profile across the fastener, because from experience, a given heat treatment of that material yields a given tensile stength, surface hardness, and hardness profile. Your recollection of the required heat treatment sounds about right from what was posted here previously.

In any event, Gregster, BY DESIGN you shouldn't get the temperature of your fasteners high enough to alter their strength or surface hardness. As I understand it, the objective of this "stress relief" process is to add enough energy to keep the hydrogen in it's atomic state so it can escape through the grain boundries, without changing the metallurgical structure of the fasteners.

[Greg L.]

Re: Rockwell Hardness of suspension bolts.

Man I should have paid more attention in chemistry class.

All I know about this is from what I've read and from what you all have said now and in the past. I found a site that mentions that hyrdogen embrittlement is only a concern if the hardness is above about C30.

http://www.mechanicalplating.com/hydrogen.htm

Thats what got me on my tangent to find out what the hardness of a grade 8 bolt is and from what I see a grade 8 bolt has a tensile strength of about 150,000 psi and a hardness of about 31-33 so it puts it just over the C30 limit. The grade 5 bolt doesn't even make the chart but I think it's tensile strength is around 130,000 psi so it should be below C30. Chuck I see that you have different figures so I'll look into those but from what I'm learning this is all ballpark stuff....heck they never even had a standard chart on baking untill 1994!

Rod I think your 400F is bang on but the length of time seems to depend on the hardness or tensile strength of the fastener so I'll have to try and figure out what would be an suitable time frame.

I have talked to ALL the local platers and they say that I'm wasting my time and money to have them baked and only one CAN actually bake them so I think I'm better off doing them myself. I did talk to a guy on the Caswell forum and he said 375F for 3 hrs. One of the platers that I talked to said that about 375F-400F for 3 or 4 hrs should be okay but they said that it wasn't required for what I was doing... If you go by the chart on the link that I posted, 8 hrs @ 400F should do the trick. At least we all have the temp right....I'm not really convinced that the extra 4 hrs will make much difference.

I'll do more research but from what I see so far hydrogen embrittlement is only a concern for grade 8 bolts and higher and even at that it just barely makes the chart which is why the platers that I spoke to probably don't think it's a concern.

I'll see what else I can learn tonight and if all else fails they will get a bake at 400F for about 6 hrs.

All this baking talk has made me hungry....time for dinner!

[Chuck S.]

Re: Rockwell Hardness of suspension bolts.

Good info, Greg. Thanks.

I'm not so sure about the Grade 5 not being affected...the ranges I posted overlap at the top end of Grade 5 and bottom end of Grade 8. But, as you said, my Rockwell ranges are broader, and slightly different from what you found.

I'm thinking the time is only relevant as the minimum time for the hydrogen to exit; if you cook them longer, it won't hurt anything or change anything...You'll just have some "hot" bolts until they cool.

[David S.]

Re: Rockwell Hardness of suspension bolts.

Running to the airport, so I did not thoroughly read all the posts. Rockwell C will not measure surface hardness or case hardness. It typically penetrates deep enough to give a very confusing reading in non-homogenous materials. It "sort of" averages out the surface/case hardness with the "core" hardness. Vickers will give you a true value of the surface/case hardness. Please keep in mind, almost all high strength fasteners will have rolled threads. This locally "works" the threads and yields a much "harder" outer layer. Too many times I have seen the wrong times and temperatures used to remove atomic hydrogen after plating based on incorrect interpretation of perceived strength. Always yield to the conservative side when it comes to single point failure items.

Dave

[Greg L.]

Re: Rockwell Hardness of suspension bolts.

Here is another good bit of info.

http://www.omegaresearchinc.com/Publications/metal.html

[Greg L.]

Re: Rockwell Hardness of suspension bolts.

Well I baked some bolts last night. Did it at 400F for 8 hours but I thought I'd mention that the Caswell plating doesn't hold up to baking like commercial plating does. I noticed that after a few hours that they were somewhat disscolored from the heat and I thought holy $Heet it's too hot. I confirmed the temp with the thermocouple on my multimeter though and it was correct so I tosed in some new zinc plated washers that I had to see if they would discolor too....they didn't.

Most of the baked plating cleaned up once they cooled and I brushed them with a stainless brush but I would say that they are only suitable apearance wise for suspension applications because if they were in your face like in the engine bay some of them would look funny.

The two standing up on the left have that yellowish tint from the baking and the ones on the right are after brushing. Better but not near as nice as just after plating without the brightener.

[Chuck S.]

Re: Rockwell Hardness of suspension bolts.

...Most of the baked plating cleaned up once they cooled and I brushed them with a stainless brush but I would say that they are only suitable apearance wise for suspension applications because if they were in your face like in the engine bay some of them would look funny...

Hey...You don't think chassis judges like to see bright shiny parts and fasteners too??!! OK...All you 68-69 chassis judges out there...be on the look out for this Linton guy!!! HaHaHa...

Greg, I'm wondering if you really have much zinc left on those "brushed" fasteners...they look pretty dull. It's possible the Caswell system is depositing no where close to the "film build" you get with commercial plating.

[Greg L.]

Re: Rockwell Hardness of suspension bolts.

Oh I'm sure they'd like to see all kinds of bright shiny things under there Chuck and I'll try not to disappoint them. Even if I do loose a few points here my authentic lacquer paint job will MORE than make up for it!

I think I'll be fine with the bright and shinys down under anyways(most were cad to be honest). That pic that I posted was only to show the relative difference between the "as cooked" and "buffed" pieces. I didn't bake the washers and they are pretty close to the "after buffed" ones on the head end. I will say though that buffing them gives them a darker tint. To get them back to a lighter cad appearing shade I quickly did a "flash plating" on the exposed areas like the heads and tips of the bolts. This took about 10 seconds each and restored the cad look so now they look proper.

As far as the thickness of the plating goes I've never measured it but after a short time you can see it building up on the copper wire and if you bend it enough it will peel like chrome on a bumper so I'm sure that the part have enough plating...especialy for the hard salty winters that the car will have to endure in years to come.