Best Bolt Material 3

[EnglishMuffin]

What is the absolute best readily obtainable bolt material for making special bolts of 20mm diameter, from both an ultimate strength and fatigue life standpoint ?

 

[TVP]

As far as I know, the highest strength commercially available fasteners use AerMet 100 from Carpenter. However, AerMet 310 produces even higher strength, as do the highest grades of maraging steels (e.g., VascoMax C-350 from Allvac). Multiphase alloys (Ni-Co-Cr-Mo) like MP35N are also used to make high strength fasteners. Use the following links for more information:

http://www.arp-bolts.com/pages/tech/matspec.html

http://www.cartech.com

http://www.spstech.com/aero/prod_lit/superalloy_brochure.pdf

http://www.allvac.com/allvac/pages/PDF/tech/VascoMaxC.pdf

 

[unclesyd]

Can you come back with a little more information as to the type/style and enviroment the fastener will see.

[/b]TVP's[/b] recommendations are very good sources but several of the materials mentioned have some limiting factors that need to be considered.

 

[EnglishMuffin]

TVP:
I have been told that maraging steel offers little advantage over medium carbon steel for this application because of the poor notch sensitivity. Any comments ?

Unclesyd: I will get back to you on the detailed operating conditions.

 

[CoryPad]

Corrosion resistance and hydrogen embrittlement are problems with maraging steels, including the AerMet alloys. The Multiphase alloys like MP35N should be suitable.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[EnglishMuffin]

Unclesyd:

The operating conditions are:

Low corrosion environment
Studs are 325mm long x 20mm dia and see 250000 lb/in^2 with a very small superimposed additional cyclic stress.

Other info:

Cannot afford to buy more than 150 feet per order.

We would like to see a material with relatively high ductility to improve the fatigue life if possible.

 

[NickE]

High ductility is not going to inherently improve fatigue life. The only way that immediately comes to mind fo rimproving fatigue life of threaded fateners is to use steel with a higher UTS.

nick

 

[unclesyd]

Please verify the load value of 250,000 lb/in^2.

I hope this is a total load for multiple studs as I don't think we can get there on one 20 mm fastener.

 

[EnglishMuffin]

Sorry - that's the required UTS. I'm asking this question for another guy in the office.

 

[EnglishMuffin]

NickE: I'm not sure where I got the impression that ductility improves fatigue life. I certainly believe it to be the case that the effect of stress concentrations is much less with ductile materials than brittle ones, so perhaps that's where my assumption came from. It is my understanding that for ductile materials, the stress concentration factor should be applied only to the alternating stress component, but for brittle materials the stress concentration factor should be applied to both the alternating and steady component, so perhaps I am mentally extrapolating from that. But probably you are correct and additional ductility beyond a certain level has no further effect, all other things being equal.

unclesyd: Perhaps I should mention that this is a rather unusual situation, where the bolts (actually studs) are being called upon to act as highly preloaded axial tension springs with very short travels. To provide maximum insurance against overstressing, it appears that it will be necessary to employ the very best material available.

 

[unclesyd]

Whew!
Cleared up the 250,000 psi and added a bear. The only thing going for you is the large L/D.

As posted by TVP there are several materials available at the company websites as posted.

From your latest posting I would contact SPS with the problem as this rather a unique application where the stud will act as working spring. As you know this imposes several additional parameters on the fastener material and will probably require a specially designed fastener, somewhat like a tensile test bar with threaded ends. At these strength levels thread fatigue, beam bending, and strip out of the nuts worries me.
I think that you will have a nut that allows internal flexing of it's threads but again this a tough design problem and SPS should be familiar with a nut of this type. The are a couple of different nuts that may be required to overcome the inherent flexing of the first threads. Anything you do will have a finite life.

We have a similar situation where we have a fastener that has to allow slight axial movement of one of the clamped components. We accomplished this with Belville Washers and Inconel 718 Studs. The studs are nowhere near the strength levels you talking about.

Is it possible to provide the following information.
What is the actual load on the stud?
How much "spring" is required?
Is the stud set in or free?
Will the clamped material be able to carry the load imposed by the stud without a hardened washer.

 

[CoryPad]

[blue]NickE[/blue],

Your last comments regarding fatigue represent only the most basic concept of fatigue (infinite life of steels assuming high cycle fatigue). Ductility is of huge importance to low cycle fatigue, as the fatigue life is determined by the amount of strain a component can provide during a cycle. Actually, the best way to think about fatigue is with fracture mechanics: a tough material (with high ductility) will have high resistance to crack propagation, and therefore, will have a high fatigue life. You may notice that materials like AerMet and MP35N have enormous strength AND fracture toughness AND fatigue strength.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[EnglishMuffin]

Thanks for your replies. I have now introduced the person that was interested in this problem to Eng-Tips (an experienced engineer by the way), and he has now posted further related questions on this thread :thread330-99671