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What are the limitations of bolts machined from stock bar?
2

What are the limitations of bolts machined from stock bar?

What are the limitations of bolts machined from stock bar?

(OP)
I am currently sourcing cap head screws and bolts by plain machining without any post treatment.
 The thread and head appear to have sharp trasitions when compared to that of a forged item.(high tensile 2.1)
 The problem is that materials such as Duplex,monel and inconel can be required, consequently there is only one method of manufacture - machining.
 Can we control the process by calling for either a standard or set criteria which already exists (we are using Metric bolting only).
 I am worried about hardening on smaller bolts, and possible compromises in performance, plus we have seen the socket head fail regularly within the socket itself i.e. the allen key looses drive.

RE: What are the limitations of bolts machined from stock bar?

Nickel-containing alloys like duplex stainless steels and monel/inconel alloys CAN be forged into fasteners, and routinely are made that way.  Often they are hot forged in order to obtain the ductility necessary to form the fastener geometry without excessive work hardening, etc.  Suppliers like SPS should be able to provide you with forged fasteners.

If you still obtain machined fasteners, then I recommend you review fastener drawings from the AN & MS series to see what types of details are specified-- head to shank radius, surface finish, etc.  Duplex and Monel alloys are not hardenable by heat treating, but Inconel alloys are strengthened by precipitation hardening, so you need to consider this as well.  AN & MS fastener drawings have mostly (completely?) been replaced by NASM standards, but AN & MS drawings are still freely available from the US Government using the following link:

http://assist2.daps.dla.mil/quicksearch/

P.S. MS 212xx type drawings would be a good place to start-- too many drawings are returned if you just type AN or MS.

RE: What are the limitations of bolts machined from stock bar?

Burdy dont forget, many hi strength fasteners have rolled threads.

RE: What are the limitations of bolts machined from stock bar?

(OP)
Boo1, thanks for reminding me of that, The current machined threads have sharp edges and ragged run out. The trouble is rolling is just not economical for small batches.
 IVP have looked at the MS etc and will use a proportion of content to control material.Ta
 
 Does any body think the strength of the bolts is being compromised by machining rather than forging or rolling?
B

RE: What are the limitations of bolts machined from stock bar?

Think about "flow lines"-how all the little non-metallic inclusions (no strength) line up in the working direction.  With a forged (upset) head, those lines flow around the body to the head very nicely, with minimum effect on the strength.

But when you cut across them, you get more area of inclusions, and a weaker bolt.  Cleaner steels have less of this effect, of course.

Also remember that material mech. properties are not as good in the center of a bar, so be careful if you start with too large a bar.

RE: What are the limitations of bolts machined from stock bar?

Burdy,
     TVP is perfectly right, Nickel-base materials such as Duplex, Monel, Inconel are very often forged into bolts, particularly in Aerospace. The requirement for forged heads is nearly always associated with a requirement for rolled threads. Each of these two characteristics provide enhanced fatigue resistance. Bolted joints are very rarely loaded in pure tension, there are very often residual shear forces in the joint which tend to tip over the bolt and produce a high bending moment in the head. The amount of leverage on the outside edge of the head often causes rupture in the underhead radius. Forging the head is a way to resist this rupture (along with cold-rolling of the fillet radius to introduce compressive stresses - which is very often a third requirement of aerospace bolts)
So, to sum up, rolled threads will give you much greater resistance in tension/tension fatogue, and a forged head will help you in tension/tension fatigue but also in bending fatigue.
      

RE: What are the limitations of bolts machined from stock bar?

(OP)
Do you have any further specifications which I can apply?

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