Rolled Threads and Fatigue 2

[macoovacany]

Hello all,
I have a specially made bolt/stud that requires remanufacture. A materials analysis of the bolt indicates that the threads have been rolled. A quick search on the internet indicates that rolled threads offer 30%-70% improvement in fatigue stress or a ten-fold increase in service life. However, these figures are from manufacturers. I was wondering if these figures are accurate?

Timbo

If your'e interested, the bolt is to hold an engine crankcase together. The bolt pretty much has to remain dimensionally similiar to the original, and fatigue will be an issue.

 

[israelkk]

High strength structural Aerospace and Military fasteners are required to have a rolled thread (at least where the size permit it). This arranges the metal grains along the thread contour in contrast to machined threads where the grains are cut. The other effect is compressive stresses induced into the thread where is as a similar effect is in shot peening thereby makes the thread more resistant to fatigue cracking (NASA Fasteners Design Manual 1990 page 13). See federal spec FF-S-86 the procurement specification for fasteners.

 

[unclesyd]

Adding a little to israelk's post. The big gain in resistance to fatigue failure comes when the threads are rolled after heat treatment. The stress to produce fatigue failure in threads rolled before heat treatment actually falls off with an increase in strength level. Threads rolled after heat treatment the stress needed to produce fatigue failure increases.
The downside is it's much harder and costly to roll threads after heat treatment.

 

[nashjp]

Not sure what you meant by remanufacture, I hope it meant replace with more durable/improved part. Threads can be rolled before heat treat (RBHT) or after heat treat (RAHT). RBHT threads have a better root geometry than cut threads and provide a little fatigue improvement. RAHT threads provide the benefit of compressive residual stresses.

As far as the sales info, many manufacturer tests were done using 0-max-0 loading. Bolts are typically installed to a very high mean stress, so you must consider how the mean stress will effect the thread fatigue life. If the manufacturers figures do not say, assume 0-max-0. The RAHT threads are sometimes very sensitive to mean stress (Use Goodman equivalencing for RAHT threads and a modified Goodman (Morrow?) for RBHT threads. If you have fasteners tested at a high mean load (proof?), processing problems like hydrogen embrittlement and carburization / decarborization are more likely to show up. For tests I carry out when I do not know the load, I use at proof as the mean - because some engineer or mechanic will use them there.

 

[CoryPad]

Rolled threads after heat treatment does improve fatigue performance. The references in faq725-600 contain more information on this subject.

Regards,

Cory

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

 

[macoovacany]

nashjp,
Remanufacture does mean replace with an improved part. The trouble being that the old parts are really strong, or more specifically, we don't know how strong.

Israelkk,
We don't have to design to the federal spec, we only have to show that the stud remanufacture meets or exceeds the performance of the original. Technically, we only have to design to the original certification requirement, but being an old British design, there isn't one, or we can't find it. Plan A, show the remanufactured part performs better than the original. As an aside where do I get a copy of FF-S-86?

All,
The matrials analysis indicates the material is Britich Standard Aircraft Material BS No 96 or 97. Hardness is 297 HV10. Approximate tensile strength is 136 ksi. No idea about the fatigue strength.
Since we have one example, testing samples is not an option.The manufacturers no longer exists, so we can't buy a new one. And we only wish to have limited production run of say one. At $8,000 to set up the tooling for thread rolling, this is something we wish to avoid.
Our local friendly materials expert recommends using 4140 heat treated to 140 ksi and have the surface nitrided.
Option B that I'm looking at is using AISI 4118. Haven't used it much so I'm not to sure about the traps of using it.
Comments?

Timbo

 

[aviat]

FF-S-86 is available at this site. It also lists what those bolts were replaced with. Have you looked into finding a replacement from a museum?

http://stinet.dtic.mil/str/dodiss4_fields.html

 

[TVP]

The following data is from a journal article written by S. Ifergane, et al ("The effect of manufacturing processes on the fatigue lifetime of aeronautical bolts", Engineering Failure Analysis 8, 2001, 227-235:

Threads rolled before heat treatment: static rupture stress = 1.29 GPa, # of cycles to failure = 13,667 +/- 1528, Knoop hardness = 456 +/- 33, depth of hardened zone = 0.1 mm.

Threads rolled after heat treatment: static = 1.33 GPa, N = 39,000 +/- 7550, Knoop = 468+/- 32, d = 0.4 mm.

Machined before HT: static = 1.33 GPa, N = 10,100 +/- 854, Knoop = 405 +/- 10.

Machined after HT: static = 1.33 GPa, N = 9500 +/- 1670, Knoop = 399+/- 16.

The fatigue test was conducted according to MIL-STD-1312-11A, which means max load = 29,200 lbs (sorry for the change in units), R ratio = 0.1, frequency = 10 Hz. All bolts were fabricated from the same lot of SAE 4340 steel, and Cd plated prior to testing.

 

[priyasachin]

Priyasachin
Friend,

In addition to these benefits explained above, the thread rolling being a cold working operation, causes the flow lines original material flow to the profile of thread. In a cut thread the flow lines are disturbed and stripping of the thread on overload is possible. The rolling operation offers additional resistance to such stripping.

MRCN

 

[macoovacany]

Maybe I asked the wrong question.
What is the simplest way to justify the remanufacture of a rolled thread bolt/stud, where the remanufactured bolt/stud does not have rolled threads.

I am aware of the advantages of using rolled threads.

 

[yates]

If you say, as in the original post, that fatigue is an issue, then clearly you cannot justify replacement of a rolled-thread bolt by one with a machined thread. Fatigue resistance in the replacement part will obviously be lower.
Another item of importance, where fatigue is an issue, is the condition of the underhead radius. Your original rolled-thread bolt will have almost certainly been cold-rolled in the underhead fillet radius to introduce compressive stress and thereby greatly improve fatigue resistance. Since thread rollers nearly always also have fillet-rolling machines, if you use a thread-rolled part as a replacement, you will also get a fillet-rolled part into the bargain. If you go for a machined-thread bolt, there is very little chance the machine shop will have a fillet rolling machine.
$8,000 seems an awful lot of money to threadroll one bolt, unless the thread concerned is a weirdo which requires a special threadroll tool. $250 sounds about the right price.