Fatigue strength of 304, cold worked versus annealed

AI Thread Summary
Cold working significantly enhances the fatigue strength of austenitic stainless steels, with 304 cold worked showing an endurance limit of 92 ksi compared to 35 ksi for annealed. However, cold worked materials experience reduced fatigue strength when notches are present, which is a concern for applications involving stress concentrations. The discussion highlights the challenge of determining the best material for designs subjected to high cycle fatigue, particularly in threaded components that fail after approximately one million cycles. Factors such as elastic range, yield strength, and creep resistance are critical in evaluating the suitability of cold worked versus annealed materials. Ultimately, understanding the specific stress conditions and potential for crack initiation is essential for making an informed decision.
Q_Goest
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This web page: http://www.hghouston.com/ss_cwp.html

states:
Cold working increases the fatigue strength of the austenitic stainless steels. However, the fatigue strength of these cold worked alloys is reduced by notches, as compared to notched fatigue strength in the annealed condition.
The web page also shows that fatigue strength can be improved dramatically by cold working. Endurance limit is listed as:
304 annealed = 35 ksi
304 3/4 hard = 92 ksi

I'm in the process of designing something that will be subject to fatigue. Cycles will quickly exceed 10^6 cycles, and even 10^8 cycles will come along all to quickly. The part is in axial tension/compression. It will have stress concentrations of about 2 or 3. But the statement makes me wonder...

Which material would be best, annealed or cold worked? How can this be quantified?
 
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Q_Goest said:
This web page: http://www.hghouston.com/ss_cwp.html

states:
Cold working increases the fatigue strength of the austenitic stainless steels. However, the fatigue strength of these cold worked alloys is reduced by notches, as compared to notched fatigue strength in the annealed condition.

The web page also shows that fatigue strength can be improved dramatically by cold working. Endurance limit is listed as:
304 annealed = 35 ksi
304 3/4 hard = 92 ksi

I'm in the process of designing something that will be subject to fatigue. Cycles will quickly exceed 10^6 cycles, and even 10^8 cycles will come along all to quickly. The part is in axial tension/compression. It will have stress concentrations of about 2 or 3. But the statement makes me wonder...

Which material would be best, annealed or cold worked? How can this be quantified?
I'm wondering under what conditions the quoted statement is made. CW metal has a greater elastic range than annealed metal, and obviously higher yield strength.

On the other hand, the uniform elongation of annealed material is much greater, so strain to failure is greater. Annealed materials are more creep resistant than CW material, and perhaps the quote is referring to creep related cracking at the tip of a notch.
Field Metallography can be used as a tool to determine remaining life, fitness for service, and damage assessment from creep mechanisms or fire damage. It is also useful in determining the cause for cracking, degree of overheating or other damage mechanisms manifested by microstructural changes.
http://www.hghouston.com/services_9.html

My question would be - what are the expected stresses as compared to yield strength of ANN and CW steels.
 
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Hi Astronuc,
Astronuc said:
My question would be - what are the expected stresses as compared to yield strength of ANN and CW steels.
That's difficult to determine because there is no really good information on stress concentration factors for threaded parts. This particular part has a 1.750-8 thread and seems to be failing regularly due to fatigue, generally after about one million cycles. The crack initiates at a thread root.

I'm trying to determine if a cold worked version of the material will improve that or not.
 
Q_Goest said:
Hi Astronuc,

That's difficult to determine because there is no really good information on stress concentration factors for threaded parts. This particular part has a 1.750-8 thread and seems to be failing regularly due to fatigue, generally after about one million cycles. The crack initiates at a thread root.

I'm trying to determine if a cold worked version of the material will improve that or not.
I have an ASTM STP about fracture of threaded fasteners. Let me see if I can find it.
 
Thanks. If you have the spec number I should be able to get it.
 

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