Fatigue strength of 304, cold worked versus annealed

In summary, the fatigue strength of 304 stainless steel is significantly improved through cold working, as it increases the material's resistance to fatigue failure. This is due to the formation of dislocations and strain hardening, which hinder crack propagation and increase the material's toughness. In contrast, annealing reduces the material's fatigue strength, as it removes the dislocations and reduces its hardness and strength. Overall, cold worked 304 stainless steel is more suitable for applications requiring high fatigue strength, while annealed 304 stainless steel may be more suitable for applications where toughness and ductility are prioritized over fatigue strength.
  • #1
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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  • #2
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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  • #3
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.
 
  • #4
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.
 
  • #5
Thanks. If you have the spec number I should be able to get it.
 

What is the difference between fatigue strength of 304 stainless steel in cold worked and annealed conditions?

The fatigue strength of a material refers to its ability to withstand repeated loading and unloading without failure. In the case of 304 stainless steel, the strength can vary depending on whether it has been cold worked (strained at room temperature) or annealed (heated and then cooled). Cold worked 304 stainless steel typically has a higher fatigue strength than annealed 304 stainless steel.

Why does the fatigue strength of 304 stainless steel increase after cold working?

Cold working involves straining the material at room temperature, which results in the dislocation of the metal's crystal structure. This process creates a more complex and irregular structure, making it more difficult for cracks to propagate and increasing the material's resistance to fatigue.

What factors affect the fatigue strength of 304 stainless steel in cold worked and annealed conditions?

The fatigue strength of 304 stainless steel can be influenced by several factors, including the degree of cold work, the type and direction of loading, the surface finish of the material, and the environmental conditions (such as temperature and corrosion).

How can the fatigue strength of 304 stainless steel be determined?

The fatigue strength of 304 stainless steel can be determined through laboratory testing using methods such as rotating bending fatigue tests or axial fatigue tests. These tests involve subjecting the material to repeated loading and unloading under controlled conditions and measuring the number of cycles it can withstand before failure.

What are the practical applications of understanding the fatigue strength of 304 stainless steel in different conditions?

Understanding the fatigue strength of 304 stainless steel in different conditions is crucial for designing and engineering products that require high durability and resistance to repeated loading, such as aircraft components, medical devices, and automotive parts. It also helps in selecting the appropriate manufacturing processes and heat treatments for specific applications.

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