Fatigue strength of 304, cold worked versus annealed 2

[iainuts]

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 made makes me wonder...

Which material would be best, annealed or cold worked? How can this be quantified?

 

[metengr]

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

As long as you select a stress amplitude that is safely below the endurance limit of 304 stainless steel, you can accommodate an infinite number of cycles. The reported endurance limit (in bending) for annealed 304 is 35 Ksi**. The endurance limit will increase with cold work**.


**Handbook of Stainless Steels by

 

[metengr]

....Peckner and Bernstein.

 

[iainuts]

Thanks meteng. Yea, I get all that. But why does it say:

However, the fatigue strength of these cold worked alloys is reduced by notches, as compared to notched fatigue strength in the annealed condition.

This makes it sound as if a stress concentration for a cold worked bar will somehow be higher than for an annealed bar. What do they mean by that statement?

 

[metengr]

iainuts;
Basically, annealed material like 304 stainless has significant capacity to absorb cold work or local tensile strain accumulation at notches with no adverse effects on fatigue life.

When you cold work material you have less capacity to absorb additional cold work and local tensile strain and as such, this could reduce fatigue life at areas of stress concentration.

As you can see you have options with using a higher strength material condition to take advantage of high strength. However, you need to understand that what comes along with this is lower capacity to absorb additional tensile strain around regions of stress concentration if not accounted for in the design of the component.

By the way you never mentioned the type of component your designing and selection of material and strength condition requires more that just fatigue strength. Also, material supplied in a cold worked condition has limitation in terms of size versus annealed material. Give careful consideration to this and other service conditions like corrosion resistance (if applicable).

 

[iainuts]

Thanks again for the explanation. Material is for a piston, 3.25" OD. Load is axial, piston is double acting, so stress is both compressive and tensile.

Given the stress is going to be X, I'll have a stress concentration of Y, is it safe to say that ultimately, the cold worked material will have better fatigue strength? In other words, comparing an annealed bar and cold worked bar without notches, the cold worked bar has higher fatigue strength. Putting a notch in the bars makes things worse in both cases. Does a notch (or stress concentration) somehow affect the cold worked bar so severely, that the annealed bar will have a higher fatigue strength?

 

[metengr]

Putting a notch in the bars makes things worse in both cases.

Not unless the increase in tensile stress (aka stress intensification) at the base of the notch exceeds the stress amplitude reported for the endurance limit.

Does a notch (or stress concentration) somehow affect the cold worked bar so severely, that the annealed bar will have a higher fatigue strength?

It can. However, this is not an easy question to answer because you need to evaluate the notch sensitivity characteristics of this material under fatigue loading to determine if the fatigue strength is reduced.

 

[CoryPad]

I think it is unlikely the annealed material would have a higher fatigue strength even considering the notch. For high stress, low life conditions, maybe the notch effect is so bad that you cannot take advantage of the stronger material. But, for low stress, high life conditions, the higher strength material should give better results.

 

[iainuts]

Thanks for the feedback. In parallel with these technical considerations, I've gone out for quote on the material. Apparantly, 304 cold worked isn't available in 3.25" round bar. Nitronic 60 cold worked is available at about $1800 for the material compared to about $200 for the 304.

I'll have to sharpen the pencil and see if the 304 annealed is suitable.

 

[unclesyd]

have you thought about looking at other stronger materials like 17/4 PH SS, Nitronic 30 or 50, or a 400 series SS.

How long a piece o 3.25" round bar do you require?

Using 304 SS or 316 SS a 1.25 " dia is about as large round bar that cold working is effective.