Heat Treating and material ductility

[yacpro13]

Hello,
I have a small cylindrical adapter D=2" L=8".
For the strength needed, according to MIL-HDBK-5H table 2.3.0.2, I should use 4340 over 4140 since 4140 has a maximum diameter of 1" for quenching & tempering.

The question I have is especially regarding material ductility.
I can establish what strength I need, and specify a heat treatment accordingly.

As clearly shown in table 2.3.1.0(e), the higher the strength obtained from the Q&T, the more brittle the material is. I also remember this from my school textbooks.

However, in the real world, how do you determine that the resulting ductility will be acceptable, other than with tests?
For example, the reduction area here can vary between 43-55%.

Are there best practices when it comes to ductilty (such ensuring 45%min reduction area)...?
What is considered "brittle", quantitatively?

Thank you.

 

[EdStainless]

Tensile elongation is usually the indicator of ductility that people refer to.
The other criteria that might be of use is impact toughness.
Less ductile does not mean brittle. There are many applications where materials with 10% elongation are used all of the time. That is too low to low cold forming operations, but if the strength and design are correct it is perfectly serviceable.
Design is more critical with lower ductility. You need to avoid notches and sharp corners, you need to consider load path very carefully.

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Plymouth Tube

 

[TVP]

The short answer is that you need a lot of experience or testing (preferably both) to decide what is an acceptable amount of ductility. As shown in Table 2.3.1.0, 4340 quenched and tempered to 260 ksi tensile strength has ~ 10% elongation. That is really the minimum amount that is safe for engineering/industrial applications. Keep in mind that notches, grooves, and other stress concentrations make things more difficult. What exactly does your cylindrical adaptor need to do? Stressed in compression only? Tension & compression? Torsion? Low temperature exposure? Is it threaded? All of these need to be understood to determine a safe limit.

 

[yacpro13]

Thanks for both of your replies.
Indeed I wanted to get opinions on what is considered safe for engineering/industrical applications. Like you said, 4340 Q&T to 260ksi tensile has approx 10% elongation (by the way, what does the '4D' refer to in elongation?). I also understand that stess concentrations should also be considered.

For this part in particular, the first third of the part is square, the second third of the part is cylindrical, and the last third is hexagonal.
There are filets for all transition faces. No threads, keyways, or any other features.
The part is an adapter for torque equipment. It will be loaded in torsion. Loading is a steady loading ramp (ie. not impact).

Without any impact loading, assuming I would need the 260ksi tensile (I don't) should the 10% elongation be worrisome?

 

[jwhit]

4D is a gage length of 4 times the diameter of the tensile specimen. For a 0.5' diameter specimen the gage length is 2", a 0.25" specimen has a gage length of 1".

 

[weldstan]

Whereas you stated that the item is a cylinder, what is its thickness? You may not need the greater hardenability of 4340. One must assume that you are not foolish enough to Q&T for high strength prior to machining 2" diameter bar stock to make a cylinder.

 

[yacpro13]

@jwhit Gauge length = 4D, got it. thanks.

@weldstan it's cylindrical in shape, but it's a solid rod. (ie not a hollow cylinder).

 

[tbuelna]

If your design requires a steel having a UTS above 220ksi, then you won't want to use 4140. Regarding the trade off between 4140 and 4340, unless raw material cost is a huge concern, then 4340 is a better choice. For your 2" dia x 8" L part the price difference between 4140 and 4340 is likely not significant in terms of the overall manufacturing cost, unless production quantities are high.