Difference In Notch Sensitivity

[teb1]

Is there a significant difference in the notch sensitivity between 4340 and 4140 when both are heat treated to an ultimate tensile strength of approximately 170ksi?

We usually specify 4340 for some of the fasteners we use in our repairs. Our supplier is having a hard time finding the 4340 but can supply the 4140. If there is a difference what can be done if anything to compensate?

Thanks,

teb1

 

[Carburize]

4340 is tougher than 4140 becasue of the addition of nickle. Heat treated to the same hardness/strength 4140 is more "notch sensitive" than 4340.

 

[metengr]

To add to the statements by carburize, 4340 is a deeper hardening steel and probably as equally important as notch sensitivity, is higher fatigue strength.

You did not mention your specific application of these fasteners. Make sure that you reduce stress concentration by using rolled threads and radius all sharp corners.

 

[teb1]

Thanks, for the quick responses. I had already told my supplier to hunt harder for the 4340. We are specifying rolled threads and polished radii. In addition we dye check and mag particle the bolt head fillet radius prior to installing.

Again,

Thanks,

TEB1

 

[israelkk]

Fasteners for federal and military specification such as FF-S-86E Title: SCREW, CAP, SOCKET-HEAD allow the use of any alloy steel that Chemically compatible with ASTM A574 and can heat treated according to MIL-H-6875 to meet the required mechanical properties. In previous versions any of the 4130, 4140, 4340, 8640, 8740 etc were specifically listed.

 

[diamondjim]

israelkk,

They all meet the minimum requirement of the spec.
It does not mean that they all are equal.

 

[israelkk]

I agree that they are not equal.

Many years ago a part made of 4340 in an aircraft had fatigue failure after some time. Anything they tried to do didn't help. Eventually they agreed to use CRO 861 from Bofors Sweden which was considered 4340 commercial equivalent but not considered aerospace grade. It had higher phosphor (I think) than allowed in 4340 military aerospace and premium quality grades.

The results were beyond anyone expectations. They could not break the part during fatigue tests and in service.

Therefore, I think that only test will reveal if 4140 is inferior.

 

[unclesyd]

israelkk,
I would think your material probably had a lower phosphorous content as Swedish iron and steel is famous for the low Phosphorous content.

It has been our experience that 4140 is far more notch sensitive, especially after exposure to temperatures above 450?F, than most of the published values. Using published physical data for 4140 vs 4340 a cursory look would suggest that 4140 would be the better alloy in respect to notch sensitivity.

One of our group generated and collected a lot of data on our inservice component failures that were made from 4140. Initially his data was so far off that he questioned his software, our machining and testing, and the PMI. After running up a rather large phone bill and consulting with others he finally agreed with our inductive observations that having no notches in 4140 components was essential. We had been increasing the use of 4340 over the years until the advent of 17/4 PH SS. 4140 parts almost dropped of the screen until we garnered problems with 17/4 PH SS. All these components operated at 600?F for long periods then cycled for several hours at 900?F.