4140 Mf and trasnsition

[metman]

SAE-AMS-H-6875 does not require cold treatment for Q&T 4140 Steel.

This leads me to conclude that the Mf temperature is above room temperature.

Question 1) True?

Question 2) Is it possible to achieve Retained Austenite in 4140 if quenched rapidly to room tempearture? This would be for a 1/2" diameter bolt.

Question 3) What is the Transition Temperature for AISI 4140?

Question 4) What are upper and lower shelf toughness values for 4140 on the transition curve?

I am asking Questions 1 & 2 because of a potential use of a 4140 bolt heat treated to 95ksi min ult shear per ...H-6875 that would be used in a critical application. In this instance, the bolt would be cold treated to shrink it for a press fit.

The press-fit would help reduce distortion occurring during fatigue testing because of leverage inherent in the design. If untempered Martensite is a possibility, it could help initiate fatigue cracks because fatigue is sensitive to notch toughness. If there is a chance for RA, we can cold treat and re-temper before shrink fit.

I am asking Questions 3 & 4 because the structure could be exposed to temperature of -80 F for long duration with fluctating stresses in some applications. Again notch toughness is a concern.

Likely ASM has the answers but unfortunately I am not able to be a member at this time although I have been in the past.

Thanks for any help!

 

[Robertmet]

4140 - Quenched from 1550 F, Ms - 640 F, Mf - 425 F.Ac1 -1375, Ac3- 1455, Ar31305, Ar1 - 1255 F.You can get RA in low carbon martensite like 4140 , even in bainite ! Significant amounts ? No as long as the hardening temperature is proper.

 

[redpicker]

You won't find any significant retained austenite in 4140, unless you carburize it, that is. The upper and lower shelf vaules depend on many factors; cleanliness, actual chemistry, steelmaking practice, heat treatment, hardness etc... Heat treated to 25-30 HRC (the range I would expect for 95KSI minimum yeild) the upper shelf can range from around 40 Ft-Lbs to over 100 Ft-Lbs. Similarly, the lower shelf can range from the single digits (below 10 Ft-Lbs) to the low 40's.

rp

 

[metman]

Thanks for the replies.

redpicker -- The 95ksi value is min ult shear. Ult UTS is 160-180ksi.

"...the upper shelf can range from around 40 Ft-Lbs to over 100 Ft-Lbs. Similarly, the lower shelf can range from the single digits (below 10 Ft-Lbs) to the low 40's."

This is a significant reduction in toughness. Do you have the upper/lower values for the harder condition?

 

[redpicker]

Sorry, I mis-read your original post.

With a UTS of 160-180 KSI, you're looking at a hardness of 37-41 Rockwell C. I've seen the upper shelf for this material at this hardness range from 40-70 Ft-Lbs and lower shelf to be, again, from the single digits to the low to mid 30's. You don't say what your section size is, but in diameters over 1 inch you are likely to see the lower end of these ranges, particularly in the core of the part. I don't like to use 4140 for critical applications if the tempering temperature is below 1000 F (1050F, actually), and I'd guess you'll be below that to hold that hardness.

If this is a critical application at -80 F and you really need 40 HRC, I'd consider using 4340 or 4330V. Either of these will give you better notch toughness at the low temperatures and, if the section size is of concern, they will give better heat-treat response, too.

 

[salmon2]

Yes, I agree with redpicker. If hardenability is not concern, shouldn't be since you mentioned it is 1/2 in. dia, using 4330v will give you much better toughness even 4130 will be better. No concern on retained austenite for 4X30 I know of.