Substituting 8637 for 4340 steel 2

[rstrecke]

Hello, there has been an issue of quench cracking that has been quite prevalent in our 4340 steel and unfortunately we are unable to alter our quenchant formulation (currently using oil with additives). I was wondering if 8637 steel would be a good substitute to eliminate the quench cracking issue while still providing similar mechanical properties/hardness. Preliminary quotes show a similar price for each of the steels so cost shouldn't be an issue (correct me if I'm wrong).

 

[WKTaylor]

rstrecke

What HT process are You using/following? What final HT Stress [strength] level?

Do machined parts undergo intermediate stress-relief operations? After HT/temper and final machining do parts recieve a second temper or a final stress relief operation?

What 4340 steel material specification [air melt, VAR, etc]?

Why 8637 exactly? What 8637 Steel material specification? [air melt, VAR, etc]?

What 'atmosphere' are You using during SHT soak? vacuum? inert gas, etc?

Are there large section differences [rough-machined to HT thickness]; or intricate details carved in the HT part?




Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[CoryPad]

8637 steel has lower hardenability, so it will not provide the same properties as 4340 if the section sizes are large. If your part can function adequately without the high hardenability of 4340, then maybe 8637 will work for you. Can you alter anything with your heat treatment process? Can you change the quenchant temperature or flow rate? Changes to those or other variables may allow you to continue using 4340.

 

[tbuelna]

There is a very extensive history of oil quenching 4340 with no problems. So I'd speculate that your quench cracking issue could be more easily remedied with design and process adjustments, rather than changing material.

Can you change the cross sections of the rough machined part blank being quenched so there are no thin sections, abrupt changes in section thickness, or sharp corners? Are all the external corners on the part blank being quenched sufficiently radiused? Is the part being dipped into the quench tank in the optimum orientation? Was the part blank properly cleaned prior to heating/quenching?

 

[rstrecke]

"What HT process are You using/following? What final HT Stress [strength] level?"

The HT process we use for 4340 is to heat to 1525 F followed by a quench in oil and then tempering at 392 F.


"Do machined parts undergo intermediate stress-relief operations? After HT/temper and final machining do parts recieve a second temper or a final stress relief operation?"

To clear things up, the part is in its final machined state and undergoing a hardening treatment. There is no machining afterwards and so no second tempering after.


"What 4340 steel material specification [air melt, VAR, etc]?"

The steel was hot rolled, I am not sure what other specifications to provide.


"Why 8637 exactly? What 8637 Steel material specification? [air melt, VAR, etc]?"

The 86xx series was recommended in literature as an alternative and 8637 had similar strength properties once properly heat treated (apparently not). I know 8637 is used to make some grade 9 fasteners.


"What 'atmosphere' are You using during SHT soak? vacuum? inert gas, etc?"

Inert gas.


"Are there large section differences [rough-machined to HT thickness]; or intricate details carved in the HT part?"

We are heat treating 3/4" diameter parts all the way to 6" occasionally.

 

[rstrecke]

The largest obstacle is that we are not able to change the quenchant as it needs to be able to heat treat a large range of steels.

 

[CoryPad]

8637 is inadequate to provide through hardening of 6" diameter parts.

 

[WKTaylor]

rstrecke... The generic way You answered my questions tends to make me believe You are not fully informed on every aspect of processing LA steels. However Your answer to the first 2-questions provide some clues as to what may be happening.

"What HT process are You using/following?
What final HT Stress [strength] level?"

I expected You to say something like... HT process per MIL-H-6875, 260--280-KSI.

Potential problem: 1525F is dead in the middle of 4340 Austenitizing Temperature range [1500--1550F] which is 'good'; however tempering at 392F is a wild card ['very bad'] per this spec: highest HT for 4340 is 260--280 per FL14 which reads as follows...

14/
a. 4340, 260 - 280 tempering must be between 425°F and 500”F.
b. 300M and Hy Tuf - tempering temperature Is mandatory.

Just a quick reference to AMS2759/2 for...

4340 [conventional steel, airmelt] 260--280-KSI, tempering temperature range: 425--490F
4340-Mod [Vacuum arc remelt premium processed aerospace steel] 280--305-KSI, tempering temperature range: 575F

Your tempering temperature [392F] is so low, it implies to me [conventional processed 4340 steel], that You are tempering way above 300-KSI, which makes this alloy exceptionally prone to the noted cracks.

NOTE. Quenching from Austenitizing soak [oil or water/polymer] generally drives the temper for most of these parts thru the roof. For this reason, moving the parts to a tempering process [to lower the strength considerably/quickly] is mandatory. IF quenched/untempered steel is allowed to remain in that condition for any significant length of time cracking is almost guaranteed.

NOTE. MIL-H-6875 mentions this quench phenomena and recommends 'snap tempering' [375--425F] for an hour] to lower the strength on an interim basis 'just enough' to minimize cracking potential... the 392F tempering temperature You use appears to be in this intermediate 'snap-temper [not permanent] range'.

3.2.2.3 Quenching. Material shall be quenched from the austenitizing
temperature In the quenchant specified In Tables IA or 15, as applicable.
Material shall be cooled to or below the quenchant temperature before
tempering. Material should be tempered within two hours after quench or within
two hours after reaching room temperature after cold treatment. If hardened
parts cannot be tempered within 2 hrs. of quenching, they can be snap tempered
for one hour at 400”F +/- 25°F or as appropriate to prevent cracking. Mill
products shall be quenched In a manner consistent with commercial practice where
Tables IA & 16 are not applicable. They shall be cooled sufficiently and
tempered within a period of time adequate to prevent quench cracking or
conditions deleterious to end product mechanical properties and corrosion
resistance.

Due to a whole bunch of reasons, 4340 has some unique properties, such as exceptionally thick section HT and the ability to process to a wide range of lower strengths... with a large leap to a fairly high/reliable strength range. CAUTION: certain gaps in permissible tempering ranges are evident in 4340 HT specs, due to blue-brittle issues at certain middle-low tempering temperature profiles [reason for wide tempering gap between 200/220-KSI....... and...... 260/280-KSI].

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[rstrecke]

Thank you so much for your help, it is greatly appreciated. My technical background is quite shallow at the moment, as this is my 6th day on the job out of college so I greatly appreciate your time.

 

[EdStainless]

If you temper any 0.40C steel at 392F I would expect cracking. This is a material with virtually no usable ductility.
Is this really what you want in your parts?

Are the cracked as soon as they come out of quench, or is it noticed sometime later?
As was mentioned above you need to temper soon after quench.
Have you looked at microstructures?
I suspect that you have some other issues (retained austenite) going on.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[tbuelna]

My technical background is quite shallow at the moment, as this is my 6th day on the job out of college so I greatly appreciate your time.

Nothing like getting thrown in the deep end your first week on the job! Don't worry though, if you provide as much detail about your situation as possible you'll get plenty of help.

Just to add to wktaylor's excellent post, I'd highly recommend obtaining a copy of AMS 2759/2 and reading thru it carefully. Pay particular attention to footnote 1 in table 3A. At the strength levels you seem to require 4340 Mod. (AMS 6417) might be a viable option.