4140 Cracking During Heat Treat 1

[Jboy333]

Can anyone provide any advice on a problem we're having with heat treating a 1/2" square 4140 rod.

We're shooting for a yield strength of 210 ksi. The first batch came back great, no problems.

The second batch, all but 2 of 15 came back terribly warped and with many cracks along the length of the rod. We sent samples of the cracked rods to the steel supplier's metallurgist and he said that there appeared to be a seam in the metal, and that was why it cracked...so we were reimbursed.

The next time we ordered a 6" wide sheet of 1/2" material and laser cut the parts out (instead of machining down a nominal 1/2" sq rod to 1/2"). We had a message from our heat treater, that these bars again cracked.

We've spoken with a few different heat treaters and they feel we should easily be able to hit 210ksi with 4140. Before we send out any more parts to be treated, I was wondering if anyone had any advice on what might be going wrong here.

Thanks,

-Jesse

 

[mog69]

Yield of 210ksi sounds extremely high for 4140. UK equivalent BS 970 708M40 indicates max achievable yield 940 N /mm2 (136 ksi) at 1/2" ruling section. I don't see a yield higher than 1235 N/mm2 (180 ksi) for any grade of BS 970, and I believe that covers an quivalent range to AISI.

 

[EdStainless]

What are temperatures and quench practice?
This sounds odd.

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[TVP]

ASM Handbook Volume 1 shows data for 13 mm round bars (1/2 inch) oil quenched from 845 C and tempered at 370 C (700 F):

yield = 1460 MPa (212 ksi)
tensile = 1590 MPa (231 ksi)
elongation = 12.5%
impact = 15 J (11 lb-ft

I agree that it is strange to have so much warpage if oil quenching was used. Perhaps the quenchant was water or water + polymer and the higher quench intensity (faster cooling) caused the distortion. Perhaps parts cut out of sheet will respond better than those hogged out of bar.

 

[arunmrao]

Have you done water quenching,. If you can change over to oil quenching the problem might just vanish.

 

[Jboy333]

Thanks for the insight so far guys.

I'm checking with our heat treater right now to see what they're doing (water or oil quench). We assumed oil because, as a few of you stated, that's what the specs call out to achieve that yield strength. But maybe they are using water. I'll post as soon as I hear back from them.

-Jesse

 

[swall]

Laser cutting may have produced small cracks at the cut edge. I just had this experience with some 6150 blanks that were laser cut. The small cracks would have become quench cracks during quenching.

 

[EdStainless]

Some laser parts are simply pickled to remove the re-cast layer on the edge. That might be enough to help you.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[Jboy333]

Our last parts were laser cut, but the previous batch were just milled down out of a 3/4"x1/2" piece of stock. Since they both failed, I'm still looking to the heat treating process as the culprit. (Still waiting to hear back from them)

 

[Jboy333]

So here is what the heat treater replied with. Let me know what you think:

We preheated to 1250 degrees F.
Raised temp. to 1520, water quenched in 120 degree water. Same process we used on first quantity of 6 that turned out good.
We did not notice the cracks at 1520, we believe during quench.
The very first ones we did, we oil quenched, we could not get the required hardness.

 

[swall]

Per Heat Treater's Guide, the recommended austenitizing temperature is 1570F. I just ran the jominy equivalent numbers on 1/2" square (.625" dia equivalent round) and this puts you at a J4.5. At the low side of the jominy curve, you should quench out at Rc 51- 52. To obtain 90% martensite, you need to quench out at Rc 51, so you should be OK with an oil quench, if you have 4140H grade. If you have regular 4140, you may not make it. How are you checking hardness? You could have some decarb present, which could give a lower surface hardness reading.

 

[Jboy333]

We were just doing a surface Rockwell C test. I think we're going to cut one of the bars and do some readings on the inner material. I was wondering too if the decarb might be lowering our hardness value and, thus our heat treater is overshooting the mark.

 

[CoryPad]

Jboy333,

You must consider more than the temperature of the quenchant. The heat capacity is another very important factor, and is one of the reasons to use oil instead of water.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Kalroy]

If you can, find out how the parts are being racked. Quenching at an angle, or even off horizontal can cause warpage and banana bend. But that wouldn't account for the cracking.

Kal

 

[unclesyd]

Based on my many rounds of specifying or actual heat treating of 4140 components you couldn't pay me enough to quench a part in water.
4140 CS has a propensity to edge crack, die crack, and center crack during the heat treatment process.
I've checked all of my remaining reference books and none even give values for water quenching 4140.

If the part was a thick section and complicated I would normally normalize it prior to heat treatment, but your part shouldn't need this.

From Bethlehem's data to achieve 210 psi yield.
Oil quench from 1550°F into agitated oil.
Temper with out delay at 700°F. I would temper twice.

I don't care to use 4140 in this hardness range. We have developed information over the years that makes the toughness values suspect.

IMO if you had preexisting cracks in your parts they would have broken into pieces.
I think the warping will be mitigated by increasing the separation and using agitated oil of sufficient volume. I like 100::1 v/v.

 

[TVP]

I agree with unclesyd-- water quenching of 4140 is going to produce cracking and distortion problems. One of the reasons I provided the impact energy previously was to highlight that this is not a great condition for toughness for this alloy, as it is just above the low point. Proper oil quenching should work, although it may require 4140H.

 

[arunmrao]

Despite all cautions during heattreatment and then quenching even with polymer quenching media cracking is common.Alternate will be to go for 4340 or oil quench ,so that you can get a good night's sleep.


I have lost many parts in heattreatment of 4140 grade!

 

[Jboy333]

Thanks for all the tips guys. We're going to get with the heat treater and see about going back to the oil quench, as well as trying to get our hands on some 4140H.

Of course, for cost reasons, we're opting not to use 4340...although that would be our preference, given the choice.

 

[Maui]

If you still experience problems with distortion and cracking during oil quenching, you might consider checking into how often they change out their quenching oil. We ran into a quench cracking problem several years ago, and discovered that the heat treater was not changing out their oil at regular intervals. It was well past its prime.

Check also to see if they monitor the oil temperature prior to quenching. What is the acceptable temperature range that they have established for their oil quenching procedure?

Maui

 

[BillPSU]

Your shape may also be giving you cracking problems. If possible chamfer or radius the corners. Using a salt water brine instead of water will improve the cooling characteristics of the quench solution.