Quench Cracking 4

[Wrenchbender]

Can a decarburized layer reduce the tendency for quench cracking?

We have forged steel cylinders 14-ft long and 16-inch diameter. The entire length is hollow with a 2.25 inch wall thickness, except for 3 feet at one end which is solid. There are no sharp corners anywhere; just very large blend radii.

We are quenching in agitated water because we need a rapid rate to develop martensite in the thru section of the wall. No problem there, but in the section that is solid, we are sometimes getting circumferential and longitudinal cracks. We are now trying remedies, e.g., eliminating time delays, keeping warm before tempering.

I thought of eliminating the anti-carb coating thinking that a decarburizied layer would be ductile and not as prone to nucleating tensile cracks (even though its only ~.03 thick). Any thoughts or comments on that?
Thanks.


Additional info:
Heat treatment is to normalize, then austenitize, quench and temper.
Quenchant is natural lake water, Seasonal temperature is 50 – 75 deg F.
Steel alloy is experimental: 0.26 C; 0.65 Mn; 1.00 ea Si, Ni, W; 2.60 Cr and some Mo, V & Cu. (No actual TTT or CTT diagrams)

 

[redpicker]

No, a decarb layer isn't going to do anything to prevent quench cracking.

Try pulling the material out of the quench at around 400-500F on the solid end and getting it into the temper before it falls below 300F. You should be below the M90 temperature and the cooling rate from 500 to 300F isn't going to make any difference in the transformation, but by slowing the cooling rate, you reduce the thermal gradient, so you reduce the tendency to crack.

Most quench cracks form below 400F.

rp

 

[Metalguy]

You might get bainite in the solid end, but martensite doesn't seem possible. Might not make much difference though.

Gold is for the mistress - silver for the maid
Copper for the craftsman cunning in his trade.
"Good!" said the Baron, sitting in his hall
But iron - cold iron is the master of them all.
Rudyard Kipling

 

[TVP]

I agree with redpicker regarding interrupting the quench. Another option would be to add a polymer to the water, like 3-5% of AquaQuench from Houghton Intl. This will reduce the quench cracking without significantly changing the quenching speed. I'm not sure that I understood the comments from Metalguy- this alloy will have substantial hardenability, so martensite formation upon quenching will certainly occur.

 

[CoryPad]

TVP,

I think Metalguy is identifying an area that won't form martensite - the 3 feet (1 m) length that is 16 inches (400 mm) in diameter. That area won't form martensite throughout.

 

[redpicker]

I do not like the term interrupted quench, or slack quench, as it implies you are not completing the quench or that you are otherwise not performing the quench correctly. On re-read of my earlier post, I suppose I wasn't clear. The process is to quench the part for long enough so that when it is pulled from the quench, the surface of the solid end "bounces back" to 450-500F (it will usually take around 2 or 3 minutes for the surface temperature to stabilize). Then, the part is allowed to air cool to 300-350F before charging into the temper. This will take an hour or so. Any austenite that hasn't transformed when pulled from the quench will transform to martensite on air cooling since, with this composition, transformation will be very sluggish at these temperatures. You will promote the transformation to occur more uniformly through the thick section, reducing the cracking. Of course, you will need to verify mechanical properties, preferably from a prolongation and surface hardness testing, from both ends if the properties of the solid end are critical.

rp

 

[Metalguy]

Even with the fairly high hardenability, that solid end probably isn't going to transform to anything but pearlite when it slowly gets down to ~1200 deg F.---EXCEPT at the surface.

Here's the likely problem. You have a hard tempered martensite surface over a huge FCC austenitic core. As that core slowly transforms to BCC, it expands. The surface must also expand plastically, and that's not going to happen with untempered martensite, so it cracks--in all directions, as you've observed.

You solve the problem by using some kind of insulating sleeve around the solid end--even a thick ring of carbon steel should work. Try for a shrink fit before heating the barrel (large gun barrel?). The idea is to prevent the surface of the solid end from cooling fast enough to transform to anything but pearlite.

Then, as the interior transforms and expands, the relatively soft surface will plastically expand (stretch) w/o cracking.



Gold is for the mistress - silver for the maid
Copper for the craftsman cunning in his trade.
"Good!" said the Baron, sitting in his hall
But iron - cold iron is the master of them all.
Rudyard Kipling

 

[Metalguy]

Alternatively, if you can quench the barrel vertically (as it should be) with the solid end up, perhaps you can keep the solid end out of the water.

Gold is for the mistress - silver for the maid
Copper for the craftsman cunning in his trade.
"Good!" said the Baron, sitting in his hall
But iron - cold iron is the master of them all.
Rudyard Kipling

 

[Maui]

A decarburized layer will not significantly reduce the tendency for this material to quench crack.

In order to minimize the occurrence of quench cracks, heat should be drawn from the bar in as uniform a manner as possible during quenching. This is especially difficult to do properly when there are large changes in cross-section or relatively thin sections are adjacent to thick sections on the same part. Water quenching can aggravate the problem due to the formation of gas bubbles at the interface between the quenchant and the part. Agitation of the water helps to reduce this tendency, but does not eliminate it. Based on your description of the problem, the components have generous radii in all of the locations where changes in cross-sectional area occur, and the quench cracks probably result from a different issue.

I ran into a similar problem with cracks in oil quenched 422 stainless steel rounds. The standard practice was to austenitize the bars at 1925 F, oil quench until the bar temperature fell between 500 F and 900 F, and then air cool the rest of the way. After the bars air cooled below 200 F, they would go back into the furnace for a temper. We experienced sporadic cracking on several orders, and I was assigned the task of identifying the root cause and to determine the appropriate corrective action.

An experiment was conducted in the heat treat department to determine if the oil quenching practice that was used actually produced bar temperatures within the specified range. When the first 422 trial order was quenched the material was pulled from the quench tank after a 4 minute soak. The temperature of the steel had dropped to 300 F, which was far below the minimum specified temperature of 500 F. The other order was run with similar results. After a soak of 1 ½ minutes in much colder oil the steel temperature had dropped to 400 F. Apparently, the orders that experienced fallout had been left in the quench tank too long. They cooled too far below the aim temperature, and cracked.

You may be experiencing a similar issue. How long are you leaving the bars in the quench tank? Have you measured the temperature of the solid cross section of the bars as a function of time during quenching? Have you monitored the temperature of the water to determine if there is a relationship between the tendency to quench crack and the initial water temperature? These are some of the questions that you should be attempting to answer in order to identify the root cause of your problem.

Maui

 

[mfgenggear]

Mar temper or Mar Quench in oil or salt 400 Deg F

works with chrome moly steels

 

[arunmrao]

maui,missed you for a long time. Were you on a sabbatical?Always enjoyed reading your posts.


If you think education is expensive, try Ignorance.
- Andy McIntyre


_____________________________________

 

[Maui]

Hi arunmrao,

Thanks for the compliment. I was occupied with a number of projects and distractions that kept me from spending any significant time here. It's good to know that you noticed I was gone. And it's good to be back.

Maui