Decarb

[smurali1]

We are doing carburising (long cycle - case depth of appx. 3mm) in pit type gas carburising furnace (endo gas). The part is taken out and quenched in seperate quenching tank which is appx. 50 mts away.

1. What should be done to control decarb on the surface?

We also apply anticarb paste in some fine pitch threaded area. Still we see some 70 to 80 micron IGO and carburising in threaded areas.

2. What could be reasons for the above.

Thanks

SPM

 

[dbooker630]

As far as the stopoff coating:
How many coats are you applying?
How much drying time before furnace entry?

I'm not sure which product you use (I have Park No-Carb Special), but in some applications I have had to apply two coats for complete stop-off. Is there any appreciable hardness increase?

 

[CoryPad]

For decarburization, do you mean in the time it takes to transport the part 50 m from furnace to quench tank?

Can you oversize the part and hard turn after carburizing? Or locally temper the threaded regions to reduce hardness and increase toughness?

Eighty micrometer depth of IGO is huge and indicates high oxygen concentration in the furnace during heat treatment. You need to control the atmosphere better.

 

[smurali1]

Here are some information:
1. We use Anticarb which is very thick in consistency. (almost like a dough used for pizza)
2. The time between anticarb application and start of HT is appx. 1 hour.
3. Hardness at the area where anti-carb is applied is 57 HRC and carburised depth is appx. 70~80 mic. (Please note the specified case depth in the intended area is 2.2 to 2.7 mm)
4. The part has to be tranported appx. 50 mts from furnace to quench tank.

Thanks

SPM

 

[redpicker]

My experience with stop-off paint is for it to be completely effective, the material has to be very clean, the application has to be completely dry, and two or three coats are needed. We used to put a heavy case (approx. the same depth as you mention) on parts that had threaded ends. Tolerances reuqired threading after heat treatment. Obviously, the threading operation could not tolerate any bleed-through of the carburization. We tried several methods, and settled on

1) burn-off treatment at 400-500 F for an hour.
2) cool and shot-blast if any residue was evident.
3) if shot-blast, repeat burn-off (shot-blast would leave contaminats)
4) apply 1st coat - dry
5) apply 2nd coat - dry
6) apply 3rd coat - dry
7) bake at 250-300 F for 2 hours
8) charge into carburize furnace before parts allowed to cool below 100F

Eventually, we got a "warmer" that would allow us to dry around 150F and then raise up to 250 for bake. Prior to that, we would just air dry for 8 hours.

Different stop-off products have different characteristics. I know of one company that performed copper plating by electrolysis to insure stop-off.

I am a little confused on your comment "The part has to be transported appx. 50 mts from furnace to quench tank." Do you mean 50 meters? If so, this doesn't make much sense since it isn't distance that is the problem, but time. If you mean 50 minutes, I don't see how you keep the parts at temperature. If it's 50 meters, you should be able to keep the transfer time to below a minute or two (thats less than 1 meter per second). Other than a little surface scaling, that shouldn't be too much of a problem.

Regarding the IGO and carburizing you are seeing in the threaded areas, this is obviously failure of the stop-off treatment. Depending on the endo gas composition, you can have both carburizing and oxidation occuring at the same time. I think this is the reason Cory suggested better control on the atmosphere.

 

[smurali1]

Sorry for the confusion. I meant 50 meters. As redpicker rightly pointed out the transport time is less than 2 minutes.

 

[flexorguy]

I don't know what carburizing steel you are using but you might consider doing a carburize and anneal first. If you leave stock in the area of the thread you do not want carburized you can bring it back and turn off insuring 100% removal of carburized layer. Then go back to heat treater for quench and temper. This avoids the unpredictability of paint off altogether.

 

[mfgenggear]

all good suggestions.

Dbooker

where do you buy your no carb from?

Smurali1

I agree with others here.

Use non-porous copper plate as carburize stop off. or Carburize anneal, Machine & Harden and Temper.
depends on the part configuration and cost.

both are good

 

[dbooker630]

mfgenggear,

I get No-Carb Special from Heatbath/Park Metallurgical in Detroit (

http://www.heatbath.com).

 

[smurali1]

Hello,

Here is one clarification:

We have to cut the threads before carburising.

But we do not want hardened threads to avoid chip off.

So, we apply anti-carb paste (Durferrit) to avoid carburising in the thread. Please also note the paste is very thick, almost like dough base of Pizza. But inspite of this, we get carburised threads(0.08mm case depth) and IGO of 0.08mm. Please note we run a cycle of 60 hours to get a case depth of appx. 3 mm.

Thanks

SPM