Heat Treating Furnaces FAQ
Heat Treating Tips with CRESS Furnaces FAQ
As an authorized seller of Cress Heat Treat Furnaces we often get questions regarding heat treating. Here are a few problems, tips, and solutions to help with your heat treating process:
Q: Why do heat treat recipes call for a pre-heat before going to the austenizing temperature? Is it really necessary?
Answer: Yes, it is necessary. It does two things. It gives the interior temperature of the part time to equalize with the surface temperature. During the heat treating, this helps relieve stresses and reduce distortion. But, it’s also essential to get the alloy’s chemistry to get ready, to transform into a proper austenite structure. This will increase the chances of forming fine-grained martensite after the quench.
Q: How do I know if my Cress furnace temperature controller is calibrated correctly?
Simple! Unless you check it, you don’t know. For that reason I always suggest customers purchase the Cress pyrometer with their furnace. It does several things.
First, with the thermocouple supplied, setting directly on the parts being heat treated, the operator will know clearly when the parts are at austenizing temperature instead of relying on ambient air temperature inside the chamber. With increased accuracy, this means less chances of over cooking, or under cooking parts.
Second, it also acts as a secondary check against the included temperature recorder. If the readings in the batch have a wide difference, that would indicate one of the thermocouples is out of calibration.
Q: I have an A2 part that has a main body that is 2” cubed, but it has a part that sticks out another 2” but is only ¼” thick on one side. The print calls for a 60 Rc but I can’t figure out how to make it work. How do I heat treat it and not over cook or under cook it?
Actually it’s not that difficult. Before you put it in your Cress Furnace, Wrap the part in SST foil and, clamp a block onto the ¼” section and trick the furnace into thinking it’s a 2” thick section. When you heat treat the part, it is all uniformly soaked as though it is a 2” thick block that is 4” long.
Q: I heat treated a piece of D2 in my Cress furnace and the part physically shrunk. Now it’s too small to use. Why?
Answer: The only reason a part will shrink in size after being heat treated is because it was over cooked. D2 is meant to be soaked at 1850F for 1 hour per inch of cross section. In many cases people will get busy and often don’t get back to the furnace when the time has expired. As little as 5 or 10 minutes too long and you will have over cooked parts.
Solution: Use the auxiliary contacts on the temperature controller of your Cress furnace to power an indicator light or ring a bell when your batch is done. Make sure you don’t over soak your parts.
If you lose size in a part from over cooking during the heat treat process, the part can be easily fixed by packing it down in dry ice or liquid nitrogen for 8 hours. This will cause the out-of-phase retained austenite to transform and the size will snap back to its original size.
Q: I heat treated a part made from A2 and when I put it on the surface grinder, it didn’t have much attracting magnetism. What causes this?
Answer: Essentially, it’s the same over cooking as we described above, only the part may not have shrunk. When steel reaches its austenizing temperature, it’s referred to as being in-solution. When that happens, the crystal structure is between stages and is essentially free floating with a loss of magnetism. If over cooked by too long, or too high a temperature, it never snaps back into phase and thus, magnetism is lost.
Solution: Similarly, packing the part in dry ice will solve the loss of magnetism.
Q: My Cress furnace is set up for the 2400F higher temperature. Can I heat treat high speed steels and powdered steels?
Yes, a Cress furnace set up for 2400F can heat treat high speed steels and powder metal tooling. But first, I generally don’t recommend heat treating these metals if you only do them occasionally. The reason I take this route is because high speed steels have extremely short austenizing soak times. Most all of them need to be soaked for 5 minutes maximum per inch of thickness once they reach austenization temperature. So, the over soaking timeframe, the duration the parts are in the furnace, is extremely critical. The margin of error is very small. The soak time literally comes down to seconds. In fact, every second of excess time takes away from the tool’s life and longevity. For that reason, I generally suggest sending the tools to a good commercial house that works on high speed tools every day.
In addition, when the parts are removed from the austenizing temperature, if you are using the oil quench method, they must be quenched in the oil immediately. Make sure the door to the furnace is closed as fast as possible. A furnace at 2400⁰F receiving an incoming draft of room-temperature air receives a heavy thermal shock. Over extended periods of time, this shock will affect the longevity of the ceramic and fire brick liner.
Q: Cress offers a Draw furnace but I can’t afford to spend the money. How important is it to have a draw furnace?
When a piece of metal comes out of the furnace and goes through the quench, whether by air, oil or water quenching, and the temperature drops below 150⁰F. It should be placed in a pre-heated tempering oven or the grain structure will be affected. That will cause premature tool failure. If the furnace is at 1750⁰F, it takes a long time for the heat to dissipate and to get it down to 400⁰F for tempering. Opening the furnace and force cooling will cut short the life of the fire brick and ceramic parts. This improper use may also affect the heating elements. Thus it makes sense to have a draw furnace that can be pre-heated and ready to complete the process.
Q: I don’t always have time to temper my parts so I put them into my Cress furnace and temper them the next day when I get to work. Is this OK?
NO! After quenching a part, it is loaded with freshly created Martensite. Martensite is a fine grain structure, but in the un-tempered condition it is extremely unstable. These parts can literally explode and send razor-sharp shrapnel pieces 25 feet in all directions. They can fail by performing a hardness test, heating, chilling, excessive vibrations as well as impact-tests.
In addition, an un-tempered part, sitting in room temperature for over 2 hours, loses much of its good attributes. Economically, it would be best to anneal the part and re-start the heat treat process from the beginning. The un-tempered part will still retain its hardness and will appear to temper properly. On the surface, it looks just fine; but the fine grain structure will have gotten coarse and refuse to transform additional Martensite as it would usually occur during the tempering process.
Solution: Never start heat treating unless you have time to complete the entire process. If you absolutely cannot finish, then quench the part to below 150⁰F, then put it back into your Cress furnace and raise the temperature back above 200⁰F. You may leave it there till the next morning. The part will temper and form the proper martensite structure. Just avoid letting it sit at room temperature. In fact, if permitted, you can temper the parts at the correct tempering temperature from the 200⁰F+ temperature set point. Remember, you can under-temper a part, but you can never over-temper a part by holding too long. Tempering temperatures will not overcook a part.
*Make sure management, the fire inspector and your insurance company approve of an unattended furnace in operation before doing this.