What does "bloom and creep" mean? I don't think that the salts used in black-oxiding are similar to the salts that I use. Neither of the salt mixtures that I mentioned in my post are horribly toxic - you ingest them daily in the food you eat (although I would not recommend eating these). Lead fumes are hazardous to breathe, which is why I greatly prefer using the salts. Wearing a helmet, face-shield, heavy welding gloves is highly recommended.
The high-temperature salt comes in a granulated form, like course table-salt, and the low-temperature salt has the appearance of small styrofoam peanuts. I don't use either of them with water. Through a hefty amount of experimentation, I know the approximate volume of high-temperature salt that I can heat to a certain temperature using a submerged bend-and-stay 1,500W incolloy heating element from McMaster-Carr. My high-temperature pot is actually a stainless-steel drinking cup that I got from a camping store. It's about 4.5 inches tall and 4 inches in diameter - quite small. I wrap it in high-temperature insulation (also from McMaster) and secure the insulation steel or stainless steel wire. I attach a stripped extension cord to the connections on the heating element, and control the voltage and temperature with a "variac" (there may be better ways to do this). I use a fluke thermometer with a K-type thermocouple to monitor the temperature, and I also have a cheapo digital lead thermometer for the low-temperature bath. I use the expensive Fluke to check the accuracy of the cheap one, so I don't need to buy two expensive ones. This pot can get up to 1,600F quite easily, indicating that I could probably make one that is considerably larger and still be able to get to the temperatures that I need.
The low-temperature salt bath can be quite a bit larger. I have one that is about 5 inches diameter x 9 inches tall, and I have no problem getting it up to 1,000F using only 100V. I used a cooking pot that I got at Bed, Bath & Beyond. It could be quite a bit larger than that and still get up to the proper austempering temperature.
Upon first melting the salt, don't just fill up your pot with salt! Add enough salt to just cover your coiled incolloy heating element, and alloy it to melt - then add more salt until the pot is full. If you completely cover the heating element, the salt in close proximity will melt, but the salt above it will form a sort of insulating cocoon that will not melt! It fuses together under the heat, and you have to hit it with a rod to break it apart, which is very dangerous.
Some important considerations:
1) Don't let the high-temperature salt exceed 1,650 degrees.
2) Don't let the low-temperature salt exceed 1,000 degrees F (it decomposes into toxic gases around 1,100F)
3) Don't introduce any oils, dirt, or organic material into either salt bath. Make sure your parts are degreased, clean, and dry.
4) Don't contaminate the high-temperature bath with low-temperature salts. I am told this will ruin the bath.
5) Don't introduce cyanide carburizing salts into the low-temperature bath. It will explode. I have no idea why you would playing with cyanide salts, but I thought I'd mention it.
6) The high-temperature salts are not soluble in the low-temperature salt. Instead, they either kind of just float in low-temp salt or fall to the bottom. My low-volume hobby work does not necessitate that I do anything about the small amount of high-temp salt collecting at the bottom of the pot, but for higher-volume work (real work), you need to scoop that crap out of there.
7) The high-temperature salt needs to be "rectified" occasionally. Click on the FAQ link that appears under the high-temperature salt for sale for instruction on how to do this.
8) I like to keep a record of the total time that my high-temperature bath has been molten, so I can have a good idea of how much "mileage" it has on it, and when I need to rectify it. My latest salt pot has only been on for a total of 1 hour - I only do this as a hobby - whereas an industrial heat-treating salt pot is left on 24/7, including weekends.
9) I like to heat-treat a test article that is representative of the volume & surface area (or maybe even larger) of the real part, so that I can have a decent idea as to whether I can pull enough heat out to get the microstructure that I want.
10) These salts are also useful for martempering, which is similar to austempering but results in a martensitic microstructure if done correctly.
The 2 ASM (American Society of Metals) books that I recommend the most are:
Heat Treater's Guide: Practices and Procedures for Irons and Steels, by Harry Chandler (editor)
Atlas of Time-Temperature Diagrams for Irons and Steels, by George F. Vander Voort (editor)
I'm not sure where your martensite is coming from. Every piece of steel is different, so the martensite start temperature of the piece you made your part out of might have a higher-than-normal start temperature. You might be forming martensite after you remove your part from the austempering bath, because if the bainite reaction isn't complete, the remaining austenite will transform to martensite - although, that doesn't sound like what you did to me.
I have never used a metallurgical microscope in my life, so I can't help you there. Sorry. I am not a metallurgist, I'm just some hillbilly.
