Austempering With Molten Lead 1

[fc60]

Greetings,

New to the forum and I have lots of questions.

I am working at Austempering 4140 Steel to a final hardness of 40-42 HRC.

Material is 0.75" diameter with a 0.313" hole through 6" in length.

I heat the part to 1600F and hold for 30 minutes.

Next, the part gets quenched in a bath of molten Lead at 650F for 30 minutes.

Resultant hardness averages 48HRC.

Returning the part to the oven at 850F for two hours brings the hardness down to 42HRC.

Here is what I hope to learn...
[ul]Am I really getting a Bainite structure?
What does tempering the Bainite do? Is it still Bainite after the 850F temper?[/ul]

Please feel free to "poke holes" in my process. I wish to learn, that is why I am here.

Cheers,

Dave

 

[EdStainless]

What about your micros? What do they look like?
Tensile tests, what about elongation?
Any impact data?
After all it is the results that really tell you if you were successful.
Most people would use molten salt for HSE reasons, but lead should work.

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P.E. Metallurgy, Plymouth Tube

 

[swall]

The hardness decrease you are seeing after the 850F post austemper treatment indicates that the austemper process did not produce 100% bainite and some untempered martensite was present. Bainite would not temper back to a lower hardness; martensite would. The holding time in the lead bath needs to be increased in order to produce a higher percentage of bainite.

 

[fc60]

Greetings,

A great start. Learning a lot.

Does molten Lead remove the heat from the steel part as quickly as molten salt?

I got into Austempering after reading the heat treat specifications on an old blueprint.

It said to Austenize 4150 steel at 1550F and quench in salt 650F-675F for 30 minutes minimum then air cool.

Lastly, it said to temper the part at 400F minimum to the desired hardness.

I am not a material scientist, I did my Apprenticeship as a Tool and Die Maker.

For a full analysis of the sample, I would have to work with one of the local Metallurgical Labs. The cost of tuition.

Thanks for all the input.

Cheers,

David

 

[BrianPetersen]

Why use lead instead of salt, especially if salt is what was recommended?

The environmental and health+safety departments would rather have you use salt, too ...

 

[Carburize22]

I've been meaning to respond to this for a while. I dabble in austempering as a hobby. I get my salts from this guy:

http://www.hightemptools.com/salts.html

He has both a high-temperature chloride mixture with a working temperature of 1,300-1,650F, and a low temperature nitrate/nitrite mixture that is suitable for austempering. I like the high-temperature salt especially, since the heat transfer rate to your part is very fast, and it protects it from scaling and decarburization as long as the bath chemistry is maintained through rectification. I have thought about using lead as an austenitization bath, but decided against it due to health ramifications.

I have an ASM book that recommends leaving your part in the austempering bath for 2 or even 3 times as long as what a TTT heat-treating diagram indicates for austempering at that temperature. I have also seen ASM references that mention parts that were accidentally left in a bath for several weeks with no ill effects.

The ASM Heat Treater's Guide for Irons and Steels has sections devoted to different quenchants and their differing cooling rates. It also has a section devoted to austempering and martempering. A nitrate/nitrite mixture with no water additions has a cooling ability that is comparable to a conventional (slow) quenching oil. Water additions to the bath increase the cooling ability a lot, but that is dangerous because of the risk of splattering and steam explosions.

I have seen references/articles stating that not only is small amount of martensite in a predominantly bainitic structure not harmful, it may be beneficial in terms of impact toughness. Is there a note on the drawing that says it needs to have a 90% bainitic microstructure?

 

[fc60]

Greetings Carburize22,

Many thanks for the post.

Since you are currently using the salts, do they have a tendency to bloom and creep? I have used water soluble salts to Black Oxide tools and I store the tank outside within a double plastic bag.

Handling Lead does not bother me as I wash frequently and wear gloves. Also, when I am done, I can pour it into ingots and sell it back to the scrap yard I bought it from. The Black Oxide salts I used had to be treated as HAZMAT and were an inconvenience to dispose of.

Would you kindly quote the ISBN number of the ASM Heat Treater's Guide for Irons and Steels? I shall start a search for a used copy.

Back to Bainite, if the part is quenched above the Ms point, where would Martensite come from? Would trace amounts occur after removing the part from the quench bath as it cools below the Ms point?

I have also started an eBay search for a good used Metallurgical Microscope. Any suggestions will be well received as to brand, type, etc.

With kind regards and appreciation,

David

 

[Carburize22]

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.