Martensite/bainite discussion continued
I realized that a couple of threads had gotten hijacked with some advanced metallurgical discussion, so I thought I'd start a new thread so we can continue this important conversation.
Twalsh, I think your understanding of bainite vs. martensite is off base. I don't have my reference texts in front of me right now, but maybe I can post some passages later.
Both martensite and bainite are end products of quenched austenite. They do not transform into one another.
Let's take 1080 steel as a simple example. If you look at the TTT diagram for 1080, it shows that you will get 100 percent martensite in a piece of steel if you quench the steel from 1450 F or so to under 800 F in about 1 second. Then you have some more time (somewhere between 10 and 60 seconds, I forget exactly) to cool the blade to the Ms point, which is about 425 F.
Once you hit Ms, you will have martensite in the finished blade. Period. That martensite will not become bainite unless you heat the steel all the way to critical and begin the process over again. Tempering does not produce bainite.
So, back to our 1080. Once you get below a certain temperature, let's say 600 F, you have a choice. You can continue to cool the blade at a normal rate and produce 100 percent martensite, or you can halt the cooling and hold the blade at a temperature of around 500 or so degrees. If you do that for enough time, you'll create bainite. This process is called austempering.
Depending on how you handle the blade between the Bs and Ms points, you could end up with a mixed bainite/martensite structure in the finished blade.
Tempering a martensite blade results in tempered martensite, not bainite. I think your confusion might be arising from the similarity between the temperatures used in tempering martensite (350 to 500 F, roughly) and the temperature where bainite can form on cooling.
Mike, correct me if I'm oversimplifying or off base.
Josh
