W2 grain size discussion

[DevinT]

Critical temperature I think refers to the temperature at which a phase is completed. I’ve heard it used mostly to refer to above Ac3 and sometimes the quench temperature.

Below Ac1 is referred to as sub-critical, above Ac1 is where austenite starts to form, above Ac3 is where you have complete transformation to austenite. Acm is the temperature at which all of the carbides are dissolved. Between Ac1 and Ac3 is called inter-critical.

Ac1, Ac3, etc. is used to note transformation temperature upon heating, Ar1 etc. is used to note transformation temperature upon cooling. A1 or Ae1 etc. is used for equilibrium temperatures.

Normalizing is done from the Acm temperature. I’ve seen studies where they air cool from Acm to form fine pearlite, and others where they quench from Acm to form fine carbides in martensite, both lead to fine grain after thermal cycling and quench and temper.

Critical temperature is reached above Ac3, so I’m not sure if decending temperatures are necessary instead of just cycling from critical temperature. I’ve never thought about this untill Larrin brought it up.

Hoss

 

[Willie71]

Critical temperature I think refers to the temperature at which a phase is completed. I’ve heard it used mostly to refer to above Ac3 and sometimes the quench temperature.

Below Ac1 is referred to as sub-critical, above Ac1 is where austenite starts to form, above Ac3 is where you have complete transformation to austenite. Acm is the temperature at which all of the carbides are dissolved. Between Ac1 and Ac3 is called inter-critical.

Ac1, Ac3, etc. is used to note transformation temperature upon heating, Ar1 etc. is used to note transformation temperature upon cooling. A1 or Ae1 etc. is used for equilibrium temperatures.

Normalizing is done from the Acm temperature. I’ve seen studies where they air cool from Acm to form fine pearlite, and others where they quench from Acm to form fine carbides in martensite, both lead to fine grain after thermal cycling and quench and temper.

Critical temperature is reached above Ac3, so I’m not sure if decending temperatures are necessary instead of just cycling from critical temperature. I’ve never thought about this untill Larrin brought it up.

Hoss

I’ve wondered this as well, but people much smarter than me have recommended the decreasing temps so I just went with it.

 

[Larrin]

So, for example-

Normalize at ~1600f - cool in still air = pearlite and carbides
Thermal Cycle at ~1460f - cool in still air = refined grain (though my question here is the austenite is formed but if cooled in still air won't it become pearlite again?)
Austenitize at ~1460f - quench?

1 thermal cycle at or around aus temp for grain refinement? Or are multiple beneficial and it's simply the progressive temperature you're taking issue with? Or is it one step further and a pre-quench like Devin is recommending with AEB-L is a grain refinement thermal cycle to harden from?

I'll admit I've broken quenched steel with grain like Paul shows in the OP, where no normalization has been done. But I've only then broken quenched pieces that have been treated with a normalizing and progressively decreasing thermal cycle all together, not at any steps in between.

If cooled sufficiently slowly pearlite will form again during the "grain refinement" cycles. Though there are carbides that are not dissolved during a hold at 1460F.

Multiple cycles would probably further refine the grain though there is a point where no further refinement will occur or even some growth.

 

[kuraki]

Interesting image that popped up in a discussion of AR15 bolt failure.

 

[Larrin]

High carbon steels don’t technically have an A3 temperature but the terminology of Ac3 and Ar3 is still often used to refer to austenite transformation finish on heating (Ac3) and start on cooling (Ar3). I’m not sure why, probably just convenient and easy to understand and remember.

Here is an example of using Ac3 and Ar3 where there is no A3: https://www.cartech.com/globalassets/datasheet-pdfs/m2-double_six.pdf

Edit: I apologize for not using subscripts but figuring out how to do that on my iPad is more trouble than it’s worth.