Austempering Core Hardness Problem 1

[Metals101]

We've been austempering a 1/4" section of 10B21 steel successfully for several years using the following process:
Heat to 1650F.
Hold 30 minutes.
Quench into 480F salt bath.
Hold in salt for 2.5 hrs.
We normally achieve core hardnesses of 40-46 HRC.
Recently we've seen core hardness drop to 25-35HRC using the same process.
If we re-run the same low-core hardness pieces using the following process, the core hardness increases to 40-46HRC:
Heat to 1650F.
Hold 15 minutes.
Quench into salt for 2.5 hrs.

We've tried increasing temperature(1680F) and time(1hr) in the first heat treatment and parts continue in the 25-35HRC range but every time we re-austemper using the 2nd process the parts fall into spec. What are we missing?

Thank you.

 

[EdStainless]

What condition are they in to start with?

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Plymouth Tube

 

[Metals101]

Cold forged then carburized and slow cooled.

 

[Metals101]

I should also add that the chemical composition of the steel is in spec.

 

[CoryPad]

Perhaps the first exposure to 1650 degrees F is not fully transforming the part to austenite, so it is not forming bainite during the quenching process. Get some metallographic images from samples at each stage to see what is happening.

 

[metengr]

I would blame the change in response from the original heat treatment cycle caused by grain size variation and carbon content gradient as a result of cold forging, and subsequent carburizing with slow cooling.

The inital austempering heat treatment seems to act similar to a pre-heat treatment conditioning (more uniform grain size) resulting in the preferred response of the second austempering heat treatment. In other words you may need to perform the heat treatment in two steps versus a single step as before.

 

[redpicker]

Your 30F increase was a move in the correct direction, it just wasn't enough. When you have a structure that is resistant to austenitization, somethimes you have to really smack it to get it to transform. Increase your austenitizaton temperature 75F.

The Slow Cool from the carburizaton temperatures results in large grains of low-carbon ferrite. While this ferrite will eventually transform at 1650F in a 0.20%C steel, but becasue of the large grains, it will take a long time since the carbon has to diffuse to the centers of these large grains. Also, understand that your furnace is probably only calibrated to +/-25F at these temperatures, so while you think you're at 1650, you could be a bit lower. At 1725F, all ferrite will have transformed to austenite and the carbon diffusivity through austenite is much higher than through ferrite, so it will result in a more homogeneous structure much more quickly. The Boron steel should have plenty of grain refiners (Al, Ti) so grain growth shouldn't be an issue. In fact, you may find a smaller average grain size with the higher austenitizaiton temperature because you will get a larger number of austentite grains forming. I'd keep the time at 30 minutes, particularly your re-runs show that if the material does austenitize, 15 minutes is really enough.

rp