Austempering Core Hardness Problem
Austempering Core Hardness Problem
(OP)
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.





RE: Austempering Core Hardness Problem
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Plymouth Tube
RE: Austempering Core Hardness Problem
RE: Austempering Core Hardness Problem
RE: Austempering Core Hardness Problem
RE: Austempering Core Hardness Problem
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.
RE: Austempering Core Hardness Problem
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