Austempered Ductile Iron

[UconnMaterials]

Hello,

We have a batch of austempered ductile iron, but the cross-section doesn't look like ausferrite. This image is at 1000x and the etchant is suoer picral. Could this be bainite? The hardness is 27-29 HRC. It's also brittle as heck; I don't need to cut samples instead I just use a vice, hammer, and file to notch. For additional context, I subjected this material to extended aging treatments at 200C,24hr, 300C,24hr, 400C,24hr, and 500C,24hr. Images from the 200C and 500C aging treatments are included. The 200C image looks like bainite of some form- a vilella etch brings out the plates better. The 500C image looks like fine-pearlite. The treatments in the 200-400C range had a similar bainitic appearance. I'm stumped on the as-received microstructure because it doesn't resemble ausferrite as I know it. I would like to get a second opinion.

Thanks




As-Recieved:

200C/24hr:

500C/24hr:

 

[mrfailure]

What hardness did you measure after each of the heat treatments?

 

[UconnMaterials]

mrfailure:

No change in hardness (HRC) until 500C. The hardness dropped a 2 points. I know the as-received hardness isn't what you want in an austempered ductile iron. Should be between 35-45 HRC- preferably above 40 HRC in a wear application. I've had other batches where the microstructure had that characteristic lathe appearance.

 

[MagBen]

I would second that the as-received is unlikely an ausferrite, but rather more likely a bainite structure. due to
1. lack of ductility
2. hardness sluggish to additional tempering (aging), meaning cementite is already formed in the matrix.
would check if the cooling rate was not high enough at austempering, and/or the temperature was too high (up to 400C?)

 

[UconnMaterials]

Thanks MagBen,

Superpicral etch also shows massive carbides for the as-received:

This seems to be somewhere between ausferrite and bainite because additional heat treatment at 200C, 24hr produces the "typical" bainite structure. I think an SEM analysis will be informative. From a practical level, this material is unusable, but I would still like to understand how this microstructure evolves with heat treatment.

 

[EdStainless]

I haven't worked with these for decades but too slow of cooling would be my first thought.
Places that do a lot of ADI have their own special chemistries in order to make HT easier and more consistent.

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P.E. Metallurgy, consulting work welcomed