20MnCr5 Duplex grains after normalizing

[23513001]

Hello,

We are seeing abnormal grain growth in 20MnCr5 material after normalizing. We see very fine grains (grain size 7/8) and in the middle of microstructure we see grain size of 3. The large grain has also transformation products inside the grain instead of normal ferrite, pearlite microstructure.

Any help in explaining underlying mechanisms for grain growth for this grade is really appreciated.

 

[CoryPad]

Can you post an image?

 

[23513001]

Hello,

Attached is the image of micro at 100X

 

[metengr]

What was the thermal processing history before normalization heat treatment? What was the temperature for normalization and hold time?

 

[23513001]

The parts are warm forged from steel bars. Normalize at 1650F for 30mins (at temperature) followed by slow cooling.

 

[CoryPad]

Mn-Cr steels like these can have chemical segregation, which can lead to microstructure variation.

How large are the parts? Are you sure the metal (especially in thick sections) is reaching 1650 [°]F?

 

[23513001]

The conducted the Temperature survey with TC inside the parts for the furnace. Parts stray at 1650F for 30 minutes. The coarse grains are observed from the surface to about 1/4" into the core.

 

[metengr]

If you are having pockets of transformation products within these large grain regions and this occurs during modest cooling, this implies local chemical segregation, as mentioned by CoryPad. You may need to perform a treatment similar to an homogenization treatment followed by hot forging to attempt to eliminate these regions.

 

[samsam1973]

Isothermal annealing would be more suitable for 20MnCr5.