CVN values on GX120Mn13 test block

[Dalinus]

Hello,

I would like to ask you for help with heat treatment.
We have serious troubles to reach mechanical properties CVN) from test block from material GX120Mn13.
Chemical composition GX120Mn13 is in range C 1,15-1,30%, Mn 12,00-13,00%, Si 0,30-0,50% and Cr max 0,50%.
Test block has size 75 x 100 x 200 mm.
Heat treatment is 5 hours on 1060°C and then quenching in water bath.
CVN results are between 60-100J, but customer demands more than 100J by 20°C.
CVN test pieces are taken almost from surface of the block.

When we cut plate 75 x 16 x 200 mm from original test block and heat treat in our laboratory small furnace with only 1,5 hour on 1060°C and quench in bucket with water, then we get CVN more than 150J!!.

Unfortunatelly I do not have pictures, but in lab test there are minimal or almost no carbides on grain boundaries or inside grains, in large test block the grain boudaries looks the same but there is something inside grains.

I have searched several sources on high Mn steels but I could not find definitive solution of this problem.
- is the holding time too short for 75 mm thicknes?
- is the holding time too long?
- is the speed of quenching water in quenching bath too low(only about 30-40 cm/s)?
- is the temperature too high or too low?
- any other possibility?

Dalin

 

[arunmrao]

Please correct if I have misunderstood your problem. Lab test of the sample qualifies the CVN values, however in actual practice the CVN values are not attained. I have following comments to make .

The problem lies somewhere in the heat treatment process . The temperature I presume is well monitored and uniform in the furnace ( Last week, I encountered a problem of heat loss near the door top, resulting in poor microstructure ( islands of carbide within the grains) .

The speed of transfer appears to be good.

My next and only concern is the water temperature as the casting enters the water tank, the rise in temperature of water and fast circulation of water through a cooling tower.What is the volume of water in the tank in relation to the plate size. At no time the temperature of water must exceed 45C.

Hope it helps.