Casting 0.2% C ,12% Mn steel

[arunmrao]

I shall be thankful if I could be given more information on a 0.2% C, 12% Mn steel. A bell casting for hopper is to be processed in this alloy. There is machining involved in the seating area and also a centre bore which needs grinding finish. The standard mentioned is a Chinese one and is incomplete(ZGMnSi) is all that it mentions.

 

[priyasachin]

priyasachin,

dear friend,

the composition is similar to high manganese steel also known as hadfield steel. It has a good wear resistance, owning its property for the work hardening tendency it has in abundance. but this material is virtually unmachineable, because as the tool digs in the material bercomes harder and further becomes difficult and finish bad. it also has damaging effect on the tool too. grinding is the standard operation on such material.
wonder what is the intended end use is.
regards,
MRCN

 

[gieter]

Virtually unmachinable is a bit to strong as we machine 12%Mn-steel occasionally. But Priyasachin has a point that it is very difficult because of the work-hardenability. As a consequence the casting should be made near net shape with a minimal machining allowance. The machining has to be done in a single pass at slow speeds. The surface finish is rough compared to other steel grades. It’s extremely difficult to attain a smooth surface finish and close tolerances.
The machinability increases with Mn content at the expense of the work hardenability. As this is the prime reason to choose Mn-steel, this normally is not an option.

 

[arunmrao]

I want to inform that the castings cracked during heat treatment,(Same given for standard manganese steel.) The customer is upset and I am having a terrible time. Can I have some clue as to why such a low carbon material should crack in heat treatment? Thanks

 

[TVP]

If you used the typical austenitizing temperature of high-Mn steels, which would be ~ 1010-1090 C, then this could be problematic. With the C content only ~ 0.2%, the austenitizing temperature is considerably lower, probably below 700 C. At these high temperatures, considerable decarburization can occur, which can cause a skin to form that is partially martensitic. Severe quenching after a very high austenitizing treatment (extensive grain growth can occur if no carbides are present) can then produce tensile stresses that cause the martensitic skin to crack. Have you sectioned any castings to observe the microstructure yet?

 

[gieter]

This steel has a low thermal conductivity and a high coefficient of thermal expansion. This means that care should be taken during heating up. Heating rate should not exceed 100°C/min. To high a rate can cause cracking.

Can you give an idea of the thickness/complexity of the casting?

 

[arunmrao]

I have discounted for the low thermal conductivity by heating slowly and then holding at 800C before raising to 1000C.
The casting is conical in shape like a bell The large dia is 675 mm and the small dia 250mm with the straight height being 900mm. The wall thickness varies from 40 mm to 20 mm at the edge.The casting is split into two halves and later the two halves are bolted.

Thanks for the interest shown in my problem.