SOS: need help - Crack under riser 1

[stanislasdz]

I have a problem :

We have casting 3 parts in GS-24Mn6 steel casting !

One part is Ok but two have a cracks under riser !!

1- The chemical composition is Ok ! Al et N are low so no rock candy formation (Al =0.032 % and N = 0.0070 %)

2- Quenching at 880 °C and tempering at 565 °C

3- The riser were sawed so no overheating !

4- 1 part / mold

5- 1000 kg / part

6- Thickhness max : around 300 mm

7- Riser diameter : 550 mm

8- Sand gravity casting

9- EAF furnace

To be honest i don't know why 1 part is Ok and the two others are imperfect !!! with the same chemical composition ?

Who can help. I try to found other tracks ...but no success for this time

Stan

 

[arunmrao]

What is GS 24Mn6 grade please?

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[arunmrao]

Yes I have the chemistry of your grade steel. It is high Manganese (1.6%) carbon steel casting.

You have mentioned that you have mechanically removed the risers from the castings.

After riser removal,did you use snag grinders to remove the extra metal on the casting?

While grinding was undue pressure exerted to cause localised overheating and assist in crack formation.

Was the surface inspected for occurrence of cracks?

Was the casting normalised prior to Q&T? In heavy section castings,especially in the riser region there is heavy segregation,which can lead to localised cracking if the casting is not homogenised.

These are some general guidelines to avoid under riser cracks.

Hope these cracks are not as a result of hot tears?



Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[stanislasdz]

hi arun :

No grinding on this surface

The inspections was made by magnetic flux and by the Utrasonic waves

yes the surface was inspected for the occurence of the cracks ?

yes all the parts were homogenizeid at 950°C before Q and T

Segregations..... so what ? ....what can i do and why 1 part is OK and 2 are not ?

I don' believe this is hot tearings....

Ps: i will send in few hours the pictures of theses cracks

 

[stanislasdz]

 

[arunmrao]

I can see typically quench cracks on the casting surface.Did you use any coolant while sawing off the riser.

I am still wondering the reason for crack to occur.

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[stanislasdz]

Aruun :

for this moment i'a m studying 2 tracks :


1- Embrittlement tempering

2- Hydrogen embrittlement (coming from lime during steelmaking !!!)


Ps: yes we use lubricant when during the sawing of the riser !

 

[arunmrao]

If there is hydrogen pickup from lime it should appear as pinholes I suppose. I am not too sure about temper embrittlement in medium carbon steel. I still have a nagging feeling of it being a process fault.

How about the test coupons that you may have cast in the heat? Use them for your first analysis.

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[stanislasdz]

what do you mean by proces fault ?

Steelmaking , molding , heat-treatment .....

 

[dbooker630]

Going back to what arunmrao said about localized overheating and coolant - Can you take a micro of the removed riser adjacent to where it was sectioned from the casting to see if there is any reformed structure? It may reveal a clue as to root cause.

 

[arunmrao]

Please correct me if my understanding is wrong. Aren't,the temper embrittlement or hydrogen embrittlement related failures time dependent? They would take some finite time for cracks to develop. In this case it has been more or less instantaneous.

I feel that it should have to do with the coolant which has been used for sawing off the large diameter riser. Riser removal by mechanical sawing is a slow process and also there is some interference by mold materials sticking onto the riser surface,making cutting difficult.

I think I have stuck my neck too far without really understanding all the details.

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[stanislasdz]

arun and others :

thanks a lot ....

I still looking for the origin of these cracks it' seems comming from the heat treatment process may be cracks quenching !

I will share the results of our investigations.

Stan

 

[mahadhatu]

Here's a couple of things I could think of...

Pouring temperature also plays important part in the cracking under risers. Were all the castings poured out of the same heat? If poured out of the same heat, see the sequence of pouring. I have generally noticed that castings poured with excessive superheat generally present a different set of problems including this cracking under risers. If poured out of differnt heats, check your pouring temperatures for good castings v/s the bad one.

As rightly mentioned in previous posts, excessive localized heating during riser removal can cause cracking under risers but from the photo posted all the cracks appear to be at the center of the riser indicating that the cracking is shrinkage related. Possibly because there was a hot spot at the junction of the riser and the casting. This again can be related back to the pouring temperature if at all it was higher for the defective casting.




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There is always some thing better out there...
Mahadhatu