Even after following specified heat treatment from knock out to final HT ,delayed intergranular cracks observed under all riser cutting locations and near chills in 40 ton P92 castings.What are factors causing cracking?
Delayed cracking is mostly related to hydrogen, and solidification stresses. Modification of casting design to minimize stress concentration at the riser corners may help. Basic melting, and other measures to minimize hydrogen will have to be considered.
Thanks a lot sir, as a matter of fact as pouring liquid metal weight is about 50 ton so 4 ladles are transferred in two ladles after vacuum degassing .Hydrogen gets picked up during transfer of hot metal and subsequently rearcing befoure pouring in mould upto 7-8 ppm level.Please advice on riser design to avoid solidification stresses.
I do not think that riser design will help avoid solidification stresses altogether. However, proper design will help minimize the stress concentration at the riser - casting interface and move the stress triaxiality to the riser itself. Hydrogen assisted cracking preferentially occurs at the triaxial stress locations because of notch effect. Large draft (chamfer) angle, and large radius of the corners will help minimize the stress concentration. My two cents (paise).....
Sir;
Whether ANTI FLAKING HEAT TREATMENT will help in avoiding the delayed cracking .I think AFT+ improved design of risers and chills will help avoiding threshold stresses.
Regards
Shivraj Singh Kasana
Yes sir, I completely agree with you on the anti flaking heat treatment. Still, you may consider ways to decrease the initial hydrogen content. Below 2 ppm may be safe. Higher the hydrogen content more will be the heat treatment time to diffuse out the hydrogen especially for a 50 ton casting. You may want to perform a cost benefit analysis between the cost of decreasing the hydrogen content, and cost of anti flaking heat treatment. All the best!
Mr. Kasana,
Am I right in presuming that the material is Carbon Steel of any carbon content?.
You must have de oxidized it using either pure Aluminum or Fero Aluminum. If the residual Al content is more than a specified limit. all the Aluminum Nitrides will seggregate to below the riser neck after completion of solidification. When you remove the risers by Arc cutting, these nitrides inclusions being highly refractory, offer resistance to cutting thereby inducing Under Riser cracks. Pl check your residual Al content in the cracked area and let me know. If you can send a sketch of your methoding of this item and the metal chemistry, perhaps I may be in a
position to give a convincing answer. Pl oblige-- A.V.Sethuraman
No sir, material is not carbon steel it martensitic steel having carbon,chromium,molybdenum,vanadium tunguston and nitrogen percent as 0.12,10,1.00,0.6,1.1 and 0.04 weight percent.
Shivraj
How about the de oxidation process/ What deoxidizer was used and what %age?
DEAR SIR, NO DOUBT MELT DEOXIDIZED BY ALUMINIUM BUT FINAL TOTAL Al IS 0.012 ,FOR SURE THEY ARE NOT ALUMINIUM NITRIDE CRACKS.
SHIVRAJ