The ASM Handbook series would be helpful for you. The term "rock candy" appears 20 times throughout. For castings, the handbook has a table of casting defects (originally created by the American Foundry Society) and lists conchoidal or rock candy as defect number C 411 and defines it as "Separation along grain boundaries of primary crystallization".
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
When excess Al is added to the melt for killing the bath, aluminium nitride is formed which results in classic rock candy fracture. You can take a small(20 lb) sledge hammer and knock off the feeders and gates easily.
" All that is necessary for triumph of evil is that good men do nothing".
Edmund Burke
stanislasdz;
I don't have my copy of the Physical Metallurgy Handbook with me today. The Handbook is an excellent resource book for metallurgical engineers, and is not too expensive. As also mentioned by CoryPad, I would investigate an on-line subscription service with ASM.
stanislasdz;
I checked in the Physical Matallurgy Handbook by Sinha, and the aluminum nitride embrittlement that can result in the "rock candy" fracture appearance in castings. The minimum amount of AlN to cause intergranular fracture (aka rock candy appearance), in carbon steel is 0.002% and for alloy steels is 0.004%.
To reduce the susceptibility to this embrittlement;
Additions of Ti,Zr,B,S,Mo,Ni or Cu to the steel
Use of the lowest possible amount of nitrogen (0.005% or less), aluminum (0.015 to 0.030%)
Increased cooling rate after solidification
Faster cooling rate in the range of 2100-1290 deg F after solutionizing to control the amount and size of AlN.
Aluminum nitride embrittlement, which is typified by a rock candy fracture surface, is dependent on (among other things): Al level, N level, and section size. There are some curves available that show the safe levels of Al for a given N and section size, but they aren't necessarily accurate particularly at large section sizes. The Steelfounders' Society of America (
to follow metengr, substitution of Ti for Al usually works well for lowering susceptibility. I have seen empirically (not sure if it is in texts or not) that AlN formation follows a trend of the Al X N product. They form on gamma grain boundaries and can often be remedied with re-austentizing. hope this helps.
