Abstract
Test coupons attached to large steel castings occasionally show the brittle fracture which is called rock candy fracture. This brittle fracture goes along the primary grain boundary. Usually this primary grain boundary disappears by heat treatment, but it often remains even after heat treatment and becomes brittle. In order to clarify the causes, the influences of dimensions of castings, heat treatment conditions and chemical composition on the mechanical properties and the state of fracture were investigated.
The results obtained were summarized as follows:
1) Rock candy fracture originated along the primary grain boundary which was appeared by an etchant of 50% HCI aqueous solution.
2) When the cooling rate in molds was 2∼5°C/hr, rock candy fracture was likely to occur.
3) The higher the annealing temperature was, the lower the rate of rock candy fracture got, and moreover the fracture of the test piece heated at 1,200°C was ductile.
4) Under a given cooling rate there were critical contents of Al and N at which rock candy fracture occured. Therefore, rock candy fracture could be prevented by decreasing Al or N contents.
5) It was concluded that rock candy fracture occurs by the precipitation of AlN at the primary grain boundary which makes the boundary brittle.
