On the casting surface observation, found that the transverse crack in the not far below the pulp cavity holebucket tooth. The crack section serious oxidation, no metallic luster, crack shape tortuous and irregular, exists in the casting surface crack width is larger, showing lacerate. Therefore, concluded that the crack is essentiallythermal cracking.
The hot cracking during solidification, casting in plastic deformation state. The liquid metal in the wall under the action of heat transfer from the surface of casting solidifies, the emergence of a large number of dendrite andbuild acomplete skeleton, produce solid contraction, then there is a layer of film is not yet solidified dendrites. If the casting solidification without any obstacles, namely the dendrite without force, tooth casting of free shrinkage, graduallyproduce stress. If the dendrite layer contraction by type, the core block, dendrite layer contraction by force, thedendrite layer tensile stress. When the stress exceeds the strength of liquid film limit,between the dendrite can be pulled apart, if around the metal liquid enough to flow into the crack, hot cracking in castings not, otherwise it will produce hot crack. The cause of its formation is mainly composed of forming components caused by uneven shrinkage. Distribution in the parts in the filling process of mold temperaturefield is not uniform, forming relatively lowpressure, can not effectively supplement, will cause the forming parts after the uneven shrinkage. As the leading part of workpiece thickness is thin, heat conduction performance is better, so the site prior to solidification of hot spot, will cause the workpiece to produce a large force of leadingcore. In the process of core pulling, generates a tensile stress, at the same time as the runner at this time has solidified, the workpiece impact certain resistance in the core pulling direction relative to the front, theresistance to tensile stress, is an opposite to the direction of the tensile stress. If the two yield stress is greater than the material under high temperature stress, will cause the bucket toothworkpiece was opened in the weak point, thermal crack formation, the so-called shrinkage crack.