Defect characteristics and detection methods
It is a straight crack on the casting. The surface of the crack is strongly oxidized, which is dark or black without metallic luster. Most of them are at the inner sharp corner of the casting, the transition part of the thick and thin section, and the joint of the inner gate and the casting.
It can be found by visual inspection, X-ray fluoroscopy and fluorescence inspection after casting or core removal.
Causes of formation
- The design structure of the casting is unreasonable. If there is sharp angle and thick section suddenly becomes thin section, uneven cooling will occur
- The strength of sand mold is too good, and the core’s collapsibility is too poor, which hinders the shrinkage of alloy liquid and causes cracks in castings
- Local overheating of mold
- Pouring temperature is too high
- Incorrect placement of cold iron
- Non feeding alloy liquid supplement for thick and large parts
- Premature casting, too intense cooling
Prevention methods and remedial measures
- Improve the design structure of the casting, eliminate the sudden change of sharp angle and thick section into thin section structure, and try to make all parts of the casting cool evenly and simultaneously
- Take all kinds of measures to make castings cool evenly and simultaneously: ① According to the shape of thick parts, choose the proper shape and size of cold iron and its reasonable setting position. ② Correctly select the position of introducing alloy liquid to prevent local overheating. ③ Proper reduction of pouring temperature. ④ Use riser and chill together to increase temperature shaving of casting.
- Try to make the castings solidify in sequence: ① The riser is placed in the thick part of the casting, and the insulation material is pasted to prolong the solidification time. ② Place the cold iron or quench material on the thick part of the casting. ③ Adopt bottom casting pouring system, match with high mold. ④ Properly reduce the pouring temperature of alloy liquid.
- Reduce the strength of sand mold and increase its yield: ① Changing dry sand mold to wet sand mold. ② Reduce the compactness of sand mold and core. ③ Replacing the strong molding sand with the collapsible molding sand ④ Reduce the core frame and outline size to prevent it from hindering the shrinkage of the casting. ⑤ Use hollow core to enhance its yielding.
- According to the shape and warping degree of the warped casting, the cold or hot rectification method is adopted to restore the casting to its original shape
- If none of the above measures can solve the problem, the alloy with high temperature and high strength with narrow crystallization interval (i.e. narrow temperature range of quasi solid phase) can be replaced