Casting Defects of Metal Materials, part 5

The main defects of casting products are segregation, pores, shrinkage and shrinkage, inclusions, cracks, cold shut and other defects.

Cracks

The cracks generated during the solidification process of the metal are called hot cracks; the cracks generated after solidification are called cold cracks.

Cracks destroy the integrity of the metal. Except for a few that can be removed by timely processing, they usually expand further along the stress concentration area during subsequent processing and use, and finally lead to rupture.

Hot cracks are formed when the ingot has not yet completely solidified or has solidified but there is still a small amount of low-melting-point phase between the grain boundary and the dendrite.

The shrinkage of the metal in liquid and solid state and solidification shrinkage are hindered. When the shrinkage stress exceeds the metal strength at that time or the linear shrinkage is greater than the elongation of the alloy.

According to the different locations where they appear, hot cracks can be divided into surface cracks, center cracks, radial cracks and side transverse cracks. Hot cracks mostly extend along the grain boundary, are tortuous and irregular, and often have branches. There may be oxide films in the cracks or the surface is slightly oxidized.

The factors affecting hot cracks include the nature of the alloy (solidification shrinkage coefficient and high temperature strength of the alloy, etc.), casting process and ingot structure. Certain elements and insoluble low-melting impurities in the alloy can significantly increase the tendency of hot cracking.

The cooling rate of semi-continuous ingots is relatively high, so the tendency of hot cracking is much greater than that of iron mold ingots. Increasing the casting speed during casting will also increase the tendency of hot cracking. From the perspective of the ingot structure, the larger the cross-sectional size, the easier it is to cause hot cracking.

Cold cracks occur when the ingot is cooled to a relatively low temperature elastic state. If there is still a large temperature difference between the inside and outside of the ingot, the shrinkage stress may be concentrated in certain weak areas. Once the stress exceeds the strength and plasticity limit of the metal, the ingot will cold crack. The characteristics of cold cracks are that they are mostly transgranular cracking, mostly linear expansion, and the cracks are relatively regular, upright and straight. Cold cracks often develop from hot cracks.

The direct cause of casting cracks is the existence of casting stress, which is caused by: inappropriate casting temperature, fast speed, excessive or too low cooling speed, uneven cooling; improper continuous casting pull-stop process; the alloy itself has hot brittleness and poor strength; unreasonable selection of covering agent or lubricant; poor design, deformation or improper installation of crystallizer, crucible, bracket, casting tube, etc.