Casting Cracks: Causes and Prevention

Cracks are the most heartbreaking defect: everything done, then a crack appears and the whole casting is scrap. Two families: hot tears and cold cracks.

Hot Tears

Features: irregular zigzag, along grain boundaries, heavily oxidized (blue-black), at late solidification, thick-thin transitions.

Causes: late solidification — outer shell solid, interior semi-solid; restrained contraction (core resistance, poor mold collapsibility) exceeds liquid film strength, tearing along grain boundaries.

Prevention: better mold collapsibility (sawdust in core sand, hollow cores); rounded transitions, fewer abrupt thickness changes; control S/P (raise tear tendency); proper pouring temperature.

Cold Cracks

Features: straight, penetrating, metallic bright fracture (no oxidation), appears after full cooling, crisp sound when struck.

Causes: internal stress (thermal + structural + contraction) exceeds tensile strength at low temperature. Thick-thin and closed-structure castings are vulnerable.

Prevention: uniform walls, rounded transitions; controlled shake-out timing; timely annealing for stress relief; slow cooling instead of water quenching.

Linked Defects

Cracks often ride on other defects: shrinkage porosity concentrates stress; inclusions weaken the matrix; coarse grains lower crack resistance. Treating cracks means treating fundamentals.

Temperature Relationship

Too hot: longer solidification, coarse columnar grains, more segregation — more hot tearing. Too cold: poor feeding, more shrinkage porosity — hidden crack seeds. A stable, correct pouring temperature window is foundational.

Summary

Hot tears = restrained solidification; cold cracks = low-temperature internal stress. Read the face, then adjust collapsibility, structure, shake-out, heat treatment and pouring temperature.