Causes and Solutions of Cracks in Steel Castings

Cracks are one of the most common and troublesome defects in steel casting production. They not only compromise the performance of the casting but may also result in scrapping. Essentially, cracks occur when internal stresses (mainly thermal stress and shrinkage stress) exceed the strength of the material at a given temperature. According to the stage of formation, cracks are generally classified into hot cracks and cold cracks.

I. Hot Cracks

Hot cracks usually appear at the final stage of solidification or shortly after solidification, when the steel is in a solid–liquid coexistence state. At this stage, the material has very low strength and ductility, making it highly susceptible to cracking.

• Temperature range: near the solidus line, approximately 1300–1450°C.

• Characteristics: fracture surfaces are oxidized, often dark or bluish, with irregular, tortuous shapes.

Main causes:

1. Casting design: significant variations in wall thickness and sharp transitions create uneven cooling and severe thermal stress.

2. Unreasonable gating system: poorly positioned or overly concentrated ingates cause localized overheating, leading to cracking during final solidification without proper feeding.

3. Poor collapsibility of molding/core sand: high sand strength prevents free contraction of the casting, generating tensile stresses.

4. Chemical composition:

• High sulfur (S) and phosphorus (P) contents form low-melting-point compounds at grain boundaries, weakening cohesion and increasing hot cracking tendency.

• Excessive carbon (C) widens the solidification temperature range and promotes coarse dendritic structures, also unfavorable for crack resistance.

5. Improper use of risers and chills: incorrect riser neck dimensions or poorly positioned chills aggravate uneven cooling.

II. Cold Cracks

Cold cracks form after complete solidification, usually when the casting cools below 600°C. At this stage, the steel is in an elastic state, and cracks are mainly caused by residual stresses.

• Temperature range: below 600°C.

• Characteristics: fracture surfaces appear clean, metallic, sometimes with light oxidation. Cracks are generally straight and continuous.

Main causes:

1. Stress factors:

• Thermal stress from uneven cooling rates.

• Shrinkage stress due to restrictions from mold, cores, riser systems, or box supports.

• Transformation stress from phase changes, such as austenite transforming into martensite with a volume expansion.

2. Poor metallurgical quality:

• High gas content, especially hydrogen, may cause “hydrogen-induced cracking.”

• Excessive inclusions act as stress concentrators, reducing crack resistance.

3. Premature shakeout: removing castings from the mold before cooling to a safe temperature (below ~400°C) may trigger cracking.

4. Improper heat treatment:

• Rapid heating or cooling introduces excessive thermal stresses.

• Quench cracks are a typical form of cold cracks, caused by martensitic transformation and the associated volume stress.

III. Prevention and Solutions

When cracks occur, the causes should be traced systematically—from material composition to process control. Key approaches include:

1. Chemical composition: strictly limit harmful elements such as S and P; adjust carbon content appropriately.

2. Refining process: adopt secondary refining to reduce gases and inclusions.

3. Casting design: avoid abrupt wall thickness changes; use smooth transitions and fillets to reduce stress concentration.

4. Process optimization:

• Design a proper gating and feeding system to achieve sequential or balanced solidification.

• Ensure molding/core sand has good collapsibility.

• Apply risers and chills correctly to control cooling sequence.

5. Shakeout and cleaning: delay mold removal until the casting cools sufficiently (below 400°C), and avoid stress introduction during riser cutting or welding repairs.

6. Heat treatment: establish proper heating and cooling rates; for complex castings or alloy steels, use step heating and controlled slow cooling.

IV. Conclusion

Cracks in steel castings are typically the result of multiple interacting factors. Correct identification of the crack type and root cause requires a combination of fracture surface observation, metallographic analysis, process review, and chemical testing. Only by optimizing every stage—from raw material selection to casting design and post-treatment—can crack occurrence be significantly reduced, ensuring higher casting quality.