What are the causes of cast steel crack defects?

The occurrence of crack defects in cast steel parts is a very common and complex problem, which involves the entire production chain from smelting, casting process to subsequent treatment. Cracks are fundamentally caused by internal stresses (mainly thermal and shrinkage stresses) exceeding the strength limit of the material at that temperature.

Usually, cracks are divided into two categories: hot cracks and cold cracks.

1、 Hot cracking occurs in the late stage or shortly after solidification of molten steel, when the metal is in a solid-liquid coexistence state with low strength and plasticity. Temperature of occurrence: usually near the solidus line (around 1300-1450 ° C). Features: The crack section is severely oxidized, appearing black or blue, with a tortuous and irregular shape.

main cause:

1. Structural design of castings: Excessive differences in wall thickness and uneven transitions at connections result in uneven cooling and significant thermal stress.

2. Unreasonable design of the pouring system: The sprue is too concentrated or improperly positioned, resulting in local overheating, which eventually solidifies in that area

Unable to receive compression and support.

3. Poor retreat of sand mold/core: The strength of the sand mold is too high, which hinders its free shrinkage during casting solidification and shrinkage, resulting in tensile stress and cracking. This is a very common reason.

4. Alloy chemical composition: High content of harmful elements such as sulfur (S) and phosphorus (P): They form low melting point sulfides and phosphides, forming liquid thin films at grain boundaries, greatly weakening the intergranular bonding force, and are extremely important factors leading to thermal cracking. Carbon (C) content: With high carbon content, the solidification temperature range becomes wider, the dendrites become coarse, and there is an increased tendency for thermal cracking. 5. Improper use of riser and cooling iron: If the riser neck is too long or too short, and the cooling iron is not placed properly, it will exacerbate uneven cooling.

2、 Cold cracking occurs after the casting has completely solidified and cooled to an elastic state, usually in the low-temperature stage below 600 ° C. Occurrence temperature: lower temperature. Features: The crack section is clean, with metallic luster or slight oxidation color, and the crack is relatively straight and continuous in a straight line shape.

main cause:

1. Excessive casting stress: Thermal stress: caused by inconsistent cooling rates of various parts of the casting. Shrinkage stress: Mechanical obstacles to casting shrinkage caused by molds, sand cores, sprue systems, and box stops. Transformation stress: The stress generated by the change in specific volume during the cooling process when there is a structural transformation (such as austenite transforming into martensite).

2. Metallurgical quality of steel: High gas content, especially hydrogen (H), can cause "hydrogen induced cracking" and reduce the toughness of the material. There are many non-metallic inclusions: as stress concentration points, inclusions can significantly reduce the strength and crack resistance of materials.

3. Premature sanding during boxing: The casting is not yet cooled to a sufficiently low temperature, and the internal stress is not fully eliminated before premature vibration and sanding can easily cause cold cracking.

4. Improper heat treatment process: Excessive heating or cooling rate: Especially during annealing and normalizing treatment, if the heating or cooling is uneven, it will generate huge heat treatment stress, which will superimpose with the original casting stress and cause cracking.

Quenching crack: This is a special form of cold cracking, which forms high hardness martensite due to the rapid cooling rate of quenching, accompanied by huge structural stress, making it very easy to crack.

Summary and solution ideas

When cracks are found in cast steel parts, the reasons should be systematically investigated from the following aspects:

1. Chemical composition: Strictly control the content of harmful elements such as S and P.

2. Smelting process: Refining methods are used to reduce the content of gases and inclusions in the molten steel. 3. Casting structure: Optimize the design to avoid sudden changes in wall thickness and use rounded transitions.

4. Casting process: sprue and riser system: Reasonably designed to achieve sequential solidification or simultaneous solidification, avoiding local overheating. Molding sand/core sand: Ensure sufficient yielding and collapsibility. Cold iron and riser: Proper use to control the cooling sequence.

5. Sand removal and cleaning: Ensure that the castings are cooled to a sufficiently low temperature (such as below 400 ° C) in the sand mold before boxing. When cutting risers and welding repairs, it is also necessary to avoid generating new stresses.

6. Heat treatment process: Develop reasonable heat treatment specifications, especially control the heating and cooling rates. For complex parts or high alloy steel parts, adopt a step heating and slow cooling method.

To accurately determine the specific cause, it is often necessary to combine macroscopic and microscopic morphology analysis of cracks (metallographic examination), process review, and chemical composition analysis to make a comprehensive judgment.