Knowledge

Cracks in castings are common problems in the casting manufacturing process. The causes are complex and diverse, mainly including internal defects in the casting, temperature stress, pressure stress, unreasonable casting design, and improper cooling measures.

Section 1: Overview

I. Causes of Cracks in Castings

1. Internal Defects of Castings:

Internal defects such as gas holes, slag inclusions, and foreign inclusions may cause stress concentration, thereby leading to cracks.

2. Temperature Stress:

During the casting process, castings undergo cooling and solidification stages. If the cooling rate is uneven or the temperature changes too rapidly, it can cause temperature stress within the casting, which may lead to cracks.

3. Pressure Stress:

Castings may be subjected to external pressure during the casting process, such as during pouring, cooling, or processing. When these stresses exceed the material's load-bearing capacity, cracks may occur.

4. Unreasonable Casting Design:

The design of the casting may have structural issues or uneven wall thickness, which can cause stress concentration and promote the formation of cracks.

5. Improper Cooling Measures:

Unreasonable cooling rates and methods can lead to uneven temperature distribution within the casting, which may cause cracks.

II. Methods for Preventing Cracks in Castings

1. Improve the structure of castings:

Strive for uniform wall thickness, and make transition fillets at corners to reduce stress concentration.

For wheel castings, the spokes can be made curved when necessary to disperse stress.

2. Enhance the melting quality of alloy materials:

Use refining and degassing processes to remove oxide inclusions and gases from the molten metal, and control the content of harmful impurities.

3. Adopt correct casting process measures:

Make the castings solidify simultaneously to reduce the tendency of hot cracks and cold cracks.

Reasonably set the positions and sizes of gates and risers to make the cooling rates of all parts of the casting as uniform as possible.

Extend the time the casting stays in the sand mold to further equalize the temperature between thick and thin sections of the casting, reducing temperature differences and thermal stress.

4. Increase the collapsibility of the sand mold and sand cores:

Remove the box pressure iron and loosen the sand box clamping device as soon as the casting solidifies to reduce the resistance to the casting's shrinkage and promote uniform cooling of all parts of the casting.

5. Perform aging heat treatment:

Timely conduct aging heat treatment for castings with large casting stress to eliminate excessive residual stress and prevent cold cracks.

6. Control the time of opening the box and the cooling rate:

Avoid opening the box too early to prevent large internal stresses in the casting and the formation of cracks.

Control the cooling rate to avoid temperature stress concentration due to too fast or too slow cooling.

7. Use appropriate pre-heat treatment:

Such as spheroidizing annealing or aging treatment, to timely eliminate the internal stress of castings and forgings.

Section 2: Hot Cracking

Hot cracking is a type of crack that occurs within the solidification range of castings. The fracture surface has a severely oxidized black appearance. The crack develops along the grain boundaries and has an irregular and tortuous shape. The outer surface of the crack is wide while the inner surface is narrow. Internal cracking occurs inside the casting, usually in the last solidified areas, and the fracture surface often shows dendritic crystallization.

I. Causes of Hot Cracking

The thickness of the casting fracture varies greatly, the fillet is too small, and there are too many forks in the overlapping part;

The gating system hinders the normal contraction of the casting, such as the sprue and riser being too close to the box band, etc.;

The collapsibility of the mold (core) sand is poor, such as too much clay in the mold sand, too high compaction, and inappropriate core bone structure and shape, etc.;

The sulfur and phosphorus content in the molten iron is too high;

Improper use of anti-cracking process ribs and chill irons;

The casting's flash hinders contraction;

Early shakeout or careless handling in the hot state, etc.

 

II. Methods to Prevent Hot Cracking

When designing castings, try to avoid sudden changes in wall thickness, and make corners rounded. Cold iron or process ribs can be used in areas prone to tensile stress and those that solidify later.

Use multiple dispersed inner gates as much as possible. The junctions between the gates and risers and the casting should have appropriate fillets. The shape and position of the gates and risers should not hinder the normal contraction of the casting.

Improve the collapsibility of the molding (core) sand, such as adding an appropriate amount of wood chips to clay sand.

Do not make the molds (cores) too compact.

Use core supports with appropriate rigidity, and ensure sufficient external sand coverage, etc.

Section 3: Cold Cracking

Cold cracking is the cracking that occurs in castings when they cool to the elastic state. The cracks are long and have a uniform width, and the fracture surface has a metallic luster or a slight oxidation color. Cold cracking often appears in the stress concentration areas of castings, especially in large cast iron parts with complex structures and significant differences in wall thickness.

 

I. Causes of Cold Cracking

Improper design of the casting, with significant differences in wall thickness, resulting in excessive variations in cooling rates among different parts of the casting;

Inappropriate design of the gating and risering system, causing large temperature differences among different parts of the casting;

High sulfur and phosphorus content in the molten iron, making the casting more brittle;

Large residual stresses within the casting, which are further exacerbated by impacts and compressions during operation;

Presence of inclusions, shrinkage cavities, and gas holes in the casting, leading to stress concentration and other issues.

 

II. Methods for Preventing Cold Cracking

Strive for uniform wall thickness in the design of castings, avoid sudden changes, and make corners rounded. Place chill irons in thick sections and set anti-cracking ribs at locations prone to tensile stress concentration.

The shape and position of the gating system should not impede the contraction of the casting.

Improve the collapsibility of the sand mold (core) to reduce contraction resistance.

Do not shake out the casting too early. After shakeout, keep it warm to avoid wind cooling and strictly prevent contact with water. Also, avoid collisions during shakeout, cleaning, and handling.

Control the sulfur and phosphorus content in the molten iron within the specified range to eliminate brittle structures.

Perform timely aging treatment on the casting to reduce or eliminate residual stresses, etc.

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