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5 Types of Metal Casting Defects and Causes

Posted on by DAWANG

As the name implies, casting defects refer to irregularities that appear in the metal casting process. In this article, we will discuss different sand casting defects and causes. Only in this way can you prevent metal casting defects and avoid defective products. This guide will start with 5 types of foundry casting defects and causes to help you identify, repair, and avoid casting problems. For cast defects prevention, we have a more detailed guide on ways to prevent defects.

Metal casting defects and causes

Common defects in metal castings are categorized into five groups: holes, cracks, surface defects, irregular shapes, deviations in sizes and weights, as well as non-conforming components, structures, and properties.

1. Perforated Casting Defects

Casting defects that could happen in steel castings include pores, shrinkage holes, shrinkage porosity, slag holes, trachoma, and iron beans.

casting defects

Stoma

Also known as gas eyes, bubbles, and holes, these casting defects are caused by gas. The characteristics of casting pores are: generally round or irregular holes, the inner surface of the holes is smooth, and the color is white or with a layer of old dark color.

The causes of casting defects are: the furnace material is wet or corroded; the surface is not clean, the steam in the furnace gas and other gases; the furnace body and the ladle are not dried after repair; the gas in the cavity and the improper pouring system; the gas is involved in the casting, Poor permeability of mold or mud core, etc.

casting defects

Shrinkage cavity

These are casting defects caused by shrinkage. The characteristics of shrinkage cavity are an irregular shape, roughness in the hole, and coarse crystal grains.

The reason for casting defects is metal shrinks during liquid and solidification. The main points are as follows: unreasonable casting structure design; improper casting system; the size, quantity, and location of the chilled iron do not match the reality; the chemical composition of the molten iron does not meet the requirements, such as excessive phosphorus content. Pouring temperature is too high, running speed is too fast, etc.

sand casting

Shrinkage

Also called looseness, pinhole honeycomb. These casting defects are caused by shrinkage resistance. Shrinkage porosity is characterized by:

  • Tiny and disjointed pores
  • Coarse crystal grains with obvious mesh holes between the crystal grains
  • Water seepage during the hydraulic test
casting defects

Scum eye

Slag hole casting defects are caused by slag inclusion, slag bag, dirty eye, low molten iron temperature, and improper casting of slag. The characteristics of the slag hole are: the shape of the hole is irregular and not smooth; the whole or part of the hole is filled with slag.

The reasons for casting defects are: insufficient purity of the molten iron; poor slag removal and poor slag retention during pouring; poor slag retention by the gating system, and the gate is not full or cut during pouring.

 

casting defects

Trachoma

Trachoma is trachoma casting defects with sand in between. The characteristics of trachoma are: the holes are irregular, and the holes are filled with molding sand or core sand.
The reasons for casting defects are: The molding sand is damaged and peeled off when the box is closed. The loose sand or sand blocks in the cavity are not cleaned. The molding sand has poor compactness, and the core is broken during pouring; improper gating system design, poor core surface coating, etc.
 

Iron beans

Iron beans, also known as an iron bead, bean eye, iron bean trachoma, etc. The characteristics of iron beans are: the holes are relatively regular, and the holes contain tiny metal beads. Such casting defects often occur in iron castings.

 

2. Cracked Casting Defects

Cracking steel casting defects include hot tear defect, warm cracking, and cold shut defect.

Hot crack (hot tear defect)

Thermal cracking casting defects occur at a higher temperature and near the solidification temperature. The characteristics of thermal cracking are penetrating or non-penetrating cracks on the casting; it is curved, and the surface of the crack is oxidized.

The reasons for metal casting defects are excessive sulfur content or excessive impurities in the iron; poor retreat performance of the sand casting mold; improper pouring temperature control; the severe transition of the casting wall thickness transition; gate, cold iron, shrinkage ribs, vent holes Improper size and location, etc.

Warm crack

Thermal cracking is also called heat treatment cracking. These metal casting defects are caused by improper cutting, welding, or heat treatment. The characteristics of warm cracking are: there are penetrating or non-penetrating cracks on the casting; the metal surface of the crack is oxidized.

Cold crack (cold shut defect)

Cold crackings are crack casting defects produced by castings at lower temperatures. The characteristics of cold cracking are penetrating or non-penetrating cracks on the casting; it is linear, and the surface of the crack is not oxidized.

The reasons for casting defects (casting defects with images) are:

    • Unreasonable casting structure, uneven thickness.
    • Poor retreat performance of sand casting mold or mud core.
    • Inconsistent cooling of various parts of casting, the high-stress tendency of the alloy itself.

3. Surface casting defects

Surface steel casting defects include sand sticking, scarring, sand inclusion, and cold isolation.

Cold Isolation

The cold compartment is also called anti-fire, receiving fire, and so on.

The characteristics of the cold partition are: there are irregular and obvious sinking linear lines on the surface of the workpiece (there are two kinds of penetrating and non-penetrating).

Its shape is small, long, and narrow, and it has a development trend under external force. Judging from the characteristics of the cold barrier, it is easy to cause damage to the mother’s body. Cracks casting defects caused by the hard barrier will not appear immediately. When it is in use, fatigue cracks are produced under the action of bearing and impact.

Reason for casting defects：

    • Inadequate metal bonding can be a common problem, occurring during both simultaneous and sequential multiple ladle pouring processes. It is often caused by various factors, including low pouring temperature, slow pouring speed, and extended pouring times. Additionally, alloy properties and specific casting geometries can contribute to this issue.

4. Metal casting defects of unqualified size, shape, and weight

Metal casting defects of unqualified size, shape, and weight castings include: fleshy, insufficient pouring, falling sand, box lifting, eccentricity, deformation, wrong box, damage, shape, size and weight inconsistency, etc.

5. Foundry casting defects with unqualified composition and performance

Steel casting defects of unqualified composition and performance include unqualified chemical composition; unqualified metallography; unqualified segregation, excessive hardness, and unqualified physical and mechanical properties.

Treatment of steel casting defects

Treatment of steel casting defects
carbon steel casting
Once you identify these defects through manual inspection of systematic quality casting inspection methods, the treatment of steel casting defects is more critical, especially when dealing with cracks. Failure to pay attention to any link in the process will cause new cracks. Therefore, we must pay attention to the processing details, strictly implement the process, and ensure quality.

1. The principle of repairing defects in foundry steel castings:

The repair of casting defects must be carried out without affecting the quality of the product. Repair should be carried out in accordance with relevant standards and relevant technical regulations. For example, whether the existing casting defects are allowed to be repaired and the relevant technical requirements for improving.

2. Treatment method of steel casting cracks

Welding repair is one of the primary production processes of steel castings. Almost all casting defects on steel castings can be repaired by welding. Arc welding is widely used.
Main points of welding repair: In order to ensure the quality of welding repair, we should carefully clean up the sticky sand, oxide scale, inclusions, etc., of the casting defects; open the groove; do pre-weld preheating and post-weld heat treatment according to the weldability of the steel.

(1) Stop cracks and eliminate cracks

The cracks can be eliminated by arc gouging, angular grinding wheel grinding, and cold elimination method of windmilling. It is suggested that the crack elimination of essential parts should be eliminated by cold as much as possible to reduce heating as much as possible. Superficial cracks may not be used as crack arrest holes. The depth and width of the crack are relatively large, and the crack stop hole of 5-8mm must be prefabricated within 10mm of the end of the crack before treatment to prevent the crack from extending.

(2) Prefabricated groove and gap

When we deal with the cracks in the castings of factory repairs, axle box tie rod seats, side bearings, etc., the welds are required to be welded through. Generally, a 60-degree V-shaped groove is used, with a clearance of 2mm. For essential parts, T-shaped joints, or corner joints with serious cracks, it is recommended to open a 30-degree V-shaped groove to the vertical plate to ensure the quality of the fusion.

(3) Preparation before welding

To deal with cracks, we recommend using alkaline electrodes to improve crack resistance and toughness. Before welding, it is required to preheat the electrode to 200-250℃, and the operating temperature should not be lower than 150℃. The ambient temperature is not lower than 5 degrees.
It is required to grind the grinding wheel within 20mm of the groove surface and the welding surface. The grooved surface is not allowed to have more than 2mm grooves. The metal surface is free of oil stains, oxides, and rust.
It is recommended to preheat the steel casting matrix before welding. It is best to use overall preheating. If overall preheating is not possible, partial preheating can be used. The range of local preheating is at least three times the width of the section thickness of the weld on both sides of the weld.

Welding process

Qualified personnel, having passed flat welding tests, are required for welding. Choose an appropriate welding specification, prevent arc damage to the casting surface, and avoid igniting the arc in non-welded areas. Minimize artificial airflow around welding parts and adopt multi-layer welding with a minimum of three layers. Use a smaller diameter electrode for the first layer to prevent thermal cracks. Remove slag between layers and maintain interlayer temperature not lower than preheating temperature. For welds longer than 200mm, use a segmented approach with staggered arc starting and ending positions to avoid blind spots.

3. Welding repair of cast iron parts

If the casting defects such as pores, sand holes, slag inclusions, cracks, and leakage on the iron castings do not exceed the allowable range of welding repairs, they can be repaired by welding. However, the welding performance of cast iron is poor. After welding, pores, deformation, easy to break, difficult to process, and other problems often appear. Therefore, you should be very cautious when welding and repairing cast iron.
(1) Welding repair method: The welding repair method of cast iron is usually classified according to the preheating temperature of the workpiece
(2) Cold welding is called cold welding if it is not preheated before welding or only preheated to below 250℃; preheating to 250～450℃ before welding is called semi-hot welding; preheating to 500～700℃ before welding is called Hot welding.

Conclusion

Here we have presented the basics of porosity, shrinkage, mold materials, pouring metal, metallurgy, and casting shape defects. Specific examples of defects that can occur in castings are included. Now that you are more familiar with the types of failures that can occur in castings, are you ready to start purchasing? 

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