As the name implies, casting defects refer to irregularities that appear in castings during the metal casting process. In this section, we will discuss different types of casting defects and their causes. Only in this way can you prevent casting defects and avoid sending defective products to your customers. This guide will start with five types of casting defects to help you identify, repair, and avoid casting problems.
Classification of 5 types of casting defects
Common defects in castings are divided into five categories: holes, cracks, surface defects, unqualified shapes and sizes and weights, and unqualified components and structures and properties. (Note: Mainly introduce the defects of steel castings that are easy to cause cracks)
1. Perforated casting defects
Hole casting defects include pores, shrinkage holes, shrinkage porosity, slag holes, trachoma, and iron beans.
Also known as gas eyes, bubbles, holes 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 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.
Also known as shrinkage, is a hole caused by shrinkage.
The characteristics of shrinkage cavity are an irregular shape, roughness in the hole, and coarse crystal grains.
The reason 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.
Also called looseness, pinhole honeycomb, and small and many holes 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.
Slag hole is also 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 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.
Trachoma is trachoma 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 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 are holes that hold iron beads. 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 casting defects include hot cracking, warm cracking, and cold cracking.
Thermal cracking occurs at a higher temperature and often occurs 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 are excessive sulfur content or excessive impurities in the iron; poor retreat performance of the sand 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.
Thermal cracking is also called heat treatment cracking. This casting defect is 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 cracking is a crack produced by castings at lower temperatures.
The characteristics of cold cracking are: there are penetrating or non-penetrating cracks on the casting; it is linear, and the surface of the crack is not oxidized.
The reasons are:
- Unreasonable casting structure, uneven thickness.
- Poor retreat performance of sand mold or mud core.
- Inconsistent cooling of various parts of casting, the high-stress tendency of the alloy itself.
3. Surface casting defects
Surface casting defects include sand sticking, scarring, sand inclusion, and cold isolation.
We mainly introduce the cold partition that can cause casting cracks.
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 and long, and narrow, and it has a development trend under the action of external force. Judging from the characteristics of the cold barrier, it is easy to cause damage to the mother’s body. Cracks 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.
multiple ladles and multiple points are poured simultaneously so that the two metal streams are connected. But incomplete fusion and no inclusions exist between, so the two-layer metal bond is fragile; the pouring temperature is too low, the pouring speed is too slow, and the pouring time is too long. The alloy has poor fluidity, too low carbon and silicon, and high sulfur content. Or multiple ladles are poured sequentially, and the cut-off time of the front and rear ladle is too long; the casting section is thin and long, the molten iron flow is unfavorable, the casting flow is interrupted, the sand mold runs out, the one-time molten iron is insufficient, and the supplementary pouring is not timely.
4. Casting defects of unqualified size, shape, and weight
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. Casting defects with unqualified composition and performance
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 casting defects
The treatment of 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 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 must be carried out by qualified personnel who have passed the flat welding test. When welding, a reasonable welding specification should be selected. When starting arc welding, prevent the arc from damaging the surface of the casting. It is forbidden to ignite the hook in the non-welded area. Welding parts should avoid artificial blowing and draughts. The butt weld cannot be filled too much at one time, and multi-layer welding shall be adopted, which shall not be less than three layers. The first layer of welding is welded with a smaller diameter electrode to prevent thermal cracks. After each layer is welded, the slag should be completely removed before soldering the next layer. During welding, the interlayer of the weld should be kept not lower than the preheating temperature. For welding with longer welds greater than 200mm, the segmented method shall be used. The arc starting and ending positions of each layer should be staggered by no less than 20mm. Try to avoid blind spots and terminal arc starting or ending.
3. Welding repair of cast iron parts
If the 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.