The main defects of gray cast iron caused by moulding core and their causes and prevention methods

Analysis and prevention of the main defects of gray cast iron caused by core molding (Qufu Longxiang Smelting and Casting Accessories Co., LTD.)

(1) Stomata

Features and discovery methods:

Local pores: local parts of the casting, the appearance of holes on the surface of the relatively clean and smooth single pores or honeycomb pores

It can be found by visual inspection, machining or magnetic inspection.

Reason analysis:

1. The setting of the pouring system is unreasonable, so that the exhaust is not smooth or the eddy current is generated, and the gas is involved

2. Sand compaction is too high, reducing the permeability

3. The sand core exhaust is poor, or the vent is blocked

Prevention methods:

1. The setting of the pouring system should consider the smooth exhaust in the cavity and the smooth flow into the mold

2. Sand compaction should be uniform and should not be too tight

3. The sand core exhaust should be smooth. When closing the box, pay attention to seal the gap between the core head, so as not to drill the iron liquid and block the vent

4. In the high part of the casting, the air outlet or air sheet can be set

5. When drawing the mold and repairing the mold, it should not brush too much water

6. For large flat castings, inclined casting can be used, and the outlet is slightly higher to facilitate exhaust

7. Core brace and cold iron must be thousand clean, no rust

(2) trachoma

Features and discovery methods:

The holes in the casting contain sand particles

It can be found by external inspection, machining or magnetic inspection

Reason analysis:

1. The position of the pouring system is not suitable, such as directly to the sand core, or the gate is too small, and the iron liquid washing force is large. Damage to local sand mould

2. Because the structural design of the model is not good enough, the adhesive mold occurs, and the sand mold is not repaired, or the corner of the casting is not rammed

3. The residence time of the wet type before pouring is too long, so that the dry bath part or the protruding part falls off

4. The falling sand when modeling and sealing box is not cleaned

Prevention methods:

1. Proper location and size of pouring system

2. Reasonable selection of drawing inclination and rounded corners, manual modeling, can be pressed out rounded corners. In batch production, the pattern should be coated with parting agent to avoid sticking, and to repair the damaged part.

3. Shorten the residence time of wet type before pouring

4. Reasonable selection of the gap between the core head and the core seat to avoid crushing when closing the box

5. Before closing the box, it is necessary to clean the inner falling sand, close the box carefully, and cover the pouring riser in time to avoid falling into the sand again

(3) sand

Features and discovery methods:

On the surface of the casting, a layer of sand is sandwiched between a layer of iron and the casting

It can be found by appearance inspection or machining

Reason analysis:

After the iron liquid enters the sand mold. The water of the mold surface layer migrates inward, and a high water zone is formed at 3~5mrn of the mold surface. The strength of this area is greatly reduced, which is easy to cause liquid iron to dive into, or due to the stress of high temperature expansion of silicon sand particles, the surface layer bulges, and liquid iron is drilled into the formation of sand.

1. The compaction degree of sand mold is too high or uneven

2. Improper pouring position; For large flat castings with horizontal casting, sometimes the sand is trapped because the liquid iron intermittently covers a place on the large flat surface

Prevention methods:

1. The compactness of sand mold should not be too tight, requiring uniform, and enhancing breathability

2. When manual modeling, local weak points can be strengthened by inserting nails

3. Try to make the large plane face down or placed on the side to reduce the baking area and baking time of the iron liquid on the upper plane

4. For large flat castings, the pouring system can be dispersed, and the inner gate section can be appropriately increased to shorten the pouring time or tilt pouring

(4) Sticky sand

Features and discovery methods:

The liquid iron on the surface of the casting is stuck with sand to form a rough surface, which can be found by appearance inspection

Reason analysis:

1. The sand compaction is uneven or too small

2. Paint is too thin

Prevention methods:

1. Appropriately improve the sand compactness, reduce the sand gap (to ensure the requirements of air permeability), and compacting evenly

2. Choose the appropriate paint (mostly graphite powder water paint), and brush with a certain thickness, which can not only improve the fire resistance, but also prevent the iron liquid drilling into the sand

(5) Hot cracking

Features and discovery methods:

Cracks with a dark or almost black oxidized surface

By appearance inspection, light transmission method, magnetic flaw detection, pressure test, kerosene penetration and other methods found

Reason analysis:

1. The retreat of sand core and sand mold is poor, and the shrinkage of casting is hindered

2. The amount of sand eaten by the core bone is too small or the sand box is too close to the casting, which prevents the shrinkage of the casting

3. The inner runner setting is too concentrated, local overheating, increasing stress

4. The flash edge of the casting is too large, and the crack at the flash edge extends to the casting

Prevention methods:

1. The compactness of the sand should be suitable, and an appropriate amount of sawdust can be added to the sand

2. Use a smaller core bone to make the amount of sand suitable and choose a reasonable sand box

3. The inner runner layout should be properly dispersed

4. Shrinkage rib can be added at the junction of thick and thin casting

5. Correctly select the parting surface position and make it close as much as possible when closing the box

(6) Deformation

Features and discovery methods:

Long or flat castings are recessed near the wall thickness to form a curved shape

Use appearance inspection, marking and other methods to find

Reason analysis:

Due to the uneven wall thickness of the casting, the cooling has been successively, resulting in thermal stress, and when its value is greater than the yield limit of the material, deformation and bending will occur

Prevention methods:

1. Set the cold iron at the thick wall or open the inner gate at the thin wall to create solidification conditions at the same time

2. Set aside pre-deformed curvature or add reinforcing ribs on the pattern

3. Improve casting structure

(7) Wrong box

Features and discovery methods:

Relative displacement of the casting occurs along the parting surface

It can be found by appearance inspection or line measurement

Reason analysis:

1. The shape size is wrong or deformed

2. The positioning of the sand box or parting plate is not accurate

3. Improper packing

4. The position of the image on the template is offset

Prevention methods:

1. Check and repair the appearance

2. Check, repair or change the sand box and parting plate

3. Close the container accurately

4. Check and adjust the position of the pattern on the mold plate

(8) Meaty

Features and discovery methods:

There are irregular burrs, slits or protrusions on the casting

Use visual inspection to find

Reason analysis:

Due to the pressure of liquid iron, the local expansion of the cavity is caused. Most of them appear in the next type, such as the sand compactness is not enough or uneven, and the local part is too loose

Prevention methods:

Appropriately improve the sand compactness, and require uniform compaction

(9) Shrinkage hole

Features and discovery methods:

The concentrated holes with irregular shape and rough surface are produced in the hot joint of the casting

It can be found by visual inspection, machining or magnetic inspection

Reason analysis:

1. Insufficient feeding of castings

2. The chiller is improperly set

3. Improper location of the inner gate

4. The compactness of sand mold is not enough, and it is generated after expansion

Prevention methods:

1. Properly increase the size of the riser

2. In the thick wall, set the cold iron to create simultaneous solidification conditions or create sequential solidification conditions when used with the riser

3. Correctly select the pouring position and pouring system to cause simultaneous solidification or sequential solidification

4. The sand tightness is required to be appropriate

(10) Lift the box

Features and discovery methods:

The appearance of the casting does not match the drawing, and it is found by appearance inspection

Reason analysis:

1. The weight of the press box is not enough

2. The force of clamping box is uneven or too loose

Prevention methods:

1. Press the box with sufficient weight or tighten it evenly with bolts

2. The parting surface should be flat, and attention should be paid to close the box to avoid leakage of iron liquid

(11) Iron bean

Features and discovery methods:

There are small iron beads in the stomata; Check the section of the casting, and the machining can be found

Reason analysis:

1. The sand is wet

2. The inner runner is too high from the low part of the casting, which causes liquid iron to splash and form iron beans when pouring. After the liquid iron is filled, it fails to melt the iron beans and pack them into the casting with gas

3. Poor permeability of sand core

Prevention methods:

1. Do not brush too much water when repairing

2. Reasonably determine the position of the pouring system

3. Strengthen the ventilation of the sand core

(12) Slag eye

Features and discovery methods:

There is slag in the holes outside or inside the casting

It can be found by visual inspection, machining or magnetic inspection

Reason analysis:

Poor slag retaining in pouring system

Prevention methods:

Reasonable selection of pouring system and strengthening of slag retaining measures

(13) Insufficient cold insulation and pouring

Features and discovery methods:

There are gaps or local missing meat that are not fully integrated on the casting, and there are round edges around it

It can be found by visual inspection

Reason analysis:

1. Improper setting of pouring system, or too small gate section

2. Parts of the casting wall are too thin

3. Improper selection of cold iron location

4. Hanging core, when the mold is misaligned, the part of the wall of the casting is too thin, or even no wall thickness at all

Prevention methods:

1. Appropriately increase the pouring system size

2. Double casting can be used for long castings; For tall parts, step pouring or dispersed gating can be used

3. The inner runner should not be too far away from the thin wall of the casting, or the thickness of the thin wall can be appropriately increased

4. When lifting the core, check the size at any time and pay attention to the accuracy of the fitting