What are the requirements for surface hardness and wear resistance of gravity casting molds?

1. Hardness requirements

Mold base hardness: The base hardness of gravity casting molds is generally required to be between HRC35-50. For small and simple molds, the hardness can be slightly lower, around HRC35-40. This is because lower hardness is conducive to mold processing and repair.

For example, some gravity casting molds for producing small aluminum crafts can meet basic use requirements within this range, and are convenient for repairing when the mold is slightly damaged, such as repair welding. For large, complex and high-pressure molds, such as gravity casting molds for automobile engine cylinders, the base hardness is usually required to be between HRC45-50 to ensure that the mold will not deform when subjected to the pressure and impact of high-temperature molten metal.

 

Surface hardness of hardened layer: After the mold surface is hardened, the hardness of the hardened layer is required to be higher. Generally speaking, the surface hardness can reach HRC55-65. For example, the mold surface treated by carburizing and quenching process can reach this range. This is because higher surface hardness can effectively resist the erosion and thermal fatigue of molten metal and increase the service life of the mold. During the casting process, high-temperature molten metal (such as aluminum alloy molten temperature can reach 700-750℃) flows and solidifies in the mold cavity. The high-hardness surface can reduce the wear of the molten metal on the mold surface and prevent defects such as scratches on the mold surface.

 

2. Wear resistance requirements

Resisting molten metal scouring: Gravity casting molds must withstand repeated scouring of molten metal during use. For aluminum alloy gravity casting molds, the flow rate and flow rate of molten metal will vary depending on the shape and size of the casting. When casting some thin-walled and complex aluminum alloy parts, the flow rate of molten metal may be faster, and the mold surface needs to have good wear resistance to resist the scouring of molten metal. It is generally required that the mold surface can withstand thousands of times of molten metal pouring without obvious signs of wear. For example, in the gravity Casting mold for the production of automotive aluminum alloy wheels, the wear resistance of the mold surface is particularly important due to the complex shape and thin wall of the wheel.

Withstand demoulding wear: After the casting solidifies, it needs to be removed from the mold, and this process will also cause wear on the mold surface. Especially for some castings with complex shapes and undercut structures, the friction between the mold surface and the casting is large during demolding. The mold surface should have sufficient wear resistance to reduce wear during demolding. For example, when casting mechanical parts with complex internal ribs, the core part of the mold is easily worn during demolding. The wear resistance of this part of the mold surface is required to be high, and it must be able to ensure that after multiple demolding operations, the dimensional accuracy and surface quality of the core can still meet the production requirements of the casting.

 

Resistance to thermal wear: During gravity casting, the mold is subjected to repeated thermal cycles. When the molten metal is injected, the mold surface temperature rises rapidly, and the mold surface temperature drops sharply after the casting is demolded. This thermal cycle will cause thermal stress on the mold surface, which in turn causes thermal wear. The mold surface needs to have good resistance to thermal wear and be able to maintain good surface quality after multiple thermal cycles. For example, in gravity casting molds for casting large steel castings, due to the high melting point of steel (generally around 1400-1500℃), the impact of thermal cycles on the mold is more serious. The material on the mold surface needs to have excellent resistance to thermal wear, such as using special hot working die steel and performing appropriate surface treatment to reduce thermal wear and extend the service life of the mold.

 

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