The performance of ductile iron castings depends on the properties determined by the microstructure on the one hand, such as the number of graphite spheres, the rate of spheroidization and the metal matrix, and on the presence of macroscopic and microscopic shrinkage holes in the castings on the other hand.In order to obtain high quality ductile iron products, the most important and critical step is to design the feeding system.Technological innovation further promotes the use of the self-feeding characteristics of ductile iron to improve the process yield of castings.It makes ductile iron become an alternative product for competing cast steel and aluminum alloy castings.This characteristic is used to describe the benefits of the new riser.
1. Solidification of ductile iron
The volume of most metallic materials (such as steel, white cast iron, nonferrous alloys) changes during cooling.First liquid shrinkage occurs on cooling, followed by contraction during solidification.In order to produce high quality castings, all liquid phase shrinkage must be supplemented throughout liquid cooling and solidification to avoid shrinkage defects in the castings.The final shrinkage of the solid phase is called the casting shrinkage.For graphite cast iron, such as gray cast iron, vermicular cast iron, and nodular cast iron, the carbide is precipitated in the form of graphite, which is less dense than liquid cast iron, and the liquid metal expands due to the density difference between the two.Expansion due to graphite precipitation can reduce the amount of supplementary metal required to obtain high-quality castings.Riserless casting has been widely used in the production of ductile iron hub parts for heavy duty wind turbines.However, in the case of tide molded sand, shell type or other conditions that are not suitable for riserless production, it is necessary to fill the metal liquid to ensure that the castings produced have no internal and external shrinkage cavity, shrinkage and porosity.
2. Optimize riser design
After the casting has solidified, the riser will become the return charge during the subsequent cleaning process, along with the gating system and the waste casting.These return charges are usually used as metal charges in subsequent production.Some foundries believe that the same electrical energy, treatment and inoculation, and metal handling are used to produce sprays, risers, and scrap castings as they are used to produce marketable castings.However, the percentage of reflux in the charge should still be strictly controlled, as they will produce more slag and have an adverse effect on the incubation process.It is necessary to optimize the process yield from economic and technical considerations.Using several large risers not only reduces the process yield rate, but also increases the production cost.Removing the risers and polishing the riser neck is an additional cost.Reducing the size and number of risers can have a positive effect on the last step in casting production, the cleaning step.
Traditionally, sand casting, riser is the only option for feeding.The shrinkage characteristics of steel castings require risers of larger volume than those of cast iron castings.For cast steel, a riser can be fed to a much smaller distance than for cast iron.Directional (sequential) solidification is almost always used for risers of steel castings.Anything that can reduce the riser size of the cast steel will save money.The first technical innovation was the use of pulp riser sleeves.These pulp riser sleeves can better ensure the shape of the riser and provide some insulation for the cooling of the feeding metal liquid.The most effective way to improve riser efficiency is to use a heating riser sleeve.It is beneficial to increase the temperature of molten iron in the riser, allowing the liquid to remain in the riser for a longer time, thus increasing the effective modulus of the riser.The shrinkage cavity formed in the riser indicates the degree of insulation of the riser sleeve or the degree of heating of the riser.
One type of riser made of this heating material is called a miniature riser, which can hold the liquid metal inside the riser for a long time.The liquid phase expansion during graphite precipitation may exceed 5% of the volume of the casting.The use of micro risers can not only provide sufficient liquid metal feeding during the cooling process of the casting, but also can receive the liquid from the expansion of the graphite precipitation process.During graphite precipitation, the expanding liquid increases the pressure in the cavity.Without an open riser neck, this pressure is difficult to release.Without the riser depressurization, the mold wall will deform and the resulting casting will be of incorrect dimensions.This is particularly common in tidal sand casting and shell casting.The riser which can provide the metal needed for liquid shrinkage and receive the expanding liquid is called the pressure controlled riser.
Two advantages of the mini-riser are that the feeding mode of the riser is the same regardless of the casting temperature, and the contact area between the riser and the casting is smaller than that of the sand riser.The difference between the riser size of sand risers, heating risers and miniature risers and the required riser neck size.Miniature risers provide the most efficient feeding system.The main advantage of using a micro riser is that it allows more models to be placed on the plate.The recessed riser neck requires less grinding and produces a better looking casting.Heating riser sleeve and miniature riser are pre-made insert risers.Extreme care must be taken in the use of these risers.If the heating riser sleeve or miniature riser is simply placed on the mold plate, it will be damaged when sand is added and the sand is compacted.If a slit is added to the insert riser, it may also be damaged during the sand compaction phase.The placement of a spring pin relieves sand pressure on the riser and ensures that the riser is positioned accurately on the casting.The sand between the bottom end of the insert riser and the surface of the mold plate can be compacted to form a slit between the insert riser and the surface of the casting.In the process of compaction, the inserted riser can move up and down, which is better than the riser sleeve with easy cut pieces.The disadvantage is that the riser neck size is too large when the spring pin is used instead of the easy cutting piece.
Casting users always require casting products to be as close as possible to the final size of the product, or even closer to the size that does not require machining.An innovative approach to improving riser casting contact is the use of miniature risers, known as “contact risers”.The thin-walled steel tube allows the liquid shrinkage to be fed from the riser, which is also placed on the spring pin, to release the pressure from the graphite expansion.The steel sleeve can be passed up to the riser cavity.The riser contact area is small and relatively clean.
Post time: Apr-16-2021