What steps and process support are required to achieve balanced solidification of ductile iron?

The core points of the balanced solidification process for ductile iron and its implementation in production are a process of perfectly transforming theory into practice. It can solve the problem of shrinkage and porosity for ductile iron in actual production. Realizing balanced solidification of ductile iron is a systematic project that requires us to do the following work:

1、 Deeply understand the core process points of "balanced solidification"

The "equilibrium solidification theory" was proposed by Professor Wei Bing, a casting expert in China. It breaks away from the traditional thinking of "sequential solidification" and its core idea is to use the graphitization expansion during the solidification process of ductile iron to compensate for shrinkage, thereby achieving the goal of castings without shrinkage and porosity.

The core points of its process can be summarized into three keywords:

The balance between "expansion" and "contraction": This is the most fundamental point. During solidification, ductile iron undergoes both "expansion" due to graphite precipitation (graphitization expansion) and "shrinkage" due to liquid and solid state shrinkage. The goal of craftsmanship is to create conditions that allow "expansion" to counteract "contraction". 2. Balance between "rigidity" and "flexibility": "rigidity" refers to the mold having sufficient strength to "hold" the pressure generated by graphitization expansion, forcing the expansion force to act in the opposite direction on the molten iron for shrinkage compensation. This is the foundation for achieving 'self replenishment and contraction'. Usually achieved through high-strength molding sand (such as resin sand, coated sand), reinforced sand boxes, and other methods. Soft "(flexible/yielding): refers to setting up an appropriate" soft "environment (such as air vents, overflow risers, soft sand layers) at the end of the path or near the hot spot where shrinkage is required, allowing the mold cavity to retreat in a controlled manner to guide the shrinkage flow field, release excess pressure, and prevent the casting from" swelling "or wall movement. 3. Balance between "hot" and "cold": Control the temperature field of the casting through a gating system. Heat ": At the thick and large hot nodes of castings, necessary liquid shrinkage and heat supplementation are provided by using hidden or side risers. Cold ": Using cold iron to accelerate local cooling at thin-walled or rapidly cooled areas of castings, eliminate hot spots, and establish a temperature gradient towards the riser.

Core mnemonic: "If it's hard, it's hard; if it's soft, it's soft; if it's hot, it's hot; if it's cold, it's cold; use expansion instead of contraction to achieve dynamic balance.

2、 The specific implementation methods of the core points in production

To translate the above theory into practical production operations, it is necessary to systematically control from the following aspects:

1. Mold process design (realization of "rigidity" and "flexibility")

Choose high-strength molding materials: resin sand (furan resin, alkaline phenolic resin) or coated sand should be preferred. These materials have high strength and can effectively resist graphitization expansion, which is the basis for achieving balanced solidification. Clay sand (wet sand) requires strict control of moisture and compaction rate, and if necessary, reinforcement of sand boxes and molds. Reasonably designed and compact pouring system: usually using semi closed (such as F straight: F horizontal: F inside=1.5:1.2:1) or fully closed pouring system. Fast filling reduces erosion and also helps the sprue cup and sprue to have a certain shrinkage effect in the later stage. Use 'small but numerous' risers: The risers do not need to be as large as cast steel. Use small-sized, mostly concealed risers (edge feeders, ear feeders, duckbill feeders, etc.) or side feeders. The design of the riser neck is key: it should be "short, thin, and wide". Its function is to smoothly compensate for liquid shrinkage in the early stage of solidification, and to quickly "self close" (solidify) at the beginning of graphitization expansion in the middle stage of solidification, locking the expansion pressure inside the casting instead of releasing it into the riser. Clever use of cold iron: Placing an external cold iron at the thick hot spot of the casting can accelerate the cooling of that area, eliminate the hot spot, and reduce its dependence on the riser. When used in conjunction with a riser, a more ideal temperature gradient can be established to guide the solidification sequence. Setting up exhaust and overflow: sufficient exhaust holes should be set up at the highest point and last filling point of the mold cavity to ensure smooth gas discharge from the cavity. An overflow riser (slag collection bag) is installed at the end of pouring or at the final flow of molten iron. It can not only collect slag but also discharge low-temperature molten iron, balancing the pressure and temperature inside the mold cavity.

2. Smelting and spheroidization control (source guarantee of "expansion")

Stable chemical composition: Carbon Equivalent (CE): Adopting a high carbon, low silicon solution. CE is usually controlled between 4.6% and 4.9%. High carbon can ensure sufficient graphite precipitation and generate sufficient expansion force; Low silicon can prevent excessive increase in eutectic temperature and prevent graphite expansion from coming too late. Residual magnesium (Mg) content: should not be too high, generally controlled at 0.03% -0.05%. Excessive height will increase the tendency of white casting, inhibit graphitization, and reduce expansion. Good spheroidization effect: Ensure that the spheroidization level reaches 1-2 levels. Only round graphite balls can provide sufficient and uniform expansion force. The more and smaller the number of graphite balls, the earlier the expansion begins, and the better the effect. Suitable pouring temperature: On the premise of ensuring complete filling, try to reduce the pouring temperature as much as possible (such as 1320 ℃ -1380 ℃). Low temperature pouring can reduce the amount of liquid shrinkage, shorten the solidification time, and enable earlier and more effective graphitization expansion for compensating shrinkage.

3. Production process control (guarantee of dynamic balance)

Adequate compaction of molding sand: Ensure that the hardness of the sand mold meets the standard (such as resin sand>90, clay sand>85), and guarantee the "rigidity" of the mold. Accurate measurement of molten iron: Ensure the accurate amount of iron in the spheroidization treatment package to ensure the precise addition of spheroidizing agent and inoculant, thereby stabilizing the spheroidization effect and chemical composition. Rapid casting: Pour as soon as possible after spheroidization treatment (usually completed within 10 minutes after "reaction settling") to prevent fertility and spheroidization decline. Reasonable boxing time: After pouring, the casting must have sufficient insulation time in the sand mold (at least after eutectic solidification is completed) before boxing and sanding. Premature boxing will lose the "rigidity" constraint of the sand mold, and the castings will deform or even swell under the action of expansion force, resulting in a sharp increase in the risk of internal shrinkage and looseness.

summary

In summary, achieving balanced solidification is not a single technique, but a systematic concept that runs through the entire process of process design, melting control, and production management. It requires producers to have a deep understanding of the solidification characteristics of ductile iron, and to achieve the ideal effect of "replacing shrinkage with expansion and balancing rigidity and flexibility" through a series of measures such as high stiffness casting, small riser, cold iron, low pouring temperature, and high-quality molten iron. In practical applications, it is recommended to conduct process experiments and section verification on typical products to optimize and determine the most suitable process parameters for specific production conditions.