In casting production, the incidence of shrinkage cavities and porosity defects in cast steel components is significantly higher than in cast iron components-the core difference lies in "graphite shrinkage compensation": cast iron has a high carbon content, and during solidification, graphitization expansion can fill shrinkage voids; however, cast steel has a low carbon content and virtually no graphite shrinkage compensation, leading to liquid shrinkage plus solidification shrinkage often exceeding solid shrinkage, ultimately resulting in shrinkage cavities and porosity.
1.Causes of Shrinkage Cavities and Porosity
Fundamental Cause: The Inevitable Result of Shrinkage Discrepancies
During solidification of cast steel components, the sum of liquid shrinkage (volume reduction due to temperature decrease) and solidification shrinkage (volume reduction from liquid to solid state) often exceeds solid shrinkage (volume reduction due to solid state cooling) - this "shrinkage difference" is an inherent characteristic of cast steel components and the root cause of shrinkage cavities and porosity.
Shrinkage Cavities: "Funnel-shaped voids" during solidification
Through decomposition of the solidification process, the formation of shrinkage cavities can be divided into three steps:
Feed stage: After pouring, as the steel liquid temperature decreases, if the internal runner has not solidified, the casting can continue to be fed with steel liquid from the runner, keeping the mold cavity continuously filled with liquid.
Hard shell closure: When the steel liquid on the mold wall cools to the liquidus line, a hard shell forms on the surface; if the internal runner solidifies at this point, the steel liquid inside the hard shell is "trapped."
Liquid level drop and shrinkage cavity formation: During subsequent cooling, the steel liquid continues to shrink due to liquid contraction and solidification contraction, while the solid contraction of the hard shell cannot compensate for this. Under the influence of gravity, the liquid level drops, separating from the top of the hard shell, ultimately forming a funnel-shaped shrinkage cavity (the hard shell at the top may also deform due to atmospheric pressure).
Shrinkage porosity: "small pores" in the last solidification zone
Shrinkage porosity originates from "the steel liquid in the last solidification zone being divided into small melt pools by dendrites":
During the late solidification stage of the casting, the temperature gradient in the last solidification zone is small, and the steel liquid's grains grow synchronously, dividing the remaining steel liquid into small, isolated melt pools. These melt pools cannot be compensated during cooling and solidification, resulting in numerous small pores.
Based on distribution, shrinkage porosity is classified into three types:
Diffuse shrinkage porosity: Fine pores are uniformly distributed, commonly found in thick, large sections with a wide crystallization temperature range (e.g., the core region of large cast steel components);
Axial shrinkage porosity: Concentrated along the central axis of plate-shaped/columnar castings;
Localized shrinkage porosity: Forms in localized thick regions (e.g., near gates or shrinkage cavities).
The "conversion relationship" between shrinkage cavities and shrinkage porosity
For cast steel components with specific compositions, the total volume of shrinkage cavities and shrinkage porosity is essentially fixed-more shrinkage cavities mean less shrinkage porosity, and vice versa. However, shrinkage porosity is more likely to form in the interstitial spaces of dendrites in the final solidification zone (due to the blockage of shrinkage channels by grains).
2.Core strategies for preventing shrinkage cavities and shrinkage porosity:
Convert "dispersed shrinkage porosity" into "concentrated shrinkage cavities," then eliminate them via risers!
The core idea is to "control the solidification sequence and establish efficient shrinkage compensation channels": allow the casting to solidify first from areas far from the riser, with the riser solidifying last, continuously compensating for shrinkage in the casting, and ultimately "transferring" shrinkage cavities to the riser for removal.
Strategy 1: Design a "sequential solidification" pouring process
Inner runner position: located in the thickest part of the casting, with the runner connected to the casting via the riser (ensuring the riser is the first to compensate for shrinkage);
Pouring parameters: reduce pouring temperature and slow pouring speed (to minimize liquid shrinkage and solidification shrinkage) while ensuring proper filling.
Strategy 2: Utilize the "iron triangle" of risers, chill blocks, and supplementary risers
The three work together to guide solidification direction and extend shrinkage compensation distance:
Riser: Position must be precise (to compensate for the last solidifying zone) and volume must be sufficient (to store molten steel for shrinkage compensation);
Cold iron: Placed at the thin-walled areas of the casting to accelerate local solidification and "guide" solidification toward the riser;
Riser: Add a "transition section" in the transition zone between thin and thick walls of the casting to extend the riser channel, allowing the riser's riser range to cover a greater distance.
Strategy 3: Enhanced riser design for special castings
For complex or large-sized cast steel components, riser design must be upgraded:
Insulated riser: Use insulating materials to slow down riser solidification and extend riser time;
Atmospheric pressure riser: Utilize atmospheric pressure to assist riser shrinkage compensation (compensate for steel liquid contraction);
Post-pour riser: Supplement the riser with steel liquid/heat after pouring to delay riser solidification.
The essence of shrinkage cavities and porosity in cast steel components is "shrinkage difference" + "insufficient shrinkage compensation." The key to prevention lies in "actively controlling the solidification sequence": by coordinating pouring processes, risers/cold irons/supplementary materials, transform dispersed porosity into concentrated shrinkage cavities, then allow the riser to "consume" the shrinkage cavities - ultimately achieving a dense cast steel component.