yvonne@nydcasting.com 
There are many factors that affect the formation of hot cracking in castings. Therefore, when considering measures to prevent hot cracking, we cannot start from a single factor, but need to conduct a comprehensive analysis based on the specific situation, so as to take corresponding measures.
1. Casting structure
Irrational casting structure design is often one of the causes of hot cracking. Therefore, the following points should be noted when designing castings:
1.1 The intersection of two sections should not be designed as a right-angle turn, but a rounded corner should be provided. The shape and size of the rounded corners are determined by the intersection of the castings.
1.2 Try to reduce and disperse the hot nodes, avoid using cross-sections, and stagger the cross-sections if conditions permit.
1.3 When it is necessary to use sections of unequal thickness on steel parts, the shrinkage of the various parts of the casting should not hinder each other.
2. Casting process design
2.1 After the liquid metal enters the mold cavity through the ingate, the casting near the ingate cools slowly, forming a weak area on the casting, which is more likely to cause hot cracking. Sometimes, the shrinkage of castings may be hindered by the gate and cause thermal cracking. Especially in order to make the molten metal pour in evenly, the danger is greater when complex and connected gates are used.
2.2 Thermal cracking often occurs at the intersection of sections with uneven wall thickness. In order to prevent such defects, process ribs can be set at these locations with the consent of the user. Anti-crack ribs enhance strength in thermal cracking zones. They also aid heat dissipation and reduce hot spots. This helps prevent thermal cracking in castings. Ribs should not be too thick—about 1/3 of wall thickness. Overly thick ribs may worsen thermal cracking. Rib shape must be well-designed. Add extra processes to ribs if needed.
3. Shell making process
Improving the yield of the shell to reduce shrinkage obstacles is conducive to reducing the tendency of thermal cracking. The factors that affect the yield are: the type and performance of the binder, refractory materials, the type of hardener and the shell making process. Therefore, the number of shell layers should be minimized while meeting the strength requirements.
4. Adjustment and refining of alloy composition and smelting process
4.1 Without affecting the performance of castings, the chemical composition of the alloy can be appropriately adjusted to narrow the solidification temperature range, reduce the shrinkage during solidification, or select a eutectic composition with good crack resistance.
4.2 Microalloying and modification of carbon steel and alloy steel can significantly improve the crack resistance of steel castings. This can be achieved by adding rare earth elements or other elements, and the amount added is generally less than 0.3%. Elements can be added alone or several elements can be added at the same time. Commonly used elements include vanadium, cerium, calcium, titanium, niobium, etc. This method refines grain size and reduces inclusions in cast steel. It improves crystallization and crack resistance. Adding 0.2% vanadium to ZG25# refines grains and removes Widmanstatten structure. Adding 0.05–0.1% cerium to alloy steels removes columnar crystals. Adding 0.15% cerium to ZG2Cr13 and ZG1Cr18N19 removes grain boundary inclusions. It also distributes sulfide inclusions evenly. These effects enhance cast steel’s mechanical properties and crack resistance.
4.3 Improving the deoxidation process of cast steel can improve the crack resistance of cast steel. Oxidation inclusions at the grain boundary of cast steel parts are one of the main causes of hot cracking. Improving the deoxidation effect, reducing oxidation inclusions and changing their distribution state can reduce the tendency of cast steel to hot crack. Experiments have shown that comprehensive deoxidation is much better than deoxidation alone. This is because the size of the deoxidation product is much larger than that of deoxidation alone, which is conducive to the removal of deoxidation products by molten steel.
Comprehensive deoxidation method:
Carbon steel and low alloy steel are first pre-deoxidized with manganese and silicon, and finally deoxidized with aluminum. Stainless steel uses calcium silicon particles and lime for diffusion deoxidation, and then uses blocky calcium silicon for precipitation final deoxidation. The most ideal method is to use a multi-component composite deoxidizer.
4.4 Control crystallization to refine primary structure and reduce hot cracking. Use suspension casting during molten steel pouring. Add fine metal powder through the gate or channels. For ZG35# steel, add 2% ferromanganese powder. For chromium-molybdenum steel, add 0.1 mm molybdenum powder. These additions refine grains and improve properties. They also lower hot cracking defects in castings.
5. Casting conditions
To prevent hot cracking, use high furnace temperature and low pouring temperature. Pour quickly at first, then slowly. For thin-walled parts, low temperature may cause cold shut or underfilling. Adjust pouring based on casting structure. Choose suitable methods for different structural features.
5.1 When pouring thin-walled and rod-shaped (wrenches, connecting rods, pliers) castings, the pouring temperature of the molten steel is controlled at about 1540℃, and the temperature of the mold shell is greater than 650℃, that is, red shell pouring is required.
5.2 For castings with a wall thickness greater than 20 mm, the pouring temperature of the molten steel is selected at about 1530℃, the temperature of the mold shell is below 400℃, and even cold shell pouring can be performed. This is conducive to improving the crust strength of the casting during the dangerous period of hot cracking.
5.3 When encountering castings with uneven wall thickness, the pouring temperature of the molten steel is 1540℃. The temperature of the mold shell is about 400-500℃, but the pouring method of first fast and then slow must be strictly implemented. After the casting is poured and the molten steel solidifies, the mold shell of the thick wall part should be removed first, and this part should be cooled first to achieve the purpose of uniform shrinkage of the casting. The above are measures taken for carbon steel castings that are prone to thermal cracking. Stainless steel castings can be carried out in the same way, but the pouring temperature needs to be appropriately increased by about 30-60℃.