Due to the often complex geometries of carbon steel castings, they are prone to deformation or cracking during production. Consequently, special care must be taken when subjecting these products to heat treatment processes.

Carbon steel castings typically possess intricate shapes. Therefore, during the heating phase—specifically when the temperature reaches the 650°C–800°C range—it is advisable to slow down the heating rate or hold the temperature constant for a certain period. At this stage, the carbon steel undergoes structural transformations, and internal stresses simultaneously shift. If the temperature is raised too rapidly during this phase, a significant temperature differential may develop between the thick and thin sections of the casting, potentially leading to cracks.

The holding period is the critical phase during which the casting undergoes its structural transformations. Only with a sufficient holding duration can the various sections of the casting complete their transformations, thereby enhancing the casting's overall performance and quality. Notably, the holding time required for the thin-walled sections of a casting is typically longer than that required for the thick-walled sections.

When annealing castings, a heat treatment process involving slow cooling to room temperature is typically employed. The primary objectives of annealing are: to reduce hardness and facilitate subsequent machining operations; to refine the grain structure and improve the internal microstructure, thereby enhancing mechanical properties; to relieve internal thermal stresses within the casting; and to increase the product's plasticity and ductility, thereby facilitating subsequent manufacturing and processing steps.

The core tasks involved in casting process design include drafting casting process diagrams, casting blank drawings, metal mold part drawings, and compiling process instruction cards. For standardized products produced in large batches—as well as for unique, critically important castings produced as single units—the casting process design is typically highly detailed and comprehensive in scope. However, for general-purpose products produced as individual pieces or in small batches, the specific content of the carbon steel casting process design can be simplified. In such simplified scenarios, creating a single casting process diagram is often sufficient. Specific Content and General Procedure for Casting Process Design

Taking the feed gearbox housing for a C6140 CNC lathe as an example, the casting process plan for the rough casting is analyzed below (approximate weight: 35 kg):

CNC Lathe Feed Gearbox Housing

This component features no surfaces requiring special quality standards; the only requirement is that reference surface D be kept free of significant casting defects to facilitate precise positioning during subsequent machining.

Material: Gray Cast Iron HT150; shrinkage compensation (feeding) does not need to be considered. When formulating the casting process plan, the primary focus is on simplifying the overall manufacturing process.

To minimize subsequent machining effort, nine bore features are to be cast directly into the part. Consequently—and to simplify core setting, mold assembly, and casting cleanup—the parting line is established along the axes of these bores, in accordance with "Plan 1." To facilitate automated molding and minimize the use of loose pieces, both the bosses and the recessed grooves are formed using cores. To mitigate the disadvantage of having the primary reference surface (Surface D) positioned on the drag side (facing downward) during casting, it is necessary to increase the machining allowance allocated to Surface D.