The forming of certain critical load-bearing components requires integral die forging to ensure that the fiber structure (flow lines) of the die-forged part is not severed; the quality of such forgings is superior to that of open-die forgings.
The direction of metal flow in high-stress areas of die-forged parts should align with the direction of the principal stress, while the direction of metal flow lines in other areas generally follows the part’s largest dimension or its external contour.
To impart the plasticity required for forging to the billet, it must be heated and held at a high temperature of 1150–1270°C. The purpose of holding the billet at this temperature is to ensure uniform temperature throughout the steel billet and to eliminate microstructural segregation through high-temperature diffusion.
During heating and forging, the surface layer of the steel undergoes oxidation and decarburization; therefore, an appropriate machining allowance must be provided to remove these layers during machining.
A certain forging ratio is required to break down the cast structure, forge out internal defects, and homogenize the microstructure.
An appropriate forging ratio ranges from 2.5 to 4, depending on the steel’s composition, smelting conditions, the workpiece’s service conditions, and the size of the steel ingot; larger steel ingots require a higher forging ratio.
An excessively high forging ratio can reduce transverse plasticity and toughness. For forgings with high transverse performance requirements, an elongation process involving one or two intermediate ups can be used, or a forging method with surface cooling can be employed to increase the degree of compression within the ingot.
Large forging manufacturers both domestically and internationally have continuously researched and developed new forging methods, such as the FM method (asymmetric flat-anvil forging), the WHF method (wide-anvil high-reduction forging), the JTS method (center-compaction forging), the SUS forging method, and the AVO forging method, all of which have been successfully applied in production.
Forging is the third major method of strengthening steel; it not only eliminates smelting defects but also provides microstructural uniformity and improves steel strength by altering the fiber structure.
It is a two-way process: preheating and holding before forging can eliminate microscopic alloy segregation; the forging process not only alters the fiber orientation, breaks up large impurities, and improves the continuity of the fiber structure, but also performs a micro-welding effect to eliminate internal micro-pores.
Forging is indispensable for improving the performance of die steel—a role that cannot be replaced by other processes. The high quality and excellent performance of many die steels are directly attributable to superior forging techniques, which serve as a closely guarded secret weapon for steel mills.
*************
Wu Dejian’s tool steel, the chief of staff of the user, bought everything he had used.