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I) The influence of forging on metal structure and properties In forging production, in addition to ensuring the shape and dimensions required for the forgings, it is also necessary to meet the performance requirements that the parts must satisfy during use. These requirements mainly include strength indicators, plasticity indicators, impact toughness, fatigue strength, fracture initiation behavior, and stress-corrosion resistance. For parts that operate at high temperatures, additional requirements include high-temperature instantaneous tensile properties, endurance properties, creep resistance, and thermal fatigue resistance. The raw materials for forging are ingots, rolled products, extruded products, and forgings. Roll-formed products, extruded products, and forged billets are semi-finished products resulting from the rolling, extrusion, and forging of ingots, respectively. In forging production, by employing appropriate processes and process parameters, the structure and properties of raw materials can be improved in the following ways: 1. Breaking up columnar crystals, reducing macroscopic segregation, transforming the as-cast structure into a forged structure; under suitable temperature and stress conditions, internal pores are welded shut, thereby increasing the material’s density ； 2. The ingot is forged to form a fibrous structure; subsequently, rolling, extrusion, and die forging are used to achieve a proper distribution of fiber directions in the forged piece ； 3. Control the size and uniformity of grains ； 4. Improve the distribution of the second phase (e.g., alloy carbides in ledeburite steel) ； 5. Cause the organization to undergo strain hardening or similar effects. Thanks to the aforementioned improvements in the organization, the plasticity, impact toughness, fatigue strength, and creep resistance of the forgings have also been enhanced. Subsequently, through the final heat treatment of the parts, desirable overall properties such as hardness, strength, and plasticity can be achieved. However, if the quality of the raw materials is poor or the forging process used is inappropriate, forging defects may occur, including surface defects, internal defects, or unsatisfactory performance. (II) The impact of raw materials on the quality of forgings: Good quality of raw materials is a prerequisite for ensuring the quality of forgings. If the raw materials have defects, it will affect the forging process as well as the final quality of the forgings. If the chemical elements in the raw materials are outside the specified ranges or if the content of impurity elements is too high, it has a significant impact on both the shaping and quality of the forgings. For example, elements such as S, B, Cu, and Sn tend to form low-melting-point phases, which can cause the forgings to become heat-sensitive. To obtain essentially fine-grained steel, the residual aluminum content in the steel must be kept within a certain range, for example, 0.02% to 0.04% (by mass) of Al. If the content is too low, it cannot prevent the growth of grains, and this often results in the grain size of the forged part not meeting the required standards ； Excessive aluminum content makes it prone to the formation of wood-grain-like fractures, tear-like fractures, etc., under conditions where fibrous structures are formed during pressure processing. Similarly, in austenitic stainless steels, the higher the contents of N, Si, Al, and Mo, the more ferrite phases are present; this makes it easier for strip-like cracks to form during forging, and it also gives the parts magnetic properties. If the raw materials contain defects such as residual shrinkage cavities, subsurface blistering, severe carbide segregation, and large non-metallic inclusions (slag inclusions), cracks are likely to occur in the forgings during forging. Defects in raw materials such as dendritic crystals, severe porosity, non-metallic inclusions, white spots, oxide films, segregation zones, and the presence of foreign metals can easily lead to a decline in the properties of the forgings. Surface cracks, folds, scabs, coarse grain rings, etc., on raw materials can easily lead to surface cracks in forgings. (III) Influence of the forging process on the quality of forgings. The forging process generally consists of the following steps: cutting, heating, shaping, cooling after forging, pickling, and heat treatment after forging. If the forging process is not carried out properly, a series of defects in the forged parts may occur. The heating process includes furnace charging temperature, heating temperature, heating rate, holding time, and furnace gas composition. If heating is not done properly, such as at too high a temperature for too long, defects such as decarburization, overheating, and overburning will occur. For billets with large cross-sectional dimensions, poor thermal conductivity, and low plasticity, if the heating rate is too fast and the holding time is too short, it often results in non-uniform temperature distribution, thereby causing thermal stresses and cracking of the forging billets. The forging process involves factors such as the mode of deformation, degree of deformation, deformation temperature, deformation speed, stress state, conditions of the tooling and dies, and lubrication conditions. If the forming process is not properly controlled, it may lead to coarse grains, non-uniform grain distribution, various types of cracks, folds, flow-through phenomena, eddy currents, and remnants of as-cast microstructure. During post-forging cooling, improper processing may lead to cooling cracks, white spots, network-like carbides, etc. (IV) Influence of the forge microstructure on the microstructure and properties after final heat treatment. In materials such as austenitic and ferritic heat-resistant stainless steels, superalloys, aluminum alloys, magnesium alloys, etc., which do not undergo allotropic transformations during heating and cooling, as well as in some copper alloys and titanium alloys, the microstructural defects that arise during forging cannot be improved through heat treatment. Materials that undergo allotropic transformations during heating and cooling, such as structural steel and martensitic stainless steels, have certain microstructural defects caused by improper forging processes or defects remaining from the raw materials, which significantly affect the quality of the forgings after heat treatment. The following are some examples: 1. Some microstructural defects in forgings can be improved through heat treatment after forging, and the forgings can still achieve a satisfactory microstructure and properties after the final heat treatment. For example, coarse grains and Widmanstätten structures in conventionally overheated structural steel forgings; slight network-like carbides in hypereutectoid steels and bearing steels caused by improper cooling, etc. 2. For some structural defects in forgings, it is difficult to eliminate them through normal heat treatment; measures such as high-temperature normalizing, repeated normalizing, low-temperature decomposition, and high-temperature diffusion annealing are required to improve them. 3. Some structural defects in forgings cannot be eliminated by conventional heat treatment processes; as a result, the properties of the forgings after final heat treatment deteriorate, or they even fail to meet specifications. For example, severe stony fractures and facet fractures, overburning, ferrite bands in stainless steel, and carbide networks and bands in ledeburite high-alloy tool steels, etc. 4. Some microstructural defects in forgings will further develop during the final heat treatment, and may even cause cracking. For example, the coarse grain structure in alloy structural steel forgings, if not improved during post-forging heat treatment, often leads to coarse martensite formation and substandard properties after carbon-nitrogen carburizing and quenching ； The large band-shaped carbides in high-speed steel often cause cracking during quenching. Different forming methods have distinct stress and strain characteristics due to their varying stress conditions, and as a result, the main defects that may arise also differ. For example, the main defects during the upsetting of billets are longitudinal or 45° cracks appearing on the side surfaces, while in the upsetting of ingots, the as-cast structure often remains at the upper and lower ends ； The main defects that occur during the drawing of rectangular-section billets are transverse cracks and corner cracks on the surface, as well as diagonal and transverse cracks inside ； The main defects in open die forging are underfilling, folding, and misalignment. Different types of materials, due to their varying compositions and structures, exhibit different structural changes and mechanical behaviors during heating, forging, and cooling processes. As a result, defects that may arise when the forging process is not carried out properly also have their own specific characteristics. For example, the defects in ledeburite high-alloy tool steel forgings are mainly coarse and unevenly distributed carbide particles as well as cracks, while the defects in superalloy forgings are primarily coarse grains and cracks ； Defects in austenitic stainless steel forgings mainly include intergranular chromium depletion, reduced resistance to intergranular corrosion, ferritic banded structures, and cracks ； The defects of aluminum alloy forgings mainly include coarse grains, folds, eddy currents, and through-flow.