Discuss the impact of forging on the metal structure and properties of forgings

In forging production, in addition to ensuring the shape and size required by the forgings, it is also necessary to meet the performance requirements of the parts in the process of use, which mainly include: strength index, plastic index, impact toughness, fatigue strength, initial fracture and stress corrosion resistance, etc., for high-temperature working parts, as well as high-temperature instantaneous tensile performance, long-lasting performance, creep resistance and thermal fatigue performance.

The raw materials used for forging are ingots, rolled materials, extruded materials and forged blanks, while rolled materials, extruded materials and forged billets are semi-finished products formed by rolling, extrusion and forging of ingots. In forging production, the use of reasonable processes and process parameters can improve the organization and performance of raw materials through the following aspects:

1. Break the columnar crystal, improve the macroscopic segregation, change the cast structure to the forged structure, and weld the internal pores under suitable temperature and stress conditions to improve the density of the material.

2. The ingot is forged to form a fiber structure, and the forgings are further distributed in a reasonable fiber direction through rolling, extrusion and die forging.

3. Control the size and uniformity of the grains;

4. Improve the distribution of the second phase (e.g., alloy carbides in lystenitic steel);

5. Make the organization deformed and strengthened, etc.

Due to the improvement of the above structure, the plasticity, impact toughness, fatigue strength and durability of the forgings have also been improved, and then the hardness, strength and plasticity required by the parts can be obtained through the final heat treatment of the parts.

However, if the quality of the raw materials is poor or the forging process used is unreasonable, it may cause forging defects, including surface defects, internal defects, or unqualified performance.

The good quality of raw materials is a prerequisite for ensuring the quality of forgings, such as defects in raw materials, which will affect the forming process of forgings and the final quality of forgings.

If the chemical elements of raw materials exceed the specified range or the content of impurities is too high, it will have a great impact on the forming and quality of forgings, such as S, B, Cu, Sn and other elements are prone to the formation of low melting point phases, making forgings prone to hot brittleness. In order to obtain intrinsically fine-grained steel, the residual aluminum content in the steel needs to be controlled within a certain range, such as 0.02%~0.04% (mass fraction) of Al acid. If the content is too small, it cannot control the size of the grain, and it is easy to make the intrinsic grain size of the forging unqualified; Excessive aluminum content is easy to form wood-grained fractures and tear-like fractures under the condition of forming fiber tissue during pressure processing. For example, in austenitic stainless steel, the more Si, Al, and Mo are contained, the more ferritic phases there are, and the more likely it is to form band-like cracks during forging, and make the parts magnetic.

If there are defects such as pipe shrinkage residue, subcutaneous blistering, severe carbide segregation, and coarse non-metallic inclusions (slag inclusions) in the raw materials, it is easy to cause cracks in the forgings during forging. Defects such as dendritic crystals, severe looseness, non-metallic inclusions, white spots, oxide films, segregation zones and foreign metal mixing in raw materials can easily cause the performance of forgings to deteriorate. Surface cracks, folds, scars, coarse grain rings, etc. of raw materials are easy to cause surface cracks in forgings

The forging process generally consists of the following processes, namely blanking, heating, forming, post-forging cooling, pickling and post-forging heat treatment. If the forging process is not properly processed, a series of forging defects may occur.

The heating process includes furnace loading temperature, heating temperature, heating speed, holding time, furnace gas composition, etc. If it is not heated properly, such as too high a heating temperature and too long a heating time, it will cause defects such as decarburization, overheating, and overheating.

For bad materials with large cross-sectional size, poor thermal conductivity and low plasticity, if the heating speed is too fast and the holding time is too short, the temperature distribution will often be uneven, causing thermal stress, and causing the forging billet to crack.

The forging process includes deformation mode, degree of deformation, deformation temperature, deformation speed, stress state, mold condition and lubrication conditions, etc., if the forming process is improper, it may cause coarse grains, uneven grains, various cracks, folding, passage, eddy current, casting structure residue, etc.

During the post-forging cooling process, if the process is improper, it may cause cooling cracks, white spots, mesh carbides, etc.

Austenitic and ferritic heat-resistant stainless steels, superalloys, aluminum alloys, magnesium alloys and other materials without allotrope transformation during heating and cooling, as well as some copper alloys and titanium alloys, etc., the microstructure defects generated during the forging process cannot be improved by heat treatment.

Materials with allotropic transformation during heating and cooling, such as structural steel and martensitic stainless steel, have a great impact on the quality of forgings after heat treatment due to certain structural defects caused by improper forging process or some defects left over from raw materials. Here are some examples:

1. The structural defects of some forgings can be improved during post-forging heat treatment, and the forgings can still obtain satisfactory structure and performance after final heat treatment. For example, coarse grains and Weiss structure in general superheated structural steel forgings, slight reticulated carbide caused by improper cooling of hypereutactic steel and bearing steel, etc.

2. The structural defects of some forgings are difficult to eliminate with normal heat treatment, and they need to be improved by high-temperature normalization, repeated normalization, low-temperature decomposition, high-temperature diffusion annealing and other measures.

3. The organizational defects of some forgings cannot be eliminated by general heat treatment process, resulting in a decrease in the performance of the final heat treatment forgings or even unqualified. For example, severe stone and edge fractures, overburning, ferritic bands in stainless steel, carbide mesh and strips in lystenitic high-alloy tool steels, etc.

4. The organizational defects of some custom forgings will be further developed during the final heat treatment, and even cause cracking. For example, if the coarse grain structure in alloy structural steel forgings is not improved during heat treatment after forging, it often causes martensitic coarsity and unqualified performance after carbon, nitrogen co-osmosis and quenching. The coarse band-like carbide in high-speed steel often causes cracking when quenched.

Different forming methods have different stress and strain characteristics due to different stress conditions, so the main defects that may occur are also different. For example, the main defect of billet upsetting is longitudinal or 45° cracks on the side surface, and only the upper and lower ends of ingot upsetting often leave cast structure. The main defects of rectangular section billet are lateral cracks and angular cracks on the surface, diagonal cracks and transverse cracks on the inside; The main defects in open die forging are dissatisfaction, folding and dislocation.

Different kinds of materials, due to their different compositions and structures, their microstructure changes and mechanical behavior are also different in the process of heating, forging and cooling, so the forging process is improper, and the defects that may occur also have their particularities. For example, the defects of lystenitic high-alloy tool steel forgings are mainly coarse, unevenly distributed and cracked carbide particles, and the defects of superalloy forgings are mainly coarse grains and cracks; The defects of austenitic stainless steel forgings include intergranular chromium deficiency, reduced resistance to intergranular corrosion, ferrite banded structure and cracks, etc.; The defects of aluminum alloy forgings are mainly coarse, folding, eddy current, throughflow, etc.