The defects of forgings include external defects and internal defects. Some forging defects will affect the processing quality of subsequent processes, while others will seriously affect the performance of forgings, reduce the service life of waste parts, and even endanger safety. Therefore, in order to improve the quality of forgings and prevent the defects of forgings, corresponding technological countermeasures should be taken, and at the same time, the quality control of the whole consumption process should be strengthened. This chapter briefly introduces three problems: the influence of forging on the structure and properties of metal and the defects of forgings; Contents and methods of forging quality inspection; General process of forging quality analysis.

(I) Influence of Forging on Metal Microstructure and Properties In forging consumption, besides the required shape and size of forgings, it is also necessary to meet the performance requirements of parts in use, which mainly include: strength indicator, plasticity indicator, impact toughness, fatigue strength, fracture toughness and stress corrosion resistance, etc. For parts working at high temperature, there are also high-temperature instantaneous tensile performance, durability, creep resistance and thermal fatigue performance. The original materials used for forging are ingot, rolled material, extruded material and forging blank. The rolled material, extruded material and forged blank are semi-waste products formed by ingot rolling, extrusion and forging respectively. In forging consumption, the structure and properties of the original data can be improved through the following aspects: 1) breaking columnar crystals, improving macro segregation, changing as-cast structure into forged structure, and welding internal pores under suitable temperature and stress conditions to improve the density of the data; 2) forging the ingot to form a fiber structure, and further rolling, extruding and die forging, so that the forging can be dispersed in a reasonable fiber direction; 3) controlling the size and evenness of crystal grains; 4) improving the dispersion of the second phase (such as alloy carbide in ledeburite steel); 5) deformation strengthening or deformation-phase transformation strengthening, 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 good comprehensive properties such as hardness, strength and plasticity required by the parts can be obtained through the final thermal treatment. However, if the quality of the original data is poor or the forging process adopted is unreasonable, forging defects may occur, including external defects, internal defects or unqualified performance.

(II) Influence of the original data on the quality of forgings The good quality of the original data is a prerequisite for ensuring the quality of forgings. If there are defects in the original data, it will affect the forming process of forgings and the final quality of forgings. If the chemical elements in the original data are out of the regular range or the content of impurity elements is too high, it will have a great impact on the forming and quality of forgings. For example, elements such as S, B, Cu and Sn are easy to form low-melting-point phases, which makes forgings prone to thermal brittleness. In order to obtain substantially fine-grained steel, the residual aluminum content in steel should be controlled within a certain range, such as 0.02% ~ 0.04% (mass fraction) of Al acid. If the content is too small, it can not control the grain growth, and it is often easy to make the real grain size of forgings unqualified; When the aluminum content is too much, it is easy to form wood grain fracture and tear fracture under the condition of forming fiber structure during pressure processing. For another example, in 1Cr18Ni9Ti austenitic stainless steel, the more contents of Ti, Si, Al and Mo, the more ferrite phases, the easier it is to form banded cracks during forging and make the parts magnetic. If there are some defects in the original data, such as residual shrinkage, blistering under the skin, serious carbide segregation and coarse nonmetallic inclusions (slag inclusion), it is easy to cause cracks in forgings during forging. Defects such as dendrites, serious porosity, nonmetallic inclusions, white spots, oxide films, segregation zones and dissimilar metal inclusions in the original data are easy to cause the performance degradation of forgings. The surface cracks, folds, scars and coarse crystal rings in the original data are easy to form the surface cracks of forgings.

(III) Influence of Forging Process on Forging Quality The forging process generally consists of the following procedures: blanking, heating, forming, cooling after forging, pickling and heat treatment after forging. If the forging process is improper, a series of forging defects may occur. The heating process includes furnace charging temperature, heating temperature, heating speed, holding time, furnace gas composition, etc. If the heating is improper, for example, the heating temperature transition is too high and the heating time is too long, it will cause defects such as decarbonization, overheating and overheating. If the heating speed is too fast and the holding time is too short, the billet with large section size, poor thermal conductivity and low plasticity will often cause uneven temperature distribution, thermal stress and cracking. Forging forming process includes deformation mode, deformation level, deformation temperature, deformation speed, stress state, tooling and smooth conditions, etc. If the forming process is improper, it may cause coarse grains, uneven grains, various cracks and folds. Cold current, eddy current, as-cast structure residue, etc. In the cooling process after forging, if the process is improper, it may cause cooling cracks, white spots, reticular carbides and so on.

(IV) Influence of the microstructure of forgings on the microstructure and properties after final heat treatment. Austenite and ferritic heat-resistant stainless steels, superalloys, aluminum alloys, magnesium alloys, etc. have no information on isomorphic transformation during heating and cooling, and some copper alloys and titanium alloys, etc., and the structural defects produced during forging cannot be improved by heat treatment. During the heating and cooling process, there are data of isomorphic transformation, such as structural steel and martensitic stainless steel. 

Some structural defects caused by improper forging process or some defects left by original data have great influence on the quality of forgings after heat treatment. 

Here are some examples to illustrate:

1) The microstructure defects of some forgings can be improved by heat treatment after forging, and satisfactory microstructure and properties can still be obtained after final heat treatment. For example, coarse grains and widmanstatten structure in common overheated structural steel forgings, fine reticular carbides caused by improper cooling of hypereutectoid steel and bearing steel, etc.

2) The structural defects of some forgings are difficult to be eliminated by normal heat treatment, and can only be improved by measures such as high-temperature normalizing, repeated normalizing, low-temperature synthesis and high-temperature diffusion annealing. For example, macrostructure, twin carbide of 9Cr18 stainless steel, etc.

3) The structural defects of some forgings can't be eliminated by ordinary heat treatment process, resulting in the performance degradation of forgings after final heat treatment, and even unqualified. For example, serious stone fracture and prism fracture, overburning, ferrite band in stainless steel, carbide mesh and band in ledeburite high alloy tool steel, etc.

4) The structural defects of some forgings will be further developed during final heat treatment, and even lead to cracking. For example, if the coarse-grained structure in alloy structural steel forgings is not improved by heat treatment after forging, martensite needles are often coarse and unqualified after carbonitriding and quenching; The coarse banded carbides in high speed steel often cause cracking after quenching. The common defects in forging process and their causes will be introduced in detail in chapter 2. It should be pointed out that the common defects in various forming methods and the main defects of various data forgings have their own laws. Different forming methods have different stress and strain characteristics, so the possible main defects are also different. For example, the main defects of billet upsetting are longitudinal or 45 cracks on the side surface, and the as-cast structure often remains at the upper and lower ends of ingot after upsetting. The main defects of rectangular cross-section blank drawing are transverse cracks and corner cracks on the outside, diagonal cracks and transverse cracks on the inside; The main defects in open die forging are insufficient filling, folding and dislocation. The common defects in each main forming process will be introduced in detail in Chapter 4. Different varieties of materials, because of their different compositions and structures, have different structural changes and mechanical behaviors during heating, forging and cooling, so the possible defects caused by improper forging process also have their own particularity. For example, the defects of ledeburite high-alloy tool steel forgings are mainly coarse carbide particles, uneven distribution and cracks, while the defects of high-temperature alloy forgings are mainly coarse grains and cracks; The defects of austenitic stainless steel forgings are mainly intergranular chromium deficiency, intergranular corrosion resistance, ferrite band structure and cracks. The main defects of aluminum alloy forgings are coarse grain, folding, eddy current, cross flow and so on.

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