The forging of the defects include surface defects and internal defects. Some forging defects will affect the processing quality of the follow-up process, some seriously affect the performance of the forgings, and reduce the life of the finished goods pieces, and even endanger the security. Therefore, in order to improve the quality of forgings, avoid forging defects produce, process measures should be taken, should also strengthen the quality control of the whole production process. This chapter provides an overview of the three aspects of the problem: forging the metal structure, performance and forging defects; forging quality content and method of inspection; forging quality analysis of the general process.
(A) forging on the microstructure and properties of metal
Forging production must ensure forging a desired shape and size, but also must meet the performance requirements of the parts during use, which mainly include: intensity pointer plastic pointer, impact toughness, fatigue strength, and fracture toughness in addition tensile properties and resistance to stress corrosion, high temperature working parts, as well as high temperature instantaneous, lasting performance, creep resistance and thermal fatigue performance. Raw materials is forging ingot, rolled products, to squeeze material and forging stock. Rolled products, extrusion material and forging stock are ingots by rolling, extrusion and forging formation of semi-finished products. Forging production, reasonable process and process parameters to improve the organization and performance of the raw materials by the following aspects: (1) broken columnar crystal, to improve the macrosegregation the cast structure becomes forging state organizations, and at the right temperature and stress conditions, the internal pore welded together to improve the density of the material; (2) ingot after forging the formation of fibrous tissue, and further by rolling, extrusion, forging, forgings get a reasonable distribution of fiber direction; ( 3) control the grain size and uniformity; (4) to improve the distribution of the second phase (eg: ledeburite steel alloy carbides); (5) enables organizations to get the strain hardening or deformation - transformation strengthening . A result of the improvement in the above-mentioned organizations, forgings plasticity, impact toughness, fatigue strength and lasting performance also will get a raise, and then through the final heat treatment of parts will be able to get a good overall hardness, strength and plastic parts required properties. However, if the poor quality of raw materials or the forging process is unreasonable, you may produce forging defects, including surface defects and internal defects or substandard performance.
(B) the impact of the raw materials of forging quality
The good quality of the raw materials are prerequisites to ensure the quality of the forging defects such as raw materials, it will affect the final quality of the process and the forging of the forging forming. Such as the raw materials of the chemical elements is beyond a predetermined range or the content of impurity elements is too high, the forging molding and quality will bring a greater influence, for example: S, B, Cu, Sn, and other elements are easy to form a low-melting phase, is easy to make forging The thermal embrittlement. In order to get the essence of fine grain steels, steel residual aluminum content to be controlled within a certain range, such as Al acid 0.02％ to 0.04％ (by mass). Content is too small, will not achieve control grain growth, often unqualified easy to make the nature of the grain size of the forgings; excessive aluminum content in the formation of fibrous tissue pressure processing conditions is easy to form a grainy fracture tear marks shaped fracture. In another example, in 1Cr18Ni9Ti austenitic stainless steel, Ti, Si, Al, Mo content is more, the ferrite phase is more forging is more easy to form a belt-like cracks, and with magnetic parts. Such as raw materials when the memory residual shrink tube, subcutaneous blistering, severe carbide segregation, coarse nonmetallic inclusions (slag) and other defects, forging easy to make forging cracks. Dendrites within the raw materials, severe osteoporosis, nonmetallic inclusions, white point, the oxide film, segregation band and dissimilar metals mixed defects easily cause forgings performance. Surface cracks of raw materials, folding, scarring, coarse-grained ring could easily lead to the forging of surface cracks.
(C) the forging process forging quality
The forging process generally consists of the following step, i.e. cutting, heating, molding, cooling, pickling and after forging heat treatment after forging. Forging process if the process will likely produce a series of improper forging defects. Heating process comprising installed furnace temperature, the heating temperature, heating rate, holding time, the furnace gas component. If heated properly, for example, the heating temperature and the heating time is too long, will cause decarburization, overheated, burning and other defects. For large cross-sectional dimensions and the poor thermal conductivity, low plasticity of the blank, if the heating rate is too fast, the holding time is too short, often so that the temperature distribution is uneven, causing thermal stresses, cracking and the dough occurs. Forging process including deformation, deformation degree of deformation temperature, strain rate, stress state, Tool & Die. Brother and lubrication conditions, improper forming process may cause coarse grains, grain uneven various cracks, folding. The cold current, eddy current, cast tissue residues. Cooling after forging process, if the process properly may cause cooling cracks, white point, carbide network.
(D) forging Organization of the organization and performance of the final heat treatment
Austenitic and ferritic heat-resistant stainless steel, high temperature alloys, aluminum alloys, magnesium alloys, such as in the heating and cooling process, allotropic transformation of materials, as well as some copper alloy and titanium alloy forging process the organizational defects means of heat treatment can not be improved. Allotropic transformation in the heating and cooling process, materials, such as structural steel and martensitic stainless steel, etc., caused by improper due to forging process, some of the tissue defect or the raw material left over from some defects, for heat treatment after forging quality greatly affected. It is described as follows:
(1) tissue defects of some forging, heat treatment after forging can be improved, forgings after the final heat treatment can still be satisfied with the organization and performance. For example, in general overheating of the structural steel forgings coarse grain widmanstatten of, too eutectoid steel and bearing steel due to the cooling caused by improper slight carbide network.
(2) the organization of some forging defects, normal heat treatment is more difficult to eliminate the need to use high-temperature normalizing, repeatedly normalizing low temperature decomposition of measures to improve high temperature diffusion annealing. For example, low coarse-grained, 9Cr18 stainless twins carbide.
(3) some forgings tissue defects, general heat treatment process can not be eliminated, and results of the final heat treatment after forging performance degradation even unqualified. For example, serious stone-like fracture and facet fracture overfiring with ferrite in stainless steel, high-alloy tool steel ledeburite carbide network with.
(4) Some forgings tissue defects in the final heat treatment will be further development and even cause cracking. For example, the coarse grain structure of the alloy structure steel forgings, if after forging heat treatment does not improve, carbon, nitrogen permeation and quenched often caused by the martensite needle coarse and performance failed; the bulky ribbon carbonation in the high-speed steel matter, quenching often cause cracking. Intro common defects in the forging process and its causes in the second chapter. It should be noted that, in a variety of forming methods common defects and major defects of the various types of material forgings are regular. Different molding method, because of its force, stress and strain characteristics is not the same, thus likely to produce the main defects is not the same. For example, the major defects in the billet upsetting is the crack of the side surface of the vertical or 45 ° direction, the lower end often residual ingot upsetting after cast organizations; major defects in the rectangular section blank stretching surface transverse cracks and corner crack internal diagonal cracks and transverse cracks; open the main defect when forging is sufficient dissatisfaction, folding and wrong shift. Common defects in the forming process will be described in detail in Chapter 4. Different kinds of materials, its components, tissue, heating, forging and cooling process, organizational change and mechanical behavior is different, thus forging process is not possible defects has its particularity. High alloy tool steel forgings ledeburite defects mainly coarse carbide particles, the uneven distribution of crack, and high-temperature alloy forgings coarse grain and crack defects; defects in austenitic stainless steel forgings mainly intergranular chromium depleted decreased resistance to intergranular corrosion banded structure of ferrite and crack; defects of aluminum alloy forgings mainly coarse-grained, the folding, eddy current, cross-flow.
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