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Effect of forging on metal structure and forging defects


Defects in forgings include surface defects and internal defects. Some forging defects will affect the processing quality of the subsequent processes, while others will seriously affect the performance of the forgings, reduce the service life of the finished parts, and even endanger safety. Therefore, in order to improve the quality of forgings and avoid the occurrence of forging defects, corresponding technical measures should be taken, and the quality control of the whole process should be strengthened. This chapter provides an overview of three aspects: the impact of forging on metal structure, properties and forging defects; the content and method of forging quality inspection; the general process of forging quality analysis.
(I) Influence of forging on metal structure and properties In forging production, in addition to the shape and size required for forgings, the performance requirements of the parts must be met, including: strength pointers, plastic pointers. , impact toughness, fatigue strength, fracture toughness and stress corrosion resistance, high temperature working parts, high temperature transient tensile properties, long-term performance, creep resistance and thermal fatigue properties. The raw materials for forging are ingots, rolled products, extruded materials and forged billets. The rolled material, the extruded material and the forged blank are respectively semi-finished products formed after the ingot is subjected to rolling, extrusion and forging processing. In forging production, using reasonable process and process parameters, the microstructure and properties of the raw materials can be improved by: 1) breaking the columnar crystals, improving the macrosegregation, changing the as-cast microstructure to the forged structure, and at the appropriate Under the condition of temperature and stress, the internal pores are welded to increase the density of the material; 2) the ingot is forged to form the fibrous structure, and further, the rolling, extrusion and die forging are used to obtain a reasonable fiber direction distribution; 3) Control The size and uniformity of the grains; 4) improving the distribution of the second phase (for example, alloy carbides in the Leysite steel); 5) obtaining deformation strengthening or deformation of the structure - phase transformation strengthening, and the like. Due to the improvement of the above-mentioned structure, the plasticity, impact toughness, fatigue strength and durability of the forgings are also improved, and then the final heat treatment of the parts can obtain a good combination of hardness, strength and plasticity required for the parts. performance. However, if the quality of the raw materials is poor or the forging process used is unreasonable, forging defects, including surface defects, internal defects or performance failure, may occur.
(II) Influence of forging process on forging quality The forging process generally consists of the following processes, namely, feeding, heating, forming, post-forging cooling, pickling and post-forging heat treatment. A series of forging defects may occur if the process is improper during the forging process. The heating process includes furnace temperature, heating temperature, heating rate, holding time, and furnace gas composition. If the heating is improper, such as too high heating temperature and too long heating time, it will cause defects such as decarburization, overheating and overburning. For billets with large cross-sectional dimensions and poor thermal conductivity and low plasticity, if the heating rate is too fast, the holding time is too short, the temperature distribution is often uneven, thermal stress is caused, and the billet is cracked. The forging forming process includes deformation mode, deformation degree, deformation temperature, deformation speed, stress state, mold and lubrication conditions of the tool and die, etc. If the forming process is improper, it may cause coarse grains, uneven grains, various cracks, fold. Cold current, eddy current, as-cast tissue residue, etc. During the post-forging cooling process, if the process is improper, it may cause cooling cracks, white spots, reticulated carbides, and the like.
(III) Influence of raw materials on the quality of forgings The good quality of raw materials is a prerequisite for ensuring the quality of forgings. For example, defects in raw materials will affect the forming process of forgings and the final quality of forgings. If the chemical element of the raw material exceeds the specified range or the impurity element content is too high, it will have a great influence on the forming and quality of the forging. For example, S, B, Cu, Sn and other elements are easy to form a low melting point phase, making the forging easy. It appears hot and brittle. In order to obtain the intrinsic fine-grain steel, the residual aluminum content in the steel needs to be controlled within a certain range, for example, 0.02% to 0.04% (mass fraction) of Al acid. If the content is too small, the effect of controlling the grain growth will not be achieved, and the intrinsic grain size of the forging is often unsatisfactory; the aluminum content is too much, and the grain-like fracture is easily formed under the condition of forming the fiber structure during the pressure processing. A tear-like fracture, etc. For example, in the 1Cr18Ni9Ti austenitic stainless steel, the more the content of Ti, Si, Al, and Mo is, the more the ferrite phase is, and the more the band-shaped crack is formed during forging, and the part is magnetic. For example, there are defects such as shrinkage of the shrinkage tube, subcutaneous foaming, severe carbide segregation, and coarse non-metallic inclusions (slag inclusion), which may cause cracks in the forging. Defects such as dendritic crystals, severe looseness, non-metallic inclusions, white spots, oxide films, segregation bands and mixed metals in raw materials are likely to cause a decline in the performance of forgings. Surface cracks, folds, crusting, coarse crystal rings, etc. of raw materials are liable to cause surface cracks in forgings.
(IV) Effect of forging structure on microstructure and properties after final heat treatment Austenitic and ferritic heat-resistant stainless steel, superalloy, aluminum alloy, magnesium alloy, etc., in the process of heating and cooling, there is no homogeneous transformation of materials As well as some copper alloys and titanium alloys, the structural defects generated during the forging process cannot be improved by heat treatment. In the heating and cooling process, there are allogeneic transformation materials, such as structural steel and martensitic stainless steel. Due to some defects in the microstructure caused by improper forging process or some defects left by the raw materials, the quality of the forged parts after heat treatment has Great impact. The examples are as follows:
1) The structural defects of some forgings can be improved in the post-forging heat treatment, and the satisfactory microstructure and properties can still be obtained after the final heat treatment of the forgings. For example, coarse-grained and Weiss structures in generally overheated structural Steel Forgings, slightly reticulated carbides and over-elected steels and bearing steels due to improper cooling.
2) The structural defects of some forgings are difficult to eliminate with normal heat treatment. It is necessary to use high temperature normalizing, repeated normalizing, low temperature decomposition, high temperature diffusion annealing and other measures to improve. For example, low-order coarse crystals, twinned carbides of 9Cr18 stainless steel, and the like.
3) The structural defects of some forgings cannot be eliminated by the general heat treatment process, and as a result, the performance of the forged parts after the final heat treatment is degraded or even unqualified. For example, severe stone fractures and facet fractures, overburning, ferritic bands in stainless steel, carbide meshes and belts in high-alloy tool steels.

4) The structural defects of some forgings will further develop and even cause cracking during the final heat treatment. For example, the coarse-grained structure in alloy structural Steel Forgings, if not improved after forging heat treatment, often causes martensite needle coarseness and unqualified performance after carbon, nitrogen co-infiltration and quenching; coarse band carbonization in high-speed steel Things, often cause cracking during quenching. The common defects in the forging process and their causes are described in detail in Chapter 2. It should be noted that the common defects in various forming methods and the major defects of various material forgings are regular. Different forming methods have different stress and strain characteristics due to different stress conditions, so the main defects may be different. For example, the main defect when the blank is upset is that the side surface produces a longitudinal or 45° crack, and the as-cast material is thickened, and the as-cast structure remains in the upper and lower ends; the main defect in the rectangular section blank is the lateral crack of the surface. And corner cracks, internal diagonal cracks and transverse cracks; the main defects in open die forging are filling, folding and misalignment. Common defects in each major forming process are detailed in Chapter 4. Different kinds of materials, due to their different compositions and structures, have different structural changes and mechanical behaviors during heating, forging and cooling. Therefore, the forging process is not correct, and the defects may be special. For example, the defects of the high-alloy tool Steel Forgings are mainly coarse carbide particles, uneven distribution and cracks. The defects of high-temperature alloy forgings are mainly coarse crystals and cracks; the defects of austenitic stainless Steel Forgings are mainly intercrystalline chromium-depleted. Resistance to intergranular corrosion, ferrite band structure and cracks; aluminum alloy forgings are mainly coarse crystal, folding, eddy current, flow through, etc.



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