Analysis and summary of influencing factors of forging heat treatment
In forging production, in addition to ensuring the required shape and size of forgings, it must also meet the performance requirements of parts in the process of use. With reasonable forging process and process parameters, the structure and performance of raw materials can be improved through the following aspects:
- (1) The results show that the columnar crystal is broken, the macrosegregation is improved, the as cast structure is changed into the forged structure, and the internal voids are welded under the appropriate temperature and stress conditions to improve the density of the material;
- (2) The fiber structure of ingot is formed by forging, and the reasonable fiber direction distribution is obtained by rolling, extrusion and die forging;
- (3) Controlling the size and uniformity of grains;
- (4) The distribution of the second phase (such as the alloy carbide in ledeburite steel) is improved;
- (5) The microstructure is strengthened by deformation or deformation transformation.
Because of the improvement of the above-mentioned structure, the plasticity, impact toughness, fatigue strength and endurance properties of the forging are also improved, and then through the final heat treatment of the parts, the good comprehensive properties such as hardness, strength and plasticity required by the parts can be obtained.
If the forging process adopted is unreasonable, forging defects may occur, including surface defects, internal defects or unqualified performance, which will affect the processing quality of subsequent processes. Some of them will seriously affect the performance of forgings, reduce the service life of finished parts, and even endanger safety.
The effect of forging microstructure on the microstructure and properties after final heat treatment is mainly shown in the following aspects:
- (1) Irreparable structure defects: austenitic and ferritic heat-resistant stainless steel, superalloy, aluminum alloy, magnesium alloy and other materials without isomerism transformation during heating and cooling, as well as some copper alloy and titanium alloy, the structure defects produced in forging process can not be improved by heat treatment.
- (2) Microstructure defects that can be improved: coarse grain and widmanstatten structure in general overheated structural steel forgings, slight network carbides in hypereutectoid steel and bearing steel due to improper cooling, etc.
- (3) The structure defects which are difficult to be eliminated by normal heat treatment, such as Macrograin, 9Cr18 stainless steel, twin carbide of H13, can be improved by high temperature normalizing, repeated normalizing, low temperature decomposition and high temperature diffusion annealing.
- (4) The structure defects that can’t be eliminated by general heat treatment process: severe stone fracture and edge fracture, overburning, ferrite strip in stainless steel, carbide mesh and strip in ledeburite alloy tool steel, etc. make the properties of forgings after final heat treatment decline or even fail.
- (5) For example, if the coarse-grained structure of alloy structural steel forgings is not improved during post forging heat treatment, the coarse-grained martensite needles and properties are often unqualified after carbonitriding and quenching; The coarse banded carbides in high speed steel often cause cracking after quenching.
- (6) Improper heating, such as too high heating temperature and too long heating time, will cause decarbonization, overheating, overburning and other defects.
- (7) During the cooling process after forging, if the process is not proper, it may cause cooling cracks, white spots, etc., and crack during heat treatment.
What are the effects of cutting on the quality of heat treated workpiece?
- (1) Under the condition of quenching and tempering, annealing and normalizing, the hardness of the workpiece is lower than 45HRC. The influence of cutting on the quality of the workpiece, including surface finish, residual stress, machining allowance, removal of decarburized and carbon poor layer, is not obvious, which will not cause the change of the potential performance of the workpiece.
- (2) Hardened steel or workpiece processing, also known as hard processing, workpiece hardness up to 50 ~ 65hrc, materials mainly include ordinary quenched steel, quenched die steel, bearing steel, rolling roller steel and high speed steel, etc., the impact of cutting is more obvious, the generation and conduction of cutting heat in the process of cutting High speed friction and wear and other factors will damage the machined surface to a certain extent. The integrity of machined surface in hard cutting mainly includes surface microstructure, hardness, surface roughness, dimensional accuracy, distribution of residual stress and generation of white layer.
The hardness of machined surface increases with the increase of cutting speed, and decreases with the increase of feed rate. Moreover, the higher the hardness of the machined surface, the deeper the hardened layer. The results show that the residual compressive stress on the workpiece surface is uniform after hard cutting, and the maximum compressive stress is mainly concentrated on the workpiece surface after grinding.
The larger the tool obtuse radius is, the larger the residual compressive stress is; The higher the hardness of the workpiece, the greater the residual compressive stress. The hardness of the workpiece has a great influence on the surface integrity of the workpiece. The higher the hardness of the workpiece, the more conducive to the formation of residual compressive stress.
Another important factor that affects the quality of machined surface in hard cutting is the formation of white layer. The white layer is a kind of structure formed with the hard cutting process. It has unique wear characteristics: on the one hand, it has high hardness and good corrosion resistance; On the other hand, it shows high brittleness, which is easy to cause early spalling failure, and even cause cracking after the workpiece is placed in a stage after processing. When cutting hardened AISIE52100 bearing steel with ceramic and PCBN tools on a high rigidity CNC lathe, it is found that the microstructure of the surface layer and subsurface layer of the workpiece changes, and the microstructure is composed of white non tempered layer and black over tempered layer.
At present, the view that the white layer is regarded as martensitic structure has been unanimously accepted, and the main controversy lies in the fine structure of the white layer. One point of view is that the white layer is the result of phase transformation, which is composed of fine grain martensite formed by rapid heating and sudden cooling during cutting. Another point of view is that the formation of white layer is only the deformation mechanism, which is only the unconventional martensite obtained by plastic deformation.
Source: China Flange Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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