Serious mixed crystal after normalizing forging steel?
Importance of reasonable selection of forging temperature
At the same time, it has a great influence on the microstructure after forging. If the initial forging temperature is too high, it will not only cause serious oxidation and decarburization, but also cause overheating and over burning, which will easily lead to coarse structure and even form stable overheated coarse structure. This kind of tissue is hereditary, and it is difficult to eliminate it by normal pretreatment. Coarse grains will reduce the plasticity and toughness of forgings, and the fatigue properties will decrease obviously.
When the final forging temperature is low, the microstructure is difficult to recover and recrystallize, then fiber structure and banded structure will be formed, and even non-equilibrium structures such as bainite and martensite will appear, which will bring about the inheritance of microstructure and make the grains grow unevenly in the later heat treatment, which will also cause the grain coarseness. In addition, the primary deformation must be controlled to avoid the formation of coarse grain structure in the range of critical deformation. If the final forging temperature is too low, the billet will crack in the forging process, or produce large residual stress in the billet, which will cause the forging to crack in the cooling process or subsequent processes.
The forging temperature range refers to a temperature interval between the initial forging temperature and the final forging temperature. In the range of forging temperature, the metal has good malleability (sufficient plasticity, low deformation resistance, etc.) and appropriate metallographic structure. In order to improve the plasticity and reduce the deformation resistance, we hope to increase the heating temperature of the metal as much as possible, but in order to ensure the product quality and avoid the defects in heating, the heating temperature is too high and not good. In order to reduce the number of fire, save energy and improve the efficiency of labor production, it is hoped that the final forging temperature will be lower and the temperature range will be expanded. However, if the final forging temperature is too low, defects will also occur. All these factors are contradictory and restricted each other, so the final forging temperature should be fully considered.
Forging process can ensure the continuity of metal fiber structure, keep the fiber structure of forging consistent with the shape of forging, and the metal streamline is complete, which can ensure that the parts have good mechanical properties and long service life.
The higher the alloy composition, the higher the final forging temperature. The final forging temperature of high alloy steel is generally much higher than that of carbon steel (about 100 ℃), which is mainly due to the sharp increase of deformation resistance and deterioration of plasticity with the decrease of temperature. When the final forging temperature is too high, coarse grains appear in the low magnification of the forging, and the impact toughness value is unqualified. This is because the temperature is too high when the forging is stopped, the grain size continues to grow, and the abnormal structure appears during cooling after forging, which reduces the mechanical properties, especially the impact toughness.
The mixed crystal is serious after normalizing
In the development of a manufacturer’s reduction gear, serious mixed crystal was found after normalizing. Based on this problem, this paper analyzes the forging process and finds out the reasons to ensure the smooth progress of the project!
Analysis of forging process
The developed reducer gear is made of 20mncr5 steel. The processing flow is as follows: sawing machine blanking → medium frequency induction heating → flattening → pre forging → punching → ring rolling → electric screw press forging 1 → electric screw press forging 2 (shaping) → air cooling → isothermal normalizing.
After investigation, the initial forging temperature is 1180 ℃± 30 ℃, and the final forging temperature is 790-830 ℃ (see Fig. 1 for the blank color after forging). We investigate that the initial forging temperature of other workpieces of the same material is similar, while the final forging temperature reaches 950-1000 ℃ (see Fig. 2 for the color of blank after forging). Only the final forging temperature is different. If the final forging temperature is high, the normalizing metallography is qualified; if the final forging temperature is low, the normalizing metallography is unqualified. Is the normalizing metallography related to the final forging temperature?
Fig.1 blank color of final forging temperature 790 ～ 830 ℃
Fig. 2 blank color at final forging temperature 950 ～ 1000 ℃
So we did several tests according to this phenomenon. The normalizing temperature is controlled at 950 ℃ ± 10 ℃ according to the customer specification, and the normalizing temperature can only be selected between 940 ℃ and 950 ℃. Since the maximum heating temperature of isothermal normalizing equipment in our company is 950 ℃, we set the maximum normalizing temperature at 940 ~ 950 ℃.
1) Normalize twice. The metallographic structure of the first normalizing is shown in Fig. 3, and that of the second normalizing is shown in Fig. 4.
Fig. 3 metallographic structure of normalizing
Fig.4 metallographic structure of twice normalizing
2) For the forged blank, 680 ℃± 15 ℃ is used for air cooling after holding for 3 h, and then normalizing. The microstructure of high temperature tempering and normalizing is shown in Fig. 5.
Fig. 5 microstructure of high temperature tempering and normalizing
3) Increase the initial forging temperature. For the forged blank, put the insulation frame with refractory asbestos wrapping temperature (see Fig. 6 for the blank color after forging). The experimental parameters are as follows: the initial forging temperature is 1200 ℃± 30 ℃, the forging process is optimized, and the final forging temperature is 820 ~ 850 ℃. High temperature forging, residual heat annealing and normalizing metallographic structure is shown in Fig. 7.
Figure 6 blank color after forging
Fig. 7 metallographic structure after annealing and normalizing
Analysis of test results
According to the test results, it can be seen that: the final forging temperature of the blank is low, the color of the blank is blue in blue, and the oxide scale is very thin and uniform; for the high final forging temperature, the blank is dark blue, and the oxide skin is very thin and uniform; after forging, it is put into the heat preservation barrel for heat preservation, and the color is gray after stress relief annealing, and the oxide skin is thick. The results show that the effect of twice normalizing is similar to that of high temperature tempering plus one normalizing; the best metallographic structure is to increase the initial forging temperature appropriately, and then heat annealing, but the microstructure of this treatment is similar to that of final forging temperature > 950 ℃.
Analysis reasons: for 20mncr5 material, when the final forging temperature is between 790 ℃ and 830 ℃, the final forging temperature is lower, which may cause that the stress after forging is not completely eliminated and the recrystallization is not complete. The test shows that high final forging temperature has good effect on relieving forging stress and recrystallization, and can ensure the microstructure after normalizing. As to why the normalizing specification of customers is 950 ℃ ± 10 ℃, we adopt the normalizing process of medium carbon steel for the products without normalizing. The normalizing temperature is 890 ℃± 10 ℃. The metallographic structure after normalizing is the same as that when the final forging temperature is 950-1000 ℃. We also find that some product blanks are purchased parts, and the microstructure, hardness and grain size of the products are qualified through the inspection of the incoming materials, but after carburizing and quenching, they are qualified It is found that the structure of the gear center is coarse and mixed crystal, and the normalizing temperature is lower than 900 ℃. High normalizing temperature has a good effect on eliminating the defects in forging process, and low normalizing temperature has little improvement on the defects formed by blank forging, and then in the long-term carburizing process, due to stress release, the microstructure is abnormal. Therefore, normalizing specification, especially normalizing temperature, should be specified when purchasing blanks.
According to the new product development, we have gained the following experience:
- 1) For developing new products and ordering new forging production lines, it should be ensured that: according to the forging materials, the initial forging temperature, final forging temperature and production rhythm are proposed. The forging equipment manufacturer will select the appropriate forging method and equipment according to the initial forging temperature, final forging temperature and production rhythm required by the manufacturer, and also recommend the appropriate medium frequency induction heating for the production enterprise according to its experience Thermal equipment.
- 2) The normalizing temperature is determined according to the carburizing temperature, rather than as long as the normalizing test is qualified.
- 3) The final forging temperature has a great influence on the subsequent normalizing microstructure. Due to the limitation of forging equipment, there are various problems in developing new products. Only by analyzing the problems and finding out the variables can we develop qualified products.
Author: Hao Fenglin
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry (www.metallicsteel.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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