Discussion on the reasons of high hardness of steel for large specification 38CrMoAl injection molding machine

38CrMoAl belongs to high aluminum steel, and the AL contained in steel is the main element of nitrogen formation. It is easy to form aluminum nitride layer on the surface after nitriding the casting, and the improvement of the surface strength and hardness of the casting can be realized under the action of diffusion reinforcement. The quality of 38CrMoAl round steel will be significantly improved after heat treatment and finishing treatment, so it is widely used in plastic mold steel, and the research on production and processing at home and abroad is also significantly deepened. In the study, the hardness of round steel used for the production of 38CrMoAl large specification injection molding machine is not allowed to exceed HB 255, but due to the influence of material characteristics and production process characteristics, it is easy to make the hardness seriously exceed the standard, which directly affects its product image and reputation, so further research on the production process is needed. The way with the highest maturity in the experiment is Gleeble thermal simulation technology, which plays a key role in the study of material properties and simulated metallurgical process. If the metallurgical components are fixed, the cooling and hot rolling temperature will affect the hardness more significantly, so the following correlation simulation analysis will be conducted with the Gleeble thermal simulation instrument for reference.

Test data and methods

In this test project, 38CrMoAl was selected as the φ100mm rod, and several samples with the same rolling number were selected for the test. The composition of test steel mainly include: C 0.38%; Si 0.30%; Mn 0.47%; P 0.012%; S 0.0007%; Alt 0.80%; Cu 0.071%; Cr 1.40%; Ni

0.04%；Mo 0.18%；V 0.012%。

Based on the field 38CrMoAl quality plan, the test scheme of Gleeble 3800 thermal simulation instrument is formulated, and the hardness and metallographic analysis is conducted according to the test results.

experimental result

Dynamic CCT curve analysis

To the 38CrMoAl dynamics with the help of a Gleeble3800 thermal simulation instrument

CCT curve expansion study, on the cooling speed of different conditions of steel organization and

The phase transition temperature is discussed, and the measurement of Brinell hardness is completed to master the influence of the organization and hardness of steel under different cooling conditions. Taking the field 38CrMoAl quality plan as a reference, the final rolling temperature shall be guaranteed at 900~1050℃ during actual rolling. During the thermal simulation experiment, the intermediate value is selected, and the temperature of the mill deformation is set to 980℃. The main process of the experiment is to choose the speed of 20℃ per second to heat the sample until the temperature increases to 1250℃. After 10 min insulation, 780℃ starts to cool, and cool at 0.1℃ per second until 490℃ deformation. This cooling speed is selected to cool and complete the phase change.

Combined with the results of metallographic test, in order to reduce the hardness of steel, the steel tissue should choose all ferrite and beads. In order to achieve this goal, the cooling rate of steel is guaranteed to be lower than 360℃ / h as far as possible.

After measuring the post-rolling cooling speed of φ100 mm round steel at the production site, it is learned that when the final rolling temperature is 1050℃ fluctuation, the cold bed temperature on the steel is 780℃, whileThe main reason for this phenomenon is in the steel rolling period

Cold speed is too fast.

According to this reason, the Gleeble3800 thermal simulation instrument is selected to further study the influence degree of the deformation temperature on the steel organization and hardness at the same cooling speed, and the thermal simulation process is specifically as follows:

The cooling speed is at 0.07~0.10℃ / s to complete the cooling to 490℃, the actual cooling speed is 0.08℃ / s to complete the cooling to 490℃, a large number of beads and ferrite, so that the hardness is reduced to HB 260, below, the measured HB 252.

Deformation temperature affects the steel organization and hardness

Combined with the above steel metallographic organization experiment, it is known that in the case that the steel deformation temperature is 910℃, a large number of ferrite and beads will be formed in the final steel tissue, which is more conducive to the reduction of steel hardness. In the case that the steel deformation temperature is 1050℃, combined with the steel metallographic organization, it is known that there are almost no ferrite and beads in the steel, and the tissue is basically belong to the bainite organization, and the steel hardness is relatively high, generally exceeding HB 330. The reason is analyzed from the energy perspective, mainly because the deformation at low temperature will increase the austenite energy, which is also beneficial to increase the driving force of the phase transition by [6]. In the analysis of microstructure, the deformation at low temperature can easily increase the incidence of defects such as deformation band, dislocation, twin and vacancy, and the ferriplasma will increase, and it is more likely to make ferrite phase change. When the dislocation density increases, the carbon atomic diffusion channel also increases, which then promotes the phase transition of ferrites.

According to the actual situation on the site, in the rolling process of 38CrMoAl, the high temperature form is generally selected, and the steel final rolling temperature is usually higher than 1050℃. The verification is completed after comparing the actual steel metallographic organization and thermal simulation test results. In order to reduce the hardness of steel, it is necessary to reduce the temperature of the condition of the existing equipment on site

conclusion

38 for CrMoAl analysis

38CrMoAl belongs to advanced nitrated steel, with high strength, high fatigue strength and wear resistance. In practice, it is often used in nitrated parts with high size accuracy or nitrated parts with high wear resistance and small shock load.

Analysis of injection molding machine

Injection molding machine is also known as injection molding machine, belongs to the molding equipment, specifically refers to the plastic products with different shapes made with the help of plastic molding mold, generally including the horizontal, vertical and all-electric three kinds. The main function of the injection molding machine is to heat the plastic, through the high pressure treatment of the molten plastic, to ensure that the shot, the mold cavity is more full. The main working principle of the injection molding machine is similar to the needle syringe, which is also to benefit the thrust of the screw rod. The sticky flow plastic is injected inside the closed mold cavity and plasticized and in the molten state, and finally obtained after curing and setting. Injection molding is a circulation process, and the slow links contained in each cycle include quantitative feeding, melting and plasticizing, pressure injection, mold filling and cooling, and mold removal. In the plasticAfter removal, the film is closed again and entered into the next cycle.

In general, the operation items of injection molding machine mainly include electrical control system operation, control keyboard operation and hydraulic system operation. And screw type injection molding machine molding process is: inside the cylinder to add granular or powder plastic, under the rotation of the screw and the cylinder wall heating makes the plastic into a melting state, and the machine mold and forward injection, guarantee nozzle and mold mouth close, then the pressure oil into the injection cylinder, and then forward the flu, with great pressure and fast speed to low temperature closed mold, under the action of specific time and pressure cooling and curing molding, finally and open mold and remove the product. The main requirements of injection molding machine are plasticization, injection and molding, and plasticization is an indispensable basis to ensure the quality of molding products. In order to achieve the molding goal, we must ensure that the injection pressure and speed reach the standard. Due to the high injection pressure, the pressure in the mold cavity will also increase, so the mold force is required to be large. Therefore, the closing molding device and the injection device play an important role in each component of the injection molding machine.

In the evaluation of plastic products, should focus on the appearance quality, size, use performance three aspects, the above quality requirements according to the use of the use of the products, the requirements of the scale are also different. Product defects generally focus on mold design, wear degree and manufacturing accuracy.

Analysis of the experimental study results

After the experimental study, it is concluded that:

When the deformation temperature is 980℃, to reduce the hardness of steel

Quantity ensures that the steel tissue is all ferrite and pearl body.

The cooling speed is at 0.07~0.10℃ / s to complete the cooling to 490℃, and to achieve this goal, the steel cooling speed should not be higher than 360℃ / h, the tissue transformation of a large number of beads and ferrite, so that the hardness is reduced

epilogue

In summary, the systematic study and analysis on the high hardness of round steel for 38CrMoAl large size injection molding machine, and the importance of combining the knowledge of 38CrMoAl and injection molding machine. In the above test study shows that the main cause of 38CrMoAl injection steel hardness is during the rolling temperature, too high, in the actual improvement process, should be committed to reduce the rolling temperature, provide necessary help to reduce the steel hardness, to continuously optimize the injection molding machine steel hardness, give full play to the value and function of mechanical equipment.