INCONEL 718 Nickel-base Alloy is a precipitation-hardened high-temperature nickel-based alloy with high strength, good fatigue resistance and corrosion resistance at -253 to 650 °C, and its instantaneous service temperature can reach 800 °C. It is widely used in aerospace, nuclear energy, petrochemical and other fields. .
The Nb segregation in the INCONEL 718 Nickel-base Alloy ingot is serious, and it needs to be homogenized first, and then forged to open the billet to refine the grains. Since forging requires multiple fires, and each fire forging requires a lot of forging to complete, resulting in The single strain is small, and the problems of mixed crystals and coarse structures are prone to occur.
Hot extrusion forming process is formed under three-dimensional compressive stress, forming one by one, with a large compression rate and a large strain rate, which is conducive to the forming of difficult-to-deform alloys, and the produced alloy structure is uniform.
In this paper, the 63MN extruder of Shanghai HY Industry Co., Ltd. is used to carry out the extrusion forming process test of INCONEL 718 Nickel-base Alloy bar stock, which provides basic data for the development of extrusion forming process of uniform fine grain bar stock.
The INCONEL 718 Nickel-base Alloy ingot smelting process is vacuum induction furnace melting + electroslag remelting. Forging process: the furnace temperature is controlled below 690°C before the ingot is put into the furnace, and gradually heated to 1120-1140°C after entering the furnace, and a large amount of Nb contained in the microstructure of the as-cast alloy will segregate, thereby forming Laves phase and coarse carbide For the material, the homogenization treatment is carried out before forging to reduce segregation. Initial forging temperature ≥ 1050°C, final forging temperature ≥ 890°C. When the final forging temperature is less than 890°C, the billet should be returned to the furnace to supplement the temperature. The holding time of returning to the furnace should be more than 60 minutes, and the forging ratio should be ≥ 3. Solution treatment is carried out after forging billet, and then processed into φ 217mm round billet for extrusion.
Test process route
After the forging billet is processed into a round billet for extrusion, it is hot-extruded on a 63MN horizontal extrusion machine. The test process is as follows: Vacuum induction furnace melting + electroslag remelting → homogenization treatment → forging billet opening → processing and extrusion Round billet → resistance furnace preheating → electromagnetic induction furnace heating → glass powder lubrication → hot extrusion → water cooling solid solution → visual inspection + ultrasonic nondestructive testing + dimension measurement → metallographic inspection.
Bar hot extrusion die structure
The die structure of hot extrusion molding is shown in Figure 1. An extrusion die and a glass pad are arranged at the front end of the extrusion cylinder, and an extrusion pad is placed at the front end of the extrusion rod. When the heated billet is coated with glass powder, it is loaded into the extrusion cylinder, and the extrusion rod pushes the extrusion pad and the billet to move forward to the front end of the extrusion die. After the front end of the billet touches the glass pad close to the extrusion die, the extrusion rod continues to push forward, pressurizing the billet through the extrusion pad, first making the billet upsetting, eliminating the gap between the outer circle of the billet and the inner wall of the extrusion cylinder, and then The metal is extruded into the circular pass of the extrusion die to form a solid finished bar. When the extruding rod advances to the limit stop, the extruding process ends, leaving 20 to 50 mm of unfinished metal pressure in the extruding cylinder. Then lock the switch of the extrusion barrel and turn it on, and the extrusion barrel will retreat, and at the same time, the metal excess, the extrusion pad, and the extrusion rod will be retreated together, and the metal excess will be cut off from the finished product with heat, and the finished product will be extruded from the extrusion machine. The export direction is transported out.
Process parameters of hot extrusion forming of INCONEL 718 alloy bar
Due to the large deformation resistance of INCONEL 718 Nickel-base Alloy and the existence of Laves phase with low melting point, the hot processing temperature range is narrow, so when the temperature is too high, it is easy to extrude and burst, and when the temperature is too low, it is easy to extrude and stuffy, which brings pressure to hot extrusion processing. To a certain degree of complexity, it is necessary to consider the temperature rise phenomenon during processing.
Key control measures for blank heating: the temperature of the blank should reach the best plastic range of the material, and the uniform preheating of the resistance furnace + electromagnetic induction heating is adopted. On the one hand, the precipitated phase is heated by the resistance furnace to dissolve and diffuse as much as possible; on the other hand, the high temperature section has good thermal conductivity and induction heating It is beneficial to increase the surface temperature, compensate for the temperature drop of the outer surface during raw material transmission and extrusion, and improve the stability of metal flow during deformation.
Key hot extrusion process parameters: the forging billet is processed into a billet for extrusion with a size of φ 217mm×(600~620) mm, the head is processed with a rounded corner of R30~R40mm, and the surface roughness value is Ra≤1.6μm; Preheat the resistance furnace to 1100°C, keep warm for 2～ 4h, and then use an electromagnetic induction furnace to quickly heat the outer surface to 1120-1140°C; then coat the outer surface with glass powder, and the glass pad is tightly squeezed
Compression die, forming a lubricating film in a molten state, separating the metal from the extrusion die; the billet is fed into the extrusion cylinder, the inner diameter of the extrusion cylinder is 225mm, the inner diameter of the extrusion die is 97.5mm, and the extrusion speed is 70-130mm/s. In the circular cavity composed of cylinder, extrusion die and extrusion pad, after the three-way compressive stress of the extruder, a round rod with a diameter of 95mm is extruded, and it is quickly cooled in water after extrusion.
Surface quality of INCONEL 718 alloy bar after hot extrusion
The hot-extruded INCONEL 718 bar was subjected to visual inspection, ultrasonic non-destructive testing and dimensional testing, and the results showed that the surface of the extruded bar was smooth and clean, without defects such as folds, cracks, delamination, and slag inclusions, and the dimensional accuracy and surface quality All are ideal, better than the surface quality of forged rods.
6. INCONEL 718 alloy bar structure and hardness
Take the metallographic structure sample at the position of 1/2 of the radius of the bar. After the sample is polished and corroded, observe the grains and precipitated phases under the metallographic microscope. It can be seen that the grain at the radius of 1/2 is measured by the grain size Degree 3.0, the overall range is not greater than 2.0, the crystal grains are relatively uniform, and there is no mixed crystal. The matrix structure is austenite, the precipitated phase is mainly the compound of Nb and Ti, and the structure of the whole section of the bar is not much different. The structure of the NbC phase is an irregular, light gray oval shape. The TiN phase has a regular, light yellow angular shape. Both NbC and TiN are formed during smelting and pouring, and are not easy to dissolve.
Due to the fast extrusion speed, the temperature drop of the billet is not large. At the same time, with the temperature rise of the extrusion deformation, the temperature of the bar after extrusion is above 1050 ° C, and the high temperature state is directly immersed in water for rapid cooling, which belongs to the solid solution state, and the strengthening phase is in the structure. Therefore, no γ″ phase, γ′ phase, and δ phase were observed. After the sample was ground and polished, the Brinell hardness test was carried out according to the standard ASTM E18, and the hardness from the outer surface to the center was 153 to 160HBW. It belongs to the hardness of ideal solid solution state.
The INCONEL 718 alloy raw material is forged and billeted, and the surface temperature is raised by uniform preheating in a resistance furnace + electromagnetic induction heating, and glass powder is used as an extrusion lubricant medium, and the extrusion speed is 70-130mm/s, and it is subjected to single-pass large-strain hot extrusion After deformation, the finished product has good surface quality, high dimensional accuracy, no defects such as folding, cracks, and delamination, and the internal structure is uniform. The grain size difference is not greater than 2.0 grades, and the recrystallization is sufficient. The surface quality and internal structure are better than the quality of forging billet.
Starting from the purpose of further refining the structure, the next research direction is to reduce the grain growth during heating through low temperature preheating + rapid induction heating, or direct induction heating, combined with large deformation and high strain rate hot extrusion forming Process, optimize the production process of fine-grained bar stock.
Shanghai HY Industry Co., Ltdhas international first-class material production equipment such as high temperature alloy isothermal forging production line, CONSARC vacuum melting furnace, VAR vacuum arc remelting furnace etc., with strong professional ability and highly responsible management team. We are a member of China Application of nuclear energy materials, Member of the International Council of the Aeronautical Sciences，China Electronic Materials Industry Association Magnetic Materials Branch. Years of experience in special alloy. When you have superalloy material inquiry, please contact us without hesitation. Shanghai HY Industry Co., Ltd is Corrosion & fatigue nickel alloy professional manufacturer,We will provide you with sincere and thoughtful service.
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