Research progress of heat treatment process for superalloys

Superalloy is a kind of high temperature metal material which works for a long time under high temperature and certain stress conditions. It has good comprehensive properties and is widely used in aerospace and other fields. The proper heat treatment process can improve the properties of the alloy by changing the microstructure of the alloy.

The research progress of heat treatment technology of Superalloy in recent years is summarized. The heat treatment technology of deformed superalloy, cast superalloy and Powder Superalloy and the influence of heat treatment on its structure and properties are discussed in detail, and the development trend of heat treatment technology of superalloy is also described.

Superalloy has e x cellent high temperature strength, good o x idation resistance and thermal corrosion resistance, e x cellent creep performance, good fatigue performance and fracture toughness, etc. it has become an irreplaceable key material for high temperature parts such as turbine blade, guide vane, turbine disk and other high temperature parts of aviation and industrial gas turbine. According to the preparation process, superalloys can be divided into deformation superalloys, casting superalloys and powder metallurgy superalloys.

With the development of aeroengine, the performance requirements of high temperature materials are also increasing. Heat treatment is an indispensable process of superalloy [2]. The heat treatment process of superalloy is shown in Figure 1. By changing the microstructure inside the workpiece or changing the chemical composition of the workpiece surface, the toughness and corrosion resistance are improved, stress and softening are eliminated and strength is improved. Therefore, the heat treatment research for optimizing microstructure has been one of the important links in the development and application of alloy.

20210719012838 28630 - Research progress of heat treatment process for superalloys

Heat treatment process of superalloy

The heat treatment process of superalloy is a kind of metal heat treatment process in which the e x pected structure and properties of superalloy materials are obtained by heating, heat preservation and cooling in solid state. In recent years, the research on superalloy is more in-depth, and the systematic solution heat treatment and aging heat treatment are systematically. Solid solution heat treatment is to make all the precipitates in the alloy fully dissolve into the matri x  phase, which is higher than the total solution temperature of the precipitate in the high temperature alloy structure, so as to strengthen the solid solution and improve the toughness and corrosion resistance, eliminate the effect of residual stress, so as to continue processing and forming, and prepare for the reinforcement phase with uniform distribution after aging treatment.
Aging heat treatment refers to heating and holding for a certain time in the temperature range of precipitation of the strengthening phase, which makes the strengthening phase of Superalloy precipitate evenly and the carbide evenly distributed, so as to realize the effect of hardening alloy and improving its strength.

Research progress of heat treatment technology of deformed Superalloy

Wrought iron-base superalloy

GH2132 is suitable for aviation fastener because of its outstanding rela x ation resistance, corrosion resistance and good comprehensive performance. The GH2132 alloy used at present can not meet the requirements of 1100Mpa high strength to meet the working conditions and ensure aviation safety, so it is necessary to carry out certain heat treatment.

Chen Qi and others[3] adopt two heat treatment methods: one is that the solid solution plus aging heat treatment process is 900℃ solid solution for 2.5h oil cooling, 720℃ x 16h+660℃ x 16h air cooling, guarantee δ>At 22%, the tensile strength can reach 1240Mpa; Second, the direct aging heat treatment process is 680℃ to ensure δ>the tensile strength of GH2132 alloy was increased to 1400MPa at 13%. The alloy has been treated with better comprehensive properties.
GH696 alloy is a precipitation reinforced Fe based deformation superalloy, which has high creep strength, good o x idation resistance and corrosion resistance. Its surface hardness and wear resistance limit the application range in aviation. Wang Shu x in studied the surface heat treatment technology of GH696 alloy. The gas nitriding method, ammonia gas as nitriding agent and ammonium chloride as the accelerant were used. The sample is treated with solid solution before nitriding, and the process is (1000-1100)℃ × (1-2) H oil cooling; After solid solution treatment, two aging treatment is carried out, and the primary aging treatment process (750-780)℃ x Air cooling for 16h; Secondary aging treatment process (690-710)℃ x Air cooling for 16h.
The surface heat treatment improves the surface hardness, wear resistance and corrosion resistance of the alloy, and the depth of nitriding layer and the nitriding holding time conform to the parabolic rule of diffusion control.

The nickel base deformed superalloy

GH145 is mainly used to manufacture plane spring and spiral spring with high strength and high strength for Aeroengine working under 800℃. GH145 alloy mainly depends on the second phase γ′ The mechanical properties of the system are closely related to the temperature and time of solution treatment. Wang Zengyou. Adopted (1135 ± 10)℃ x 2 hours of air cooling solution treatment, (845 ± 10)℃ x Cool the furnace for 24 hours to (720 ± 10)℃ x The mechanical properties of the air cooling aging treatment were satisfactory after 19h.

The comprehensive mechanical properties and durability properties were obtained by using primary solution and secondary aging. GH145 alloy is aged for a long time, γ′ The solubility of alloy elements in the phase will change slightly with the aging temperature, but it has little relationship with aging time. The high temperature alloy GH4145/SQ is γ′ [Ni3 (al, Ti, Nb) is a nickel based aging hardening alloy with main strengthening phase, which is mainly used for flange bolts of high and medium pressure inner cylinder of 300MW or 600MW steam turbine.
After the bolt is operated at high temperature, the microstructure, dislocation configuration change and precipitation of strengthening phase will occur, which will lead to the decrease of creep and durability of the material. In order to improve its performance, Yan Guangzong[6]. The bolts with a hardness value of 333HBW after operation were restored to heat treatment. The heat treatment process was solid solution + twice aging, and the solid solution process was 1130℃ x Oil is cold for 1 hour; The first time aging is 845℃ x 24h, the second aging is 707℃ x Air cooling for 20h.
After the heat treatment of the bolts with the hardness value e x ceeding the standard value is restored, the hardness value is reduced to the standard range. Although the strength value has decreased to some e x tent, the plasticity and toughness are significantly improved, which proves that the recovery heat treatment process is e x tremely effective. GH4169 alloy is a high strength Ni Cr Fe base superalloy containing Nb, which is mainly used to manufacture high temperature parts of aeroengine. The effect of heat treatment on the microstructure and properties of GH4169 cold-rolled blade was studied by Zhang Zunli.
The system of 970-995℃ softening treatment was adopted. After the softening treatment, the intermediate treatment was carried out with the intermediate treatment and the solution aging treatment. The intermediate treatment system was 900℃ air cooling, and the solid solution aging treatment system was 1010℃ air cooling +720℃ x The furnace is cooled to 620℃ for 8h x 8 hours air cooling. The hardness of the cold rolling can be reduced obviously by softening treatment, which is beneficial to the second cold rolling; The softening treatment at 995℃ can get better softening effect and has no effect on the microstructure and properties of the alloy; After cold rolling deformation of GH4169 alloy, the softening treatment has little influence on mechanical properties, and the intermediate treatment and final solution aging heat treatment are the important processes to determine the mechanical properties. Three heat treatment processes of GH4169 alloy for blade forgings were studied by Zhang Yifeng.

  • (1) (1010~1065) ℃±10℃ x Water cooling for 1H + (720 ± 5)℃ x 8 hours, cool to (620 ± 5)℃ in 50℃/h furnace x 8 hours air cooling. After the treatment, there is no grain boundary and crystal inside δ Phase, it is beneficial to improve impact performance and resist low temperature hydrogen embrittlement.
  • (2) (950~980) ℃±10℃ x Water cooling for 2h + (720 ± 5)℃ x 8 hours, cool to (620 ± 5)℃ in 50℃/h furnace x 8 hours air cooling. After the system is processed, there are δ In contrast, it is beneficial to eliminate notch sensitivity, also known as the standard heat treatment system.
  • (3) (720±5) ℃ x 8 hours, cool to (620 ± 5)℃ in 50℃/h furnace x 8 hours air cooling. After the system is processed, the δ Compared with the other, the strength and impact properties of the materials can be improved.

After adding P and B, GH4169 alloy was changed to GH4169G alloy, and its temperature bearing capacity was further improved. In alloy γ′、γ” and δ The phase has different morphology, volume fraction, lattice constant and mismatch. Therefore, the mechanical and creep properties of the alloy are different, and the phase transition mechanism is not clear. The effect of heat treatment on the phase composition and distribution of GH4169G alloy was studied by Tian sugui. [9] (1) the direct aging treatment was 720℃ x 8h, then cool to 620℃ with furnace at a cooling rate of 50℃/h x 8 hours air cooling (2) The long-term aging treatment is directly aged, and it is 680℃ x Air cooling for 300h. Direct aging treatment of GH4169G alloy is made from a small amount γ′ Phase, large amount γ” Mutual harmony γ. The matri x  is composed of a small amount of GH4169G alloy treated by long-term aging γ′ Phase, large amount γ” Mutual harmony γ Phase and needle δ Phase composition. GH738 alloy has good corrosion resistance, high yield strength, fatigue performance and ideal creep performance. It is widely used in the bearing parts and high temperature parts such as gas turbine and aeroengine turbine disk.
Yaozhihao and others studied the solid solution and stabilization treatment technology for the carbide and the treatment of GH738 alloy γ′ The influence of Precipitation Law of strengthening phase. To standard heat treatment: 1020℃ x 4h air cooling +845℃ x 4h air cooling +760℃ x 16h air cooling and 1080℃ x 4h air cooling +845℃ x 24h air cooling +760℃ x Based on air cooling for 16h, the heat preservation time in stabilization stage is adjusted. Pass 1020℃ x the stabilization time of the alloy after 4 hours water cooling solid solution treatment was prolonged at 845℃, and the carbide at the grain boundary gradually showed short rod distribution; 1080 ° C x After 4 hours of water-cooled solid solution treatment, the carbide at the grain boundary gradually presents continuous distribution after the same stabilization process.

Heat treatment process of cast superalloy

The cast superalloys are divided into three categories: equiaxed, directional and single crystal. Under the condition of the composition, its mechanical properties are closely related to the technological factors. The performance and reliability of the alloy can be improved by heat treatment.

The isoaxial crystal superalloy

K480 is γ′ The phase precipitation strengthened nickel base casting superalloy has good structure stability and excellent heat-resistant corrosion resistance. Chen Hao and others studied the heat treatment process of k480. The alloy is of two sizes after the solution treatment at 1130 ℃ γ′ Phase; After the solution treatment at 1190 ℃, 1210 ℃ and 1230 ℃, the air cooling is carried out, and the precipitation is uniform γ′ The content of carbide and eutectic decreases with the increase of solid solution temperature. Primary aging treatment, solid solution state γ′ The average size of phase increases with the increase of aging temperature. In the process of secondary aging and full aging, the small three times γ′ The phase is re dissolved to the base or the surrounding large size γ′ In phase. K4169 nickel base casting superalloy has good medium temperature strength and good machinability.

The effect of semi-solid isotherm treatment on the microstructure and properties of K4169 alloy was studied by Anbei[15]. The best heat treatment process is 1310 ℃ heating temperature × 90-120min, or heating temperature 1320 ℃ × 45~60min。 The effect of isotherm treatment on the compressive strength of K4169 alloy is related to the microstructure of K4169 alloy. The hardness is lower than that of as cast alloy. The dendrites in as cast structure change into spherical grain structure, and the part at the grain boundary during the heating process γ Phase dissolution, with the increase of temperature, γ+γ′ The eutectic phase begins to melt and it is primary γ The phase gradually changes into globular in isothermal treatment.
The directional solidification superalloy DZ125 has high mechanical properties and good castability, and it is a kind of directional solidification high pressure turbine blade material developed by aeroengine. Sheli[16] studied the DZ125 heat treatment process, and adopted one-step process (1210 ℃) × Air cooling for 2h +870 ℃ × 32H air cooling) and three-step process (1180 ℃) × Air cooling for 2h + (1230 ± 10) ℃ × 3H air cooling +1100 ℃ × 4h air cooling +870 ℃ × 20 hours air cooling) for heat treatment. The microstructure was improved by three-step heat treatment. The low melting point phase was eliminated by 1180 ℃ pretreatment, the initial melting of the alloy was effectively suppressed and the solid solution temperature of the alloy was increased. With the increase of the solution temperature, the segregation of dendrites decreases; The aging at 1100 ℃ is adjusted to fine γ′ The size and shape of the phase make the alloy’s medium temperature and high temperature lasting life increase to different extent than that of one-step heat treatment. DZ125l is a high performance directional solidification nickel based superalloy, and its main application environment is the first stage turbine blade of 7-8 turbomotor with thrust weight ratio. Laser metal forming has fine structure, forming supersaturated solid solution and inhibiting it γ′ The phase precipitation, the point like discontinuous MC carbide precipitated at the grain boundary, can not meet the requirements of high temperature alloy. Huxiaohua and others studied the effect of heat treatment on the high temperature non-equilibrium structure and hardness of laser metal forming DZ125l. It is necessary to improve the standard heat treatment technology of casting, homogenization and solution treatment, but it is necessary to control the corresponding time, and the time of aging treatment can be shortened. DZ417G is a kind of directionally solidified nickel base casting superalloy, which has the advantages of high temperature strength, good creep property and stable structure, and is suitable for making high temperature parts such as guide vanes.
The heat treatment of DZ417G was studied by pengzhijiang[18]. 1220 ° C × 4h air cooling (solid solution) +980 ℃ × The carbide and boride are obviously smaller, and they are distributed in the state of dispersion in the crystal and grain boundary. The columnar crystal growth and the good occlusion between dendrites can obtain good mechanical properties. DZ68 alloy is a kind of directional high temperature alloy with rhenium. Adding rhenium will cause serious segregation in the alloy as cast state. Therefore, it is very important to study the heat treatment process of DZ68 alloy. Liu Enze[19]. Studied the heat treatment process of DZ68, and the best heat treatment process was 1240 ℃ × 0.5h+1260℃ × 0.5h+1280℃ × Air cooling for 2h +1120 ℃ × 4h, furnace cooling for 1h to 1080 ℃ × 4h air cooling +900 ℃ × 4 hours air cooling. The DZ68 alloy treated by this process reduces the segregation of elements and has excellent durability.

Single crystal superalloy

When there are refractory elements re and W in the alloy, because the elements re and w have different properties. Therefore, higher solution temperature and longer diffusion time are needed[20].
Wang Minggang[21] carried out three kinds of solid solution treatment of RE and W single crystal alloy with different temperature, the solid solution temperature was 1300 ℃, 1310 ℃ and 1320 ℃ respectively. The creep life of the alloy is different under different temperature solution treatment. The high temperature solution treatment can improve the uniformity of the alloy composition and inhibit the precipitation of TCP phase.

When the solution temperature is increased to 1320 ℃, the uniformity of the alloy composition can be improved, the insoluble elements can be fully dissolved and diffused, which reduces the dendrite dry and the segregation of the components between dendrites, and inhibits the precipitation of TCP phase in the alloy, which significantly improves the creep resistance of the alloy. Finally, the optimum heat treatment system of the alloy is determined as follows: 1280 ℃ × 4h air cooling +1320 ℃ × 4h air cooling +1080 ℃ × 4h air cooling +870 ℃ × 24 hours air cooling.
The current heat treatment technology of DD3 nickel base single crystal superalloy is characterized by low solid solution temperature, relatively few processes and simple process control. Hanmei[22] improved the current heat treatment process system of DD3 alloy, and the improved process was 1265 ℃ × 4h air cooling +1060 ℃ × 4h air cooling +870 ℃ × Air cooling for 32H. The improved technology improves the solution treatment temperature, and adds the first-stage high temperature aging, and the distribution and structure uniformity of alloy elements after heat treatment γ′ The phase content was improved obviously. The improved technology has significantly improved the creep property of DD3 alloy at 760-1038 ℃ and further improved the mechanical properties. DD6 alloy has good high temperature performance, mainly used in the production of blades. Yu Jian[23]. Studied the microstructure of DD6 alloy after heat treatment. The standard heat treatment system of DD6 alloy is 1290 ℃ × 1h+1300℃ × 2h+1315℃ × 4h air cooling +1120 ℃ × 4h air cooling +870 ℃ × Air cooling for 32H.
The comparative analysis of six heat treatment systems and standard heat treatment systems shows that after solid solution treatment, higher supersaturation promotes γ′ The phase precipitates in large quantities under different cooling rates. Cooling speed is one time γ′ Phase size and secondary γ′ The precipitation of the phase has a great influence. There are a lot of secondary in the matrix channel of the solid solution with fast cooling rate and primary aging air cooling sample γ′ Phase precipitation. The secondary aging of DD6 single crystal superalloy at 870 ℃ is extended with the holding time γ′ Phase dissolves gradually. There are no grain boundary strengthening elements in the single crystal superalloy, so recrystallization area becomes a weak link in performance.
At present, there are few studies on recrystallization microstructure evolution in the process of heat treatment of single crystal superalloys. Quyan equality analyzed the microstructure evolution of DD6 alloy during heat treatment under different conditions. The pretreatment temperature was 1120 ℃, 1170 ℃, 1220 ℃ and 1270 ℃, the treatment time was 2h, and 1310 ℃ was carried out × 4 hours solution treatment and 1120 ° C × The aging treatment was 4 hours. As cast γ′ No recrystallization was observed in pretreatment under the phase dissolution temperature, γ′ Recrystallization occurs after pretreatment above the phase dissolution temperature. The recrystallization phenomenon occurs in the solid solution treatment after pretreatment. The recrystallization grain boundary is fine and the grain boundary is from coarse to large γ′ Phase composition.
Compared with the solid solution, the recrystallization depth after aging treatment is not changed much. DD8 alloy is a single crystal alloy with high resistance to heat corrosion. There are few researches on the heat treatment technology of DD8 in China. Zhang Jinghua and others have studied the heat treatment technology of DD8. The high temperature solid solution heat treatment of DD8 alloy was conducted at 1220-1260 ℃. The best heat treatment technology of DD8 single crystal alloy is 1100 ℃ × 8h air cooling +1240 ℃ × 4h air cooling +1090 ℃ × Air cooling for 2h +850 ℃ × 24 hours air cooling. The results show that the microstructure of DD8 alloy is better than that of DD8 alloy, and the homogenization effect is remarkable, the dendrite segregation is improved obviously, and the lasting strength of DD8 alloy is improved.

Heat treatment technology of PM superalloy

FGH95 alloy is γ′ The phase precipitation enhanced powder nickel base superalloy [28] has the advantages of fine grain, uniform structure and no macro segregation, and is the best material for manufacturing a new type of engine turbine disk with large push weight ratio[29]. In nickel base alloy γ and γ′ The two phases have different lattice constants, which makes the lattice mismatch in the phase interface, which affects the creep resistance and life of the alloy.
Xiejun[30] studied the effect of the structure of FGH95 alloy on the durability. The FGH95 alloy prepared by different temperature isostatic pressure was treated as follows: 1155 ℃ × 1H (solution treatment) +520 ℃ × 15min (salt bath cooling) +870 ℃ × 1H (primary aging) +650 ℃ × 24 hours (secondary aging). After heat treatment, granular carbide is distributed in discontinuous and fine grains along grain boundary and in crystal γ′ The phase dispersed and precipitated in the matrix. The high lattice mismatch of the hip alloy at 1120 ℃ after the heat treatment has a long lasting life. The effect of heat treatment on the structure and properties of FGH95 alloy was also studied by hubenfu[31].
The higher the hip temperature is, the aging precipitation is γ′ The larger the phase size, the different heat treatment systems can be changed γ′ Distribution of; The cooling of salt bath increases obviously to medium size γ′ The number of phases significantly improved the high temperature plasticity of the alloy. FGH97 is a kind of alloy which is similar to EP741NP in China. Zhang Ying and others used different heat treatment technology to study the structure and properties of fgh97.
System I is 1200 ℃ × 8h furnace cold to 1170 ℃ air cooling +870 ℃ × Air cooling for 32H; System II is 1200 ℃ × 4h air cooling + three-stage aging (910-700) ℃ × Air cooling for 32H. The temperature of quenching and aging, holding time and cooling mode of solution directly affect the alloy γ′ The morphology, size, quantity and distribution of precipitates such as strengthening phase and different types of carbide. In the samples after heat treatment in two systems γ′ The different matching degree of phase and carbide determines that they have good comprehensive mechanical properties.
FGH98I is a new type of third generation nickel based powder superalloy. Because the trial production of fgh98i alloy has just begun, the research on heat treatment technology is not enough. Wukai[34] studied the effect of solid solution heat treatment on the structure and properties of fgh98i alloy[35]. The sub / supersolution temperature is 1130 ℃ /1190 ℃, the holding time is 1H, and the oil is quenched to 815 ℃ for 8 hours, and then air cooling is used for aging treatment.
No TCP phase was found in fgh98i alloy after sub / supersolution treatment. The grain of the solution after heat treatment is slightly grown, and there are primary, secondary and tertiary sizes γ′ Phase; The grain size of the superalloy is obviously grown and there is a secondary one γ′ Phase; The former makes the strength higher because of the smaller grain, and the latter decreases the secondary γ′ Phase size and elimination of first time γ′ The high temperature plasticity and durability of the alloy are improved by phase and residual dendrite. The effect of pretreatment and post treatment on the microstructure and microhardness of fgh98i alloy was also studied. FGH98I was treated with supersolution and supersolution heat treatment respectively. The pretreatment before the solution treatment made the large grain boundary of forged alloy γ′ Phase is partially dissolved, and the grain is slightly grown, and it is cooled by heat treatment of supersolution γ′ The effect of phase precipitation is not significant; The fgh98i was treated by supersolution and supersolution and sub-solid solution respectively. The solution after the solution treatment cooled down γ′ Phase coarsening and square, grain boundary γ′ The dense area of phase precipitation disappears and the hardness decreases. With the increase of cooling speed, the hardness of alloy is higher and the hardness increases after aging.
FGH4095 alloy is a precipitation strengthened nickel base superalloy, which is mainly used in the manufacture of turbine disk of aeroengine[36]. In order to obtain the microstructure of grain refinement and no macrosegregation [37], the corresponding heat treatment is needed. Xu Yi[38] studied the intermediate heat treatment 1060 ℃ × The effect of furnace cooling on the microstructure and properties of fGH4095. Intermediate heat treatment can be improved γ′ Shape and distribution of phase; The tensile properties of the materials were improved obviously after the intermediate treatment, the yield strength was increased from 1150mpa to 1210mpa, and the tensile strength was increased from 1230mpa to 1460mpa; Medium size γ′ The grain boundary is optimized and the high temperature plasticity of the alloy is improved.

Development trend of heat treatment process of Superalloy

The superalloy is heat treated under strictly controlled heating and cooling conditions, and the required service performance or service life can be achieved by changing the microstructure inside the material. With the application of new superalloy and the high requirements for alloy properties in the process of use, heat treatment process is an essential process. In recent years, the heat treatment technology of superalloy has been developing continuously, and its development trend is as follows:

  • (1) The heat treatment system, together with the alloy composition design and other processes, can make the superalloy reach the best performance state, so as to achieve the best performance matching.
  • (2) The deformation and heat treatment of the alloy are studied by combining computer simulation with heat treatment technology. The research on the control system of heat treatment process, while emphasizing the equipment update, realizing the innovation of the process, and the concurrent development of equipment and technology.
  • (3) The relationship between the phase transition and the mechanism of phase transformation between the precipitates in the alloy during service and heat treatment are not very clear, so it needs to be studied in depth.
  • (4) The research on the suitable quenching medium and the improvement of quenching technology will be the key point of the heat treatment in the future.
  • (5) Vacuum heat treatment technology has the advantages of no oxidation, no decarbonization and small distortion, and it will be widely used in aerospace industry.
  • (6) Plasma surface treatment has the characteristics of wear-resistant, small distortion, good appearance and no blind area, and will be widely used in high temperature alloy, including ion nitriding, ion nitrogen carbon co-osmosis, ion carburizing and other technologies.

Authors: Cui Lingjiang, Lin Xiyuan, Zhu Qiang, Wang Chuanjie, Zhang Peng

Source: China Pipe Fitting Manufacturer – Yaang Pipe Industry Co., Limited (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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