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1)Baowu Special Metallurgy Co.,Ltd.,Shanghai 200940,China;2)High Temperature Material Research Division,Central Iron and Steel Research Institute,Beijing 100081,China

Abstract: Superalloy is developed to meet the stringent requirements of materials used in aeroengine hot-end parts.It has become an irreplaceable key material for military and civil engines.With the increasing contents of Al and Ti,the hot-working properties of superalloy materials are significantly reduced.The metallurgical technology and product quality of GH4720Li alloy produced by triple-melted process were introduced.The results showed that the PESR ingot structure was shrinkage-free,compact,and had high-degree purity,and the VAR process was obviously stable.The sulfur content of GH4720Li alloy was only several 10-6 by the triple-melted process,which was propitious for forging the alloy.The uniform heat treatment technology was also studied,and the results showed that the hot-working plasticity of GH4720Li alloy ingot was significantly improved after uniform heat treatment.The properties of GH4720Li alloy by the triple-melted process were good.

Key words: GH4720Li alloy; structure; tensile properties; stress-rupture properties

1 Introduction

GH4720Li alloy is a Ni-Cr-Co-based precipita-tion-hardening superalloy with the service temper-ature below 750 ℃.It has high-temperature strength,fatigue resistance,creep resistance,and long-term microstructural stability[1].This alloy is one of the most practical alloys with the highest level of com-prehensive mechanical properties among the wrought superalloys,which is suitable for manufacturing turbine disks and turbine blades of aeroengines.

The alloying element content of GH4720Li is extremely high,especially the sum of precipitated strengthening elements,such as Al and Ti,is up to 7.5%.The volume fraction of the precipitated strength-ening phase γ′ exceeds 40%.Because of the poor thermoplasticity of alloy ingots,it is easy to produce cracks during deformation,which resembles the most typical characteristic of refractory superalloy.Generally,this alloy is manufactured by triple smelt-ing,which consists of vacuum induction melting (VIM),protection atmospheric electric slag-remelt-ing (PESR),and vacuum arc remelting (VAR).

The superalloy in aeroengines is usually manu-factured by VIM+electroslag remelting(ESR) or VIM+VAR.When using VIM+PESR,a proper selec-tion of slag compositions[2-3]can effectively remove S and improve alloy purity.VIM+VAR can reduce the gas content in the alloy and avoid the adverse effect of slag skin during heat dissipation.A shallow melting tank[4-5]can be formed using helium-cooling gas.VIM+VAR gives good metallurgical quality but cannot effectively remove sulfur.

LIU Y M,CHEN G S,and WANG Q Z et al.studied the effect of different melting rates on the microporosity ofφ508-mm GH4169 alloy ingot[6].WANG Q Z,CHEN G S,and SUN W R studied the as-cast microstructure and homogenization treatment ofφ508-mm GH4169G alloy ingot[7].CHEN G S et al.studied the metallurgical quality of GH4169 alloy ingot by VIM+PESR+VAR triple process.Com-pared with general ESR and VAR processes,the VIM+PESR+VAR process has better removal of S and O[2].There were no metallurgical defects,such as freckles and white spots,but the ingot shape was <457 mm.

Thus,the triple smelting process consisting of VIM+PESR+VAR combines the advantages of PESR and VAR.The GH4720Li alloy ingots pro-duced by Baowu Special Metallurgy Co.,Ltd.using VIM+PESR+VAR have no macroscopic segre-gation.Homogenization diffusion annealing is used to reduce microscopic segregation and improve alloy workability.This article focuses on the aspects of metallurgy and mechanical properties of GH4720Li alloy smelted by VIM+PESR+VAR,and testing the microstructural and mechanical prop-erties of theφ150-mm trial production rod.

2 Experimental procedures

VIM of GH4720Li alloy was conducted in a 6.0-t furnace of Baowu Special Metallurgy Co.,Ltd..PESR was conducted in a 5.0-t Ar-protected electroslag furnace.VAR was conducted in an 8.0-t furnace with functions of droplet control,He gas auxiliary cooling,and computer automatic control.The ingot was 508 mm in diameter.

A 300-mm length test material was cut from the head of the consumable ingot to analyze the as-cast microstructure and study the homogenization process.

The GH4720Li alloy ingot smelted by VIM+PESR+VAR was cogged in a 2 000-t fast-forging machine and then into aφ150-mm forging bar.Specimens for microstructural and mechanical properties investigations were cut from the forging bar.

Solution heat treatment of the forging bar occurred at 1 110 ℃±5 ℃ for 4 h and oil-cooled.

Aging of the forging bar occurred at 650 ℃±5 ℃ for 24 h,760 ℃±5 ℃ for 16 h and air-cooled.

3 Results and discussion

3.1 Effect of the triple smelting process

Table 1 presents the chemical composition of GH4720Li alloy smelted by VIM+PESR+VAR.

Table 1 Chemical composition of GH4720Li alloy %

3.2 Cast structure analysis

A 25 mm×508 mm×300 mm test material was cut from the head of the consumable ingot,polished,and etched for macrostructural observation (Fig.1).It can be seen from Fig.1 that the macro-structure of the ingot head is dense without macrosegregation,and it exhibits a typical columnar crystal structure.

The microsegregation in the ingot and the eutectic precipitation between dendrites adversely affect the thermoplasticity and microstructural uniformity of the bar and forging.A proper homogenization diffusion annealing should be conducted to elimi-nate the adverse effects.The specimens were cut at 1/2Rfrom the macrostructure shown in Fig.1 and analyzed.Fig.2 shows a typical microstructure of GH4720Li alloy ingot before and after homogeniza-tion diffusion annealing.

Fig.1 Macrostructure of the head of φ508-mm GH4720Li alloy ingot smelted by VIM+PESR+VAR

Fig.2 Typical microstructures of GH4720Li alloy ingots at different stages

3.3 Macrostrucre of the forging bar

After homogenization,the ingot was cogged in the fast-forging machine and then into aφ150-mm forging bar.Fig.3 shows the cross-sectional macro-structure of the forging bar.Specimens that were heat treated and aged were cut at the center,1/2R,and the edge for longitudinal microstructural obser-vations.

Fig.3 Cross-sectional macrostructure of the forging bar

3.4 Microstructure of the forging bar

The primary microstructure of GH4720Li alloy after standard heat treatment consists of γ,γ′,MC,M23C6,and M3B2[1].The longitudinal microstructures of theφ150-mm GH4720Li-forging bar at the center,1/2R,and near the edge after solution heat treatment and aging are shown in Figs.4 and 5,respectively.Both figures reveal that the microstructures of the forging bar are composed of fully recrystallized grains.

3.5 Tensile properties

The tensile properties of the triple-smelted GH4720Li-forging bar at room temperature and at 650 ℃ are shown in Table 2.The test results show that the tensile strength,yield strength,elongation,and area reduction under two testing conditions meet the standard requirements.

3.6 Stress rupture properties

The stress rupture properties of the triple-smelted GH4720Li-forging bar under 680 ℃/830 MPa and 730 ℃/530 MPa are shown in Table 3.The test results show that rupture time and elongation under two testing conditions meet the standard require-ments.

Fig.4 Microstructures of GH4720Li-forging bar after solution heat treatment

Fig.5 Microstructures of GH4720Li-forging bar after solution heat treatment and aging

Table 2 Tensile properties of GH4720Li-forging bar

Table 3 High-temperature stress rupture properties of GH4720Li-forging bar

4 Conclusions

(1) GH4720Li alloy smelted by VIM+PESR+VAR is stable,effectively reduces the sulfur con-tent,and improves the ingot’s thermoplasticity.

(2) The transverse macrostructure of the trial production rod is relatively uniform.The micro-structure is composed of fully recrystallized grains.

(3) The mechanical properties of the GH4720Li alloy rod meet the requirements of the trial production.