Nickel-based Alloy Bars

Ways to Improve the Purity and Homogenization of Nickel-base Alloy Bars (HY-industry technical centre)

Abstract:

This article introduces the Nickel-base alloys supplied by Shanghai HY Industry Co., Ltd for domestic non-aerospace applications and the difficulties in the preparation process, and discusses the main ways to improve the purity and homogenization of Nickel-base alloy billets: Optimizing pure charge And return materials; optimizing the vacuum melting process; limiting the amount of oxygen and deoxidizer used for VOD; preventing the pollution of refractory materials in the pouring system; ESR is protected by an inert atmosphere. Optimizing VAR and ESR ingot types and melting speeds to reduce the degree of segregation of ingots; determine the heating and forging temperature range according to the second dissolution analysis law; for ingots, fast forging machines are preferred for multiple upsetting to increase the forging ratio; diameter forging machines are selected The rapid strain to achieve dynamic recrystallization of the material.

Introduction

In the past decade or so, the demand for Nickel-base alloy materials for high-end equipment manufacturing outside of China’s aerospace industry has increased rapidly with the increase in industrialization. At the same time, high-end equipment manufacturing has higher and higher requirements for the quality of Nickel-base alloy materials. High purity, homogenization, and long service life have become the eternal theme of the development of Nickel-base alloy materials. Moreover, in the fields of nuclear energy, electric power, and petrochemicals, the demand for Nickel-base alloy materials to make super-sized parts has continued to increase, which objectively makes it difficult to improve the purity and homogenization of Nickel-base alloy materials. For this reason, Shanghai HY Industry Co., Ltd has carried out the technical transformation of equipment such as VOD furnace, vacuum induction furnace (VIM), vacuum electric arc furnace (VAR), inert atmosphere electroslag furnace (ESR), fast forging machine, and radial forging machine. Process development to meet the market demand for high-quality Nickel-base alloy materials. As a result, Shanghai HY Industry Co., Ltd’s Nickel-base alloy materials have entered China’s nuclear power equipment, flue gas turbines, industrial gas turbines, oil and gas exploration, refining and chemical engineering, transportation, environmental protection, 700℃ ultra-supercritical power generation project selection and application. Considering that the quality of forged Nickel-base alloy bar stock determines the structure and performance of downstream products such as discs, blades, ring forgings, rotors, valves, and seamless pipes, this paper studies ways to improve the purity and homogenization of Nickel-base alloy materials.

Nickel-base alloy materials for non-aerospace applications

Nickel-base alloys are based on nickel (or adding part of iron and cobalt) as the austenitic matrix, selectively adding chromium, tungsten, molybdenum, and niobium elements to solid solution strengthening, and selectively adding aluminum, titanium, and niobium to form the second Phase precipitation strengthening, a type of special metal material that selectively adds trace elements such as boron, cerium, lanthanum, magnesium, and zirconium to purify grain boundaries. Nickel-base alloys are divided into deformed alloys and cast alloys according to the forming method; according to the strengthening methods, they are divided into solid solution strengthened alloys and age strengthened alloys. In the high-end equipment industry other than aerospace, the use environment of Nickel-base alloys is usually divided into two categories. One type of use is service under high temperature oxidation conditions above 600 ℃, and the other type of use is use in corrosive media below 600 ℃.

The components in service under high temperature oxidation conditions include industrial gas turbine turbine discs and blades, steam turbine rotors, blades and bolts, flue gas turbine turbine discs and blades, and gas valves for diesel locomotives and diesel engines. In addition to oxidation resistance, Nickel-base alloy materials for this purpose also require high room temperature or high temperature tensile strength and plasticity, long-term durability or small creep elongation, and good fatigue resistance. . Based on the determination of chemical composition, the high-temperature mechanical properties of Nickel-base alloys depend on the purity of the material and the uniformity of the microstructure.

The parts used in corrosive media include steam generator heat transfer pipes, oil well pipes, oil well drilling and testing protection pipes, submersible pump shafts, oil refining and chemical equipment, etc. In addition to certain strength requirements for Nickel-base alloys for this type of use, special attention is paid to the corrosion resistance of the material. Based on the determination of chemical composition, inclusions and segregation phases (such as the σ phase in Nickel-base alloys) are the main culprits that damage the service life of Nickel-base alloys in corrosive media.

Shanghai HY Industry Co., Ltd has a history of producing Nickel-base alloys for more than 20 years. It has successively produced more than 150 Nickel-base alloys, and has accumulated rich Nickel-base alloy alloying, melting, forging, rolling, heat treatment, and testing technologies. Engineering experience. For Nickel-base alloys used in different fields, the main products supplied by Shanghai HY Industry Co., Ltd are forged bar stocks. The production process design of Nickel-base alloy bar billets is more complicated. First, select the smelting process route according to its chemical composition characteristics and performance requirements. Then select thermal processing equipment according to the shape and size of the ordered product.

In the development process of Nickel-base alloy bar stock and its preparation process, Shanghai HY Industry Co., Ltd uses its engineering experience to promote technological innovation, continuously improve the purity and homogenization level of materials, and solve the problem of various Nickel-base alloys. Process performance, mechanical performance or corrosion resistance issues.

Case I:

The nuclear power steam generator heat transfer tube uses Inconel 690 alloy. The base material smelted in a vacuum induction furnace is initially remelted in an ordinary electroslag furnace. Its hydrogen, oxygen, nitrogen and other gas content and inclusions cannot meet the technical specifications. In addition, the Inconel 690 alloy bar produced by the fast forging machine + diameter forging machine has a band-like grain structure. Ultrasonic detection noise is serious, and the bottom wave loss exceeds the standard. Switch to inert gas-protected electroslag furnace to remelt the ingots, use pre-melted slag, the gas content is qualified, and the B and D inclusions are within 1.0 level.

The fast forging machine uses upsetting + drawing to open the billet, and the diameter forging machine uses high-frequency forging to obtain a uniform grain structure, which is qualified by ultrasonic inspection. It has provided Inconel 690 alloy bar stock for steam generator heat transfer tubes of 4 units for K2 and K3 units of Karachi Nuclear Power Plant. These technologies have been promoted to produce Inconel 690 alloy rods for nuclear reactor control rod drive mechanisms, and Inconel 690 alloy rods for blasting valve shear caps of the AP1400 project. The 350 ℃ tensile strength of Inconel 690 alloy bar billet body reaches 570Mpa-580Mpa, and the yield strength is 240Mpa-265Mpa, which meets the requirements of corresponding specifications.

Case II:

HASTELLOY G3 and Alloy 28 alloy materials are used for oil well pipes for the exploitation of high-acid oil and gas fields in China. Initially, non-vacuum induction furnace was used to smelt HASTELLOY G3 alloy base material, electroslag furnace was used to remelt the ingot, and after the rapid forging machine opened the billet, the extruder produced the waste tube. Although the inclusions of type B and type D reach 1.5, but the extruded waste pipe has many intermittent delamination defects. Later, it switched to electric arc furnace or non-vacuum induction furnace + LF+VOD process to smelt the base material, especially when VOD furnace blowing oxygen to remove carbon, analyze the exhaust gas of VOD furnace to prevent overblowing of oxygen and avoid adding too much deoxidizer to form a large number of inclusions Things. The electroslag furnace remelting uses the self-designed argon protection device, and the inclusions of type B and D are not more than 1.0 level. The vacuum induction furnace + electroslag furnace process is used to smelt HASTELLOY G3 alloy, and the B and D inclusions are also controlled within the 1.0 level.

These process technologies have also been promoted and applied to the production of Alloy 28 alloy. However, after the purity of Alloy 28 is improved, the strength of cold-rolled oil well pipe cannot meet the requirements of 125Ksi steel grade, and microalloying technology is needed to improve the strength. A large number of sigma phases appeared in the Alloy 28 alloy bar stock, and later the sigma phase was eliminated by the diffusion annealing process. At present, oil well pipes made of HASTELLOY G3 and Alloy 28 alloy materials have been widely used in China’s Puguang, Yuanba, Moxi, Tahe and other oil and gas fields.

Case III:

In the process of oil drilling, geological information needs to be measured. Inconel 718 alloy is used for the protective tube of the measuring device. Inconel 718 alloy contains γ’, γ” and δ phases in addition to the austenite matrix. In addition, Inconel 718 alloy has a high niobium content, which is easy to form NbC and NbN. Inconel 718 alloy melted by VIM+ESR was originally used. The ingot was passed through Diffusion annealing, fast forging machine + diameter forging machine hot processing, solution + aging treatment of bar materials, slow strain rate tensile (SSRT) test in corrosive medium. Due to residual δ phase in grain boundaries and inclusions in the material The stretching time, elongation and reduction of area cannot meet the technical specifications. By reducing the nitrogen content, reducing the number of inclusions, reducing the size of the inclusions, and curbing the precipitation of grain boundary delta phase, Inconel 718 alloy material passed the SSRT test.

Case IV:

REFRACTALOY 26 alloy and Nimonic 80A (NiCr20TiAl) alloy are used as the material of steam turbine high-pressure rotor blades. Both are smelted by the VIM+ESR process, and square flat profiles are produced after forging hammers. REFRACTALOY 26 alloy profile measures the endurance life of 566℃/740Mpa stress and 650℃/540Mpa stress. The problem encountered at first is that the zirconium content does not meet the range of 0.01%-0.10%, the inclusions are 1.5 grades, and the individual grains reach 0 grades, and the difference in coarse and fine grains is large. The long-lasting life is less than 100 hours. Try to cancel the 815℃ aging treatment, which can improve the durability to reach the standard, but the room temperature hardness of the blade will exceed the upper limit of the specification.

By improving the microalloying technology of zirconium, reducing the number of inclusions and reducing their size, improving the uniformity of crystal grains, in accordance with the standard three-stage heat treatment, the endurance life meets the technical specifications. Nimonic 80A (NiCr20TiAl) alloy flat bar also has a structure with a coarse and fine grain size difference of 6 or more, resulting in a 750°C/310Mpa endurance life less than 100 hours. By improving the grain uniformity, the durability of Nimonic 80A alloy flat material meets the requirements of technical specifications. Inconel 783 alloy bars used for high-strength bolts of steam turbines are smelted by the VIM+VAR process, ingot diffusion annealing, and then produced by fast forging machine + diameter forging machine. Initially, the β-phase distribution in the microstructure of the bar is not uniform, and the β-phase presents a large block or long strip morphology. Although the endurance life of Inconel 783 alloy at 650°C/586Mpa stress exceeds 23 hours, it does not reach the 100-hour life expectancy of users. According to the dissolution law of β phase in Inconel 783 alloy, the improved forging process eliminated the massive and long β phases, and the durability life exceeded 100 hours.

In Shanghai HY Industry Co., Ltd, microalloying technology, VOD furnace refining technology, VIM+VAR vacuum melting technology, protective atmosphere ESR melting technology, VIM+ESR+VAR triple melting technology, homogenization treatment technology, fast forging machine upsetting Drawing technology, high-frequency radial forging technology, and two-phase zone forging have all been widely used in the production of Nickel-base alloys. The GH750 alloy Type B and Type D inclusions developed for the 700℃ ultra-supercritical coal-fired generating set are controlled within the 1.0 level, and satisfactory long-term durability performance is obtained.

In addition, C700R1 alloy adopts VIM+ESR+VAR triple process to melt Φ820mm ingot, which is used to manufacture the high pressure rotor of steam turbine of 700℃ ultra-supercritical unit. The GH706 alloy used for the turbine disk of the F-class gas turbine adopts the triple process of VIM+ESR+VAR to melt Φ920mm ingots. The fast forging machine produces Φ750mm billets, which are finally forged into Φ2000mm disc forgings. Incoloy 825 alloy used in refining and chemical industry uses IM+LF+VOD+ESR to melt Φ1235mm ingots, and fast forging machine produces Φ1300mm bars. Gradually accumulated experience in smelting and forging processes for super large ingots and bar stocks.

Improve purity

The purity of Nickel-base alloys not only refers to information such as the number, type, morphology, size and distribution of non-metallic inclusions, but also contains information such as the content of material impurity elements, residual elements and harmful elements. Nickel-base alloys choose different smelting equipment and process combinations, and the sources of inclusions, impurity elements, residual elements, and harmful elements are different. The number, type, morphology, size and distribution of inclusions in Nickel-base alloys need to be controlled, and gas and impurity elements are the root causes of inclusions. Residual elements refer to metal elements that are not included in the material composition range. After the residual elements reach a certain content, it is difficult to determine their role in a specific material, so it also needs to be limited. Harmful elements will not only cause the deterioration of the material’s thermal processing plasticity, but also promote the substantial reduction of the medium temperature plasticity, and damage the material’s creep, durability, fatigue and other properties. Therefore, Nickel-base alloys must limit harmful elements.

To improve the purity of Nickel-base alloys, we must first start with raw materials and refractory materials. Nickel-base alloys contain a variety of elements, and the raw materials are composed of metal materials, iron alloys and return materials of the corresponding elements. The returned materials include soup channels, injection residues, metal electrode tails produced in the smelting process, cutting heads and leftovers produced by forging and rolling products, and chips produced by turning processing. The same element can also have multiple choices of metal materials, iron alloys, and master alloys. The selection of raw materials and return materials should consider the selected smelting process. In terms of raw material purity, pure metal materials are better than ferroalloy materials. The metal materials produced by different methods also have differences in purity. The nickel beads produced by the hydroxyl method are significantly better than the electrolytic nickel; the electrolytic chromium is better than the metal chromium produced by the aluminum-silicon reduction; and the niobium-nickel alloy or the niobium-iron alloy is better than the metal niobium.

Secondly, we must choose the appropriate smelting process. In the primary smelting method, the gas content of VIM is significantly lower than that of the IM+LF+VOD process. VIM can reduce the content of harmful elements to a certain extent; but the desulfurization ability of VIM is worse than that of the IM+LF+VOD process. High purity Nickel-base alloys should be smelted by the VIM process. Considering that inclusions such as Al2O3 and TiN cannot be decomposed under the conditions of VIM smelting, the metal materials or master alloys selected for VIM also require high purity. The return material used by VIM cannot include soup, surplus, and ingot cutting heads; the machined scraps need to be sorted and cleaned before use. The construction and baking process of the VIM crucible needs special control to avoid the refractory material on the crucible wall from peeling off. The melting temperature and stirring should also avoid the reaction between the molten pool and the crucible. VIM has two pouring methods: chute and hopper. No matter the chute or the funnel, it is necessary to choose anti-scouring refractory materials. The slag retaining structure of the chute or the funnel must promote the alloy fluid to have a smaller dead zone ratio and a longer residence time in it.

The IM+LF+VOD process cannot be used to smelt Nickel-base alloys with high aluminum and titanium content. Aluminum and titanium have a relatively large affinity with oxygen, and are easy to oxidize and burn, and it is difficult to accurately control the content of aluminum and titanium. The cleanliness of the raw materials used in the IM+LF+VOD process can be lower than that of the VIM materials, but raw materials with harmful elements that exceed the corresponding material specifications cannot be used. No matter what kind of smelting process, raw materials with a residual element content of more than 0.03% cannot be used. VOD needs to control the amount of oxygen used to ensure that the carbon content is reduced to the internal control level and no longer overblowing, reduce the amount of aluminum used for deoxidation, and prevent excessive deoxidation products from polluting the metal bath. The gating system needs to choose anti-scouring refractory materials. In the pouring process, an argon gas protection device should be used to prevent the alloy fluid from inhaling. Nickel-base alloys should avoid using electric arc furnaces to melt the charge to avoid excessive burning of precious elements; Nickel-base alloys should also avoid using non-vacuum induction furnace (IM) smelting without ladle refining and directly pouring metal electrodes in Nickel-base alloys. Higher levels of chromium will adsorb nitrogen during the melting period. All in all, Nickel-base alloys require primary smelting to obtain high-purity base materials.

Furthermore, the ingot type should be selected according to the shape and size of the product, and the remelting process should be selected in combination with the characteristics of the chemical composition of the material and the technical requirements. Considering the strict limitation of the gas content and the uniformity of the overall aluminum and titanium content of the ingot, the vacuum consumable electric arc furnace (VAR) should be preferred for smelting Nickel-base alloys. Vacuum consumable electric arc furnace can further remove harmful elements. In addition, there is no slag barrier between the ingot and the mold during the smelting process of the vacuum consumable electric arc furnace. Filling with helium gas can effectively improve the cooling conditions. Therefore, the same material is smelted in the vacuum consumable electric arc furnace, and the ingot type is comparable to electricity. The slag furnace (ESR) is larger, and will not produce macro-segregation such as freckles. Of course, the vacuum consumable electric arc furnace also has shortcomings. If the shrinkage of the metal electrode is serious and the purity of the metal electrode is poor, it will be affected by the shrinkage of the electrode, the crown of the ingot on the inner wall of the mold, and the edge of the molten pool. Affected, it is easy to produce white spots defects. For materials that have been in service for a long time at high temperatures, the VIM+ESR+VAR triple smelting process is used to eliminate shrinkage holes in the metal electrode. Even the triple smelting process can only reduce the risk of dirty white spots by 50%, so it is particularly important to improve the purity of the metal electrode (base material).

The electroslag furnace for smelting Nickel-base alloys adopts constant melting rate control and has an inert atmosphere protection function. Choose pre-melted slag to avoid variable valence oxides or gases carried by the slag.

Improve homogenization

The homogenization of Nickel-base alloy materials includes the uniformity of the chemical composition of the material, the uniformity of the grain size and the uniformity of the second phase distribution, so that the mechanical properties of all parts and directions of the material are the same. The composition, grain structure, second phase distribution and uniformity of properties of Nickel-base alloy materials are affected by many factors during smelting and hot working.

In essence, easy segregation is a characteristic of Nickel-base alloys. Nickel-base alloys contain a variety of elements, and the total amount of alloying elements is relatively high. Regardless of the remelting process of VAR or ESR, selective crystallization is inevitable during the solidification of the ingot, and there are significant differences in the element content between dendrites and dendrites. . But this is only the component segregation in the onlooker area, and the micro segregation can be eliminated by the homogenization treatment of the ingot (ie diffusion annealing). However, the black spots, white spots, and annual ring-like segregation on the bar at low power are macro-segregations, which are related to the melting rate of the VAR or ESR process and the cooling conditions of the solidification of the metal bath. The rapid melting rate of the metal electrode, poor cooling conditions of the crystallizer, deep metal molten pool, and widening of the mushy zone at the bottom of the molten pool will cause macro-segregation. Macro segregation cannot be eliminated by the homogenization treatment of the ingot. Only choose the appropriate ingot type, choose a reasonable melting rate, and improve the cooling conditions to avoid risks.

There are many reasons for the uneven grains of Nickel-base alloy materials. One reason is the uneven distribution of carbides caused by the smelting process, which is also the consequence of selective crystallization during the solidification of Nickel-base alloys. The pinning effect of carbides on the grain boundary promotes a significant difference in the grain size of the densely distributed areas of carbides from other areas. This uneven carbide distribution can be eliminated by diffusion annealing. However, the improper diffusion annealing process and the excessive dissolution of carbides into the matrix will also bring catastrophic damage to the subsequent thermal processing. Another reason is that complete dynamic recrystallization does not occur during thermal processing. The material is not completely dynamically recrystallized during the forging or rolling process. Its microstructure may be abnormally coarse grains, or it may be abnormally coarse grains surrounded by fine grains. The large grains appear along the direction of metal flow. A prolate morphology. Unfinished dynamic recrystallization grains cannot be refined 100% by subsequent heat treatment, and the temperature of refining unfinished dynamic recrystallization grains is much higher than the given solid solution temperature of Nickel-base alloys. And the refined grain size will exceed the technical specification requirements. The dynamic recrystallization of Nickel-base alloys during hot working requires two conditions, a sufficiently high temperature and a sufficiently high strain rate. In terms of Nickel-base alloy recrystallization conditions, the diameter forging machine is better than the fast forging machine, and the fast forging machine is better than the electro-hydraulic hammer.

No matter what kind of Nickel-base alloy there is a second phase. The second phase of Nickel-base alloys includes MC, M6C, M7C3, M23C6 and other types of carbides, including Laves phase, σ phase, δ phase, β phase, γ’and other intermetallic compounds. The precipitation and distribution of these second phases in Nickel-base alloys are significantly different. The second phase in Nickel-base alloys has its own law of dissolution and precipitation. In order to obtain the ideal number, morphology and distribution area of the second phase, the heating temperature and deformation temperature range of the ingot must be determined according to the law of the second phase dissolution analysis and its function in the material and the material application.

The smelting ingot of Nickel-base alloys is relatively small, and the forging of unidirectional drawing deformation is relatively small, which easily leads to large differences in performance in both vertical and horizontal directions. In addition, due to uneven structure, ultrasonic detection noise is serious. This requires multiple upsetting + elongation deformation methods to eliminate anisotropy. The multiple upsetting deformation of Nickel-base alloy ingots can usually only be realized on fast forging machines. The free forging hydraulic press is only suitable for the roughness of the pier, and cannot meet the dynamic recrystallization conditions of the Nickel-base alloy ingot elongation.

Conclusion

1) The main ways to improve the purity of Nickel-base alloys: optimizing pure charge and return materials; optimizing vacuum melting process; limiting the amount of oxygen and deoxidizer used in the ladle refining process to prevent contamination of refractory materials in the pouring system; ESR uses inert atmosphere protection.
2) The main way to improve the homogenization level of Nickel-base alloys: optimize the VAR and ESR ingot type and melting rate, reduce the degree of segregation of the ingot; determine the heating and hot working deformation temperature range according to the second phase dissolution analysis law. For ingot casting, fast forging machine is the first choice for multiple upsetting to increase forging ratio; rapid strain of radial forging machine is selected to realize dynamic recrystallization of material.

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