Preparation of high temperature alloy solder powder by argon gas atomization

(HY-industry technical centre)

Abstract:

This paper discusses the principle of preparing high-temperature alloy powder by liquid atomization full-protection gas atomization method, and the trial production results, showing that argon gas protection, gas atomization process, vacuum ingot non-vacuum melting, atomization nozzle improvement, and improvement of molten steel Temperature and other measures to obtain nickel-based superalloy welding powder, can obtain accurate chemical composition and low oxygen content, which is equivalent to the result of high-temperature alloy welding powder usually obtained by vacuum atomization, and can be mass-produced.

Preface:

With the rapid development of the petrochemical industry, there are more and more equipments operating under high pressure, high temperature and corrosive conditions, and there is a trend toward large-scale and high-parameter development. Stainless steel and nickel-based materials (Ni-Mo, Ni- Cr-Mo) equipment is the material of choice. If all of the above materials are used, the cost is too high. In order to reduce production costs and save expensive materials, as long as a layer of nickel-base superalloy or stainless steel is deposited on the surface or inner wall of the equipment, the corrosion resistance of the equipment can be increased. In particular, nickel-based superalloys are important corrosion-resistant materials. They are used in various corrosive environments, even chemical and electrochemical corrosion, compared to ordinary stainless steels, other corrosion-resistant metals, and non-metallic materials. It has the ability to withstand various forms of corrosion and damage, and has good mechanical properties and machinability. Its comprehensive corrosion resistance is stronger than ordinary stainless steel and other corrosion-resistant metal materials, especially suitable for the contemporary industry with harsh media environment.

The mature gas atomization milling process and the argon gas protection steelmaking method, that is, the raw materials are made into a vacuum ingot in a vacuum furnace, and then remelted in a non-vacuum furnace, and the molten steel and gas are poured under the protection of liquid argon. Atomization, argon gas protects the molten steel in the whole process, and the same quality product is obtained by the atomization method of the vacuum furnace.

1. Argon gas protection gas atomization method to prepare high temperature alloy welding powder

1.1 Principle of gas atomization milling:

After the chemical composition of the alloy grade is compounded, the metal material is heated and melted by the intermediate frequency furnace, and the molten metal is passed through the atomizing nozzle at a certain casting speed in the sealed atomization chamber (the atomization chamber is filled with pure nitrogen gas, and the oxygen content is less than 0.02). Below %) impacts the alloy flow column by high-speed nitrogen gas to form fine alloy droplets. The alloy droplets fly freely in the atomization chamber, and the molten metal or alloy liquid is pulverized by a special nozzle with a supersonic gas jet. The process of metal or alloy powder, the flying alloy droplets are solidified into fine powder at 0.5-1.0 seconds.

This method can produce various metal and alloy powders with a melting point lower than 1700 ° C. The obtained powder is spherical, the oxidation degree of the powder surface is also lower than that of water atomization (only about 1/10), and the powder particle size distribution is wide (median position) The diameter is 10-100 μm). Commonly used atomizing media are nitrogen, argon or air.

Gas atomization to produce metal or alloy powders has the following characteristics:

  • Various alloying elements can be added during the smelting process to produce a wide range of alloy powders with various chemical compositions.

  • The obtained powder has the same chemical composition and uniformity, no segregation, less non-metallic inclusions, and high purity.

  • The powder particles are spherical or spheroidal, and the particle size and particle size distribution can be adjusted by changing the atomization process and adjusting the process parameters.

  • The oxide of the powder is mainly concentrated on the surface of the particle, and the degree of oxidation can be adjusted by adjusting the atomization process parameters, and a high-quality alloy powder having an extremely low oxygen content can be obtained.

  • The powder has high repeatability and is suitable for mass production of powders with consistent properties.

1.2 argon full protection

Argon is an inert gas that does not react with molten steel to isolate the air and ensure the stability of the molten steel. It ensures that the alloy is not contaminated by oxygen or nitrogen during melting, casting, etc., and the oxygen content is low. Powder. The argon protection system consists of the following parts:

1.2.1 The top of the furnace is blowing argon

The vacuum ingot is heated and melted in a non-vacuum state, and after being reddish, it is oxidized by air to form oxides and increase material inclusions. To this end, the company has specially designed an argon gas shield. During the heating and melting process, the furnace is filled with argon gas to completely isolate the air. After the furnace material is melted, the protective cover is removed, the slag is protected, and the furnace bottom blowing argon device is activated.

1.2.2 Bottom argon blowing The gas is turned at the bottom of the furnace.

When the molten steel is melted, the argon valve is opened. The argon gas is blown into the molten steel from the bottom of the furnace and floated up. During the floating process, other gases and impurities in the molten steel enter the argon bubble. The argon gas floats together and the molten steel is purified.

1.2.3 Argon gas protection during tapping

In order to prevent the secondary oxidation of molten steel during the tapping process, a tapping protective cover was specially designed and installed in the nozzle. During the tapping process, the molten steel flow was completely covered in an argon atmosphere to isolate the air.

1.3 Strictly adopt gas atomization milling process and liquid argon full-process protection steelmaking method to assist reasonable atomization device (nozzle) and steel temperature

1.3.1 Atomizing device (nozzle)

The nozzle is a component of the atomizing device that enables the atomizing medium to obtain high energy and high speed, and plays an important role in atomization efficiency and stability of the atomization process. The company’s patented “combined nozzle and its alloy powder vertical gas atomization device” ensure the most reasonable injection angle between the atomization medium and the metal liquid flow; the metal liquid flow becomes obvious turbulent flow; the middle is the main nozzle Conical jetting is carried out, auxiliary nozzles are added at the periphery, annular parallel jet is applied, secondary crushing and secondary cooling of metal droplets are simultaneously performed, and lateral flight of the metal droplets is restrained, the powder falling speed is accelerated, and droplets and high temperature can be prevented. The small metal balls adhere to ensure a good spherical shape of the powder and greatly reduce the “satellite ball powder”. The secondary crushing and secondary cooling performed by the auxiliary nozzles have obvious effects on improving the overall quality of the powder:

  • The large particles are reduced, so that the proportion of the applicable powder is increased, that is, the yield is improved;

  • The satellite ball powder is reduced, the surface of the powder is smoother, the friction between the powders is reduced, and the flow performance is improved;

  • The hollow ball powder is reduced, the bulk density of the powder is increased, that is, the looseness ratio is increased; the hollowness ratio of the powder is reduced from 5% to less than 1%; the powder yield is obviously increased to 50-70%, It saves atomizing gas, electric energy and labor costs.

1.3.2 Increase material temperature

The high-temperature alloy has a large viscosity, and the spheroidization rate of the powder is improved by increasing the temperature of the metal liquid flow, that is, increasing the superheat of the metal liquid flow. When the atomization temperature is 1300 ° C, the sphericity of the powder is not high, the powder is mainly spherical and the surface of the powder is rough. This is because the viscosity of the metal liquid flow is low, and the viscosity of the metal powder is difficult to make the metal powder into a ball. In addition, the lower metal flow temperature shortens the solidification time of the metal powder, and the powder solidifies before it shrinks into a ball. When the atomization temperature reaches 1500 ° C, the powder has a high degree of sphericity and the surface of the powder is smooth. Therefore, increasing the superheat of the metal liquid can increase the sphericity of the powder because it can shorten the solidification time of the metal powder and lower the viscosity of the metal liquid, thereby shortening the spheroidizing time of the metal powder, that is, in practice. The superheat degree of the production metal liquid flow can be controlled at 200-300 ° C to obtain a high-quality powder with high sphericity and smooth surface.

Production practice shows that increasing the superheat of the metal stream can reduce the average particle size of the powder. This is because the liquid volume expands when the temperature rises, and the molecular distance increases, which weakens the force of the bulk molecules on the surface layer molecules. At the same time, when the temperature rises, the vapor pressure of the gas phase becomes larger and the density increases, and the action of the gas phase molecules on the surface molecules of the liquid increases, so that the surface tension and viscosity of the molten metal of the metal decrease with the increase of the temperature of the metal and gradually decrease. The reduction of the surface tension and viscosity of the molten metal causes the molten metal to be easily torn into atomic droplets by the atomizing gas during the atomization process, which is favorable for the breakage of the metal liquid flow.

2, trial production of nickel-based superalloy welding powder

2.1 Nickel base superalloy welding powder

Nickel-based superalloy powders have been widely used in mechanical, chemical, aerospace and other fields. The surfacing layer has good oxidation resistance and corrosion resistance, and its surfacing temperature range is heat treated with stainless steel and superalloy. The system is consistent with ERNiCrMo-3 (Inconel 625) and ERNiCrMo-10 (Hastelloy c22). Moreover, with the rapid development of the automobile industry and civil products in recent years, the use of nickel-based superalloys in civilian products is also increasing. Therefore, nickel-base superalloy welding powder is suitable for surfacing welding on carbon steel, stainless steel, heat-resistant steel and high-temperature alloy parts working under high temperature and corrosion. In the surfacing process, the chemical composition of the solder powder is required to be uniform and stable, and has a low oxygen content and low impurities, so that the process of the surfacing process is good, the powder and the substrate are sufficiently wetted, and the oxide inclusions are reduced. Defects in the welding head (surfacing layer), but the nickel-base superalloy powder produced in China has a high oxygen content, difficult to control powder shape, too small batch size, or requires more sophisticated production equipment, resulting in a heap. The weld has defects and cannot work properly, so it has long relied on imports.

2.2 Test content

The ERNiCrMo-3 (Inconel625) solder powder was produced by trial production. The ball shape was controlled according to the working conditions, and the oxygen content was controlled below 0.08%.

2.3 Production principle

Ni-based superalloy powder gas atomization production is based on electrolytic nickel plate, adding Cr, Si, Mn, Fe, Mo, Nb and other metals or alloys, smelting into a chemical composition-compliant melt in an intermediate frequency induction furnace. The preheated tundish is placed above the nozzle and is centered with the nozzle, and then the molten metal stream is injected into the leaking crucible (the tundish), and a gas atomizing nozzle is placed under the tundish when melting When the metal liquid flows through the nozzle through the nozzle, the high-pressure nitrogen gas flow device is simultaneously turned on to maintain a certain pressure, and the high-speed atomizing gas flow through the nozzle impacts the alloy flow flowing through the leaking nozzle through the tundish to the alloy liquid at a certain angle. Drops, the alloy droplets fall into the atomization barrel and solidify into an alloy powder. The entire milling process operates under non-vacuum conditions. See below:

2.4 Test results and analysis

2.4.1 Control of chemical composition

The stability and accuracy of the chemical composition of the powder directly affect the performance of the weld head and the choice of the surfacing temperature. Therefore, the following measures are taken during the production process to ensure reasonable alloy composition:

1) For the appropriate raw materials, each batch of raw materials shall be inspected according to the raw material inspection specifications. It is strictly forbidden to enter the melting furnace with the wet or rusted charge or return material, and all raw materials are properly treated. The thickness of the nickel plate is not less than 3 mm, the edge is free of dendritic agglomerates and dense pores, and the plate surface shall not have pores larger than 2 mm in diameter, and the total area of ​​dense pores having a diameter of 0.5-2 mm shall not exceed 10% of the single-sided area of ​​the nickel plate.

The surface of the metal chrome should be grayish white, and the section should be dense and free of pores.

The surface of the molybdenum strip should be smooth and shiny, with a silver-gray or silver-gray metallic luster. It must not have severe yellowish or dark black oxidation and staining.

2) Select suitable lining materials to avoid pollution caused by unreasonable lining materials

3)Smelting in an acidic medium frequency induction furnace, the quartz sand crucible should be kept in good condition. The new crucible should be washed once or twice with metal nickel or the same alloy before use.

4) Strictly control the melting temperature and time

These conditions affect the amount of burning of the alloying elements and the degree of alloying of the nickel-base superalloy powder. In the production process, the general heat of the melt is between 200 ° C and 300 ° C, and after the furnace material is completely melted and slag is slag, it is calmed for 3-5 minutes, and then transferred to the atomization process.

2.4.2 Oxygen content of the powder

The oxygen content in the powder is one of the main factors affecting the quality of the welded joint. When the oxygen content in the powder is too high, the oxide in the powder will hinder the wetting between the powder and the metal matrix, making the two not better. Fusion; and because of the high melting point of the oxide, oxide inclusions are formed in the weld joint to cause defects, causing stress concentration of the parts under high temperature operation and damaging the components. Controlling the oxygen content of the powder is therefore the key to measuring the quality of the powder. Since all processes in this production process are carried out in a non-vacuum environment, in the production process, we have taken many measures to prevent oxidation of the powder for different production processes.

Effect of smelting method on oxygen content of powder: Vacuum atomization method is melting, remelting and atomizing of raw materials in vacuum medium frequency induction furnace; instead of vacuum melting, smelting, remelting and atomization in atmospheric environment. As can be seen from the table below, in the case where the other atomization process parameters are the same, the oxygen content of the solder powder obtained by the vacuum atomization method is slightly lower than that of the atmospheric smelting, which is only 0.004% lower.

3. Conclusion

The mature gas atomization milling process and the argon gas protection steelmaking method are used to prepare alloy powders having a relatively high viscosity such as a nickel-based superalloy powder or an iron-based superalloy powder. Namely: the raw material is made into a vacuum ingot in a vacuum furnace, and then remelted in a non-vacuum furnace, and the molten steel and gas atomization are realized under the protection of argon gas. The liquid argon protects the molten steel in the whole process, and the ERNiCrMo- is successfully produced. 3 (Inconel 625), ERNiCrMo-10 (HC-22), ERNiFeCr-1 (Incoloy 825), Monel 400, HC-276 and other high-temperature alloy solder powders with high viscosity, the chemical composition meets relevant standards, and the oxygen content is obtained with vacuum The furnace atomization method produces the same quality product.

Effective control of the quality of welding is guaranteed. At the same time, high-temperature alloy welding powder can be mass-produced according to this process, which promotes the localization of high-temperature alloy welding powder and accelerates the development of efficient welding process.

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