Vacuum Melting and Remelting Processes - ASM International

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ASM Handbook Volume 15: Casting (#05115G) materials in terms of material quality, size, and shape • The different methods of material processing available • Product quality, size, and quantity Contamination-free environment and crucible Material evaporation.-- , and splattering Reflected \..~__.~, electron beam X-ray emission Flexible melting rate and refining dwell time Feedstock Fig. 1 Schematic of the electron beam melting process (a) li' i ~! (f) Contamination of the product is avoided by melting in a controlled vacuum and in water-cooled copper crucibles (Fig. 1). The energy efficiency of electron beam processing exceeds that of competing pro- Electron beam gun Flexible power and power distribution Vacuum Melting and Remelting Processes / 411 Scanning electron beam Drip melt area Refining in the pool zone i ( , ' ) ~ (g) (h) Water-cooled copper crucible Continuous casting and solidifying ingot Fig. 2 Examples of electron beam melting and casting processes. (a) Button melting with controlled solidification for quantitative determination of low-density inclusions. (b) Consolidation of raw material, chips, and solid scrap to consumable electrodes for vacuum arc or electron beam remelting. (c) Drip melting of horizontally or vertically fed feedstocks. (d) Continuous flow refining/melting. (e) Floating zone melting. (f) Investment casting. (g) Pelletizing (manufacture of pellets from scrap and other materials for scrap recycling). (h) Atomization and granulation of refractory and reactive metals (e) (b) /~ Copyright © 2008 ASM International ® All rights reserved. www.asminternational.org cesses because of the control of the beam spot dwell time and distribution at the areas to be melted or maintained as liquid. In addition, unnecessary heating of the ingot pool, as occurs in vacuum arc remelting, for example, is avoided. Power losses of the electron beam inside the gun and between the gun nozzle and the target are very small, but approximately 20% of the beam power is lost because of beam reflection, radiation of the liquid metal, and heat conductivity of the water-cooled trough and crucible walls. Electron Beam Melting and Casting Processes From the large variety of electron beam melting and casting processes shown in Fig. 2 only the processes illustrated in (a), (c), (d), and (f) are related to processes used in foundry technology: • Button melting processes for the quality control of steel and superalloy cast parts to control the content of low-density inclusions • Drip melting process for the preparation of refractory and reactive metal feedstock material for electron beam and VAR skull melting and casting • Continuous flow melting process for the feedstock refining of superalloys for VIM and electron beam casting • Electron beam investment casting process Electron Beam Heat Source Specifmcat,ons For all electron beam melting and casting processes, except for the crucible-free floating zone melting process, Pierce-type electron beam guns with separately evacuated beam generating and prefocusing rooms are the key components of the furnaces used. The essential features of these guns are: • Large power range of 0 to 1200 kW • Long free beam path of 250 to 1500 mm (10 to 60 in.) and the adjustable beam power distribution • Beam deflection angle of ---45 ° and spot frequency up to 500 Hz Schematics of electron beam consolidation and drip melting processes. (a) Consolidation of coarse and solid scrap. (b) Continuous consolidation of raw material, chips, Fig. 3 and solid scrap by direct feeding into a continuous casting crucible. (c) Drip melting of horizontally fed compacts, sintered bars, or consolidates for initial melting of reactive and refractory metals. (d) Drip melting of vertically fed vacuum induction melted or conventionally melted electrodes. (e) Drip melting of horizontally and vertically fed materials for the production of alloys from feedstocks with very different melting points
ASM Handbook Volume 15: Casting (#05115G) 412 / Foundry Equipment and Processing Table 2 Melting and refining data of refractory and reactive metals and alloy steels gained in laboratory and pilot production furnaces Electron beam Total specific Interstitial elements Ingot Ingot Integral power of Operating vacuum melting in feedstock Feedstock size, diameter, weight, melting rate, second melt, pressure of last melt, energy, Material and final ingot, ppm Hardness, Metal mm (in.) mm (in.) kg (Ih) kg/h (Ib/h) kW Pa (tort) kW • h/kg yield, % C O N H HB Tungsten .......... 40 (1.6) diam 55 (2.2) diam, 100 (4) long Tantalum ......... 60 (2.4) diam 60 (2.4) square, 160 (6.3) long Molybdenum ...... 100 (4) diam 100 (4) diam 120 (4.7) square, 180 (7.1) long Niobium ......... 80 (3.2) square, 150 (6) long 120 (4.7) square, 180 (7. I) long Hafnium ......... 60 (2.4) square 80 (3.2) diam 100 (4) diam 100 (4) diam Zirconium ....... 60 (2.4) square, 100 (4) long 60 (2.4) square, 150 (6) long Vanadium ....... 60 (2.4) diam, 80 (3.2) long Titanium ........ 100 (4) diam Ti-6AI-4V ....... • • • Ti-8Al-lMo-IV .... 100 (4) diam 4340 steel ......... 80 (3.2) diam 60 (2.4) 115 (4.5) 80 (3.2) 160 (6.3) 100 (4) 180 (7.1) 150 (6) 180 (7.1) 80 (3.2) 130 (5.1) 173 (380) 100 (4) 19.5 (43) 150 (6) 179 (395) 80 (3.2) 7.7 (17) 100 (4) 28.2 (62) 100 (4) 28.2 (62) 150 (6) 31 (70) 150 (6) 31 (70) (a) The reproducibility of the refining data could not be confirmed. • Usable vacuum pressure range between I and 0.0001 Pa (10 -2 and 10 -6 mbar, or 7.5 × 10 -3 and 7.5 x 10 -7 torr) • Reliability of the gun and cathode system The power control and distribution system allows a very accurate distribution of beam power and energy for achieving the required heating for material melting, superheating, refining, and electrothermal effects. Button Melting for Quality Control The button melting process (Fig. 2a) serves to control the quality of feedstock materials for investment casting and to produce casting samples• In contrast to the conventional electron beam melting process, this process is not used for refining, but only for flotation and concentration of low-density inclusions• Dur- ing the eight-step process, the sample is heat- ed and drip melted. Low-density inclusions are floated to the surface and concentrated in the center of the pool of molten metal during controlled solidification by computer-controlled reduction of beam power and circular electrothermal stirring. The concentrated impurities can be identified and evaluated by conventional metallographic methods, but the size of the raft gives the first indication of the quantity of impurities in the metal. 37 (80) 10.5 (23) 200 (440) 20 (44) 65 (145) 16.7 (37) 523 (1150) 38.4 (85) 64 (140) 12.5 (27.5) 408 (900) 50.2 (111) 227 (500) 17.6 (39) 326 (720) 13.2 (29) 40 (90) 1.7 (3.7) 7.5 (16.5) 14.3 (31.5) 73 (161) 2.8 (6.2) 45.2 (100) 22.5 (50) 10 (22) 80 (176) 119 3O0 130 371 130 290 240 218 80 110 80 250 80 87 6O 52 80 8 x 10 -3 (6x 10 -s) 2 x 10 -2 (1.5 × 10 -4 ) 8 X 10 -3 (6 × 10 -5) 3 × 10 -3 (2 × 10 -s) 8 × 10 -4 (6 X 10 -6) 10 -3 (7.5 x 10 -6) 10 -2 (7.5 x 10 -5) 5 x 10 -3 (3.8 X 10 -s) 5 x 10 -3 (3.8 X 10 -s) 4 X 10 -3 (3 x 10 -s) 8 x 10 -3 (6 x 10 -5 ) 2 x 10 -2 (1.5 x 10 -4 ) 8 x 10 -3 (6 x 10 -5 ) 0.8 (6 x 10 -3) 0.4 (3 x 10 -3 ) 4 x 10 -3 (3 × 10 -5 ) 2.0 (0.015) Most button melting furnaces are com- pletely automated and microprocessor controlled to guarantee process reproducibility• Melting is usually carried out in the pressure range of l to 0.001 Pa (10 -2 to l0 -5 mbar, or 7.6 × 10 -3 to 7.6 × 10 -6 torr). Drip Melting The drip melting processes (Fig. 3c and d) are primarily used for the production of clean, mostly ductile ingots of refractory and reactive metals or of specialty steels• The feedstock for the first melt (Fig. 3c) can be compacted sponge, granular, powder, or scrap, which might be presintered in a vacuum heating furnace• In some cases, loose raw materials can be consolidated in a water-cooled copper trough (Fig. 3a). The consolidated ingot can then be fed horizontally for drip melting• Raw material that is con- tinuously consolidated in a water-cooled copper crucible with a retractable bottom plate (Fig. 3b) can be fed horizontally or vertically for drip melting. In both consolidation processes, only 20 to 80% of the material is melted• Refining and losses of material by splattering are negligible• Drip melting of horizontally fed compacts is the most frequently used process for the production of ingots from refractory or reactive metals• The resulting ingot is of suf- 10.3 9.9 6.0 8.30 10.4 5.2 12.3 15.9 38 14.7 4.6 2.6 3.1 1.91 2.66 1.67 1.0 Copyright © 2008 ASM International ® All rights reserved. www.asminternational.org • .. 70 4100 30 10 93.1 10 8 5 1 • .. 200 5000 50 20 90 10 8 7 1 • .. 100 1200 140 10 92 10 75 30 1 ...... 650 13 10 91.8 6 15 13 2 - • • 170 810 50 10 95 12 10 10 2 • • • 200 750 60 10 96.8 10 12 11 2 ... 160 5220 554 35 87 12 106 60 5 • .. 80 4500 330 40 96.8 6 111 52 8 ...... 1870 95 2 93.1 • • • 170 25 I • • • 500 900 100 2 93.5 100 200 50 1 ...... 950 95 30 88.2 • • • 545 30 3 ...... 950 95 30 ...... 735 48 9 ...... 1045(a) 210(a) 16(a) 91 ... 235(a) 95(a) 13(a) ...... 2180 100 30 99 • • - 1850 80 16 ...... 890 70 ' • • 98 • • • 730 50 • • • • • • 3900 63 100 0.3 93 4300 2.4 26 0.08 99 3622 10 78 0.10 200 210 65 69 140 140 77 66 160 170 120 125 30-100 ficient purity, but has an area of inhomoge- neity caused by the shadow of the horizontally fed bar. Two or more electron guns are used in drip melting to make use of reflected electron beams and to reduce evaporation and splattering• The end of a compact is welded to the front of the fol- lowing one to avoid dropping semisolid material into the pool. Table 2 lists processing parameters that have been successfully used to electron beam melt various reactive and refractory metals and 4340 alloy steel. Vertical Feeding of Ingots. Refractory and reactive metal ingots of high purity, homogeneity, and smooth surface are re- melted by vertical feeding (Fig. 3d). The molten metal droplets run down the conical, rotating electrode tip, are refined, and then drop into the pool center• The crucible pool is normally of the same diameter as the electrode but is sometimes smaller or larg- er. It is kept in the liquid state to allow final refining and to guarantee ingot homogeneity. Because two or more electron guns are used, the entire pool can be equally bom- barded; thus, shadow effects of the electrode can be eliminated• Simultaneous melting of horizontally and vertically fed electrodes (Fig. 3e) can be used for the production of critical alloys. In this case, the feedstock should be of the desired purity•
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