Foseco Ferrous Foundryman''''s Handbook Episode 7 doc

140 Foseco Ferrous Foundryman’s Handbook withdrawn so the steel is generally cleaner than from a lip pour ladle. The disadvantage is that the narrow ‘spout’ may occasionally permit the liquid steel to freeze if the heat is tapped cold or pouring is prolonged. Figure 11.6b Section through a teapot ladle. (From Jackson, W.J. and Hubbard, M.W., Steelmaking for Steelfounders, 1979, SCRATA. Courtesy CDC.) With both lip pour and teapot ladles, it is only necessary to invert the ladle to remove all slag and metal before refilling or reheating. Bottom pour ladles The ladle is fitted with a pouring nozzle in its base, closed by a refractory stopper rod (Fig. 11.6c). The metal is drawn from the bottom and is therefore slag-free and non-metallics such as deoxidation products are able to float out of the melt. The metal stream flows vertically downwards from the ladle so that there is no movement of the stream during pouring. The Lifting bail Safety catches in open position Liquid metal Mild steel shell Refractory lining Pouring stream Slag Molten metal handling 141 disadvantage is that the velocity and rate of flow change during pouring as the ferrostatic head changes. Figure 11.6c Section through a bottom pour ladle. (From Jackson, W.J. and Hubbard, M.W., Steelmaking for Steelfounders, 1979, SCRATA. Courtesy CDC.) Slag Liquid metal Refractory lining Mild steel shell Slide Pouring handle Stopper rod Stopper rod covers Stopper rod end Pouring nozzle Unless a reusable system is fitted, the nozzle and stopper rod assembly must be changed after each use, thereby increasing the turn-round time, costs and the number of ladles required to handle a given output of metal. The stopper rod may also distort or erode making it impossible to shut off the stream completely. It is not practical to handle less than 100 kg of steel due to the chilling effect of the stopper rod assembly. The flow from a bottom pour ladle depends on the size of the nozzle and the height of metal in the ladle so the flow rate and velocity of the metal stream reduces as the ladle empties. The nozzle/stopper rod is an excellent on-off valve but is not an effective flow control valve. Attempts to use it to control flow result in breakup of the metal stream with consequent risk of reoxidation of the steel. However it is possible to calculate the discharge rate and metal velocity for each ladle and nozzle and its variation throughout the pour so that ladles can be suited to the mould sizes that are being cast (see Chapter 18). 142 Foseco Ferrous Foundryman’s Handbook Ladle linings The ideal lining is highly refractory, non-reactive with metal or slag and of low thermal conductivity and heat capacity. Fireclay is the traditionally used material either in the form of pre-fired bricks or as a plastic ramming grade. Brick linings must be installed by skilled personnel to ensure tight joints. Monolithic refractories are easier to install and eliminate the weak spots associated with bricks. Although fireclay is the lowest cost refractory, it is possible to raise the level of as-cast quality considerably by using better lining materials. High alumina monolithic linings are popular because of their better refractoriness and their longer life. Whether bricks, rammed monolithic refractories or castables are used, there is a long preparation and drying time needed. New ladle linings must be dried before use to remove moisture. After preliminary air drying, gas heaters are used to heat the lining to 600–800°C. Final firing of the lining occurs during its first use. Steel rapidly loses temperature when held in ladles. A 5-tonne bottom pouring ladle lined with fireclay will lose 20°C in 5 minutes, while the same ladle lined with high alumina refractory will lose as much as 50°C in 5 minutes. The KALTEK ladle lining system The KALTEK range of products are disposable, insulating, refractory ladle linings for steel, iron and non-ferrous applications. The low thermal mass and insulating properties of KALTEK linings eliminate the need for pre- heat in virtually all ladle sizes and with most alloys. KALTEK inner linings can be quickly stripped and replaced when necessary. KALTEK one-piece ladle linings are used for lip pouring in a wide range of ladle sizes. A FOSCAST castable alumina dam board can be integrated with the KALTEK liner to create a teapot ladle. Larger ladles, up to 15 tonnes, are lined with KALTEK in the form of board segments and bottom boards designed to suit individual ladles. KALTEK boards and linings are supplied in silica based refractory and also alumino-silicate for severe applications and high magnesia for special alloys. The ladle to be lined with a pre-formed one piece lining must be free from holes. The KALTEK one-piece lining is fitted inside the shell with a minimum of 15 mm backfill of coarse, dry silica sand or BAKFIL, a coarse graded aggregate (AFS 35). The top exposed ring between lining and shell should be capped with a suitable mixture such as silicate bonded core sand and adequately vented with holes every 10–12 cm formed using 6 mm wire (Fig. 11.1). Larger ladles to be lined with KALTEK boards must first be stripped and then furnished with a permanent base lining of alumino silicate bricks or Molten metal handling 143 castable refractory to act as a safety lining. The permanent lining must be dried and fired carefully. Vent holes in the ladles must be cleaned or new holes bored where necessary of 6–8 mm diameter at 20 cm centres. The KALTEK segments and base boards are then carefully fitted inside the permanent lining, the joints between them being filled with KALSEAL refractory cement. The gap between the refractory and backing brick safety lining should be filled with BAKFIL or coarse, dry silica sand (Fig. 11.2). Refractory nozzle assemblies can be fitted into bottom pouring KALTEK lined ladles using special KALPACK ramming material (Fig. 11.7). The nozzle must be pre-heated to dry the KALPACK product before use. (3) Modified ladle nozzle is raised so that its top is flush with KALTEK lining (4) Ladle with nozzle well-block as (3) but KALTEK is butted to well-block rather than nozzle Permanent lining Backfill Nozzle KALPACK The four methods available for a bottom pour ladle are shown here: (1) Normal ladle nozzle is flush with permanent lining (2) Normal ladle nozzle raised using packing pieces Figure 11.7 Methods of installing nozzles in bottom pour ladles. KALTEK ladle linings are used under cold start conditions, pre-heating is unnecessary and would destroy the binder components of the boards. The thermal capacity of a KALTEK lined ladle is lower than conventional refractories and the thermal insulation twice as good so that while there is some initial chill when steel is tapped into the cold ladle, the superior insulation properties soon compensate so that lower tapping temperatures are possible (Fig. 11.8). Initially, the concept of the KALTEK cold ladle lining system for steel foundries was based on single use of the disposable lining, but now in most cases, multiple life is possible. Pot ladles can be used many times as long as they are not allowed to cool between fillings. In many cases, bottom pour ladles can be used more than once as long as multi-life stopper rods such as the Roto-rod isostatically pressed one-piece alumina-graphite rod are used. With KALTEK there is: Various configurations in bottom pour ladles 144 Foseco Ferrous Foundryman’s Handbook Temperature °C 1700 1650 1600 1550 Heat loss comparison 5 ton bottom pour ladle (steel tapped from AOD vessel) Tapped at 1693°C Tapped at 1650°C KALTEK No preheat 70% AI 2 O 2 Brick-Preheated to 800°C 0 5 10 15 20 Time (minutes) Figure 11.8 Heat loss comparison, 5 ton bottom pour ladle, KALTEK v. brick-lined. No pre-heat Faster ladle turnaround Easier ladle maintenance Better temperature control Better working environment Lower inclusion levels Pouring temperature for steels The temperature at which steel castings are poured is at least 50°C above the liquidus temperature. Further superheat is needed to allow for cooling Table 11.1 Variation of liquidus temperature with carbon content for Fe-C alloys Carbon Liquidus temperature Carbon Liquidus temperature (%) (°C) (%) (°C) 0.05 1533 0.55 1490 0.10 1528 0.60 1486 0.15 1524 0.65 1483 0.20 1520 0.70 1480 0.25 1515 0.75 1477 0.30 1511 0.80 1473 0.35 1507 0.85 1470 0.40 1502 0.90 1466 0.45 1498 0.95 1463 0.50 1494 1.00 1459 Molten metal handling 145 Table 11.2 Depression of liquidus temperature caused by the presence of 0.01% of alloying elements Element Depression (°C) Element Depression (°C) P 0.300 Mo 0.020 S 0.250 Si 0.080 Mn 0.050 Cu 0.050 Cr 0.015 Sn 0.080 Ni 0.040 V 0.030 Example: a steel containing 0.06C, 1.0Si, 1.2Mn, 0.03P, 0.02S, 18Cr, 2.0Mo, 10.5Ni would have a liquidus temperature of 1532 – (8.0 + 6.0 + 0.90 + 0.50 + 27 + 4 + 42) = 1532 – 88.4 = 1444°C The pouring temperature should be at least 1444 + 50 = 1494° say 1500°C in the ladle during casting, which can be as high as 10°C per minute in high alumina lined ladles though much lower for KALTEK lined ladles. The liquidus temperature of a steel can be estimated from Tables 11.1 and 11.2. Chapter 12 Sands and green sand Silica sand Most sand moulds and cores are based on silica sand since it is the most readily available and lowest cost moulding material. Other sands are used for special applications where higher refractoriness, higher thermal conductivity or lower thermal expansion is needed. Properties of silica sand for foundry use Chemical purity SiO 2 95–96% minimum The higher the silica the more refractory the sand Loss on ignition 0.5% max Represents organic impurities Fe 2 O 3 0.3% max Iron oxide reduces the refractoriness CaO 0.2% max Raises the acid demand value K 2 O, Na 2 O 0.5% max Reduces refractoriness Acid demand value 6 ml max High acid demand adversely to pH 4 affects acid catalysed binders Size distribution The size distribution of the sand affects the quality of the castings. Coarse grained sands allow metal penetration into moulds and cores giving poor surface finish to the castings. Fine grained sands yield better surface finish but need higher binder content and the low permeability may cause gas defects in castings. Most foundry sands fall within the following size range: Grain fineness number 50–60 AFS Yields good surface finish at Average grain size 220–250 microns low binder levels Fines content, below 2% max Allows low binder level to be 200 mesh used Clay content, below 0.5% max Allows low binder levels 20 microns Size spread 95% on 4 or 5 screens Gives good packing and resistance to expansion defects Specific surface area 120–140 cm 2 /g Allows low binder levels Dry permeability 100–150 reduces gas defects    Sands and green sand 147 Grain shape Grain shape is defined in terms of angularity and sphericity. Sand grains vary from well rounded to rounded, sub-rounded, sub-angular, angular and very angular. Within each angularity band, grains may have high, medium or low sphericity. The angularity of sand is estimated by visual examination with a low power microscope and comparing with published charts (Fig. 12.1). Figure 12.1 Classification of grain shapes. High sphericity Medium sphericity Low sphericity Very angular Angular Sub-angular Sub-rounded Rounded Well rounded The best foundry sands have grains which are rounded with medium to high sphericity giving good flowability and permeability with high strength at low binder additions. More angular and lower sphericity sands require higher binder additions, have lower packing density and poorer flowability. Acid demand The chemical composition of the sand affects the acid demand value which has an important effect on the catalyst requirements of cold-setting acid- catalysed binders. Sands containing alkaline minerals and particularly significant amounts of sea-shell, will absorb acid catalyst. Sands with acid demand values greater than about 6 ml require high acid catalyst levels, sands with acid demand greater than 10–15 ml are not suitable for acid catalysed binder systems. Typical silica foundry sand properties Chelford 60 Sand (a sand commonly used in the UK as a base for green sand and for resin bonded moulds and cores) 148 Foseco Ferrous Foundryman’s Handbook Grain shape: rounded, medium sphericity Bulk density, loose: 1490 kg/m 3 (93 lb/ft 3 ) GF specific surface area: 140 cm 2 /g Angle of repose: 33° Chemical analysis: SiO 2 Fe 2 O 3 Al 2 O 3 K 2 ONa 2 O CaO TiO 2 Cr 2 O 3 LOI 97.1 0.11 1.60 0.73 0.15 0.10 0.06 15 ppm 0.3 Acid demand (number of ml 0.1N HCl): to: pH3 pH4 pH5 pH6 pH7 ml: 2.0 1.8 1.4 1.0 0.8 Sieve grading of Chelford 60 sand Aperture size (µm) BSS mesh No. % wt retained 1000 16 nil 700 22 0.4 500 30 2.3 355 44 10.0 250 60 25.7 210 72 23.8 150 100 28.7 105 150 7.6 75 200 1.3 –75 –200 0.2 AFS Grain Fineness No. 59 Base permeability: 106 Table 12.1 gives size gradings of typical foundry sands used in the UK and Germany. Safe handling of silica sand Fine silica sand (below 5 microns) can give rise to respiratory troubles. Modern foundry sands are washed to remove the dangerous size fractions and do not present a hazard as delivered. It must be recognised, however, that certain foundry operations such as shot blasting, grinding of sand covered castings or sand reclamation can degrade the sand grains, producing a fine quartz dust having particle size in the harmful range below 5 microns. Operators must be protected by the use of adequate ventilation and the wearing of suitable face masks. Sands and green sand 149 Segregation of sand Segregation, causing variation of grain size, can occur during sand transport or storage and can give rise to problems in the foundry. The greatest likelihood of segregation is within storage hoppers, but the use of correctly designed hoppers will alleviate the problem. 1. Hoppers should have minimum cross-sectional area compared to height. 2. The included angle of the discharge cone should be steep, 60–75°. 3. The discharge aperture should be as large as possible. Measurement of sand properties Acid demand value Acid demand is the number of ml of 0.1 M HCl required to neutralise the alkali content of 50 g of sand. Weigh 50 g of dry sand into a 250 ml beaker Add 50 ml of distilled water Add 50 ml of standard 0.1 M hydrochloric acid by pipette Stir for 5 minutes Allow to stand for 1 hour Table 12.1 Typical UK and German foundry sands Sieve size Sand type UK sands German sands microns BSS No. Chelford 50 Chelford 60 H32 H33 F32 1000 16 trace nil 700 22 0.7 0.4 500 30 4.5 2.3 1.0 0.5 1.0 355 44 19.8 10.0 15.0 7.5 7.0 250 60 44.6 25.7 44.0 30.0 30.0 210 72 21.6 23.8 39.0 60.0 60.0 150 100 8.2 28.7 100 150 2.6 7.6 75 200 nil 1.3 1.0 2.0 2.0 75 –200 nil 0.2 nil nil nil AFS grain fineness No. 46 59 51 57 57 Average grain size 0.275 mm 0.23 0.27 0.23 0.23 Note: Haltern 32, 33 and Frechen 32 are commonly used, high quality German sands. German sieve gradings are based on ISO sieves. The German sands have rounder grains and are distributed on fewer sieves than UK sands, they require significantly less binder to achieve the required core strength. [...]... size No Grain shape Specific gravity Bulk density (kg/m3) (lb/ft3) Thermal expansion 20–1200°C Application 60 rounded 2.65 1490 93 1.9% non-linear general 102 rounded 4.66 277 0 173 0.45% 74 angular 4.52 2 670 1 67 0.6% 65 angular 3.3 170 0 106 1.1% refractoriness resistance to Mn steel chill metal penetration chill Zircon, ZrSiO4 Zircon sand has a high specific gravity (4.6) and high thermal conductivity... etc.) 3–4% 70 –100 kPa 10–15 psi 45–52% 80–110 5.0–5.5% 2.5% 7. 0 7. 5% 2.5–3.2% 150–200 kPa 22–30 psi 38–40% 80–100 6.0–10.0% 2.0% 6.0% Steel foundries use sand having similar properties except for reduced volatiles and LOI since coal dust is not used A typical grading of a sand suitable for iron or steel castings is: Sands and green sand 161 Sieve mesh size (µm) % 71 0 500 355 250 212 150 106 75 pan 0.14...150 Foseco Ferrous Foundryman’s Handbook Titrate with a standard solution of 0.1 M sodium hydroxide to pH values of 3, 4, 5, 6 and 7 Subtract the titration values from the original volume of HCl (50 ml) to obtain the Acid Demand Value Grain size See Section I, pp 15,... of the sand The base sand should have the same size grading as the core sand used in the foundry, so that burnt core sand entering the system does not alter the overall size grading 154 Foseco Ferrous Foundryman’s Handbook Clay: The best bonding clays are bentonites which can either have a calcium or a sodium base Sodium bentonites occur naturally in the USA as Wyoming or Western bentonite and also... green sand mixtures (Table 12.4) The main benefits to moulding properties obtained by adding a cereal binder are: Providing greater toughness to the mix and, therefore, the compacted 156 Foseco Ferrous Foundryman’s Handbook Table 12.4 The effect of starch and dextrin on green sand mixes Property Starch addition Dextrin addition Green strength Toughness Green deformation Pattern stripping Mould compaction... sand, clay, coaldust and water are made to the returned sand Sand mills may be continuous or batch but batch mills are nowadays preferred because better sand control is possible Several 158 Foseco Ferrous Foundryman’s Handbook designs of mill are available, their purpose is to mix the sand and spread the moistened clay over the surface of the sand grains in order to develop the bond Older sand mills used... Technical Liaison Ltd.) Muller wheel Plough blade Figure 12.4 Vertical wheel batch muller (Sixth Report of Institute Working Group T30, Mould and Core Production Foundryman, Feb 1986.) 160 Foseco Ferrous Foundryman’s Handbook particles and water sprays are used to effect some preliminary cooling before elevation, since very hot sand can damage the elevator Screen The return sand is elevated to a screen... moulds to cast dimensionally accurate castings The high cost of zircon sand makes reclamation necessary and thermal reclamation of resin bonded moulds and cores is frequently practised 152 Foseco Ferrous Foundryman’s Handbook Zircon sands contain low levels of naturally occurring radioactive materials, such as uranium and thorium Any employer who undertakes work with zircon mineral products is required,... cycle represent only about 10% of the total active clay or coal present in the system It is not possible to change the total clay or coal dust content quickly since any change in the 162 Foseco Ferrous Foundryman’s Handbook addition rate takes about 20 cycles to work its way fully into the system For example; an addition of 0.3% clay is usually sufficient to maintain the total clay level at 3.0% If... reconstituted after casting by adding a compensating amount of LUTRON binder and milling The LUTRON binder can be used with any suitable fine, dry sand; additions of 10–12% are normally needed 164 Foseco Ferrous Foundryman’s Handbook Green sand moulding machines The moulding machine must compact the green sand evenly around the pattern to give the mould sufficient strength to resist erosion while liquid metal . 16 trace nil 70 0 22 0 .7 0.4 500 30 4.5 2.3 1.0 0.5 1.0 355 44 19.8 10.0 15.0 7. 5 7. 0 250 60 44.6 25 .7 44.0 30.0 30.0 210 72 21.6 23.8 39.0 60.0 60.0 150 100 8.2 28 .7 100 150 2.6 7. 6 75 200 nil. size (µm) BSS mesh No. % wt retained 1000 16 nil 70 0 22 0.4 500 30 2.3 355 44 10.0 250 60 25 .7 210 72 23.8 150 100 28 .7 105 150 7. 6 75 200 1.3 75 –200 0.2 AFS Grain Fineness No. 59 Base permeability:. 1533 0.55 1490 0.10 1528 0.60 1486 0.15 1524 0.65 1483 0.20 1520 0 .70 1480 0.25 1515 0 .75 1 477 0.30 1511 0.80 1 473 0.35 15 07 0.85 1 470 0.40 1502 0.90 1466 0.45 1498 0.95 1463 0.50 1494 1.00 1459 Molten