Foseco Ferrous Foundryman''''s Handbook Part 2 ppsx

Tables and general data 15 Calculation of average grain size The adoption of the ISO metric sieves means that the old AFS grain fineness number can no longer be calculated. Instead, the average grain size, expressed as micrometres (µm) is now used. This is determined as follows: 1 Weigh a 100 g sample of dry sand. 2 Place the sample into the top sieve of a nest of ISO sieves on a vibrator. Vibrate for 15 minutes. 3 Remove the sieves and, beginning with the top sieve, weigh the quantity of sand remaining on each sieve. 4 Calculate the percentage of the sample weight retained on each sieve, and arrange in a column as shown in the example. 5 Multiply the percentage retained by the appropriate multiplier and add the products. 6 Divide by the total of the percentages retained to give the average grain size. Example ISO aperture Percentage Multiplier Product (µm) retained ≥ 710 trace 1180 0 500 0.3 600 180 355 1.9 425 808 250 17.2 300 5160 212 25.3 212 5364 180 16.7 212 3540 150 19.2 150 2880 125 10.6 150 1590 90 6.5 106 689 63 1.4 75 105 ≤63 0.5 38 19 Total 99.6 – 20 335 Average grain size = 20 335/99.6 = 204 µm 16 Foseco Ferrous Foundryman’s Handbook Calculation of AFS grain fineness number Using either the old BS sieves or AFS sieves, follow, steps 1–4 above. 5 Arrange the results as shown in the example below. 6 Multiply each percentage weight by the preceding sieve mesh number. 7 Divide by the total of the percentages to give the AFS grain fineness number. Example BS sieve % sand retained Multiplied by Product number on sieve previous sieve no. 10 nil –– 16 nil –– 22 0.2 16 3.2 30 0.8 22 17.6 44 6.7 30 201.0 60 22.6 44 1104.4 100 48.3 60 2898.0 150 15.6 100 1560.0 200 1.8 150 270.0 pan 4.0 200 800.0 Total 100.0 – 6854.2 AFS grain fineness number = 6854.2/100 = 68.5 or 68 AFS Foundry sands usually fall into the range 150–400 µm, with 220–250 µm being the most commonly used. Direct conversion between average grain size and AFS grain fineness number is not possible, but an approximate relation is shown below: AFS grain fineness no. 35 40 45 50 55 60 65 70 80 90 Average grain size (µm) 390 340 300 280 240 220 210 195 170 150 While average grain size and AFS grain fineness number are useful parameters, choice of sand should be based on particle size distribution. Tables and general data 17 Recommended standard colours for patterns Part of pattern Colour As-cast surfaces which are to be left unmachined Red or orange Surfaces which are to be machined Yellow Core prints for unmachined openings and end prints Periphery Black Ends Black Core prints for machined openings Periphery Yellow stripes on black Ends Black Pattern joint (split patterns) Cored section Black Metal section Clear varnish Touch core Cored shape Black Legend “Touch” Seats of and for loose pieces Green and loose core prints Stop offs Diagonal black stripes with clear varnish Chilled surfaces Outlined in Black Legend “Chill” 18 Foseco Ferrous Foundryman’s Handbook Dust control in foundries Air extraction is used in foundries to remove silica dust from areas occupied by operators. The following table indicates the approximate air velocities needed to entrain sand particles. Terminal velocities of spherical particles of density 2.5 g/cm 3 (approx.) BS sieve Particle Terminal velocity size dia. (µm) m/sec ft/sec ft/min 16 1003 7.0 23 1400 30 500 4.0 13 780 44 353 2.7 9 540 60 251 1.8 6 360 100 152 1.1 3.5 210 150 104 0.5 1.7 100 200 76 0.4 1.3 80 For the comfort and safety of operators, air flows of around 0.5 m/sec are needed to carry away silica dust. If air flow rate is too high, around the shake-out for example, there is a danger that the grading of the returned sand will be altered. Buoyancy forces on cores When liquid metal fills a mould containing sand cores, the cores tend to float and must be held in position by the core prints or by chaplets. The following table lists the buoyancy forces experienced by silica sand cores in various liquid metals, expressed as a proportion of the weight of the core: Liquid metal Ratio of buoyant force to core weight Aluminium 0.66 Brass 4.25 Copper 4.50 Cast iron 3.50 Steel 3.90 Core print support Moulding sand (green sand) in a core print will support about 150 kN/m 2 (21 psi). So the core print can support the following load: Tables and general data 19 Support (kN) = Core print area (m 2 ) × 150 1 kN = 100 kgf (approx.) Support (kgf) = Core print area (m 2 ) × 15 000 Example: A core weighing 50 kg has a core print area of 10 × 10 cm (the area of the upper, support surface), i.e. 0.1 × 0.1 = 0.01 m 2 . The print support is 150 × 0.01 = 1.5 kN = 150 kgf. If the mould is cast in iron, the buoyancy force is 50 × 3.5 = 175 kgf so chaplets may be necessary to support the core unless the print area can be increased. Opening forces on moulds Unless a mould is adequately clamped or weighted, the force exerted by the molten metal will open the boxes and cause run-outs. If there are insufficient box bars in a cope box, this same force can cause other problems like distortion and sand lift. It is important therefore to be able to calculate the opening force so that correct weighting or clamping systems can be used. The major force lifting the cope of the mould is due to the metallostatic pressure of the molten metal. This pressure is due to the height, or head, of metal in the sprue above the top of the mould (H in Fig. 1.1). Additional forces exist from the momentum of the metal as it fills the mould and from forces transmitted to the cope via the core prints as the cores in cored castings try to float. Figure 1.1 Opening forces of moulds. Ladle h H Bush F Cope Area A Drag The momentum force is difficult to calculate but can be taken into account by adding a 50% safety factor to the metallostatic force. 20 Foseco Ferrous Foundryman’s Handbook The opening metallostatic force is calculated from the total upward-facing area of the cope mould in contact with the metal (this includes the area of all the mould cavities in the box). The force is: F AHd (kgf) = 1.5 1000 ××× A is the upward facing area in cm 2 H (cm) is the height of the top of the sprue above the average height of the upward facing surface d is the density of the molten metal (g/cm 3 ) 1.5 is the “safety factor” For ferrous metals, d is about 7.5, so: F AH (kgf) = 11 1000 ×× For aluminium alloys, d is about 2.7, so: F AH (kgf) = 4 1000 ×× Forces on cores The core tends to float in the liquid metal and exerts a further upward force (see page 18) In the case of ferrous castings, this force is 3.5 × W (kgf) where W is the weight of the cores in the mould (in kg). In aluminium, the floating force can be neglected. The total resultant force on the cope is (for ferrous metals) (11 × A × H)/1000 + 3.5 W kgf Example: Consider a furane resin mould for a large steel valve body casting having an upward facing area of 2500 cm 2 and a sprue height (H) of 30 cm with a core weighing 40 kg. The opening force is 11 × 2500 × 30/1000 + 3.5 × 40 = 825 + 140 = 965 kgf It is easy to see why such large weights are needed to support moulds in jobbing foundries. Tables and general data 21 Dimensional tolerances and consistency achieved in castings Errors in dimensions of castings are of two kinds: Accuracy: the variation of the mean dimension of the casting from the design dimension given on the drawing Consistency: statistical errors, comprising the dimensional variability round the mean dimension Dimensional accuracy The major causes of deviations of the mean dimension from the target value are contraction uncertainty and errors in pattern dimensions. It is usually possible to improve accuracy considerably by alternations to pattern equipment after the first sample castings have been made. Figure 1.2 The average tolerance (taken as 2.5 σ ) exhibited by various casting processes. (From Campbell, J. (1991) Castings, Butterworth-Heinemann, reproduced by permission of the publishers.) Tolerance (2.5 σ ) (mm) 100 10 1 0.1 0.01 Sand casting Steel AI + malleable White cast iron Grey cast iron AI Pressure die Zn Pressure die Investment AI and steel AI Precision sand core assembly AI Gravity die and low-pressure die 10 100 1000 10 000 Casting dimension (mm) 22 Foseco Ferrous Foundryman’s Handbook Dimensional consistency Changes in process variables during casting give rise to a statistical spread of measurements about a mean value. If the mean can be made to coincide with the nominal dimension by pattern modification, the characteristics of this statistical distribution determine the tolerances feasible during a production run. The consistency of casting dimensions is dependent on the casting process used and the degree of process control achieved in the foundry. Figure 1.2 illustrates the average tolerance exhibited by various casting processes. The tolerance is expressed as 2.5σ (2.5 standard deviations), meaning that only 1 casting in 80 can be expected to have dimensions outside the tolerance. There is an International Standard, ISO 8062–1984(E) Castings – System of dimensional tolerances, which is applicable to the dimensions of cast metals and their alloys produced by sand moulding, gravity diecasting, low pressure diecasting, high pressure diecasting and investment casting. The Standard defines 16 tolerance grades, designated CT1 to CT16, listing the total casting tolerance for each grade on raw casting dimensions from 10 to 10 000 mm. The Standard also indicates the tolerance grades which can be expected for both long and short series production castings made by various processes from investment casting to hand-moulded sand cast. Reference should be made to ISO 8062 or the equivalent British Standard BS6615:1985 for details. Chapter 2 Types of cast iron Introduction The first iron castings to be made were cast directly from the blast furnace. Liquid iron from a blast furnace contains around 4%C and up to 2%Si, together with other chemical elements derived from the ore and other constituents of the furnace charge. The presence of so much dissolved carbon etc. lowers the melt point of the iron from 1536°C (pure iron) to a eutectic temperature of about 1150°C (Fig. 2.1) so that blast furnace iron is fully liquid and highly fluid at temperatures around 1200°C. When the iron solidifies, most of the carbon is thrown out of solution in the form either of graphite or of iron carbide, Fe 3 C, depending on the composition of the iron, the rate of cooling from liquid to solid and the presence of nucleants. If the carbon is precipitated as flake graphite, the casting is called ‘grey iron’, because the fractured surface has a dull grey appearance due to the presence of about 12% by volume of graphite. If the carbon precipitates as carbide, the casting is said to be ‘white iron’ because the fracture has a shiny white appearance. In the early days of cast iron technology, white iron was of little value, being extremely brittle and so hard that it was unmachinable. Grey iron, on the other hand, was soft and readily machined and although it had little ductility, it was less brittle than white iron. Iron castings were made as long ago as 500 BC (in China) and from the 15th century in Europe, when the blast furnace was developed. The great merits of grey iron as a casting alloy, which still remain true today, are its low cost, its high fluidity at modest temperatures and the fact that it freezes with little volume change, since the volume expansion of the carbon precipitating as graphite compensates for the shrinkage of the liquid iron. This means that complex shapes can be cast without shrinkage defects. These factors, together with its free-machining properties, account for the continuing popularity of grey cast iron, which dominates world tonnages of casting production (Table 2.1). Greater understanding of the effect of chemical composition and of nucleation of suitable forms of graphite through inoculation of liquid iron, has vastly improved the reliability of grey iron as an engineering material. Even so, the inherent lack of ductility due to the presence of so much graphite precipitated in flake form (Fig. 2.2) limits the applications to which grey iron can be put. A malleable, or ductile form of cast iron was first made by casting ‘white 24 Foseco Ferrous Foundryman’s Handbook iron’ and then by a long heat treatment process, converting the iron carbide to graphite. Under the right conditions the graphite developed in discrete, roughly spherical aggregates (Fig. 2.3) so that the casting became ductile with elongation of 10% or more. The first malleable iron, ‘whiteheart iron’ Figure 2.1 The iron–carbon phase diagram. (From Elliott, R., Cast Iron Technology, 1988, Butterworth-Heinemann, reproduced by permission of the publishers.) Temperature (°C) 1600 1200 800 400 0 2 4 6 wt. %C α + Fe 3 C or α + G α D γ + Fe 3 C or γ + G γ α + γ B δ + γ δ δ + liquid liquid γ + liquid A G + liquid Fe 3 C + liquid C Fe – G system Fe – Fe 3 C system ABCD (––) Fe–G 2.09 4.25 0.68 %C 1154 1154 739 °C ( ) Fe–Fe 3 C 2.12 4.31 6.68 0.76 %C 1148 1148 1226 727 °C Table 2.1 Breakdown of iron casting tonnages 1996 (1000s tonnes) Total iron Grey iron Ductile iron Malleable iron Germany France 6127 3669 (59.9%) 2368 (38.6%) 84 (1.37%) UK USA 10 314 6048 (58.6%) 4034 (39.1%) 232 (2.25%) Data from CAEF report The European Foundry Industry 1996 US data from Modern Castings    [...]... gutters Refrigerator compressors Tractor axles gear boxes Valves, low pressure (gas) water hydraulic Water pumps 20 0 25 0 20 0 25 0 20 0 20 0 150 20 0 20 0 20 0 20 0 20 0 20 0 150 20 0 100–150 20 0 25 0 20 0 25 0 25 0 20 0 150 20 0 20 0 25 0 100–150 100 20 0 20 0 25 0 150 20 0 150 20 0 150 25 0 150 20 0 20 0 150 20 0 20 0 25 0 20 0 iron Without sufficient Mn, iron sulphide forms during solidification and deposits around grain boundaries... 150 20 150 15 150 2 150 150 25 175 18 180 12. 9 20 0 30 20 0 20 20 0 3 20 0 20 0 22 5 Minimum tensile strength (N/mm2 ) 16 .2 250 35 25 0 25 25 0 4 25 0 25 0 40 27 5 27 5 19.4 300 45 300 30 300 5 300 300 325 22 .7 350 50 350 35 350 6 350 350 60 400 40 400 32 Foseco Ferrous Foundryman’s Handbook Table 3 .2 HB30 hardness grades of grey iron (EN 1561:1997) Grade EN-GJLHB30 HB155 min max HB175 HB195 HB215 HB235 HB255... ranges (10–6 per °C) 20 –100°C Ferritic flake or nodular Pearlitic flake or nodular Ferritic malleable Pearlitic malleable White iron 14 22 % Ni austenitic 36% Ni austenitic 20 20 0°C 20 –300°C 20 –400°C 20 -500°C 11 .2 11.1 12. 0 11.7 8.1 16.1 4.7 11.9 11.7 12. 5 12. 2 9.5 17.3 7.0 12. 5 12. 3 12. 9 12. 7 10.6 18.3 9 .2 13.0 12. 8 13.3 13.1 11.6 19.1 10.9 13.4 13 .2 13.7 13.5 12. 5 19.6 12. 1 Table 2. 5 Specific heat capacity... temperature range (J/kg.K) 20 –100°C 20 20 0°C 20 –300°C 20 –400°C 20 –500°C 20 –600°C 20 –700°C 20 –800°C 20 –900°C 20 –1000°C 515 530 550 570 595 625 655 695 705 720 Typical mean values for grey, nodular and malleable irons, for 100°C ranges Mean value for each temperature range, (J/kg.K) 100 20 0°C 20 0–300°C 300–400°C 400–500°C 500–600°C 600–700°C 700–800°C 800–900°C 900–1000°C 540 585 635 690 765 820 995 750 850 Iron... resistivity of cast irons Grey iron Tensile strength (N/mm2) Resistivity at 20 °C (micro-ohms.m2/m) 150 180 22 0 26 0 300 350 400 0.80 0.78 0.76 0.73 0.70 0.67 0.64 Ductile iron Grade Resistivity at 20 °C (micro-ohms.m2/m) 350 /22 0.50 400/ 12 0.50 500/7 0.51 600/3 0.53 700 /2 0.54 600/3 0.41 700 /2 0.41 Malleable iron Grade Resistivity at 20 °C (micro-ohms.m2/m) Table 2. 4 350/10 0.37 450/6 0.40 550/4 0.40 Coefficient... at 20 °C (g/cm3) 350 /22 7.10 400/ 12 7.10 500/7 7.10–7.17 600/3 7.17–7 .20 700 /2 7 .20 600/3 7.30 700 /2 7.30 Malleable iron Grade Density at 20 °C (g/cm3) 350/10 7.35 450/6 7.30 550/4 7.30 Other cast irons Type Density at 20 °C (g/cm3) White cast irons Unalloyed 15–30%Cr Ni-Cr 7.6–7.8 7.3–7.5 7.6–7.8 Austenitic Grey (Ni-hard) high-Si (6%) 7.4–7.6 6.9–7 .2 28 Foseco Ferrous Foundryman’s Handbook Table 2. 3... the matrix Tables 2. 2, 2. 3, 2. 4, 2. 5 and 2. 6 show, respectively, the density, electrical resistivity, thermal expansion, specific heat capacity and thermal conductivity of cast irons The figures in the tables should be regarded as approximate Table 2. 2 Density of cast irons Grey iron Tensile strength (N/mm2) Density at 20 °C (g/cm3) 150 180 22 0 26 0 300 350 400 7.05 7.10 7.15 7 .20 7 .25 7.30 7.30 Ductile... where Types of cast iron 29 Table 2. 6 Thermal conductivity of cast irons Grey iron Tensile strength (N/mm2) Thermal conductivity (W/m.K) 100°C 500°C 150 180 22 0 26 0 300 350 400 65.6 40.9 59.5 40.0 53.6 38.9 50 .2 38.0 47.7 37.4 45.3 36.7 45.3 36.0 Ductile iron Grade Thermal conductivity (W/m.K) 100°C 500°C 350 /22 400/ 12 500/7 600/3 700 /2 40 .2 36.0 38.5 35.0 36.0 33.5 32. 9 31.6 29 .8 29 .8 Malleable iron Grade... only the outlines and not the structure of the graphite 36 Foseco Ferrous Foundryman’s Handbook Dimensions of the graphite particles forms I to VI Reference number Dimensions of the particles observed at ×100 (mm) True dimensions >100 50– 100 25 – 50 12 25 6– 12 3– 6 1.5– 3 1 0.5–1 0 .25 –0.5 0. 12 0 .25 0.06–0. 12 0.03–0.06 0.015–0.03 . 1.9 425 808 25 0 17 .2 300 5160 21 2 25 .3 21 2 5364 180 16.7 21 2 3540 150 19 .2 150 28 80 125 10.6 150 1590 90 6.5 106 689 63 1.4 75 105 ≤63 0.5 38 19 Total 99.6 – 20 335 Average grain size = 20 335/99.6 =. nil –– 16 nil –– 22 0 .2 16 3 .2 30 0.8 22 17.6 44 6.7 30 20 1.0 60 22 .6 44 1104.4 100 48.3 60 28 98.0 150 15.6 100 1560.0 20 0 1.8 150 27 0.0 pan 4.0 20 0 800.0 Total 100.0 – 6854 .2 AFS grain fineness. each temperature range (J/kg.K) 20 –100°C 20 20 0°C 20 –300°C 20 –400°C 20 –500°C 20 –600°C 20 –700°C 20 –800°C 20 –900°C 20 –1000°C 515 530 550 570 595 625 655 695 705 720 Typical mean values for grey,