Castings 2 Episode 1 potx

[...]... and temperature When the partial pressure of hydrogen P = 1 atmosphere, it is immediately clear from this equation that k is numerically equal to the solubility of hydrogen in the metal at that temperature Figure 1. 1 shows 500 600 Temperature ("C) 700 800 90 010 0 011 00 1. 3 1 .2 1. 1 1. 0 0.9 0.8 0.7 Reciprocal absolute temperature (K-' x 10 3) Figure 1. 1 Hydrogen solubility in aluminium and two o f its alloys,... But the presence of zinc in copper alloys does not cause boiling until higher temperatures because, of course, 8 Castings 20 00 - 15 00 ISI I E E Y 0 _ N c 0 2 l0OC 2 I n v) 2 Q L 3 Q 0 9 50C ( 0 10 00 11 00 Temperature ("C) 12 0 0 the zinc is diluted (strictly, its activity is reduced) Figure 1. 4 shows the effects of increasing dilution on raising the temperature at which the vapour pressure reaches one... by H2 = 2[ H] (1. 3) The equation predicting the partial pressure of hydrogen in equilibrium with a given concentration of hydrogen in solution in the melt is [HI2 = kPH2 (1. 4) where the constant k has been the subject of many experimental determinations for a variety of gasmetal systems (Brandes 19 83; Ransley and Neufeld 19 48) It is found to be affected by alloy additions (Sigworth and Engh 19 82) and... sources of moisture are to be avoided.) In summary, the gases which can be present in the various copper-based alloys are: cup0 PbO -20 0 FeO L x -600 a a ?? c U L U 5 -800 5 CaO -1 000 Y I 0 20 0 I 400 I I I I I 9 I 600 800 10 00 12 0 0 14 00 18 00 Temperature ("C) Figure 1. 5 The Ellbzghnr?i diagrcrrn, illicrtrnfing the free eiiergy of forrmtior7 oxides AS A fiiiz~tioi7o f temperatiire of The melt boundary... atom for a I 0-5 I I I I I 10 4 I 0-7 10 -8 I 0-9 1o-'O lo-" lo-" 1o-' 10 -74 I 0-75 10 -'6 similar-sized matrix atom This process is more difficult (Le has a higher activation energy) because the solute atom has to wait for a gap of sufficient size to be created before it can jostle its way among the crowd of similar-sized individuals to reach the newly created space Figures 1. 6 to 1. 8 show the rates of diffusion... are in general more complicated as we shall see 1. 1 .2 Copper alloys Copper-based alloys have a variety of dissolved gases and thus a variety of reactions In addition to hydrogen, oxygen is also soluble Reaction between these solutes produces water vapour according to (where square brackets indicate an element in solution) 2[ H] + [O] = H20 2[ H] + CUZO= ~ (1. 5) Thus water vapour in the environment of molten... the copperbased alloy in Figure 1 .2) Further consideration of Figure 1. 3 indicates that where the liquid aluminium is in contact with wet refractories or wet gases, the environment will effectively be close to one atmosphere pressure of 1. 1 .1 Aluminium alloys Considering first the reaction of liquid aluminium with oxygen, the solubility of oxygen in aluminium is extreme1 small; less than one atom in... effectively in operation inside the liquid 20 0 r l i I t High initial gas content Y c ' 5 ? 3 > - Medium initial gase content l n % (0 $ 50 - Low initial gas content Figure 1 .2 Hydrogen content of liquid aluminium I 0 50 I 10 0 Time (rnin) I 15 0 bronze held in a gas-fired furnace, showing how the melt equilibrates with its surroundings Data from Ostrorn et al (19 75) The mclt effect of water vapour in... this ratio the modulus Notice that it has dimensions of length For instance, a 10 tonne holding furnace would have a volume of approximately 4 m3, and a surface area in contact with the atmosphere of perhaps 10 m2, giving a modulus of 4 /10 m = 0.4 m = 400 mm A crucible furnace of 20 0 kg capacity would have a modulus nearer 20 0 mm These values around 300 mm for large bodies of metal contrast with those... Figure 1. 3 is calculated from Equation 1. 4 illustrating the case for hydrogen solubility in liquid aluminium It demonstrates that on a normal day with 30 per cent relative humidity, the melt at 750°C should approach about 1 ml.kg-' (0 .1 ml.lOO g-I) of dissolved hydrogen This is respectably low for most commercial castings (although perhaps just uncomfortably high for aerospace standards) Even at 10 0 per . OX2 8DP 22 Wheeler Road, Burlington MA 018 03 First published 19 91 Paperback edition 19 93 Reprinted 19 93, 19 95 (twice), 19 97, 19 98, 19 99, 20 00 Second edition 20 03 Copyright 0 19 91, 20 03,. mechanisms 2 12 7.5 Initiation of shrinkage porosity 22 2 7.6 Growth of shrinkage pores 22 6 7.7 Final forms of shrinkage porosity 22 7 8. Linear contraction 23 2 8 .1 Uniform contraction 23 2 8 .2. 1. The melt 1 1 .I 1 .2 Transport of gases in melts 10 1. 3 Surface film formation 12 Reactions of the melt with its environment 2 2. Entrainment 17 2. 1 Entrainment defects 20