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vacuum by this technique , since industrial vacuum in vacuum induction melting ( VIM ) are insufficiently good to prevent the formation of the oxide film on the liquid metal . The oxide films formed in vacuum conditions are , of course , much thinner than those formed in air melting and casting . However , they still appear to make excellent bifilms but their extreme thinness simply makes them harder to detect .
Ductile iron Turning now to the possibility of using contact pour for ductile iron , this alloy is known to be susceptible to turbulent filling systems , in which magnesia-rich films can be entrained . The turbulence in the filling of the casting and the consequent generation of magnesia-rich bifilms in the metal is the reason for poor nodularity counts and the associated fall in properties . The graphite prefers to grow on the bifilms in suspension and fractures the symmetry of the austenite envelopes around the growing nodules , eliminating the spherical symmetry . In addition , the presence of bifilms as cracks in the alloy reduces the mechanical properties ( notice that the properties are hardly influenced by the shape of the nodules , which are too small to have much effect ). The huge unknown in application to ductile iron is the fact that late stream inoculation would become impossible . This might appear to be a death blow to contact pouring for ductile iron . However , this might not be the case . When it is considered that contact pouring eliminates the 80 % air content of the flow , this huge source of oxidation loss of inoculant additions is now removed . Thus inoculation in the ladle might now be sufficient , because the normal huge oxidation loss in the running system , probably deleting most or all of the late stream addition , now no longer occurs . The author has never been a fan of in-mould inoculation . The in-mould geometries are seriously unfavourable with respect to creating turbulence , creating entrainment defects and eliminating inoculation additions . The elimination of in-mould disturbance of the flow of metal would allow a naturally pressurised filling system to operate with maximum effectiveness , having the potential to deliver castings with very few if any defects , together with high nodularity and high properties .
Contact pour applied to ductile iron , avoiding late stream and avoiding inmould additions , might be proven to be an at least adequate system . It is to be expected that the reduction or elimination of bifilms , if used together with a good filling system design , might at least regularise the structure and properties of the alloy and may deliver an excellent product .
Safety Particularly in steel foundries , the pouring of steel is regularly associated with minor misalignment issues when attempting to position the ladle . The result is a spectacular pyrotechnic display by the spray of steel droplets . Thus burns are a common feature , difficult to avoid during normal operation . In contrast , contact pouring has a delightful aura of safety . No metal is seen . There can be no splash or spray of hot metal . The bystander is not even aware that anything is happening .
Chemical analysis Checking the chemistry of the melt by introducing a sampling spoon into the metal stream is , of course , no longer possible during the filling of the mould . Sampling of the stream , if necessary at all , can perhaps be carried out at the end of pour , as the ladle starts to be lifted clear . The metallostatic pressure in the ladle will be lowest at this time , reducing the strength of the stream and making the sampling that much safer .
Conclusions The world ’ s castings currently employ conical basins , whose use ensures the degradation of practically all metals by massive amounts of entrained air , oxidising the metal and bequeathing a legacy of a dense population of cracks in the form of bifilms . Defective castings are therefore to be expected as the norm . Contact pouring is a first but major step towards the elimination of entrained air and therefore a major step towards improved casting quality and reduction of scrap . For this reason , the process is welcomed by the company accountant . Contact pouring has yet to be proven for slide-gate ladles and for ductile iron ( noting that late stream inoculation will not be possible but possibly not required ). In all cases where employed so far , the system appears to be a great advance in the safety of the foundry and has been welcomed by both the safety officer and by operators .
Cast Metal & Diecasting Times July / August 2016