Cracking solidification

In view of the poor mechanical properties of the high-silicon irons, the development of any stresses in the castings during solidification is very dangerous, since they may cause the casting to crack in subsequent service. To overcome this risk, it is often desirable to strip the castings from the moulds while they are still red hot and to anneal them at 850°C for 4-5 h, followed by slow cooling ... 

Hot cracks Large solidification range More crack-proof filler ... 

Of the weldability problems, nickel and nickel-based alloys are particularly prone to solidification porosity, especially if nitrogen is present in the arc atmosphere, but this may be controlled by ensuring the presence of titanium as a denitrider in the filler and maintaining, a short arc length. The other problem that may be encountered is hot cracking, particularly in alloys containing Cr, Si, Ti, Al, B, Zr, S, Pb and P. 

Figure 19.25 Shape of heterogeneous nucleus (ABCD) for solidification formed at the root of the crack in the mold wall, (a) Oblique view, (b) Cross section through the nucleus midplane. The crack extends normal to the page.

The result of the solidification of a liquid metal is mostly an unperfect packing, which means a crystal lattice with imperfections. These imperfections have already been discussed in chapter 4. An imperfection is a deviation from the perfect packing. For instance a vacancy is an empty place in the lattice which should be occupied. It is also possible for a particle to end up in a place where it should not be. A foreign metal ion which is bigger than the own ions can upset the move of parts of the lattice in relation to each other. And then there are the faults which we call dislocations and which concern parts of the crystal lattice. For example when it seems as if a crystal is partly cleaved or both parts have shifted over one atom distance in relation to each other. Or the crystal is in fact partly cleaved, both halves move and the crack is filled with a layer of atoms. 

When solids deform almost to the breaking point, they exhibit brittle behavior the stress at fracture is several orders of magnitude lower than the computed strength. The loss is ascribed to the presence of minute cracks probably formed during solidification. Compressive stress can induce crack propagation the magnitude of the stress at... 

Tin organohalogenides can be used as catalysts to produce ethers of phosphoric acid and to polymerise lactons forming colourless polyesters. Various halogen derivatives of dibutyltin have been put forward as catalysts for the solidification of silicone elastomers, as agents preventing the cracking of polystyrene, as inhibitors of metal corrosion in silicone polymers. 

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