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Why is the Ti:N ratio specified in steel specifications?


Frequently Asked Questions

At any given temperature, there is an equilibrium grain size which is dependent on the temperature. Therefore, grains will attempt to coarsen at elevated temperatures. Some restriction on grain growth by pinning the grain boundaries is provided by aluminium nitride particles, but above a certain temperature these are taken into solution, thus leaving the grain boundaries unimpeded. The presence of titanium nitrides (TiN) extends the temperature range over which the grain boundary pinning is effective, since these particles are taken into solution at a significantly higher temperature than other particles known to restrict grain growth [1,2] .

The optimum effect occurs with a particle size of ~0.05nm and a titanium content of ~0.015% [3] at around the stoichiometric composition (Ti/N = 3.42). It should be noted that at high temperatures, TiN particles coarsen (known as Ostwald ripening) and hence become less effective in the prevention of grain coarsening. It should also be recognised that the correct Ti:N ratio is not a guarantee of a suitable distribution of particles. In particular, the cooling rates during solidification of ingot cast steels are too slow, and result in too coarse a particle size distribution. TiN also promotes the formation of intragranular acicular ferrite by acting as nuclei during the transformation from austenite. This provides the steel with much improved notch toughness.

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Further information


1 Svensson L-E: 'Control of Microstructures and Properties in Steel Arc Welds', CRC Press 1994, p 97-99.
2 Easterling K: 'Introduction to the Physical Metallurgy of Welding', Butterworth Heinemann 1992, p145.
3 Lancaster J F: 'Metallurgy of Welding', Abington Publishing, pp 233-235.

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