Local film boiling takes place when initial heat flux density, during immersion of heated to high temperature steel part into liquid, is equal to the first critical heat flux density qcr1. In this case vapor blanket sporadically covers part surface especially at beginning stage of cooling. This results in non-uniform cooling of part surface: slower cooling under vapor blanket in local areas and faster cooling where vapor blanket is absent .Local film boiling is more dangerous as compared with the full film boiling because full film boiling provides uniform cooling, but is very slow.
Martensite is formed in areas where transient nucleate boiling takes place providing in these areas very high hardness. In areas where local film boiling exists, intermediate phases appear with the lesser hardness that results in spotty hardness.
Due to local boiling, martensite forms non-uniformly throughout part surface layer resulting in excessive part distortion. It is explained by higher specific volume (4%) of the martensite.
When local film boiling takes place between teeth of a gear, double distortion during quenching is observed. The left- hand tooth of a gear moves to the right and the right -hand tooth moves to the left. More information one can find in the paper:
N,I,Kobasko, M.A.Aronov, B.L.Ferguson, and Z.Li, Local film boiling and its impact on distortion of spur gear during batch quenching, Materials Performance and Characterization, Vol.1, No 1, 2012, pp.1 - 15, doi: 10.1520/MPC104533. ISSN: 2165-3992.
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