Reasons for the formation of structural heterogeneities during surface quenching of high-carbon structural steel

Conclusions

• 1. The structural heterogeneities occurring at a certain distance below the surface during surface quenching of high-carbon structural steel result from the formation of an upper bainite structure with less stability and greater tendency to softening during tempering than the granular sorbite layer above it and the lamellar sorbite layer below it.

• 2. During tempering bainite is more softened than the sorbite because of the greater effect of the fine structure of the α-phase on the strength of the bainite and the lesser effect of the carbide particle size. With increasing tempering temperatures the intensity of the growth of blocks and the relief of the second-order stresses in bainite occurs earlier than the coagulation of carbides into sorbite, which also leads to greater weakening of the bainite than the lamellar sorbite.

• 3. The elimination of structural heterogeneities during surface quenching of high-carbon structural steel requires either suitable alloying to obtain continuous hardenability or a decrease of the cooling rate during quenching in order to eliminate the possibility of the martensitic and bainitic transformations in the surface layers.