In HSLA, high yield stress (strength) is attained due to dispersion hardening of carbides/carbo-nitrides, and their grain refinement effect. But, same carbides/carbo-nitrides makes ferrite, free from interstitial alloying elements (IF steels) to reduce yield stress (strength) and improve ductility - formability. Isn't it bit controversial and confusing !! whether carbides/carbo-nitrides improves yield stress or reduces it?
The HSLA steels attain their improved performance mainly due to the optimization of the material design (adding the alloying elements in the appropriate quantity) and mainly due to their technology of production (TMCP, QT etc). To my knowledge, they do not encourage the formation of carbides or carbo-nitrides, due to their low carbon content. Their yield strength is augmented also do to their reduced grain size (see Hall-Petch equation).
Dear Harshal
I agree with mr Kazasidis about formation of carbides and carbonitrides in HSLA steel.
Each strengthening mechanism of a steel is related to presence some obstacles that hinder dislocation movement. Generally speaking carbides/carbo-nitrides can have direct or indirect effect on hindering of dislocation sliding, that is on steel strength. Direct effect is related to presence of fine and coherent particles of carbides/carbo-nitrides on small distance in matrix that significantly retain dislocation sliding. Indirect effect is related to presence of the stable cabides/carbo-nitrides on grain boundaris that hinder growth of austenitic grains at elevated temperatures and ensures attaining of finer feritte grains at room temperatures (grain refinement strengthening mechanism). By creation of carbides / carbo-nitrides the number of carbon/nitrogen atoms in matrix are reduced and because of that the effect of solid-solution strengthening is reduced also. If precipitated particles of carbides/carbo-nitrides in feritte matrix of HSLA or other steel type are not fine and nicely distributed (non-coherent particles) they will not be significant obstacle to dislocation sliding and because of that the yield strength (strength) will not significantly increase.
reduction in grain size by tuning the process parameter (thermo-mechanical rolling process) yield better property for HSLA.
- Badges
- Science topic
- Similar topics
- Engineering
- Materials Engineering