Effect of interstitial elements on Hall-Petch coefficient of ferritic iron

Yield strength was investigated in iron with different grain size and different contents of carbon and nitrogen. The Hall-Petch coefficient; k(y) is originally very small in high purity iron (about 100 MPa.mu m(1/2)) but it increases with increasing the amount of solute carbon content. The value of k(y) is enlarged to around 550 MPa.mu m(1/2) by 60 ppm of solute carbon and levels off at around 600 MPa.mu m(1/2) in the region above 60 ppm solute carbon. On the other hand, nitrogen hardly influences the k(y) value. The mechanism of the change in Hall-Petch coefficient was then discussed in terms of the grain boundary segregation of carbon and nitrogen atoms.