Yield strength was investigated in iron with different grain size and different contents of carbon and nitrogen. The Hall-Petch coefficient; k yis originally very small in high purity iron (about 100 MPa.μm1/2) but it increases with increasing the amount of solute carbon content. The value of ky is enlarged to around 550 MPa.μm1/2 by 60 ppm of solute carbon and levels off at around 600 MPa.μ1/2 in the region above 60 ppm solute carbon. On the other hand, nitrogen hardly influences the ky 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.
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