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Several grain sizes were obtained by heat treatment at different temperatures for interstitial-free (IF) and 2.25Cr-1Mo steels. Samples of the steels with different grain sizes were aged at 600 and 680 • C for IF steel and 520 and 560 • C for 2.25Cr-1Mo steel for sufficient time to achieve their equilibrium grain boundary segregation. The grain boundary concentrations of phosphorus were examined using Auger electron spectroscopy. At the same aging temperature, the boundary segregation of phosphorus increased with increasing grain size. The effect of grain size on equilibrium grain boundary segregation thermodynamics was analyzed based on the information of both grain size and phosphorus boundary concentration. The segregation enthalpy increased with increasing grain size and simultaneously the segregation entropy became less negative. Moreover, the segregation entropy (∆S) and enthalpy (∆H) of phosphorus in both IF and 2.25Cr-1Mo steels exhibited a unified linear relationship, being expressed as ∆S = 0.85∆H − 38.06, although it segregated to different types of grain boundaries (ferrite grain boundaries in IF steel and prior austenite grain boundaries in 2.25Cr-1Mo steel). With the aid of the acquired thermodynamic parameters and grain boundary segregation theories, the equilibrium segregation concentrations at different aging temperatures were modeled under different grain sizes for both steels.

The dependence of fracture appearance transition temperature (FATT) on phosphorus grain boundary segregation, yield strength, and grain size is experimentally evaluated for a 2.25Cr-1Mo low-alloy steel. Both the phosphorus boundary segregation and yield strength are directly correlated to the FATT of the steel. However, the prior austenite grain size is not directly linked to the FATT although it influences phosphorus boundary segregation and yield strength. The FATT exhibits linear relations to phosphorus segregation concentration and yield strength, but there is no synergistic effect between these two entities. Based on the measurements, an embrittlement equation is established, which is expressed as FATT( C) ¼ 0.25σ s þ 2.64C P -270, where σ s is the yield strength in MPa and C P is the phosphorus boundary concentration in at%. The calculated FATTs have a good agreement with the measured ones.

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