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.

When performing fluorescent magnetic particle testing in steel welding-repaired zones, as traditional viewpoint usually ascribes magnetic particle indications to discontinuous welding defects such as cracks, incomplete fusion, etc., welding-repaired zones showing indications of these defects are always judged to be unqualified. In this study, a novel reticular phase was identified on the weld fusion line of 06Cr13Ni4Mo. By selected area electron diffraction, it was proved to be an austenite. By roasting test, it was proved to be induced by the thermal effect of welding. Non-ferromagnetic reticular austenite reduces the overall magnetic permeability, leading to the presence of fluorescent magnetic particle indication. Though the mechanical properties of the welding repaired zone are changed by the reticular austenite with the yield strength, tensile strength, and microhardness decreasing to 571 MPa, 752 MPa, and 279, respectively, they still exceed the required values of standard specifications. 06Cr13Ni4Mo welding-repaired zones showing magnetic particle indications induced by reticular austenite are qualified and should be accepted.