Tempering of 2.25 Pct Cr-1 Pct Mo Low Carbon Steels

Thermodynamic Analysis for the Magnetic-Field-Induced Precipitation Behaviours in Steels

Alloy carbide M23C6 plays a significant role in the creep strength of reduced activation steels. Experiments have proven that a magnetic field accelerates the precipitation of M23C6 at intermediate temperature. A scheme that combines first-principle calculations, Weiss molecular field theory and equilibrium software MTDATA is proposed to investigate the thermodynamic features of magnetic-field-induced precipitation. The calculated results reveal that the origin of the magnetic moment is the NaCl-like crystal structure. The magnetic field enhances the exchange coupling and stabilizes the ferromagnetic phase region. The external field influences the Curie temperature, thereby changing the magnitude and position of the maximum magnetic heat capacity, magnetic entropy and enthalpy. The strong magnetic field improves the stability of M23C6, and the theoretical results agree well with the previous experiment. The study provides a theoretical basis for the magnetic-field-induced precipitation behaviours in steels.

Formation of (Fe,Cr) carbides and dislocation structures in low-chromium steel studiedin situusing synchrotron radiation

The evolution of the size distribution of (Fe,Cr) carbides and the dislocation structure in low-chromium steel is studied during quenching and rapid heating byin situsmall-angle X-ray scattering (SAXS). The two-dimensional SAXS patterns consist of streaks on top of an isotropic SAXS signal. The evolution of the size distribution of the (Fe,Cr) carbides during heat treatment is determined from the isotropic component of the SAXS patterns. The isotropic part of the SAXS patterns shows that, after austenitization and quenching to room temperature, the average precipitate radius is 4.74 nm and the dispersion parameter for the lognormal size distribution is 0.33. Subsequent rapid heating to 823 K results in an average precipitate size of 5.25 nm and a dispersion parameter of 0.26. Bright-field transmission electron microscopy and high-resolution transmission electron microscopy reveal the nearly spherical morphology of the precipitates. The microstructural evolution underlying the increase in the average precipitate size and the decrease in the dispersion parameter after heating to and annealing at 823 K is probably that at room temperature two types of precipitates are present,i.e.(Fe,Cr)23C6and (Fe,Cr)7C3precipitates according to thermodynamic calculations, and at 823 K only (Fe,Cr)7C3precipitates are present. Additional measurements have been carried out on a single crystal of ferrite containing (Fe,Cr) carbides by combining three-dimensional X-ray diffraction (3DXRD) and SAXS during rotation of the specimen at room temperature, in order to investigate the origin of the streaks at low angles in the SAXS pattern. From simulations based on the theory of SAXS from dislocations, it is shown that the measured streaks, including the spottiness, in the two-dimensional SAXS patterns correspond to a dislocation structure of symmetric low-angle tilt boundaries, which in turn corresponds to the crystallographic orientation gradient in the single crystal of ferrite as measured by 3DXRD microscopy.