The Precipitation Behavior of Titanium Carbide on the Surface of SUS 321 Stainless Steel

Abstract

The surface composition of SUS 321 stainless steel at high temperatures was observed in vacuum with Auger electron spectroscopy.
The precipitation of titanium carbide was found on the surface of SUS 321. The thickness of precipitated titanium carbide layer increased in proportion to the square root of annealing time and became about 0.05 μm after heated at 1100 K for 432 ks. The precipitated titanium carbide was not replaced by the most surface active element sulfur, and remained stable on the surface. The precipitated layer, however, was not even and had many holes about 1 μm in diameter. The bottom of a hole was SUS 321, on which phosphorus, oxygen and sulfur segregated. As the annealing time was prolonged, these segregants were replaced one by one in the order of the surface activity, and finally the most surface active element, sulfur, remained on the bottom of the hole. Moreover, sulfur diffused over the outside of the hole.
The titanium carbide precipitated on the surface was not removed by the argon ion sputtering (acceleration voltage; 3 kV, current density; 0.6 A/m²) at 1100 K. This was because titanium and carbon were supplied to the surface at sufficient velosity by the diffusion during the sputtering at high temperature.
The precipitation of titanium carbide on the surface occurred according to the following processes: (1) The titanium and carbon which had been dissolved in the bulk diffused onto the surface of the stainless steel. (2) The titanium carbide which had been precipitated in the bulk dissolved because the concentration of titanium and carbon fell under their solubility limits in the bulk. (3) The titanium and carbon diffused onto the surface which was exposed to vacuum. (4) The titanium and carbon recombined into titanium carbide and precipitated on the surface. The growth rate of the thickness of the precipitated layer was controlled by the diffusion of titanium and carbon in the precipitated titanium carbide.
Thermodynamic consideration based on the Gibbs dividing surface model showed that the compound which was precipitated in the bulk came out onto the surface in order to decrease the strain and the interface energies.