Yong Jiang

This study examined the effects of low-temperature carbonitriding on the microstructure and corrosion resistance of heavily pre-strained metastable 304 austenitic stainless steel. Electron backscatter diffraction (EBSD) and electrochemical measurements were employed. Plastic deformation produced abundant deformation-induced α′-martensite, dense dislocations and slip bands, increased kernel average misorientation (KAM), and a high fraction of low-angle grain boundaries (LAGBs). The α′-martensite enhanced electrochemical activity and formed micro-galvanic couples with untransformed austenite, accelerating local anodic dissolution. Moreover, defect-rich regions associated with dense dislocations and slip bands impaired corrosion resistance. Although the increased LAGB fraction was beneficial, it could not offset the detrimental effects of α′-martensite and high KAM values. Low-temperature carbonitriding formed an approximately 23 μm thick layer with an N-rich surface zone containing about 6 wt.% N. It also caused nearly complete reversion of α′-martensite to austenite, recovery of deformation structures, and a further increase in the low-angle grain boundary (LAGB) fraction. These changes eliminated micro-galvanic couples and high-energy defect sites, whereas supersaturated nitrogen further improved corrosion resistance. The treatment effectively restrained the initiation of localized corrosion. After electrochemical testing, initial and pre-strained specimens displayed pits of various sizes, whereas carbonitrided specimens showed shallow grooves along slip lines.

Oxygen boost diffusion (OBD) is an effective technology for improving the surface hardness of titanium and its alloys. In this present paper, the effect of temperature, vacuum condition and surface roughness on oxygen boost diffusion of Ti–6Al–4V alloy are studied. Test results show that OBD processing can be achieved at a low temperature and over long times, as well as at a high temperature and over short times. By comparing processing efficiency and mechanical properties, high temperatures and short times are preferred for OBD treatment. The influence of vacuum conditions on oxygen vacuum diffusion is significant. Under low vacuum degree conditions, relatively high oxygen content not only corrodes the OBD layer but also leads to spalling of the outmost surface of the OBD layer and the remaining oxide layer. High surface roughness can induce cracking not only in the oxide layer during the oxidation process but also in the outmost surface of the OBD layer during the vacuum diffusion process.

In this present paper, the mechanical properties of σ-phase and its effect on the mechanical properties of 304H austenitic stainless steel after servicing for about 8 years at 680–720 °C were studied by nano-indentation test, uniaxial tensile test, and impact test. The results showed that the nano-hardness (H), Young’s modulus (E), strain hardening exponent (n), and yield strength (σ_y) of σ-phase were 14.95 GPa, 263 GPa, 0.78, and 2.42 GPa, respectively. The presence of σ-phase increased the hardness, yield strength, and tensile strength, but greatly reduced the elongation and impact toughness of the material.