Jianming Gong

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.

Postbuild heat treatment is an important component in optimized manufacturing processing for laser beam powder bed fusion (PBF-LB) Ti-6Al-4V. The development of predictive modeling, based on the understanding of the relationships between process parameters, microstructure evolution, and mechanical properties, is a potentially key ingredient in this optimization process. In this paper, a process-structure-property (PSP) model is developed to predict the effect of postbuild heat treatment on yield strength, which is a key tensile property for PBF-LB Ti-6Al-4V. The process-structure part is developed with a focus on the prediction of solid-state phase transformation, especially dissolution of martensite during the heating phase. Subsequent tensile properties are quantified by a microstructure-sensitive yield strength model based on the predicted microstructure variables. The integrated PSP model is validated via experimentally measured phase fraction, α lath width and monotonic tensile tests on PBF-LB Ti-6Al-4V with different heat treatment temperatures, for identification of optimal process parameters.

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.

The impact of NaCl impurity on corrosion of stainless steel (SS) 316 L in binary nitrate salts (60 wt% NaNO₃ + 40 wt% KNO₃) was systematically investigated at 565°C via electrochemical measurements. SS 316 L exhibits active dissolution behavior in salts containing 0–1.4 wt% NaCl. The influence of NaCl impurity shows a strong time dependence according to the negligibly affected corrosion current density obtained from a short‐time potentiodynamic polarization sweep. The corrosion process of 316L in salts with NaCl‐free is controlled by ions transport through the oxide layer. With the presence of NaCl impurity, it is controlled by higher rate of ions transport and the diffusion of metal chlorides. The alternation in basicity in the corrosion front caused by chloride ions increases the solubility of chromium oxide and hence the dissolution of chromium oxide, which can lead to the formation of a less protective oxide layer.

The microstructures and tensile properties of fresh and aged reformer furnace tubes and a fresh-to-aged welded joint were investigated to assess the weldability of fresh-to-aged reformer furnace tubes. Damage evaluation of the fresh-to-aged welded joint was also carried out using the modified Kachanov–Rabotnov model. The experimental results showed that M₇C₃ carbide transforms into M₂₃C₆ carbide and secondary carbides precipitate in the matrix after aging treatment. With continuous exposure, the interdendritic precipitates coalesced and coarsened and the number of secondary carbides reduced gradually. Microdefects were absent in the fresh-to-aged welded joint, and the tensile properties of the welded joint were close to the as-cast alloy, which confirms the weldability of fresh-to-aged furnace tubes. According to the results of the simulation, stress redistribution occurred during the creep process and the peak damage of the welded joint was located in the aged tube. The maximum damage of the fresh-to-aged welded joint reached 34.01% at 1.5 × 10⁵ h.