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In order to reduce the segregation of Cr and Mo and inhibit the precipitates, we added a small amount of B and Ce to traditional S31254 steel. Using an air-cooling and low-temperature diffusion treatment, the purpose was to control B and Ce at the grain boundary. The heat-treatment process could prompt co-segregation of B, precipitate-forming elements, and Ce at the grain boundary at 950 °C. After aging at 950 °C for different amounts of time, the diffusion treatment had an obvious inhibitory effect on the precipitates that caused them to become discontinuous, fine, and serrated. The B-containing serrated precipitates were only rich in Mo, while Cr was homogeneously distributed in the probed volume. A uniform distribution of Cr reduced the Cr-depleted zone in the area adjacent to the phase interface. Ce was observed to be segregated at the grain boundary. This showed that Ce could inhibit the formation of precipitates at the grain boundary. The serrated precipitates had an obvious resistance to intergranular corrosion. [ABSTRACT FROM AUTHOR]

Focusing on the serious volatilization of MoO3 in super austenitic stainless steel with a high Mo content, the influence of B on the formation of oxide film and the distribution of Cr and Mo was investigated at 900 °C and 1000 °C. Without the addition of B, Mo tends to diffuse to the surface, forming porous Cr/Mo-rich oxides, causing the volatilization of Mo. The addition of B can inhibit the diffusion of Mo to the surface, facilitate the diffusion of Cr to the surface and combines with O, providing conditions for the nucleation of Cr2O3. A large amount of Cr2O3 accumulated on the surface to form a dense passive film, which inhibited the diffusion of Mo to the surface, reduced the loss of Mo, and formed Mo/Cr-rich precipitates at grain boundaries that are close to the surface. However, it was difficult to form Mo-rich precipitates at the grain boundaries of a sample without B, which aggravated the volatilization of Mo from grain boundary to surface. Therefore, the addition of B can improve the oxidation resistance of 254SMO and inhibit the volatilization of Mo. [ABSTRACT FROM AUTHOR]

S is a common corrosion medium for austenitic stainless steels. The severe intergranular fracture of austenitic stainless steels occurs in sulfur environments. In this paper, the permeation of S at different atomic positions for three symmetric tilt grain boundary types, i.e., Σ5(210), Σ5(310), and Σ9(114) have been computed using first-principles calculations. S has the strongest segregation tendency in the Σ5(210) grain boundary. A high content of S at the grain boundary indicates harm to the grain boundary. Sulfur segregation in the grain boundaries can weaken the strength of the metallic bond. When Mo and Cr are present at the Σ5(210) grain boundary, the sulfur-induced embrittlement is inhibited. With increased S concentration at the grain boundary, the coexistence of Mo and Cr can suppress the intergranular fracture of S on the grain boundary. The reason why high-Mo stainless steel has excellent sulfur-induced intergranular corrosion resistance is explained at the atomic level. [ABSTRACT FROM AUTHOR]

The precipitation of the σ phase at the grain boundary (GB) has an important effect on the properties of super austenitic stainless steel. This study used density functional theory (DFT) calculations to assess the segregation of Mo and B atoms at the GB and their effects on GB strength. The results demonstrated that Mo and B atoms tend to converge to grain boundary positions and B atoms are easier to segregate than Mo atoms. In addition, B can restrain the segregation of Mo to grain boundary positions, making it difficult for the positions to form an σ phase. An analysis of the inhibitory effect of different contents of B on Mo indicated that higher B content is better for restraining Mo segregation to grain boundary sites. These results are beneficial for improving the shortcomings of super austenitic stainless steel. B can also increase the GB strength while inhibiting the segregation of Mo to grain boundary sites and a charge density analysis demonstrated the results. [ABSTRACT FROM AUTHOR]

The addition of B can inhibit the precipitation of σ phases at the grain boundary to improve the hot workability and corrosion resistance for super austenitic stainless steel with high Cr and Mo content. This study focused on the interaction between B and Mo at the Fe–Cr(111)/Cr2O3(0001) interface and its effect on interfacial adhesion by employing the first-principles method, especially the effect of B on the segregation behavior of Mo. The most stable O-terminated Fe/Cr2O3 interface was chosen as the basic configuration. The segregation energy and the work of separation were calculated for the metal/chromia interface with Fe–Cr as the substrate. It has been demonstrated that B can promote the diffusion of Mo atoms into the oxide layer to increase the content of Mo in the passive film. In addition, the interfacial adhesion is higher at the most segregated sites. However, it is more difficult for two or more Mo atoms than a single Mo atom to diffuse into the oxide part with the effect of B, indicating that B can only improve the Mo content of the passive film to a small extent. The electronic properties were also further discussed to analyze the interactions and the binding characters between doped atoms and their surrounding atoms and to explain the underlying reasons for the variation of interfacial adhesion. [ABSTRACT FROM AUTHOR]

The second phases in the S31254 super austenitic stainless steel is easily precipitated, and the solution treatment plays an important role on the microstructure and performance. In this work, the solution treatment is performed on S31254 steel at 1180, 1200, and 1220 °C for 1, 2, and 4 h, respectively. Given the grain size and precipitation of second phase, the 1200 °C for 4 h or 1220 °C for 2 h may be suitable as the solid‐solution parameters. The corrosion resistance and impact toughness of the steel after solution treatment at 1200 °C are further investigated by means of microstructural analysis and electrochemical experiments. The sample after solution treatment for 4 h, where no second phase can be observed, exhibits a lower corrosion rate and a better mechanical properties, compared to that after annealed for 1 or 2 h. The electrochemical impedance spectroscopy, Mott–Schottky, and XPS results indicate that the passive films of the samples, with a significantly decreased donor and acceptor densities and increased Cr2O3 and Mo‐riched oxides, may be responsible for the improvement of corrosion resistance. The impact toughness results show that the samples after solution treatment for 4 h has the highest impact energy among the studied samples. [ABSTRACT FROM AUTHOR]

To get the atomic scale understanding of impurities induced intergranular embrittlement or enhancement and the control of these effects on austenitic stainless steels, behavior of impurities B and S at γ-Fe grain boundary (GB) and effects of alloying elements (Cr, Ni, Mn, Co, Mo) additions on the cohesive properties of the γ-Fe GB doped by impurities are investigated by performing density functional theory (DFT) calculations. The results showed that, as the GB enhancer and embrittler respectively, impurities B and S tend to stay at the interstitial sites of γ-Fe GB, while alloying elements also can segregate at the GB studied. The presence of all the alloying elements considered can inhibit B GB segregation to some extent due to the higher segregation energies and can impair the enhancement effect of B on GB cohesion. However, these elements almost have no impact on GB segregation and embrittlement effect of S, except for Ni, Mn and Mo:Ni and Mo inhibit and improve S GB segregation, respectively; Mn and Mo strongly enhance and reduce S-induced GB embrittlement, respectively. Moreover, the remedial effect of (Cr + Ni), (Cr + Mn) on S-induced GB embrittlement is also discussed. Additional atomic structure and total valence charge density analysis demonstrated and explained our conclusions. [ABSTRACT FROM AUTHOR]

• B addition of S31254 stainless steels promotes the redissolution of σ phases. • B addition of S31254 stainless steels improves the pitting corrosion resistance. • B addition of S31254 stainless steels improves the intergranular corrosion resistance. • The optimum solid solution condition for B-containing S31254 is at 1200 °C for 2 h. • The passive films of S31254 with B have increased Cr 2 O 3 and Mo-riched oxides. The effects of B on microstructure, pitting and intergranular corrosion in solution-annealed S31254 super austenitic stainless steels were systemically investigated. After solid solution treatment at 1200 °C for different time, the precipitation and grain size of S31254 with and without B steels were analyzed. Addition of B to S31254 promotes the redissolution of detrimental sigma (σ) phases, and the optimum solid solution condition for B-containing S31254 is at 1200 °C for 2 h. The S31254 steels with B after solid solution treatment exhibited much better corrosion resistance than that without B, which is relative to the elimination of destructional σ precipitates and the decrease of chromium-depleted region near the grain boundary. Moreover, the passive films of samples with B have increased Cr 2 O 3 and Mo-rich oxides, which may be responsible for the improvement of corrosion resistance. [ABSTRACT FROM AUTHOR]