The addition of boron without the formation of any boron nitrides during normalizing heat treatment at high temperature minimizes the degradation in creep strength of both base metal and welded joints of 9Cr steel at 650 °C and long times. The enrichment of soluble boron near prior austenite grain boundaries (PAGBs) by the segregation is essential for the reduction of coarsening rate of M 23 C 6 carbides in the vicinity of PAGBs, enhancing boundary and subboundary hardening, and also for the production of same microstructure between the base metal and heat-affected-zone (HAZ) in welded joints, indicating no Type IV fracture in HAZ. Excess addition of boron and nitrogen promotes the formation of boron nitrides during normalizing, which reduces the soluble boron concentration and accelerates the degradation in creep rupture ductility at long times. 9Cr- 3W-3Co-VNb steel with 120 - 150 ppm boron and 60 - 90 ppm nitrogen (MARBN) exhibits not only much higher creep strength of base metal than Gr.92 but also substantially no degradation in creep strength due to Type IV fracture at 650 °C. The pre-oxidation treatment in Ar gas promotes the formation of protective Cr 2 O 3 scale on the surface of 9Cr steel, which significantly improves the oxidation resistance in steam at 650 °C.

In advanced ultra-supercritical (A-USC) power plants, which operate at steam temperatures of 700 °C or higher, there is a need to replace 9 to 12Cr martensitic steels with high-strength nickel-base superalloys or austenitic steels for components exposed to the highest temperatures. However, due to the high cost of nickel-base superalloys, it is desirable to use 9 to 12% Cr martensitic steels for components exposed to slightly lower temperatures, ideally expanding their use up to 650 °C. Key challenges in developing ferritic steels for 650 °C USC boilers include enhancing oxidation resistance and long-term creep rupture strength, particularly in welded joints where resistance to Type IV cracking is critical for constructing thick-section boiler components. The current research aims to investigate the creep deformation behavior and microstructure evolution during creep for base metals and heat-affected-zone (HAZ) simulated specimens of tempered martensitic 9Cr steels, including 9Cr-boron steel and conventional steels like grade 91 and 92. The study discusses the creep strengthening mechanisms and factors influencing creep life. It proposes an alloy design strategy that combines boron strengthening and MX nitride strengthening, avoiding the formation of boron nitrides during normalizing heat treatment, to improve the creep strength of both base metal and welded joints.

This paper investigates the combined effect of shot peening and pre-oxidation treatment in air on the subsequent steam oxidation resistance of Modified 9Cr-1Mo steel with different sulfur contents. Cast steel balls (50-180 μm diameter) and pure Cr (50-230 μm diameter) were used for shot peening durations of 5-50 seconds. After shot peening, pre-oxidation was performed in air at 973K for 3.6ks. Then, oxidation testing was conducted in steam at 923K for up to 3.6Ms. Only the combination of Cr shot peening and pre-oxidation treatment facilitated the formation of a protective Cr-rich oxide scale on the specimen surface during pre-oxidation. This Cr-rich oxide scale remained stable during subsequent steam oxidation, resulting in excellent oxidation resistance of the steel.

For high-strength steels developed at the National Institute for Materials Science (NIMS) in Japan, a dispersion of nano-sized MX nitride particles along boundaries and in the matrix is achieved by reducing carbon concentration below 0.02%. This structure results in excellent creep strength at 923K, approximately two orders of magnitude longer rupture time than P92. Additionally, adding a large amount of boron exceeding 0.01% combined with minimized nitrogen effectively improves creep rupture strength by stabilizing the martensitic microstructure during creep. Efforts have been made to enhance the steam oxidation resistance of these 9Cr steels strengthened by boron and fine MX nitrides. A combination of 0.7% Si, 40-60 ppm S, and pre-oxidation treatment was applied. Steam oxidation tests were conducted at 923K for up to 4000h. Pre-oxidation treatment in argon gas at 973K for 50h significantly improved oxidation resistance in steam at 923K by forming a protective Cr-rich oxide layer. The pre-oxidized steels exhibited much lower mass gain in steam at 923K than Mod.9Cr-1Mo steel at 873K, and lower than T91 at 873K after 1000h. After 4000h, their mass gain was about zero, much lower than P91 at 873K and 923K. SEM/EDS analysis and low mass gain suggest a protective Cr-rich oxide scale formed on the pre-oxidized steel surface, exhibiting excellent oxidation resistance in steam at 923K.

This study investigated the creep rupture strength and microstructure evolution in welded joints of high-boron, low-nitrogen 9Cr steels developed by NIMS. The welds were fabricated using the GTAW process and Inconel-type filler metal on steel plates with varying boron content (47-180 ppm). Creep rupture tests were conducted at 923K for up to 10,000 hours. Despite their higher boron content, these steels exhibited good weldability. Welded joints of the boron steel displayed superior creep properties compared to conventional high-chromium ferritic steel welds like P92 and P122. Notably, no Type IV failures were observed during creep testing. Welding introduced a large-grained microstructure in the heat-affected zone (HAZ) heated to the austenite transformation temperature (Ac3 HAZ). This contrasts with the grain refinement observed in the same region of conventional heat-resistant steel welds. Interestingly, the grain size in this large microstructure was nearly identical to that of the base metal. Analysis of the simulated Ac3 HAZ revealed crystal orientation distributions almost identical to those of the original specimen. This suggests a regeneration of the original austenite structure during the alpha-to-gamma phase transformation. Simulated Ac3 HAZ structures of the boron steel achieved creep life nearly equivalent to the base metal. The suppression of Type IV failure and improved creep resistance in welded joints of the boron steels are likely attributed to the large-grained HAZ microstructures and stabilization of M 23 C 6 precipitates. The optimal boron content for achieving the best creep resistance in welded joints appears to lie between 90 and 130 ppm, combined with minimized nitrogen content.

Effects of Ni content and heat treatment condition on impact toughness and creep strength of precipitation strengthened 15Cr ferritic steels were investigated in order to discuss a possibility of improvement in both mechanical properties. Both creep strength and impact toughness of the developing steels were improved drastically by solid solution treatment with water quenching. However, an addition of Ni reduced the long-term creep strength of the steels, though Ni was effective in improvement in impact toughness. It was found that water quenching suppressed formation of coarse block type particles and precipitate free zones around them, and precipitation of plate type fine particles and thermal stability of them within ferrite phase were promoted by solid solution treatment with water quenching. However, martensite phase with sparsely distributed coarse block type particles were formed in the Ni added steels, and such microstructure reduced the precipitation strengthening effect slightly. On the other hand, increase in impact values of the steel indicated no relation to volume fraction of martensite phase. It was supposed that the impact toughness of ferrite phase itself was improved by solid solution treatment and addition of Ni.

Tempered martensitic 9-12%Cr steels bearing tungsten, such as P92 and P122 showing higher creep rupture strength than the conventional steel P91, have been developed for thick section components in ultra-supercritical (USC) boilers. However, their creep strength is not sufficient for applying at the steam condition of 650°C/35MPa or above, which is a recent target condition in order to increase plant efficiency. The research and development project in NIMS on advanced high-Cr steels which can be applied at the steam condition of 650°C/35MPa as boiler components with large diameter and thick section has been carried out since 1997. In this project, it has been revealed that the addition of boron more than 0.01 mass% to the 0.08C-9Cr- 3W-3Co-V,Nb-<0.00ЗN steel remarkably improves creep strength. The boron enriched in M 23 C 6 carbides near prior-austenite grain boundaries suppresses coarsening of these carbides during creep deformation, leading to excellent microstructural stability and creep strength. Further improvement of creep strength is achieved by the addition of appropriate amount of nitrogen which enhances precipitation of fine MX. Excess addition of nitrogen to the high-B containing steel reduces creep rupture lives and ductility. The highest creep strength is obtained in the 0.08C-9Cr-3W-3Co-0.2V-0.05Nb-0.0139B-0.0079N (mass%) steel, resulting in excellent creep strength in comparison with that of P92 and P122. This steel shows good creep ductility even in the long term. It is, therefore, concluded that this high-B bearing 9Cr-3W-3Co-V,Nb steel with the addition of nitrogen in the order of 0.008 mass% is the promising candidate which shows superior creep strength without impairing creep ductility for thick section components in the 650°C-USC plant.

This study investigates the behavior of boron nitride (BN) inclusions in high-chromium ferritic heat-resistant steels like P92 and P122. Boron is added to improve creep resistance, but its role is not fully understood. Here, the formation and dissolution of BN inclusions during high-temperature heat treatment were examined. Microscopic analysis revealed coarse BN inclusions (2-5 μm) alongside smaller alumina inclusions. Annealing experiments showed that these BN inclusions only dissolved at temperatures exceeding 1200°C, suggesting they form during casting or forging processes below 1150°C. Chemical analysis identified a critical boron and nitrogen concentration threshold (below 0.001% B and 0.015% N) for BN inclusion formation.

In our earlier studies we reported the excellent steam oxidation resistance of 50Ni-50Cr coatings produced by high velocity oxyfuel (HVOF) spray process. The coating produced by atmospheric plasma spray (APS) for the thickness of 40 µm yielded the scale incorporation into the coating structure on the steam oxidation due to its porous structure compared to HVOF process. The steam could penetrate into the pores and resulted in the scale initiation at the coating-substrate interface. To improve the steam oxidation resistance of the coating, thick coatings (~450 µm) of 50Ni-50Cr with sealant layer were tested in the steam. The sealant layer was applied to fill the pores produced by the APS process. The results showed that the thick coating of 50Ni-50Cr with sealant was able to protect against the steam oxidation till 3000 hours of tested duration.

HVOF coatings of 80Ni-20Cr and 50Ni-50Cr powders were carried out on 9Cr-1Mo steel substrate. The coating thickness was around 60 µm. The coated specimens were steam oxidized in four different temperatures, ranging from 600 to 750°C. The steam oxidized specimens were taken out from the chamber at 10, 100 and 1000 hours to examine their protectiveness against scale growth. X-ray diffraction, scanning electron microscope and electron probe microanalysis were carried out on the steam-oxidized specimens in reference with as-coated conditions. Both 80Ni- 20Cr and 50Ni-50Cr coatings show neither scale growth at the interface nor de-lamination in the coating structure. Fe, and Ni diffusion was found in the case of 80Ni-20Cr coatings. The diffusion increased with increase in the temperature and test duration. On the other hand, 50Ni-50Cr coatings, showed an excellent performance in all tested temperatures. The effect of Cr content in the coating, change in phase and compound formation during oxidation, and their influence on the diffusion process will be discussed in detail.

Thermal spray of Al was carried out on the modified 9Cr-1Mo steel to evaluate the steam oxidation resistance of the sprayed Al coating. Atmospheric plasma spray process (APS) was used to coat aluminum on sandblasted 9Cr-1Mo steel substrate. The coating thickness was around 40 µm. The coated specimens were steam oxidized in four different temperatures, ranging from 600 to 750°C. The results show that the scale growth occurred in the interface between coating and substrate subsequently it penetrated into the coating structure. Al diffused into the alloy substrate with high solubility. The diffusion increased with increase in the steam temperature and test duration. Diffused aluminum formed the high hardness intermetallic compound in the substrate near the coating/substrate interface. With increase in the test duration, the intermetallic compound moved towards the bulk and at prolonged aging, it became dissolved. This was identified from the decrease in the micro hardness values at coating/substrate interface at prolonged duration. The scale growth at the substrate surface of Al sprayed steel was much controlled compared to the uncoated specimens.