Alloy Design of Creep- and Oxidation-Resistant 9Cr-Boron Steel Beyond Grades 91, 92 and 122

Abstract

The addition of boron and nitrogen without the formation of any boron nitrides during normalizing heat treatment at high temperature improves long-term creep strength and rupture ductility of base metal and suppresses Type IV fracture in welded joints of 9Cr ferritic power plant steel at 650 o C. The enrichment of soluble boron near prior austenite grain boundaries (PAGBs) by the segregation is essential for the stabilization of fine distribution of M 23 C 6 carbides along boundaries in the vicinity of PAGBs, enhancing the boundary and subboundary hardening.9Cr-3W-3Co-VNb steel (MARBN) with 120-150 ppm boron and 60-90 ppm nitrogen, in which no boron nitride formed during normalizing, exhibits not only much higher creep strength and rupture ductility of base metal than Gr.92 but also substantially no degradation in creep strength due to Type IV fracture in welded joints at 650 o C. The formation of protective Cr 2 O 3 -rich oxide scale is achieved on the surface of MARBN by pre-oxidation treatment in argon gas, which significantly improves the oxidation resistance in steam. The present results suggest that MARBN is superior to Gr.91, Gr.92, and Gr.122 in terms of creep strength of base metal and welded joints and can be applied to boiler components in next-generation USC power plant. EXPERIMENTAL PROCEDUREThe 9Cr-3W-3Co-0.2V-0.05Nb steel with different boron concentrations of 0, 48, 92 and 139 ppm was used to investigate the effect of boron on creep strength of 9Cr boron steel base metals [2]. Nitrogen was not added to the steel to avoid the formation of BN during normalizing heat treatment at high temperature. The 9Cr-3W-3Co-0.2V-0.05Nb steel with 140 ppm boron but different nitrogen concentrations was used to investigate the effect of nitrogen on creep strength of 9Cr boron steel [3]. The steels were basically prepared by vacuum induction melting to 50 kg ingots. Hot forging and hot rolling were performed to produce plates of 20 mm in thickness. Creep tests were carried out at 650 o C for up to about 7 x 10 4 h, using specimens of 10 mm in gauge diameter and 50 mm in gauge length.The welded joints were prepared by means of multi-layered Gas Tungsten Arc (GTA) welding [4]. The simulated heataffected-zone (HAZ) specimens were also prepared. Finally, the post-weld heat-treatment (PWHT) was carried out for each specimen including the base metal at 740 o C for 4.7 h. RESULTS AND DISCUSSION Effect of boron on creep strength and microstructure1Published with permission.

On the Relationship between the Chromium Concentration, the Z‐Phase Formation and the Creep Strength of Ferritic‐Martensitic Steels

“…Table shows the chemical composition of some existing heat resistant steels listed in order of their Cr level, and decade in which the steel was introduced in the market is reported . For the commercial grades, the specification ranges of their alloying elements are listed as well.…”

Development of a New High-Performance Martensitic Heat-Resistant Steel for Boiler Applications

“…Small additions of B can effectively reduce the coarsening rate of M 23 C 6 at and in the vicinity of PAGBs, hence strengthen them and consequently, delay the recovery of the subgrain microstructure next to the PAGBs [8]. However, in order to unfold the beneficial effect of B on coarsening of M 23 C 6 carbides, it is necessary to limit the nitrogen content and to avoid BN formation [9].…”