Mechanisms for boron effect on microstructure and creep strength of ferritic power plant steels

2009 ◽  
Vol 4 (4) ◽  
pp. 166-174 ◽  
Author(s):  
F Abe ◽  
M Tabuchi ◽  
S Tsukamoto
Keyword(s):  
Power Plant ◽  
Creep Strength

Weld Hardness Study of P91 Welded Pipes with Post Weld Heat Treatment for Elevated Temperature Application in Power Plants

2015 ◽  
Vol 1115 ◽  
pp. 503-508 ◽  
Author(s):  
Muhammad Sarwar ◽  
Mohd Amin bin Abd Majid
Keyword(s):  
Heat Treatment ◽  
Power Plant ◽  
Weld Metal ◽  
Creep Strength ◽  
Power Plants ◽  
Thermal Power ◽  
Base Material ◽  
Welding Process ◽  
Post Weld Heat Treatment ◽  
Weld Heat

The creep strength-enhanced ferritic (CSEF) steels are undergoing an encouraged use around the world especially in power plant construction. On construction sites, it has always been the target to have no problems in welded joints but premature failures are being encountered. The primary reason of these premature failures is found to be the improper heat treatment that is mandatorily carried out to achieve the required weld hardness. Weld hardness has close relationship with creep strength and ductility of the welded structures. Hence it is important for any weld to achieve certain level of weld hardness. This study aims at ascertaining the importance of Post Welding Heat Treatment (PWHT) in achieving the required hardness in creep-strength enhanced ferritic (CSEF) materials.The study was carried out on the welding of alloy steel ASTM A335 Gr. P-91 with the same base material (ASTM A335 Gr. P-91) by Gas Tungsten Arc Welding (GTAW) process using ER90S-B9 filler wire with pre-heat of 200oC (min) and inter-pass temperature of 300oC (max). After welding, the joints were tested for soundness with Radiography testing. Induction heating was used for heat treatment of P91 pipes during welding and post weld heat treatment. The effect of Post Weld Heat Treatment (PWHT) was investigated on the Weld metal and the Heat Affected Zones (HAZ) by hardness testing. It is perceived that the scattered and higher hardness values, more than 250HB in 2” P91 pipes in the weld metal and in the heat affected zones, can be brought into the lower required level, less than 250HB, with an effective post weld heat treatment at 760°C for 2hrs.It is concluded that PWHT is the most effective way of relieving the welding stresses that are produced due to high heat input in the welding process and to achieve the required level of hardness in the weld as well as in the heat affected zones (HAZ) in thermal power plant main steam piping.

Estimation of long-term creep strength in austenitic power plant steels

2016 ◽  
pp. 1-6 ◽  
Author(s):  
A. Zieliński ◽  
G. Golański ◽  
M. Sroka ◽  
J. Dobrzański
Keyword(s):  
Power Plant ◽  
Creep Strength ◽  
Term Creep

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

2014 ◽  
Author(s):  
Fujio Abe ◽  
Masaaki Tabuchi ◽  
Susumu Tsukamoto ◽  
Hiroyuki Kutsumi
Keyword(s):  
Power Plant ◽  
Base Metal ◽  
Creep Strength ◽  
Welded Joints ◽  
Boron Steel ◽  
Rupture Ductility ◽  
Type Iv ◽  
Term Creep ◽  
M23c6 Carbides

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°C. The enrichment of soluble boron near prior austenite grain boundaries (PAGBs) by the segregation is essential for the stabilization of fine distribution of M23C6 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°C. The formation of protective Cr2O3-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. Paper published with permission.

Critical Issues for Development of High-Cr Ferritic Steels for USC Power Plant at 650°C

2007 ◽  
Author(s):  
F. Abe
Keyword(s):  
Heat Treatment ◽  
Power Plant ◽  
Oxidation Resistance ◽  
Base Metal ◽  
Creep Strength ◽  
Welded Joints ◽  
Ferritic Steels ◽  
Critical Issues ◽  
Term Creep

Critical issues for the development of high-Cr ferritic steels for USC power plant at 650°C are described in terms of long-term creep strength and oxidation resistance. Creep tests were carried out for 9Cr-3W-3Co-VNb steel at 650°C for up to about 6 × 104 h. The addition of boron as high as 100ppm or more in combination with minimized nitrogen, which causes no formation of boron nitride (BN) during normalizing heat treatment, is essential for the improvement of long-term creep strength of base metal and welded joints. The addition of nitrogen causes the precipitation of fine MX carbonitrides, which significantly improves the creep strength. However, excess addition of nitrogen promotes not only the formation of BN during normalizing heat treatment and of Z phase during creep in base metal but also the formation of fine grained heat affected zone in welded joints. Therefore, the boron and nitrogen contents should be optimized for the improvement of long term creep strength of base metal and welded joints. On the other hand, the candidate steels for boiler at 650°C should exhibit oxidation resistance in steam at 650°C better than that of Gr.91 in steam at 600°C, because Gr.91 is being now used for long duration in power plants operating at 600°C. However, existing 9 to 12Cr steels, such as P92 and P122, cannot satisfy the criterion for oxidation resistance in steam at 650°C. The formation of Cr-rich oxide scale is achieved by pre-treatments such as pre-oxidation treatment and coating, which significantly improves oxidation resistance in steam at 650°C and satisfies the criterion for oxidation resistance. The assessment of resistance to exfoliation is a key issue for the Cr-rich oxide scale.

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