Improving the creep properties of 9Cr-3W-3Co-NbV steels and their weld joints by the addition of boron

Abstract

New ferritic steels with a controlled addition of boron have been developed recently for ultrasupercritical fossil power plants. These steels possess excellent creep resistance compared to conventional steels like P91, P92, P122, etc., and this has been attributed to the delay in coarsening of the carbides during creep owing to partial replacement of carbon by boron in these carbides. However, the susceptibility of the weld joints of the boron-containing ferritic steels to type IV cracking, which significantly brings down the rupture life of the weld joints, has not been investigated so far. In the present work, the creep properties of recently developed 9Cr-3W-3Co-NbV steels with boron contents varying from 47 to 180 ppm and of their weld joints have been studied. Creep tests were carried out at 923 K in the stress range of 140 to 80 MPa. Specimens were examined for particle coarsening using field-emission scanning electron microscopy, and the boron content in the precipitates was estimated using field-emission auger electron spectroscopy (FE-AES). The grain size of the parent metal and the heat-affected zone (HAZ) were estimated using electron backscattered pattern (EBSP) imaging. Results showed that the creep properties of the steels with 90 and 130 ppm boron and of their weld joints are superior to those of the P92 steels and its weld joints. Further, no weld joints exhibited type IV cracking. No significant coarsening of the carbides was observed, not only in the parent metal but also in the HAZ of the steels with Ն90 ppm of boron. In addition to the delay in carbide coarsening, the large prior-austenite grain size of the parent metal and the absence of a conventional fine-grained HAZ (FGHAZ) in the weld joints also seem to have a beneficial effect on improving the creep properties of these steels and their weld joints.

Effect of Tempering on Microstructure and Mechanical Properties of Boron Containing 10%Cr Steel

“…An inspection of experimental results showed that the tempering embrittlement is attributed to the formation of the film-like M 23 (B·C) 6 phase on martensite/retained austenite boundaries in the temperature interval of 450-525°C. The 10%Cr steel exhibits very low toughness in a normalized state.…”

“…1) A key issue for achieving a steam temperature ranging from 600 to 650°C and a steam pressure of about 30 MPa is the development of 10-12%Cr martensitic steels with a high creep strength and sufficient fracture toughness for the components of steam turbines such as rotors and rotating buckets/blades. 1) Boron containing 10-12%Cr steels with Co, W and Mo additives are attractive candidates for these crucial parts; [1][2][3][4][5][6][7][8] boron is considered as a very effective agent to enhance the creep strength. It is known [2][3][4][5][6][7][8][9][10] that the addition of boron decreases the coarsening of M23C6 carbides and leads to increased number density of these carbides because boron substitutes for carbon in M23C6, i.e.…”

“…The microstructures of the boroncontaining 9-12%Cr steels were extensively studied in tempered and crept conditions. [2][3][4][5][6][7][8][9][10] However, a number of studies dealt with the examination of microstructure evolution during tempering is very limited. 7,10) There is no sufficient information on redistribution of boron during tempering, mechanisms of boron enrichment for M23C6 carbides located on prior austenite grain boundaries and lath boundaries.…”

Microstructural Evolution of Boron Nitride Particles in Advanced 9Cr Power Plant Steels

“…The recently developed ferritic 9Cr-3W-3Co-V-Nb steels with controlled additions of B and N have shown superior creep resistance at elevated temperatures compared to other conventional 9 wt.% Cr steels and are considered to be future candidates for 923 K (650°C) applications [3][4][5][6]. The microstructure of the 9Cr-3W-3Co-V-Nb steels after the pre-service heat treatment is generally similar to that of conventional 9-12 Cr steels, which typically exhibit a tempered martensite matrix with a fine dispersion of secondary phase particles [3,[7][8][9].…”

“…Due to the high normalising temperature used with the 9Cr-3W-3Co-V-Nb steels, the prior austenite grain size of these steels after heat treatment is much larger compared to conventional 9-12 Cr steel. The large grain size is believed to be beneficial to the creep resistance of weld joints [4]. In addition to the large grain size, two of the important alloy design concepts behind the ferritic 9Cr-3W-3Co-V-Nb steels are: a) B is added to stabilise both Cr-rich carbides (M 23 C 6 ) against coarsening and grain boundaries during creep [10,11], and b) the addition of N is carefully controlled to produce a fine dispersion of MX (where M is V or Nb, and X is N or C) particles to strengthen the martensitic microstructure [10,11].…”