Microstructure of a Creep-Resistant 10 Pct Chromium Steel Containing 250 ppm Boron

Abstract

The microstructure of a trial martensitic chromium steel containing a high content of boron (250 ppm) was characterized in detail in the as-tempered and aged conditions. This steel has a similar composition and heat treatment as the TAF steel that still is unsurpassed in creep strength among all 9 to 12 pct chromium steels. Characterization was performed by using scanning electron microscopy, energy-filtered transmission electron microscopy, secondary ion mass spectroscopy, and atom probe tomography. Focus was placed on investigating different types of precipitates that play a key role in improving the creep resistance of these steels. The low tempering temperature of 963 K (690°C) is enough for the precipitation of the full volume fraction of both MX and M 23 C 6 . A high boron content, more than 1 at. pct, was found in M 23 C 6 precipitates and they grow slowly during aging. The high boron level in the steel results in metal borides rather than BN with the approximate formula (Mo 0.66 Cr 0.34 ) 2 (Fe 0.75 V 0.25 )B 2 . Two families of MX precipitates were found, one at lath boundaries about 35 nm in size and one dense inside the laths, only 5 to 15 nm in size.

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

“…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.…”

“…Any increase in coarsening resistance of M23C6 particles and/or a decrease in their size is considered as the mechanism for the increase in creep strength of martensitic 9-12%Cr steels. 4,9) On the other hand, a high boron content impairs hot workability and welding properties of heat-resistant steels 8) due to the formation of metal borides or boron nitrides (BN). In addition, boron containing low alloy steels exhibit inferior room temperature Charpy V-notch impact toughness because of the precipitation of large, more or less continuous carbide films on lath boundaries under tempering 12) or M23(B⋅C)6 particles situated on prior austenite boundaries.…”

“…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.…”

Effect of Boron on Carbide Coarsening at 873 K (600 °C) in 9 to 12 pct Chromium Steels

“…Mo 3 B 2 was identified in a trial high boron (250 ppm) content steel. [16] BN is found only when the boron and nitrogen contents exceed a certain limit (e.g., 120 ppm B and 150 ppm N) according to Sakyraya et al [33] Almost all the rest of boron is incorporated in M 23 C 6 . Formation of BN is thus possible in all the four steels.…”

“…[8][9][10][11][12][13][14][15][16] In the current work, recent results obtained using atom-probe tomography (APT), secondary ion mass spectroscopy (SIMS), and energyfiltered transmission electron microscopy (EFTEM) are presented, and together with first-principle calculations, a deeper insight is given into the mechanisms behind the beneficial effects of boron to restrict the coarsening of carbides.…”