The Effect of Micro-Segregation on the Quantification of Microstructural Parameters in Grade 91 Steel
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TRODUCTION
SINCE the international energy crisis in the 1970s, increasing efforts have been made to improve the efficiency of power generation.[1] An outcome of the drive for increased Fossil fuelled plant efficiency was the adoption and widespread design and fabrication of components using Grade 91 steel. This alloy is a creep strength enhanced ferritic (CSEF) steel that was originally developed over approximately a ten-year period beginning in the mid-1970s by the Oak-Ridge National Laboratory (ORNL) and Combustion Engineering.[2] Extensions of this effort, and similar parallel studies in other parts of the world, have resulted in a family of martensitic CSEF steels otherwise referred to as 9 to 12 wt pct Cr CSEF steels.[3] Gr.91 is the most widely used steel from this family of alloys due to its combination of corrosion and oxidation resistance, high thermal conductivity, a low thermal expansion coefficient and availability in the necessary product forms to fabricate complex components.[4,5] It is commonly produced in a
SHARHID JABAR, MARTIN STRANGWOOD, and GEOFF D. WEST are with the WMG, University of Warwick, Coventry, CV4 7AL, UK. Contact e-mail: [email protected] JOHN A. SIEFERT is with the EPRI, 1300 W W.T.Harris Blvd, Charlotte, NC 28262. Manuscript submitted August 6, 2020; accepted October 11, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
wide variety of product forms including seamless pipe (ASTM A335 P91),[6] seamless tubes (ASTM A213 T91),[7] forgings (ASTM A182 F91),[8] plate (ASTM A335 Gr. 91),[6] and castings (ASTM A217 C12A),[9] among others. After shaping, P91 components are subjected to a series of normalizing and tempering heat treatments carried out in the temperature ranges of 1040 C to 1080 C and 730 C to 800 C, respectively.[6] The intent of this heat treatment cycle is to form a uniform tempered martensitic structure strengthened by three primary mechanisms: 1) solid solution strengthening via Mo alloying, 2) dislocation strengthening via the tempered martensite matrix, and 3) precipitation strengthening via (Fe,Cr) M23C6-type carbides and (V,Nb)(N,C) MX-type carbo-nitrides.[10] When P91 and other 9 to 12 wt pct Cr components are subjected to elevated operating temperature (~ > 550C), the precipitation of other phases may occur (which are not formed during tempering). These include the Mo-rich Laves intermetallic phase (Fe2X) where X signifies Mo/W and the modified Z-phase (Cr(V,Nb)N). Z-phase in these steels is well known for its detrimental effect on creep performance as it forms at the expense of smaller MX particles, which results in net lower precipitation strengthening. Z-phase is not often reported in 9 wt pct Cr steels, this is likely because it is only observed after extended aging times (> 30,000 hrs) even at relatively high temperatures (e.g., 650 C).[11–13]
The intermetallic Laves phase is known to precipitate and grow early during high-temperature exposure through the segregation of Mo and Si to martensite lath boundaries located next to M23C6 carbides.[14] Similarly, it
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