Continuous Cooling Transformations in Nuclear Pressure Vessel Steels
- PDF / 2,866,625 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 50 Downloads / 262 Views
I.
INTRODUCTION
NUCLEAR power has over the last 40 years provided a usually reliable source energy, currently providing around 13 pct of the electrical power world-wide.[1] Low-alloy steels, such as the so-called SA508, have in this context been key materials for the manufacture of reactor pressure vessels. The steel is generally given a multiple heat treatment involving austenitization followed by water quenching and tempering at temperatures as high as 923 K (650 C). Within the broad specification of SA508, there is a particular variant, Grade 3, which exhibits better mechanical properties than earlier versions, and is the material of choice for pressure vessels in Generation III plants. There have been many studies of the final microstructure following the complete heat-treatment, with the conclusion that the quenching results in upper bainite, whilst the tempering leads to the formation of molybdenum-rich M2C* precipitates in addition to *‘M’ refers to metal atoms.
residual cementite.[2–15] Reactor pressure vessels (RPVs) are large components, so in spite of the water quenching, the cooling rate following austenitization varies significantly as a function of depth relative to the surface. Cooling rates of 0.3 K/s are common at the 1/4 thickness position, and 0.2 K/s has been measured at the mid-wall of a 340HECTOR POUS-ROMERO, Ph.D. Student, and HARRY K.D.H. BHADESHIA, TATA Steel Professor of Metallurgy, are with Materials Science and Metallurgy, University of Cambridge, Cambridge, U.K. Contact e-mail: [email protected] Manuscript submitted December 19, 2013. Article published online July 10, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
mm-thick forging component during the water quenching process.[8,16] Such variations introduce gradients of microstructure, and the atomic mechanisms by which the structure is generated.[8,12,13,17,18] Those structures generated by displacive transformation are preferred because they tend to be fine in scale and are subjected to severe tempering that relieves any excess carbon to precipitate desirable carbides. It has been reported that cooling rates of 0.2 K/s lead to fully bainitic microstructures.[17] However, modern pressure vessels can be 700 mm in thickness.[18] The question then arises whether the desired microstructures are in fact present in regions where the cooling rate is slowest. Previous studies also did not include the effects that the grain size may have on the hardenability of the steel. Therefore, the purpose of the present work was to undertake a detailed characterization of SA508 Gr.3 steels to establish conditions under which only the finer microstructures might be generated.
II.
EXPERIMENTAL PROCEDURE
The three slightly different SA508 Gr.3 commercialsteels studied are listed in Table I. Alloys A and C in their as-received state had been subjected to multiple heat treatments: austenitization between 1133 K and 1153 K (860 C and 880 C) for approximately 12 hours, water quenched and tempered between 908 K and 928 K (635 C and 655 C) for 10 hours. Steel B
Data Loading...
