Effect of Thermomechanical Treatment on the Grain Boundary Character Distribution in a 9Cr-1Mo Ferritic Steel
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of ferritic steels is widely used as structural material in the production and transport of steam in power plants, because these steels possess good high-temperature strength, oxidation/corrosion resistance, and thermal properties. Due to its excellent void swelling resistance against neutron irradiation compared to the austenitic stainless steels, ferritic steel is also being considered as a candidate material for wrappers in nuclear core applications of fast reactors.[1] The bcc ferrite structure, however, exhibits a considerable reduction in fracture toughness below the ductile-to-brittle transformation temperature (DBTT). Temper embrittlement generally enhances the intergranular mode of fracture, leading to an increase in DBTT or reduction in upper shelf energy of the steel, wherein impurity
T. KARTHIKEYAN and V. THOMAS PAUL, Scientific Officers E, S. SAROJA, Scientific Officer G, and M. VIJAYALAKSHMI, Scientific Officer H, are with the Physical Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, India. Contact e-mail: [email protected] S.K. MISHRA, Research Scholar, and I. SAMAJDAR, Professor, are with the Department of Metallurgical Engineering and Materials Science, IIT Bombay, Powai, Mumbai-400076, India. Manuscript submitted March 18, 2009. Article published online July 24, 2009 2030—VOLUME 40A, SEPTEMBER 2009
elements such as P, S, As, and Sb tend to segregate to the grain boundary and lower its cohesive strength and promote intergranular fracture.[2] Precipitation of secondary brittle phases such as carbides and intermetallics along the grain boundaries can also reduce the grain boundary cohesive strength. In addition, displacement damage under the radiation environment in the nuclear reactor can lead to irradiation hardening, causing a further shift of DBTT to higher temperatures. Thus, it is important to develop the ferritic steel with a low DBTT and enhanced resistance to temper and irradiation embrittlement. The high-energy ‘‘random’’ boundaries are preferential sites for segregation and precipitation of embrittling phases, compared to the low-energy ‘‘coincident site lattice’’ (CSL) boundaries. Grain boundary engineering seeks to increase the fraction of low-angle, CSL boundaries, and thereby reduce the susceptibility to failure by intergranular fracture.[3] In fcc alloys, the twinning process helps to increase CSL fraction and break the random boundary network. Multiple cycles of strain annealing significantly improve the CSL fraction to as high as 70 pct in fcc alloys.[4] In contrast, no such generic thermomechanical treatment (TMT) has been reported for bcc systems. Gupta et al.[5] carried out several sets of TMTs on modified 9Cr-1Mo steel, and a TMT consisting of 5 pct reduction in thickness followed by normalizing and tempering (N and T) treatment resulted in the highest CSL fraction of 0.36 compared to 0.28 of the starting material. The modified 9Cr-1Mo steel is generally used in the N and T condition (1333 K/1 h/air cool and 1033 K/2 h), which results in a tempered martens
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