Effects of rare-earth micro-alloying on microstructures, carbides, and internal friction of 51CrV4 steels

The effects of rare-earth (RE) micro-alloying on the microstructures, mechanical properties, and internal friction (IF) of 51CrV4 steels after quenching and tempering were investigated. The addition of RE elements was beneficial for refining the size of the martensite and increasing the fraction of twins in the 51CrV4 steels. The RE-51CrV4 steel possessed a high dislocation density and a large fraction of low-angle grain boundaries after quenching. Cr7C3 carbides of RE-51CrV4 steel tended to precipitate along the lath martensite interfaces and were easily spheroidized during tempering. With the addition of RE elements, the Snoek–Köster–Kê (SKK) peak was increased after quenching but decreased after different tempering times. The results of the IF and high-resolution transmission electron microscopy analyses indicated that RE atoms formed micro-segregations at the lath martensite interfaces and resulted in a pinning effect on the carbon atoms. Therefore, the hardenability of the RE-51CrV4 steel was reduced, whereas its elongation improved significantly due to the presence of high-density twins and spheroidized carbides.