Carbide precipitation kinetics of a 2000 MPa grade highstrength spring steel

School of Materials Science and Engineering, Jiangsu University, Jiangsu Zhenjiang 212013, P.R. China
e-mail: xiaobo1814@163.com

Received: 17 November 2015
Accepted: 21 April 2016

Abstract

The carbide strain-induced precipitation kinetics of a 2000 MPa grade Nb-V high-strength spring steel during isothermal deformation is quantitatively investigated using the stress relaxation method. The stress relaxation technique was employed to determine the precipitation start (P_s) and finish (P_f) times. The evolution of carbides during the isothermal holding is observed by Transmission Electron Microscope (TEM). The results show that the precipitation-time-temperature (PTT) diagrams of the carbides in the experimental steel exhibit a C-curve, with a nose temperature of 900 °C. The precipitation kinetics of carbides in the experimental steel is slower than that of plain carbon steels, which is attributed to the reduced activity of Nb. The result indicates that the precipitate is a Nb, V, Ti and C-containing complex carbide at deformation temperatures ranging from 850 °C to 1050 °C. Large parts of the precipitates are (Nb,V)C, and there is also a tiny amount of TiC. The carbides are dispersed below 900 °C and distributed in clusters near dislocations and grain boundaries above 950 °C.