Relation between γ–α transformation kinetics and mechanical properties of vanadium steels

Abstract

A-study has been made of the influence of additions of Al and Ni on the γ–α transformation in steels based on 1V–0·2C. The kinetics have been determined using high-speed dilatometry and the resulting structures examined by thin foil electron microscopy. The mechanical properties of a systematic series of alloys have been determined after isothermal transformation in the range 650–900°C, and after continuous cooling at rates between 0·25 and 200 K min⁻¹. Under conditions of isothermal transformation it has been shown that Al, which raises the γ–α transformation temperature and accelerates the reaction, results in the highest strengths, whereas Ni, by lowering the transformation temperature and slowing down the reaction, gives the lowest strengths. In contrast, after continuous cooling treatments, Ni leads to the highest strength levels, whereas Al results in the lowest strengths obtained, alloys containing both Ni and Al falling between these two limits. A further more complex series of alloys with additions of AI, Si, Mn, and Ni showed similar trends in so far as the strengths obtained could be related to the effects that the alloying additions had on the γ–α transformation kinetics. Likewise, ductility was found to behave systematically with respect to the kinetics of reaction, and decreased with increasing yield strength. With cooling rates between 20 and 100 K min⁻¹ yield strengths in the range 800–1000 MN m⁻² could be achieved with ductilities between 10 and 20%; in these conditions Mn (or Mn + Ni) was the most effective alloying addition.