Microstructure evolution of hot-work tool steels during tempering and definition of a kinetic law based on hardness measurements

Abstract

Microstructural evolutions of the 55NiCrMoV7 steel during tempering were investigated by transmission electron microscopy, scanning electron microscopy and X-ray diffraction in order to describe the main mechanisms of softening. The softening resistance is strongly associated with evolution of obstacles to the movement of dislocations (prior austenitic grain boundary, lath boundary, secondary carbides, etc.). Only the average size of carbides was found to be influenced by tempering conditions. Moreover, a strong correlation observed between the hardness measured after tempering and the average size of carbides showing that this easy test could in this case partially characterize the state of the microstructure after tempering. Performing hardness measurements at the as-quenched, tempered and annealed states, a kinetic law of tempering based on the work of Johnson, Mehl and Avrami has been proposed. This law was validated in the case of complex tempering and for other steels and can well describe the evolution of hardness during tempering.

Brittle Fracture Behaviors of Large Die Holders Used in Hot Die Forging

“…Figure 11 is the bright-field images of region 4 and region 6. The microstructure of the two regions are both lath martensite and carbides [11].…”

“…The large die holder studied in this paper is made of 55NiCrMoV7 hot-work tool steel, which is similar to 5CrNiMo in chemical composition except with higher content of Cr and V. Compared with 5CrNiMo, tool steel, which is similar to 5CrNiMo in chemical composition except with higher content of Cr and V. Compared with 5CrNiMo, the 55NiCrMoV7 has better hardenability and abrasion resistance. Furthermore, it has been widely used in aviation and motor industries [11]. Zhang et al [12] studied the low cycle fatigue behaviors of 55NiCrMoV7 under different working temperatures.…”

Identifying the dominant failure mode in the hot extrusion tooling used to forge nickel based superalloy

“…Doege et al (2004) modelled the resultant phase transformation that may occur upon applying lubrication to a hot tool surface. Zhang et al (2004) measured and modelled the softening of tempered martensitic hot work tool steel during thermal exposure representative of tooling used in a hot forming environment. Bernhart et al (1999) investigated the plastic flow behaviour of tempered martensitic tool steel under fatigue and found the material to soften under cyclical loading.…”

Efeito do Nb na cinética de revenimento de aços de baixo carbono

“…Contudo a dureza não pode indicar, de forma clara, o amaciamento do estado temperado para o estado de equilíbrio. Assim, a razão de revenimento, τ, é o parâmetro adequado para se avaliar a cinética de revenimento, a partir dos valores da dureza das amostras no estado revenido, temperado e recozido [11] As Figuras 5 e 6 mostram a razão de revenimento como uma função do tempo de revenimento, para os aços I e INb, respectivamente. A razão de revenimento aumenta com o aumento do tempo de revenimento para o aço I, para as temperaturas de revenimento utilizados.…”