Effect of Microstructure and Precipitates on Mechanical Properties of Cr–Mo–V Alloy Steel with Different Austenitizing Temperatures

Abstract

The mechanical properties of as-quenched and tempered steels are affected by austenitizing temperature. The present work has investigated the effect of austenitizing temperature on martensitic microstructure, carbide precipitates and mechanical properties of 30NiCrMoV12 alloy steel for the axle of high-speed train. The martensitic microstructure and carbide precipitates were studied using OM, FE-SEM, TEM, EBSD and EDS. Thermodynamic calculation of equilibrium precipitation were carried out by Thermo-Calc software. The results showed that the prior austenite grains, martensitic packets, blocks and laths were coarsening with increasing austenitizing temperature. Besides, with increasing austenitizing temperature, after tempering the amount of large size carbides precipitated at martensitic lath boundaries decreased while the amount of small size carbides precipitated in matrix increased. Meanwhile, phase transformation from M 23 C 6 to M 7 C 3 during tempering was enhanced with increasing austenitizing temperature. Coarse grains and wide martensitic laths were beneficial to reducing the amount of strip-like M 23 C 6 carbides precipitated at martensitic lath boundaries due to the reduction of boundary area and thereby obtaining more fine precipitates in matrix. The strength and impact toughness could be improved to a certain extent by refining carbides in tempered steel with higher austenitizing temperature. However, the degree of favorable influence on impact toughness resulting from refining carbides was lower than the negative effect from coarse martensitic structures. Therefore, the toughness is deteriorated and the strength is improved with increasing austenitizing temperature.

High-Temperature Precipitation Design-of-Experiments Simulation in Low-Alloy Cr–Mo–Ni Hot Forging Steel

“…The lamellar spacing indicated in Figure 6, as a function of the distance from the surface of the hot deformed bar, showed a higher difference for the middle zone where, due to the larger difference in the strain and cooling rate, the spacing was expected to increase. However, Figure 3 shows that the variation in hardness was larger in the upper zone, which means that lamellar spacing is not the only factor to determine the mechanical properties and that PAGS is also important [12]. It is also important that the 95% confidence interval at the center of the bar was large for both zones, as this is a zone where the cooling rate is lower and the effect of strain in lamellar spacing is stronger.…”

“…Grain growth models (steels with Ti and Nb), which consider the previous austenite grain size (PAGS), indicate that between 950 and 1200 • C, small precipitates (~300 nm) may form [11]. Moreover, austenitizing treatments promote the M 23 C 6 to M 7 C 3 transformation in Cr-Mo-V steels, and the packet size of martensite is linearly related to PAGS with trip-like M 23 C 6 precipitate at the martensite lathe boundaries [12].…”

“…In the as-received form, 1020 has a 23 µm grain size, while 8620 has 10 µm [13]. As impact toughness is significantly deteriorated by increasing the PAGS size [12] and these steels are used in automotive hot forging applications because of their high toughness, it is evident that PAGS controlling mechanisms must be active at 1000 or 1100 • C during deformation, with only Cr, Mo or Mn being responsible for the nanometric, short-term precipitation slowing down static and dynamic recrystallization and austenite grain growth. Table 1 summarizes the simulations or analyses of precipitates at high temperatures by the studies just mentioned.…”

“…For steels that require high toughness in the martensite state, PAGS must be low enough to ensure that cracks will not propagate. Reduction of the lamellar spacing in martensite is an additional way to maintain or increase this property [12]. This work analyzed the resulting microstructure of quenched steel after deformation at 1100 • C, in order to verify that the austenite grain size was small and also that the resulting lamellar spacing as a function of strain, being the deformed bar, was large enough to differentiate the microstructure and hardness characteristics at clearly separated points.…”

Evaluation of Mechanical Properties and Microstructures of Direct‐Quenched and Direct‐Quenched and Tempered Microalloyed Ultrahigh‐Strength Steels

“…Many researches have shown that coarse precipitates tend to decrease the impact toughness of steels. [ 26,27 ] Previously presented TEM investigation showed that NbC precipitates existed in the matrix of Mo–Nb steel.…”