Effect of Ti on Evolution of Microstructure and Hardness of Martensitic Fe–C–Mn Steel during Tempering

Abstract

The effect of the addition of 0.042 wt.% of titanium on the relation between the evolution of the microstructure and the softening kinetics of quenched martensite in high-purity Fe-C-Mn steel has been studied during tempering at 300 and 550°C. The evolution of the microstructure is characterized by measuring the cementite particle size, the martensite block size, the area fraction of martensite regions which contain a high dislocation density, the macroscopic hardness, the nano-hardness of martensite blocks boundaries, the nano-hardness of the matrix and the TiC-precipitate size during tempering. Nucleation of TiC-precipitates take place during annealing at 550°C and starts earlier in regions close to the block boundaries, after 5-10 minutes, and thereafter in the matrix, after 10-30 minutes, due to the higher dislocation density in the regions close to the block boundaries. The TiC-precipitates slow down the recovery in regions of high dislocation density compared to the alloy without TiC-precipitates. The TiC-precipitates increase the macroscopic hardness of the steel after 30 minutes annealing at 550°C. The growth of TiC-precipitates in martensite is simulated in good agreement with experimental observations by a model that takes into account: 1) capillarity effects, 2) the overlap of the titanium diffusion fields between TiC-precipitates, and 3) the effect of pipe diffusion of titanium atoms via multiple dislocations. The average, experimentallyobserved, TiC-precipitate size is 69 ± 48 Ti atoms.

Modelling the Evolution of Multiple Hardening Mechanisms during Tempering of Fe–C–Mn–Ti Martensite

“…The value that we find for α = 0.401 is in good agreement with the literature reports ranging from 0.24 to 1. 18,40) We note that the experimental work of 10) show that the concentration of carbon atoms in true solid solution (between laths) and the concentration of carbon atoms segregated to lath boundaries (high dislocation density) are 0.005 wt% and 0.045 wt% respectively, after 60 minutes of tempering at 550°C. This indicates that the majority of carbon atoms in the martensite have segregated to dislocations and that Eq.…”

“…This value corresponds well to our fitted value of 1.34 GJ/m 3 or 4.74 kJ/mol. Figure 3 shows the calculated TiC precipitate size distribution together with the TiC precipitate sizes measured by Ohlund et al 10) The precipitate size as measured by APT is based on the APT atom count for each precipitate 1 . The KWN model predicts that 0.27% of the equilibrium volume fraction of TiC forms and that the average TiC precipitate radius is 1.38 nm after 60 minutes of isothermal tempering at 550°C.…”

“…26) Studies have furthermore shown that both TiC nucleation and growth is enhanced by the presence of dislocations. 10) Literature furthermore reports that the presence of TiC precipitates in martensite affects the evolution of the dislocation density in steel during the heat treatment. 10,18) The modelling of TiC nucleation and growth in martensite must therefore be linked to dislocation density and recovery in martensite during tempering.…”

“…9) The simulation is following the same temperature profile as applied directly after quenching during the experiments of Ohlund et al 10) …”

“…The SEM-measurements in Ref. 10) are used to determine the Fe 3 C-particle width, w, and length, l, which are used to calculate the volume fraction of Fe 3 C precipitates. We calculate the volume of each Fe 3 C-particle that is measured within a square area of SEM images using The total volume of all particles is summarized within each sample.…”