*3.4. Kinetics of Precipitation*

In Figure 3 it was shown that precipitation of Ti(C, N) can start from the liquid. So, it is difficult to control the size and distribution of the Ti(C, N) particles in the structure. However, SEM pictures of the Q&P-treated sample shows small precipitates as well (Figure 7). EDS study of the particles confirmed the Ti content of these large and sharp-edged particles.

Beside these very large precipitates form during casting, there are some other particles that can nucleate, grow, or coarsen during welding and Q&P treatment. For example, in Figure 7 different particle types can be seen in the HAZ. For example, in region (b) a Ti(C, N) particle nucleated and coarsened during welding and Q&P; region (c) shows accumulation of nucleated carbides at the grain boundary and (d) shows transition carbides that are created during tempering of martensite laths in the partitioning stage.

**Figure 7.** SEM pictures of S(640, 50) displays secondary precipitates at different size and shapes.

As reported by Gustafson [35], there are two sizes of TiC particles, one with sparsely distributed particles of micrometer size and a second with densely distributed particles with sizes of a few tens or hundreds of nanometers. In this work, precipitates with the smaller size were studied, under the assumption that the large primary particles are so sparsely distributed that they will not affect the coarsening of the secondary ones, since the coarsening of the large particles is expected to follow a much slower process and has no important influence on the mechanical properties.

Since the precipitates (especially the small ones) from casting will melt in FZ during welding, the most critical area will be HAZ. In order to model the nucleation and growth of precipitation using TC–Prisma, isothermal heating at 1350 ◦C for 5 s is considered. Figure 8, shows that the equivalent average diameter of Ti(C, N) particles is around 150 nm.

**Figure 8.** Prediction of precipitate size in HAZ considering isothermal heating at 1350 ◦C for 5 s, resulting in a particle size of around 150 nm.

Comparison of the precipitate coarsening rates in corresponding matrix phases using the Thermocalc database shows that increasing the temperature from 540 ◦C to 640 ◦C will increase the coarsening rate of Ti(C, N) 1000 times to 1.27 × <sup>10</sup>–35 m3/s and of MoC 100 times to 3.875 × 10–31 <sup>m</sup>3/s.

Quantification of the precipitate size distribution in base material (BM) and in those partitioned at 540 ◦C and 640 ◦C was carried out by STEM from carbon replicas of the samples.

Figure 9 shows the particle size distribution for both partitioned samples (at 540 ◦C and 640 ◦C) and for the BM. The average particle size was calculated to be 0.263 ± 0.108 μm, 0.207 ± 0.089 μm, and 0.07 ± 0.11 μm for S(540, 5), S(640, 50), and BM, respectively. The precipitates quantification in the BM became more complicated since it presents particles over a large range of sizes. SEM analysis of precipitates show a few large particles (between 0.2 and 0.65 μm) at relatively low magnification (20,000×). However, at higher magnifications (e.g., 80,000×), a large quantity of small particles (<0.2 μm) can be distinguished. For this reason, the analysis of the precipitates in this sample was performed at two different magnifications.

**Figure 9.** Measured size distribution of precipitates vs. number of particles for post-welding heat-treated samples (**a**) at QT = 355 ◦C and PT = 540 ◦C for 5 s; (**b**) at QT = 355 ◦C and PT = 640 ◦C for 50 s; (**c**) base metal.

Figure 10 shows STEM images from C replicas corresponding to the samples. An apparently larger particle density can be observed in the sample partitioned at 640 ◦C, when comparing the same area of both samples (Figure 10a,b). However, the particle density has not been systematically analyzed in this case. Two hundred six and 388 particles were considered for S(540, 5) and S(640, 50), respectively, and this was compared with the base metal (see Figure 9).

Results indicate that the approximate average size of particles in treated samples are 0.263 ± 0.108 μm for the sample partitioned at 540 ◦C and 0.207 ± 0.089 μm for the one partitioned at 640 ◦C, while the number density of particles after partitioning at 640 ◦C is around 1.5 times higher than for 540 ◦C. This can be seen when comparing Figure 10a,b.

**Figure 10.** Comparison between distribution, number density, and shape of the particles in carbon replicas made from the sample: (**a**) S(540, 5) and (**b**) S(640, 50).

From EDS measurements, it was determined that the particles in samples partitioned at 540 ◦C and 640 ◦C are principally MoC (round shaped particles) and Ti(C, N) (rectangular shaped particles).
