The Study of Local Effect of Manganese on Microstructure Development of Admixed Fe-Mn-C Sintered Steels

Microstructure of FeMn particles in specimens Fe-3 Mn

The corresponding changes of FeMn particles after annealing at the temperatures of 750–1,200°C are presented in Figure 1(a)–(e).

Figure 1:

(a)–(f) Structure of FeMn particles in specimens Fe-3Mn; (a) as-pressed; (b) 750°C; (c) 900°C; (d) 1,050°C; (e) 1,150°C; (f) 1,200°C.

Figure 1(a) shows specimens surface in the initial state after compaction where regularly shaped FeMn particle can be distinguished. Figure 1(b) shows the FeMn particles after sintering at 1,023 K indicating some changes in particle morphology even at this low temperature. These little changes of the FeMn particles surface are the consequence of the starting of manganese sublimation that increases considerably after ~1,013 K, causing an important redistribution of Mn in the vapor phase. This results in appearance of alloyed rims around FeMn particle even at such low temperature, see Figure 2 (a), (b).

Figure 2:

(a), (b) Microstructure of Fe-3Mn and Fe-3Mn-0.3C, respectively, after sintering at 750°C.

This effect was firstly found and reported by Šalak [4, 23, 24] and represents an active alloying mechanism described before as diffusion-induced grain boundary migration (DIGM) [25]. This phenomenon of alloying with the presence of gas phase for iron-manganese system was firstly described by Navara [26] and further developed by other authors [27, 28]. DIGM is a recently recognized phenomenon that leads to unexpected motion of grain boundaries. Vastly enhanced mass transport characterizes the low-temperature aspect of the phenomenon, since the grain boundaries provide easy paths for diffusional redistribution of atoms in the regions traversed by the boundaries [29]. The measured values of mentioned thickness of alloyed regions at 1,173, 1,323 and 1,473 K are presented in Table 2.

The fine “sponge” morphology of the surface of FeMn particles occurs after the annealing at 1,173 K (Figure 1(c)). The annealing in the temperature in range of 1,323–1,473 K results in the intensive changes of FeMn particles (Figure 1(d)–(f)). During annealing at temperature of 1,473 K FeMn particle undergo significant changes in shape, size and surface morphology.

Microstructure developments

The microstructures of investigated materials are complex and heterogeneous, consisting of austenite, martensite, bainite and fine pearlite depending on the local Mn and C contents.

Microstructure of sintered at 1,173 K specimen, see Figure 3 (a), (b), consists mostly of the ferrite (original Fe particles) and the pearlite regions close to the particles edges indicate start of the pearlite formation, see Figure 3(a).

Figure 3:

(a), (b) Microstructure of Fe-3Mn and Fe-3Mn-0.3C, respectively, after sintering at 900°C.

Small pearlite islands are the result of carbon diffusion into the austenite. More extended Mn-enriched areas around FeMn particles indicate intensive manganese transport due to DIGM, see Figure 3(b). Intensive sublimation of manganese with temperature increasing promotes Mn redistribution in the vapor phase within the powder compact and increases Mn content on the prior powder surface of iron particles. The gradient of Mn concentration in iron matrix becomes a driving force for accelerated alloying which involved DIGM mechanism.

It should be noticed that investigated microstructures present the most alloyed regions around the pores in the places of the Mn sources. Manganese from the FeMn particles is concentrated around the pores and in the necks volume. It appears that the primary porosity inside iron powder is also relatively permeable. As the temperature increases, Mn vapor reacts with oxygen and water vapor. Formed oxides are accumulated in residual pores, in accordance with other authors [1, 6, 9, 11, 28]. Salak [15] suggested that to utilize to the maximum extent the effect of sublimation and condensation of manganese vapors as a primary alloying mechanism, iron compacts must have mainly open pores, at least at the start of sintering, this ensures that by filling these pores the manganese vapors can affect the surface of all particles. Manganese vapor is supposed to be flowing, mainly through the secondary porosity. Ciaz et al. [7] postulated that manganese vaporized (sublimed) and was trapped in the pores. With the presence of oxygen and water vapor, even in the reduced dew point cases, it reacts with this vapor and/or resublimed into the pores seen in these measurements. The secondary porosity is originated from the sublimation of ferromanganese particles.

The ferromanganese donors, especially with high carbon content, are able to generate a liquid phase at sintering temperature of 1,393 K according to Castro et al. [28,30]. Due to the progressive diffusion of Fe atoms into the ferromanganese particles and of C atoms away from them and into the Fe matrix, lower stability of liquid phase in the case of low-carbon ferromanganese is expected. Analysis of this phenomenon was presented by Šalak [27]. A bulk diffusion equation for D_Mn in Fe_α and D_Mn in Fe_γ is presented by Nohara and Hirano [31, 32], as well Wells and Mehl [33] and Kučera and Stránsky [34]:

and

where R [J · mol⁻¹ · K⁻¹] is the gas constant, and T [K] is the absolute temperature.

Calculations of diffusion coefficients and diffusion distances for Mn in Fe matrix after sintering at different temperatures with 180 s; along with experimental measured value are summarized in Table 3.

It is clearly evident that the data of apparent diffusion coefficients in bulk manganese diffusion in Fe matrix are lower than data for sintered manganese steels. The highest values are registered due to mentioned characteristics of sintered system concerned with manganese evaporation, its condensation on iron powder particles surface and further transport by diffusion and diffusion-induced grain boundary migration mechanisms. The results shows that for investigated manganese sintered steels manganese travels into iron matrix up to 18×10⁻⁶ m in 180 s at the temperature of 1,473 K for both investigated systems. Amicarelli et al. [35] studied with the manganese diffusivity phenomena, compared sintered and wrought steels and determined that carbon migrates toward the manganese-rich zone due to the affinity of manganese for carbon. Manganese diffusion is enhanced by alloying elements. For equal times and temperatures the manganese diffusion depth of sintered steels is larger than in wrought steel of similar composition. Mitchell et al. [36] analyzed changes of manganese concentration in “three-layer” sintered specimens. Specimens were prepared on the basis of sponge iron powder NC 100.24 that formed border layers and medium layer was composed on the basis of ferromanganese. Specimens were sintered at temperature of 1,393, 1,423 and 1,453 K, respectively. The present results along with previous mentioned results [26–28, 35, 36] are in agreement with conclusions that DIGM is the dominant alloying mechanism in sintered manganese steels.

Manganese penetration depth

Mn penetration depth of scale analyses was calculated on the basis of microanalysis of Mn content along a distance from Mn source, represented by FeMn particles, using:

where D^b is the coefficient of grain boundary diffusion; δ is the width of the grain boundary; a is the Mn penetration depth.

Consequently, the penetration depth a was calculated according to eq. (3) using l from experiments and the coefficient of grain boundary diffusion, as [26]:

and the width of the grain boundary:

and the distance l from experiments at the time t=180s (Table 3).

Therefore, the results for sintering temperature of 1,173, 1,323 and 1,473 K in Fe-3Mn are listed in Table 4.

DIGM mechanism, along with intensive sublimation of manganese (occurring at high temperatures and promoting Mn redistribution in the vapor phase) and liquid phase presence at temperatures of 1,393 K, caused improved manganese distribution around the original Fe particle boundaries. Therefore, Mn distribution depths in the iron matrix are greater than those calculated for Mn volume diffusion.