Cementite particles in AISI 1045 steel


Figure 1: The changes in the distribution of cementite particles for different tempering time at 973 K in martensite. (a) quenched (b) 5 min(c) 30 min (d) 2 hr (e) 10 hr (f) 50 hr. Scale bar: 3 µm.


Figure 2: The coarsenining behaviors of cementite particles for different tempering time at 973 K in bainite. (a) quenched (b) 5min (c) 30 min (d) 2 hr (e) lO hr (f) 50 hr. Scale bar: 3 µm.


Figure 3: The chtmges in the matrix in martensite during tempering at 973 K. (a) quenched (b) 30min (c) 2 hr (d) 10 hr. Scale bar: 50, 100 nm.


Figure 4: The changes of bainitic microstructure during tempering at 973 K. (a) quenched (b) 30 min (c) 2 hr (d) 10 hr. Scale bar: 20, 80, 100 nm.

Carbide name: Cementite
Record No.: 1092
Carbide formula: Fe3C
Carbide type: M3C
Carbide composition in weight %: No data
Image type: SEM, TEM
Steel name: AISI 1045
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: AISI 1045
Other designation: No data
Steel group: Medium carbon steels
Steel composition in weight %: 0.45% C, 0.22% Si, O.62% Mn, 0.004% P, O.0038% S.
Heat treatment/condition: To produce the different initial microstructures, the rods with lO mm diameter were austenitized at 1173K for 30 min followed by quenching to water or the salt bath with the temperature of 653K for 1 hr. For temperlng treatments, the heating rate was lO C/min, and heating temperature was 973 K. By varying holding time at 973 K, the samples were quenched into water to investigate the coarsening behavior of cementite particles.
Note: The effect of initial microstructures, martensite and bainite, on the coarsening behavior of cementite particles during tempering at 973K for medium carbon steels has been investigated. The coarsening of cementite particles in bainite proceeded more slowly than in martensite, due to the thermal stability of cementite particles in bainite. The coarsening of cementite particles proceeded by a combination of the different coarsening mechanisms. The observed coarsening kinetics in martensite were found as acombination of boundary diffusion and diffusion along dislocation for cementite particles at boundaries, and a combination of boundary diffusion and matrix diffusion for cementite particles within laths. In bainite, the coarsening was controlled by a combination of boundary diffusion and diffusion along dislocation for intergranular particles, and controlled mainly by diffusion along dislocation for intragranular particles.

Figure 1 shows the changes in the distribution of cementite particles during tempering at 973 K. Characteristic features after short tempering time less than 2 hr (Figs. 1(b)-1(d)) are the appearanceof elongated regions which are almost denuded of cementite partlcles. The similar shape and size of the denudedareas in Fig, l(b), tempered for 5 min, to that of martensite needles in Fig. l(a), indicate that cementite particles are preferentially located at the martensite needle boundaries and that the denuded areas are associated with needles. At the very beginning of tempering martensite, the nucleation of cementite starts with the concurrent eiimination of epsilon-carbide within martensite laths, retained austenite decomposesin to ferrite and cementite at lath or prior austenite boundaries. Particles at boundaries have the lower interfacial energy than equivalent sized particles in the matrix. Since the dissolution of smaller particles within martensite laths occurs due to the solute diffusion from the matrix to boundaries, the number of intragranular particles would decrease with the progress of spheroidization and coarsening. Consequently, the coarsening of cementite particles proceeds by the growth of large cementlte particles at boundaries with the dissolution of smaller particles within martensite laths and results in the formation of the denuded areas. Further increment of tempering tlme up to 2 hr (Figs. l(c)-1(d)) shows the increase of the size of denuded areas, although they still retain the elongated shape, as well as the slze of cementite particles at boundaries. The observed non uniformity in the particle size maybe due to an initial growth of boundary particles during the early stage of tempering. In addition, small boundary particles may dissolve and allow grain boundary diffusion to favor the growth of larger neighbors during tempering. Accordingly, cementite partlcles at boundaris are always larger than intragranular particles. After lO hr tempering (Fig. 1(e)), the presence of equiaxed ferrite grains is observed and most cementite particles are located at ferrite boundaries. A few cementite particles remainlng inside ferrlte grains would come from the result of the migration of ferrite grain boundaries during grain growth. Longer temperlng time of 50 hr, Fig. l(f), produces little change in the microstructure, except for the growth of ferrite grains and cementite particles.

The coarsening behavior of cementite particles in bainite at 973 K in Fig. 2 is different from that in martensite. Since the transformation of austenite into bainite occurs at relatlvely high temperatures, cementite particles tend to be coarser and more thermally stable than those associated with tempered martensite. Furthermore, since most cementite particles with the similar size are located at lath boundaries, lath boundaries, or prior austenite boundaries, the reduction of concentration gradients of solute atom would slow down the coarsening behavior of cementite particles during tempering. Accordingly, unlike martensite there has been little change in the distribution and size of cementite particles up to tempering tirne of 2 hr (Fig. 2(b)-2(d)), although the size of cementite particles located at boundaries are slightly larger than those located inside. The only difference detected in the micorgraphs is the change In the morphoiogy of cementite particles from initially rod-shaped to rather spheroid-shaped. Consequently, only mlnor changes in the morphology of cementite particles are observed. Thus, it can be said that bainitic mlcrostructures are muchless sensitive to tempering. However, after lOvhr tempering (Fig. 2(e)), the nonuniformity in the dlstribution and size of cementite particles, which is associated with the coarsening of cementite particles, can be observed. This morphology maybe due either to the growth of larger neighboring particles at boundaries with the dissolution of smaller boundary particles, or to boundary plnning being more effectlve for large particles durlng tempering. When tempering time incre'ases up to 50 hr (Fig. 2(f)), this difference becomes more evident. The microstructure consists of mainly coarse cementite particles located at feerrite boundaries, and fine particles inside boundaries.

The changes in the matrix structure have an influence on coarsening behavior of cementite particles during tempering of martensite. After 30 min tempering the substructure in the elongated form is well developed and all cementite particles are associated with subboundaries or dislocations, as shown in Fig. 3(b). Although, in the present investigation, a quantitative analysis regarding the effect of tempering time on the substructure size and the dislocation density has not been performed, the apparent increase of the substructure width and decrease of dislocation density from martensite in Fig. 3(a), indicate that the polygonization and the annihilation of dislocations are operative as a recovery process. As tempering proceeds up to 2 hr (Fig. 3(c)), the presence of more equiaxed subgrains is detected, although the stringers of cementite particles are still remained. For longer tempering time of 10 hr in Fig. 3(d), the microstructures are similar to those observed in Fig. 3(c), except for the growth of subgrains, the more randomly distributed and large sized cementite particles.

Figure 4 shows the changes of bainitic microstructure during tempering at 973 K. When tempered for 30 min in Fig. 4(b), the well developed substructure in the elongated form indicates the progress of recovery. All the observed cementite particles, which are either rod shaped or spheroid-shaped, are located at subboundaries or dislocations. However, unlike martensite, most particles observed in Fig. 4(b) shows the little variatlon in the size and the width of the substructure is much narrower than those observed in martensite (Fig. 3(b)). This implies that the higher thermal stability and fine distribution of cementite particles in bainite would effectively hinder the movementof dislocations and slow downthe polygonizatlon and the annihilation of dislocations during recovery. When tempering time increases up to 2 hr (Fig. 4(c)), any noticeable change in the microstructures, such as the size of cementite particles and the shape of substructures is not detected. Only the changes observed in Fig. 4(c) are the shape of cementite particles from the rod-type to spheroid-type, although they still exist as the stringers of particles, and the increase of the width of substructure. For longer tempering time of 10 hr in Fig. 4(d), the presence of equiaxed subgrains can be detected and the size of cementite particles connected with subboundaries Is increased. Although the general features are similar to those observed in Fig. 3(d), the size of subgrains and cementite particles are smaller than those in martensite.
Links: No data
Reference: Won Jong NAM, Effect of Initial Microstructure on the Ooarsening Behavior of Cementite Particles, ISIJ lnternational, Vol. 39 (1999), No. 11, pp. 1181-1187.

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