Effect of the Microsegregation on Martensitic and Bainitic Reactions in a High Carbon-High Silicon Cast Steel

Abstract

Casting processes show some weaknesses. A particular problem is presented when the workpiece needs to be subjected to heat treatments to achieve a desired microstructure. This problem arises from the microsegregation phenomena typically present in cast parts. The effect of the microsegregation on the martensitic and bainitic transformations has been investigated in a high carbon-high silicon cast steel, with the approximate composition Fe-0.8C-2Si-1Mn-1Cr (in wt. %), which was poured into 25 mm keel block-shaped sand molds. The microsegregation maps of Cr, Si, and Mn characterized by electron probe microanalysis (EPMA) show that interdendritic regions are enriched while dendrites are impoverished in these elements, implying that their partition coefficients are lower that the unity (k < 1). As-quenched martensitic and austempered bainitic microstructures (at 230 °C) were obtained and analyzed after applying an austenitization heat treatment at 920 °C (holding for 60 min). The thermal etching method used to reveal the prior austenite grain size showed a bimodal grain size distribution, with larger grains in the dendritic regions (≈22.4 µm) than in the interdendritic ones (≈6.4 µm). This is likely due to both the microsegregation and the presence of small undissolved cementite precipitates. Electron Backscatter Diffraction (EBSD) analysis carried out on the martensitic microstructure do not unveil any differences in misorientation distribution frequency and block size between the dendritic and interdendritic zones related to the microsegregation and bimodality of the austenite grain size. On the contrary, the bainitic transformation starts earlier (incubation time of 80 min), proceeds faster and bainitic ferrite plates are longer in the dendritic zones, were prior austenite grains are larger and impoverish in solute. The presence of these microsegregation pattern leads to the non-uniform development of the bainitic reaction in cast parts, modifying its kinetics and the resulting microstructures, which would probably have a major impact on the mechanical properties.

Effect of Microsegregation and Bainitic Reaction Temperature on the Microstructure and Mechanical Properties of a High-Carbon and High-Silicon Cast Steel

“…In Figure 8b, which corresponds to the sample austempered at 330 • C, it is clear that the formation of bainite (black needles) is associated particularly to the low-alloyed areas or dendritic zones. As it was reported in previous works by the authors [29,46], this cast steel (with the same chemical composition and casted in the same mold sizes) presents a considerable microsegregation that results in dendritic areas with low amounts of alloying elements (≈1.6 wt.%Si, ≈0.8 wt.%Mn and ≈0.7 wt.%Cr) and interdendritic areas with a high concentration of alloying elements (≈2.6 wt.%Si, ≈1.6 wt.%Mn and ≈1.3 wt.%Cr). As Mn is an austenite stabilizer, high concentrations of this element promote slower bainitic transformation kinetics, while dendritic areas present faster reaction kinetics.…”

“…As it was described, the dendritic regions, which have the lower concentration of solutes, showed a larger average prior grain size than the interdendritic areas (≈22 and ≈6µm, respectively). As it was discussed in a recent paper by the authors [46], this difference in grain size is attributed to the combined influence of the following: (1) the presence of undissolved cementite in segregated areas would pin the grain boundaries, inhibiting grain growth; and (2) differences on transformation temperatures between dendritic and interdendritic areas associated to different concentration of alloying elements. In zones with a smaller ∆T=Tγ − Ac cm , austenite grains would have less driving force to grow and would remain smaller.…”

The Effect of Compositional Heterogeneity on the Martensite Start Temperature of a High Strength Steel During Rapid Austenitisation and Cooling

“…IOP Publishing doi:10.1088/1757-899X/1249/1/012061 7 PAGS affect found by Basso et al [12], albeit in a very different cast steel after an hour's austenitisation. Although carbides did appear in the 1100 °C samples, the grain boundaries appear to have enough time and/or enough mobility to move past the pinning carbides and form equiaxed grains in the duration of the experiment.…”

“…In most cases, the literature suggests that effect of banding appears to be reduced where the PAGS is greater than the width of the bands [1,10]. It was reported by Basso et al [12] that banding can create a bimodal PAGS distribution in a high carbon and silicon cast steel austenitising at 920 °C. This effect was attributed to the presence of undissolved carbides in one band.…”

“…It follows that local compositional changes could have a significant effect on phase transformation behaviour and various authors have shown that this is the case, although they often focus on situations where different microstructures/phases form in different bands within the same block of material [1,6,7,8,9,10]. In cases where compositionally banded steel is rapidly quenched to a martensitic or bainitic microstructure in both enriched and depleted bands, evidence of the banding can usually still be found after etching [1,[11][12][13]. In most cases, the literature suggests that effect of banding appears to be reduced where the PAGS is greater than the width of the bands [1,10].…”