Thermal Analysis Experiments in Titanium and Magnesium Additions to 4130 Steel
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JMEPEG https://doi.org/10.1007/s11665-020-05065-5
Thermal Analysis Experiments in Titanium and Magnesium Additions to 4130 Steel Robert Tuttle (Submitted July 2, 2020; in revised form August 4, 2020) Titanium and magnesium additions singly and together were examined in 4130. A thermal analysis system measured the cooling rate of molten steel samples. These curves were analyzed for changes in phases evolved and phase reactions. The samples were also sectioned to determine the solidification structure. All additions refined the structure; however, magnesium and titanium additions together resulted in the finest structure. The peritectic reaction temperature was found to decrease with the additions. This decline may indicate an increase in the d-ferrite stability due to favorable nucleation of that phase. The solidus temperature also appeared to decline. Inclusion analysis found TiN particles in the magnesium/titaniumcontaining sample. Only titanium-containing oxides were observed in the titanium-only samples. It appeared that the addition of magnesium made the formation of TiN more favorable. Keywords
grain refinement, steel, thermal analysis, TiN
1. Introduction Solidification-based grain refinement has been gaining interest in steel research for the past decade (Ref 1-5). This interest stems from the fact that refining the microstructure continues to be the only known strengthening mechanism where strength and ductility improve together. Solidificationbased routes can include higher mold cooling rates, grain growth restriction, and increased nucleation during solidification (Ref 6). Practicalities of mold making limit the ability to increase cooling rates via mold material changes, and growth restriction can cause unwanted phases at the end of solidification that decrease ductility (Ref 7). Thus, solidification-based refinement using heterogeneous nuclei has been found to be the most effective method for reducing the as-cast grain size. Interest in titanium-based refinement stems from the formation of TiC and TiN. Early work on nucleation theory by Bramfitt discovered that the predictions of the Turnbull– Vonnegut equation were correct (Ref 8). He found that the undercooling required to begin solidification decreased as the lattice disregistry decreased. TiC and TiN had the smallest disregistry with d-ferrite and produced the lowest undercooling. Figure 1 depicts the crystal structures of TiC and TiN. Subsequent work has uncovered that the presence of Ti(C,N) to refine the structure of as-cast steels (Ref 5, 9-11). There is some debate as to how titanium carbonitrides reduce the grain size. Work in microalloy steels and some grain refining research have revealed that a grain pinning mechanism may cause the observed refinement (Ref 5, 12). Earlier work by the author observed that Ti(C,N) particles can form in the last 5% of the solidification range of a steel. Formation in the late stage of
Robert Tuttle, Saginaw Valley State University, University Center, MI. Contact e-mail: [email protected].
Journal of Materials Engineering and
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