타이타늄 중공마더빌렛 주조재의 열처리공정 최적화 연구

Study for Heat Treatment Optimization of Titanium Hollow Casted Billet

Abstract

${\alpha}$-titanium alloy has a relatively low heat treatment characteristic and it is mainly subjected to heat treatment for residual stress, recovery or dynamic recrystallization. In this study, commercially pure titanium hollow castings was fabricated by gravity casting. Heat treatments were carried out at $750^{\circ}C$, $850^{\circ}C$ and $950^{\circ}C$ to investigate the effect of post-heat treatment on microstructure and mechanical properties. Beta-transus temperature ($T_{\beta}$) was about $913^{\circ}C$, and equiaxed microstructure was shown at temperature below $T_{\beta}$ and lath-type microstructure at temperature above $T_{\beta}$. Microstructure and mechanical properties did not show any significant difference in the direction of solidification for titanium hollow billet, so it can be seen that it was a well-made material for extrusion process. The optimum heat treatment condition of hollow billet castings for the seamless tube production was $850^{\circ}C$, 4 hr, FC, indicating a combination of equiaxed microstructure and appropriate mechanical properties.

Fig. 1. Microstructures of gravity casted titanium; (a) center (b) middle (c) side (×100).

Fig. 2. Optical micrographs of the heat-treated CP Ti samples for (a) 750°C, vertical (b) 750°C, horizontal (c) 850°C, vertical (d) 850°C, horizontal (e) 950°C, vertical (f) 950°C, horizontal (×50).

Fig. 3. Engineering strain-stress curves of the as-casted and heat-treated samples.

Fig. 4. Tensile properties of the as-casted and heat-treated samples.

Fig. 5. Vickers hardness variation of CP Ti with different heat treatments; (a) as-casted, (b) 750°C, (c) 850°C, and (d) 950°C.

Fig. 6. Schematic diamgran of new seamless tube manufacturing process.

Table 1. Chemical compositions of CP Ti (Grade 2)