No abstract

The number of steel-frame buildings is increasing as a result of development of social economy. However, the fire-resistant property of steel-frame buildings is much weaker than that of brick-frame buildings and RC-frame buildings. In response to such demand, fire-resistant steel has been developed since the last two decades. Mo is one of the most effective strengthening elements for the high-temperature strength of steels. With the increase of the Mo content in steels, there is a dramatic increase in costs which is unacceptable for the cost-sensitive construction area. Therefore, a fire-resistant steel with a low Mo content is highly desired. Two fire-resistant steels with low Mo content (<0.3%) were designed in this paper. The interdependence of microstructure, properties and temperature was studied and analyzed in hot-rolled testing steels. The results show that the low-Mo fire-resistant steels have improved elevated temperature strength (The yield strength of Nb content steel is 240MPa, which can met the requirements of grade Q345 fire-resistant steel), low yield ratio (<0.6) and good welding performance (Ceq≈0.40). The bainite and fine grains are mainly beneficial to the elevated temperature UTS. Nb is an alloying element effective in increasing elevated temperature strength (especially for YS) as well as Mo.

Alumina-forming austenitic (AFA) heat-resistance steels firstly developed by Yamamoto et al. at Oak Ridge National Laboratory have been reported as a new promising class of steels with potential for use in high temperature applications in recent years. The creep resistance of AFA steels is improved mainly by precipitation strengthening. Besides modifying the typical existing precipitates, i.e. MC and M23C6 type carbides, B2-NiAl and Fe2Nb-type Laves phase, introduction of coherent L12-ordered precipitate is highly desired. L12-ordered phase gamma prime (γ’) is the most important precipitate for high-temperature strengthening in Ni-based superalloys. In the present work, we demonstrate that addition of 2.8 wt. % Cu to an AFA steel promotes the formation of an L12-ordered phase with the dominating elements Ni, Cu and Al. TEM characterization after slow rate tensile tests indicated there were the different precipitation behaviours at 700°C and 750°C. It was revealed that the occurrence of L12-ordered Ni-Cu-Al phase depends on temperature and Ni content. This opens up new opportunities to promote the formation of L12-ordered phase in Fe-based austenitic heat-resistance steels and benefit high-temperature mechanical properties.

NbSi2 is an attractive material for high temperature applications due to its high melting point, low density and good oxidation resistance. The high-temperature strength of NbSi2 is expected to be further improved by incorporation with Nb5Si3, which performs a high creep resistance and strength at high temperature due to its complex crystal structure. In this paper, directionally solidified NbSi2/ Nb5Si3 in-situ composites have been prepared using an optical floating zone method. Scanning Electron Microscopes (SEM) and X-ray diffraction (XRD) have been used to investigate the phase constitution and microstructure. The orientation relationship between Nb5Si3 and NbSi2 is investigated by transmission electron microscopy (TEM). High-temperature properties of alloys are tested by compression at the strain rate of 1×10-4/s at 1673K and 1773K. It was found that high temperature strength and phase constitution of directionally solidified alloys depended on the addition of Mo.

In this paper, high-quality single crystals of (Mo0.85Nb0.15)Si2, around 8 mm in diameter and 90 mm in length, have been grown by optical heating floating zone method. Effects of chemical composition and growth rate on forming C40 structured single crystal were studied. Aligned C40/C11b lamellar structured can be formed in the as-grown crystals after post annealing at temperatures between 1473 and 1873 K. Chemical composition as well as annealing temperature are found to be two important factors to form C11b lamellae in the C40 matrix. Fully C40/C11b lamellar structure was formed after annealing at 1873 K in the present work. The aligned C40 and C11b lamellae follow a crystallographic orientation relationship of (0001)C40//(110)C11b. Dislocations were observed in some coarse C11b lamellae but never in C40 lamellae of the duplex structure. This is probably due to accumulation of misfit strain during formation of C40/C11b lamellae.