Effect of Tempering Conditions on Secondary Hardening of Carbides and Retained Austenite in Spray-Formed M42 High-Speed Steel.

In this study, the effect of tempering conditions on microstructure, grain size, and carbide phase compositions of spray-formed high-speed steel after quenching at 1180 °C was studied. The influence of carbide phase, size of carbides, and retained austenite content on secondary hardening of the steel was analyzed by field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), electron backscattered diffraction (EBSD), and differential scanning calorimetry (DSC); the hardness, microhardness of carbide, and bending strength were tested. The results show that M3C, M6C, M7C3, and MC carbides may precipitate at different tempering temperatures and the transformation of the retained austenite can be controlled by tempering. The phase composition of carbides, microstructure, and retained austenite content strongly influences the performance characteristics of M42 high-speed steel after tempering. In contrast, the secondary carbides produced by tempering thrice at 540 °C are mainly M6C carbides rich in W and Mo elements, and the content of retained austenite is effectively reduced. At this stage, the Rockwell hardness reaches 67.2 HRC, bending strength reaches 3115 MPa, and the properties and microstructure are optimal.

Superstable Magnetic Nanoparticles in Conjugation with Near-Infrared Dye as a Multimodal Theranostic Platform.

Near-infrared (NIR) dyes functionalized magnetic nanoparticles (MNPs) have been widely applied in magnetic resonance imaging (MRI), NIR fluorescence imaging, drug delivery, and magnetic hyperthermia. However, the stability of MNPs and NIR dyes in water is a key problem to be solved for long-term application. In this study, a kind of superstable iron oxide nanoparticles was synthesized by a facile way, which can be used as T1 and T2 weighted MRI contrast agent. IR820 was grafted onto the surface of nanoparticles by 6-amino hexanoic acid to form IR820-CSQ-Fe conjugates. Attached IR820 showed increased stability in water at least for three months and an enhanced ability of singlet oxygen production of almost double that of free dyes, which will improve its efficiency for photodynamic therapy. Meanwhile, the multispectral optoacoustic tomography (MSOT) and NIR imaging ability of IR820-CSQ-Fe will greatly increase the accuracy of disease detection. All of these features will broaden the application of this material as a multimodal theranostic platform.