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A study of microstructure evolution and mechanical performance of vacuum electron beam welded IN738LC joint was performed. The joint obtained a typical bowl-like weld profile. No macro-cracks occurred in the fusion zone (FZ). The results of EPMA showed that obvious elemental segregation formed in the FZ. Microstructure characterization of FZ exhibited the existence of secondary micro-constituents, comprised of the ordered γ′ phase, MC carbide, γ/γ′ and γ/MC eutectic. The solidification processing of the precipitations in the FZ was adequately expounded. In the heat affected zone (HAZ), the γ′ particles were dissolved partially. The distance to fusion line and the size of γ′ particles affect the extent of dissolution of γ′ particles. liquation cracking was also observed, mainly resulting from the constitutional liquation of γ′ precipitates. The micro-hardness of the joint from base metal (BM) to FZ increased gradually, related to the formation of more MC carbides in the FZ and high-density reprecipitated γ′ phase in the FZ and HAZ. All the joints fractured in the BM at room temperature (RM) and 900 °C. Fracture analysis indicated that the joint tested at RM failed with a quasi-cleavage fracture mode, and the tensile fracture at 900 °C showed a dominant ductile rupture.

The weldability of K465 nickel-based superalloy by electron beam weld (EBW) was studied, and the microstructural evolution and cracking characteristics were also investigated. The microstructure of fusion zone (FZ) were mainly γ dendrite, Chinese-script MC carbides, the interdendritic γ/γ′ eutectic and fine γ′ particles precipitated from dendritic and interdendritic region. The solidification cracking of FZ and intergranular liquation cracking in heat affected zone (HAZ) were observed in the joint. Solidification cracking was caused by the residual liquid metal originated from segregation of Ti, Nb and Al elements in interdendrite region at the last stage of solidification. And the HAZ liquation cracking was associated with the constitutional liquation of γ′, MC carbides, and melting of Cr-rich boride. With the increased heat input, the susceptibility of solidification cracking and liquation cracking presented different variation. And this variation of cracking susceptibility was discussed in this paper.

An economical route has been developed to prepare high purity titanium carbide from Panzhihua ilmenite, which has high contents of Mg and Si. In this paper, high purity (>99 wt%) TiC particle was prepared by vacuum carbothermal reduction followed with acid leaching and alkali leaching processes. The current process has a lower reaction temperature, and a shorter reaction time, meanwhile the used raw materials are much cheaper than the industrial method of preparing TiC which used TiO2 and C as the raw materials. Effects of atmosphere, reduction temperature and carbon ratio were studied and the purification mechanisms of elements Mg, Mn, Ca, Si, Al were put forward. Mg and Mn could be separated in gaseous state during vacuum carbothermal reduction. Al was reduced to Al4C3 after reduction and then was removed in acid leaching process. Meanwhile, CaO was also removed in the acid leaching process. SiO2 was reduced to the dissolved silicon in liquid iron firstly, then formed amorphous silicon after acid leaching, and finally was removed in alkali leaching process. The as-prepared TiC powder had a narrow particle size distribution within 1–20 μm and Dv50 was 5.29 μm.

IN738LC joint by vacuum electron beam welding (VEBW) exhibited high strain-age cracking (SAC) susceptibility and SAC mainly propagated along grain boundaries (GBs) of fusion zone (FZ) after post-weld heat treatment (PWHT). SAC occurred in the temperature range of 700 °C ∼1000 °C when IN738LC joints were subjected to PWHT with air atmosphere. The time to crack initiation decreased rapidly as the holding temperature rose. The investigation of influence factors of SAC indicated that the precipitation of γʹ particles, age hardening, oxidation-induced embrittlement and residual stress had a significant impact on the formation of SAC in FZ. Age-contraction stress induced by the precipitation of γʹ particles and residual stress acting on FZ were the essential factors to induce SAC. In addition, age hardening originating from the precipitation of γʹ particles and surface oxidation of weld metal exacerbated the formation of SAC during PWHT.

The carbothermic reduction of Panzhihua ilmenite with activated carbon in vacuum was investigated by isothermal experiments over the temperature range of 1473–1773 K. The reaction mechanism of the impurity elements in vacuum was investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) and thermodynamic calculation. It was found that when the reaction temperature was higher than 1673 K and the reaction time was longer than 4 h, the main phases of the final products obtained at 10 Pa were Fe and TiC, without the presence of MgO and MgAl 2 O 4 which existed when roasting at 1 atm in argon atmosphere. The contents of Mg and Mn in the products decreased with increasing the reaction temperature or lowering the pressure. After the carbothermic reduction process, the main impurity elements of Si and Al were dissolved in the iron.

Influence of pre-weld heat treatment on the microstructure and mechanical properties of electron beam welded IN738LC superalloy was investigated systematically. The microstructure of as-cast base metal (BM) mainly consisted of “ogdoadically” diced cube γ′ particles, fanlike γ-γ′ eutectics, MC carbides, Cr–Mo borides and Ni–Zr intermetallics. Grain-boundary liquation cracking was observed in the HAZ of as-cast joint, resulting from the segregation of elemental boron. Pre-weld heat treatment had a significant influence on the size, volume fraction and distribution of γ′ particles. It also decreased the non-equilibrium segregation of boron during cooling, which inhibited the formation of grain-boundary liquid film and liquation cracking. The precipitation of MC carbides and nanoscaled γ′ particles in the fusion zone (FZ) improved the micro-hardness of the FZ. The micro-hardness of the BM and the HAZ were influenced by the volume fraction and the size of γ′ particles. All the joints fractured in the BM at RM and 900 °C. The as-cast and water quenched (WQ) joints failed with a quasi-cleavage fracture mode at RM and 900 °C while the tensile fracture of furnace cooled (FC) joint showed a ductile rupture at RM and 900 °C.

Cyclic plasma oxynitriding and cyclic plasma nitriding catalyzed by rare earth La of AISI 420 martensitic stainless steel were performed and compared with conventional plasma nitriding. The nitrided layers were investigated by means of an optical microscope, microhardness tester, Auger electron spectroscopy (AES), X-ray diffraction (XRD), wear machine, scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The results show that the wear resistance of AISI 420 martensitic stainless steel is improved significantly by the two new rapid and deep plasma nitriding techniques. The new techniques increase the surface hardness of the nitrided layers and make the microhardness profiles gentler, which are consistent with the nitrogen concentration depth profiles. Meanwhile, the nitrided effect improves with increasing cycles. It was also found that the optimum phase compositions of nitrided layers with more γ′ phases and less ɛ phases for long-term service conditions can be obtained by the two new techniques, which is in agreement with the microstructure. In addition, traces of Fe3O4 were found in the cyclic plasma oxynitrided sample. Combining the SEM and EDS analysis indicated the existence of La in the nitrided layer of the sample under cyclic plasma nitriding catalyzed by rare earth La.