Microstructure performance in the welding zone of T91 heat-resistant steel under the condition of TIG welding was researched by means of metallography, X-ray diffraction and scanning electron microscope (SEM). Experimental results indicated that microstructure of T91 weld metal was austenite + a little amount of 5 ferrite when using TGS-9cb filler wire. Substructure inside the austenite grain was crypto-crystal lath martensite, on which some Cr23C6 blocky carbides were distributed. The maximum hardness (HRC44) in the welding zone is near the fusion zone. There existed no obvious softening zone in the heat-affected zone (HAZ). For T91 steel tube of φ63 mmx5mm, when increasing welding heat input (E) from 4.8kJ/cm to 12.5kJ/cm, fracture morphology in the fusion zone and the HAZ changed from dimple fracture into quasi-cleavage fracture (QC). Controlling the welding heat input of about 9.8kJ/cm is suitable in the welding of T91 heat-resistant steel.

The fine structure in the Fe-Al alloy layer of a new hot dip aluminized steel (HDA) was examined by means of X-ray diffractometry (XRD), electron diffraction technique, etc. The test results indicated that the Fe-Al alloy layer of the new aluminized steel mainly composed of Fe3Al, FeAl and a-Fe (Al) solid solution. There was no brittle phase containing higher aluminum content, such as FeAl3 (59

Microstructure, precipitates and fracture morphology in the coarse grained heat-affected zone (CGHAZ) of a new high-purity 0Cr18Mo-Ti ferritic stainless steel were studied by means of optical metallography, SEM, TEM, X-ray diffractometer, etc. Experimental results indicated that grain coarsening resulted in brittle fracture in the CGHAZ of 0Cr18Mo-Ti steel. The reduction of impact toughness in the CGHAZ due to change of cooling rate can be attributed to the increase of nitrides (TiN, Cr-N, etc). These nitrides in the CGHAZ promote initiation and propagation of brittle cracks. The precipitated Cr2N nitrides in the grain boundaries decrease impact toughness in the CGHAZ of 0Cr18Mo-Ti steel by promoting crack initiation. In practical applications, the welding heat input (E) should be as low as possible to prevent toughness reduction in the CGHAZ.

The distribution of hydrogen in the welding zone of HQ130 high strength steel is calculated by using finite element method (FEM). The finite element program of hydrogen in the welding zone is worked out. In the program, the effects of welding heat input (q/v), temperature and surface-escaping coefficient of hydrogen, etc., are taken into account. Crack morphology in the fusion zone and effect of diffusion hydrogen on cracks are analyzed. Cracks originated in the partially melted zone at about 20-60m from the melting interface line and propagated parallel to the fusion zone or turned into the weld metal. Hydrogen accumulates seriously near the fusion zone, particularly at the root fusion zone. This is one of the importance reasons for the forming of cracks in this zone. The test results and analysis indicate that welding heat input should be controlled about q/v=16.0kJ/cm to prevent cracks being produced and propagated for HQ130 steel.

The brazing parameters and microstructure in the interface of WC-TiC-Co hard alloy and the brazing filler were investigated by means of inside-furnace brazing, scanning electron microscopy (SEM) and X-ray diffraction. Test results indicated that the brazing joint with which the interface combines excellently can be obtained by using Cu-Zn-Ni brazing filler alloy and controlling the heating temperature 940-960℃, the heat preservation time 10-15min and a suitable cooling. The crystal microstructure of the brazing filler alloy is α+β eutectic. The interface microstructure of the hard alloy and the brazing filler alloy distribute evenly. The interface zone consists of WC, TiC, CuZn (s-phase). There are no microcracks, inclusions, etc. nearby the interface. The interface zone is formed by mutual diffusion of the hard alloy and the brazing filler alloy under high temperature.