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.