In this paper, SEM, EPMA, and micro shear experimental method were applied to analyse the structure, inclusion morphology and composition, micro-area strength and toughness of the X70 pipeline steel girth weld joint. The results show that inclusions are mainly nonmetallic oxides, sulfides and calcium compounds. The length of inclusions can be up to 1mm, which have substantially reduced the strength and toughness of weld joint. By the combining effect of internal and external force, cracks tend to initiate around inclusions and propagate along the direction of inclusions, and finally reduce the impact toughness of girth weld.

The TiC/Ni composite coating was prepared by Laser Cladding In-situ Synthesis on the surface of damper plate with Ni--Ti-Mo-C -LaF3 powder．Microstructure and Wear resistance have been studied throughout EPMA、SEM and M—2000 Friction and Wear Tester．It was found that the microstructure got more homogenized grain fineness became finer, TiC distributed better and heterogeneous phase decreased, as adding moderate rare earth LaF3. When the content of LaF3 was 1%, the highest microhardness was got. While the optimal wear resistance properties were obtained with 2% LaF3. The laser cladding with excessive LaF3 has more TiC particle segregation which induces hardness and wear resistance of cladding layer.

Have studied microstructure of the gas nitriding layer and the laser quenching/ nitriding layer using the X-ray diffractometer and the transmission electron microscope to the brittl of nitriding layer. The results show that the content of Fe3N in composite nitrided layer increased from 14.74% to the 69.45%, while the crisp hard ξ-Fe2N content dropped from 79.95%to 25.03%, Especially the Cr2N particles are much finer and the depth of the nitrided layer increases. There have phase relation, coherence and compatibility between γ'-Fe4N and α-Fe. spherical particles of cementite was formed in lower strip bainite, while the γ′ of gas nitriding diffusion layer is big, the carbide assumes the long strip off and with distributing certain directivity. Brittleness of laser hardening/ nitriding layer reduces.