The distribution of the residual stress in the weld joint of HQ130 grade high strength steel was investigated by means of finite element method (FEM) using ANSYS software. Welding was carried out using gas shielded arc welding with a heat input of 16 kJ/cm. The FEM analysis on the weld joint reveals that there is a stress gradient around the fusion zone of weld joint. The instantaneous residual stress on the weld surface goes up to 800～1000MPa and it is 500～600 MPa, below the weld. The stress gradient near the fusion zone is higher than any other location in the surrounding area. This is attributed as one of the significant reasons for the development of cold cracks at the fusion zone in the high strength steel. In order to avoid such welding cracks, the thermal stress in the weld joint has to be minimized by controlling the weld heat input.

High strength steel; heat-affected zone; M-A constituent; fine structure.

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

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.