The hardfacing alloys with different concentrations of titanium were deposited on carbon steel substrates by shielded metal arc welding, and the effect of titanium content on the microstructure characteristics of the hardfacing alloys was investigated. The wear resistance test of the hardfacing alloys was carried out by using a slurry rubber wheel abrasion test machine, and the wear behaviour was also studied. The results indicate that the addition of titanium can effectively promote the precipitation of the complex carbides of Nb and Ti due to the prior precipitation of titanium carbide which acts as nucleation sites for complex carbides. With the increase of titanium content, the wear resistance of the hardfacing alloys is increased gradually resulting from the refinement of microstructure and dispersive distribution of fine carbide precipitates. And the wear mechanism is mainly minimum plastic deformation with interrupted grooves due to the strengthening and protecting effects of carbide precipitates.
The 304 stainless steel strips were deposited one layer on carbon steel base metal by electroslag strip cladding （ESC） and submerged arc cladding （SAC）, respectively. The solidification microstrueture of ESC metal was analyzed by the optical microscopy, scanning electron microscope and energy dispersive spectroscopy. The corrosion resistance studies of strip cladding metals were carried out in 10% oxalic acid electrolytic etching test. The results showed that the cladding metal obtained by ESC presented low content of C, high content of Cr and enough alloying element of Ni in the chemical composition. The transition zone of ESC with small width was almost parallel with the base metal, leading to a lower dilution. There are three types of solidification modes （ A→AF→FA ） occurred in the ESC metal due to the decrease of cooling rate and degree of dilution from the transition zone to the top of ESC metal. As a result, the microstructure of ESC metal exhibited mainly austenite with a small amount of ferrite, contributing to achievement of better corrosion resistance.
Niobium, as the most effective second-phase forming element, was added in the Fe-Crl3-C-N hard- facing alloy to get carbonitride precipitates. Morphology and composition of carbonitride in the hardfacing alloy were studied by optical microscopy, scanning electron microscopy, and electron probe microanalyzer. The ther- modynamics and the effect on the matrix of the formation of carbonitride were also discussed. It was found that niobium carbonitrides are complex Nb（C, N） precipitate distributed on grain boundary and matrix of the hardfacing alloy. Under as-welded condition, primary carbonitride particles were readily precipitated from the hardfacing alloy with large size and morphology as they were formed already during solidification. Under heat treatment condi- tion, a large number of secondary carbonitrides can pre- cipitate out with very fine size and make a great secondary hardening effect on the matrix. As a result, addition of niobium in the hardfacing alloy can prevent the formation of chromium-rich phase on grain boundaries and inter- granular chromium depletion.
Magnetically impelled arc butt welder is mainly used in the automotive industry for butt welding tubes.To extend the range of the welding applications,a new type of magnetically impelled arc butt welder for tube to plate welding had been developed.The welder consisted two parts of mechanical device and electrical control system,in which the mechanical device with an appropriate clamping,location and upsetting control was specially designed for simple operation.The electrical control system involved the power supply,magnetic field and logic control of the welding.So the starting and rotating behaviour of the arc in the welding region of tube and sheet can be accurately controlled for the quality of tube to sheet joints.As a result,the welder for tube to sheet welding could generate a stable rotating arc by adjusting the welding parameters,and had a constant high-quality joint.
The most effective carbide-forming elements titanium and niobium were added into hardfacing alloy.Formation and composition of carbides in the hardfacing alloy were investigated by means of optical microscope（OM）,scanning electron microscope（SEM）,X-ray diffraction（XRD）and energy-dispersive spectrometer（EDS）.Hardness and impact toughness of the hardfacing alloy were measured.The thermodynamics and formation mechanism of carbides were also discussed.It is found that the carbides consist of TiC and NbC which are able to form directly from welding pool during the welding process.The formation mechanism of carbides involves nucleation of TiC followed by epitaxial precipitation of NbC on the surface of TiC.The formation of titanium and niobium carbides can obviously refine the microstructure and deplete the carbon in the matrix.The micros tructure transforms to well-distributed carbides and a tough martensite matrix,contributing to a good combination of high hardness and high toughness in the hardfacing alloy.
