| In recent years, due to the rapid growth of the price of Ni element, explore and research a kind of ferritc stainless steel which owns the equivalent corrosion resistance with austenitic stainless steel is an inevitable trend. Super ferritic stainless steel has excellent corrosion resistance. In addition, it also owns high thermal conductivity, high strength and excellent resistance to stress corrosion, which enable it to be widely used in the petroleum, chemical industry and building. However, there will be grains coarsening and precipitated phase embrittling in the joints during welding, which will lead to the deterioration of mechanical properties and corrosion resistance of the materials. Hence, it is quite necessary to search for a more suitable welding method as well as optimized welding parameters. As a kind of high energy-beam welding method, laser welding can achieve a bigger weld dimension in a relatively low heat input. Therefore, the coarsening of grains in the welded joint of ferritic stainless steel can be effectively avoided.The author of the present paper studied the continuous laser welding of 30Cr-4Mo and 26Cr-3.5Mo super ferritic stainless steel which developed by the Shanghai Bao steel Co. LTD., and researched the effect of lasers power and welding speed on the appearance, microstructure, mechanical and corrosion resistance properties of the welded joint.Results show that the appearance of welded joint is good, and there are no defects such as cracks and porosity using the welding parameters hereby. The welded joint is mainly composed of the central equiaxial crystal and columnar crystal at the edge of the weld. With the increase of the power or the decrease of the welding speed, the weld width, amount of central equiaxial crystal, grain size of the central equiaxial crystal and the width of the columnar crystal all increase gradually.For 30Cr-4Mo super ferritic stainless steel, there are many ? phase and ? phase on the base metal. But in the welded joint, there are only tiny Ti and Nb carbonitrides. The tensile strength and the percentage elongation after fracture of the joint are both lower than the base metal. The fracture happened in a ductile-brittle mixed mode. With the increase of the power, the pitting corrosion resistance of the welded joint increases firstly and then decreases, and keeps increase with the welding speed.For 26Cr-3.5Mo super ferritic stainless steel, the precipitates in the base metal and the welded joint are mainly Ti and Nb carbonitrides, and no ? phase and ? phase found. Moreover, the welded joint owns less precipitates. Under the experimental condition, the tensile strength of the welded joint of 26Cr-3.5Mo super ferritic stainless steel is similar to the base metal(it changes within the range of 2% under various welding parameters), but the percentage elongation after fracture decreased by 20%. The fracture mode of the welded joint is the microporous gathered type, and the precipitation of Ti and Nb carbonitrides leaded to the decrease of toughness of the material. The pitting potential of the base metal is higher than that of the 30Cr-4Mo super ferritic stainless steel. And the Ti and Nb carbonitrides which were found at the bottom of the corrosion hole played the role of inducing the happening of pitting corrosion. |
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