Fatigue strength of steel AISI 321 laser welded seams

The main difficulties in austenitic steels welding are associated with the need to increase the resistance of the weld metal and heat affected zone to formation of hot cracks, which are usually divided into crystallization and subsolidus ones. Increased resistance of metal to formation of crystallization cracks is achieved by suppressing the columnar crystallization and structure refining by increasing the cooling rate, alloys purity, using the doping elements-modifiers or elements contributing to the formation of high-temperature second maximally plastic phases (e.g. δ-ferrite). These methods narrow the temperature range of brittleness and increase the plasticity reserve. To increase the resistance of austenitic steels to formation of subsolidus hot cracks during welding, the following methods are recommended: alloying with elements that contribute to creating a fragmented cast structure, increasing the purity of the base metal of interstitial impurities, reducing the time spent by the metal at a high diffusion mobility (increasing the cooling rate of the weld metal), restricting deformations by selecting a rational design of joints, etc. The methods listed above are realized in laser welding, which is characterized by high rates of heating and cooling, a little time of stay of the metal in the molten state. It reduces the diffusion interaction and contributes to formation of fine fragmented cast structure of the joint material. Intense convective stirring of the melt in the weld pool helps to remove non-metallic inclusions. A special role can be played by adding refractory nanopowders (NP) into the forming material the welds. Specially prepared well-wettable refractory nanopowder particles, being introduced into the melt, form a dispersed system in which the solid phase serves as the core of each suspension particle. As a result, each nanoparticle becomes a potential seed for the emergence of the new phase. Due to this, during cooling of the melt, a fine crystalline structure is formed in it, thereby increasing the mechanical characteristics of the solidified alloy. The paper addresses the problem of increasing the strength of the weld on the example of AISI 321 (12Kh18N10T) steel. One-piece welded joints are made by laser welding with the use of nanopowder additives. The values of fatigue strength of the welded joints of the steel under investigation produced with a CO 2 laser and additives of nanopowders TiN and Y 2O 3 clad with titanium and iron are determined. The role of the microstructure, grain size, the nature of distribution of microhardness in the formation of the fracture surface under chosen test conditions is studied. It is found out that the average value of tensile strength for the weld is 690 MPa, which exceeds its value for the steel itself (650 MPa). Even the presence of micropores in the material of the welds did not reduce the mechanical properties compared to the base ones. The relief of the sample fractures corresponds to the viscous failure. The additives of nanopowders increased durability of the material of the joints obtained 2.8 times at the maximum cycle stresses above 460 MPa. At that, the zones of complete fracture by the mechanism of viscous failure constituted 65% of the total area of samples fractures with nanopowders and 78% without them. At lower values of the maximum cycle stresses, share of the complete fracture zone was about 50% of the area of sample fractures.

laser welding, strength, fatigue, microstructure, microhardness, fractography