Development of Friction Flash to Tube (F2T) and application to S355 grade steel

Friction Flash to Tube (F2T) is an innovative friction based manufacturing technique to produce seamless tubes based on open die forging, invented at Aalto University. These tubes can be produced economically in small sizes and batches, envisaging applications of high value materials that are not available in the market. The objective of this Master thesis was to develop the experimental condition of F2T as well as the proper parameters in F2T by approaching Taguchi method. The pre-defined parameters to investigate in Taguchi method were established as forging force, tool rotation and initial transient plunging depth and the investigating of geometrical and metallurgical characteristics were done. Cold rolled high strength and low alloy structural steel S355 is the material used in this research work. The parameters of the F2T process were developed based on design of experiments, with geometrical and hardness properties as performance parameters. The optimized conditions and parameters were applied to produce tubes for extensive evalua-tion of the mechanical and metallurgical material properties. The F2T process has specific components and control demands that cannot be met by the existing manufacturing systems. This challenge was overcome by developing one first version of a dedicated system based on an existent Friction Stir Welding equipment. One additional challenge was to produce tubes longer than 40 mm because of buckling. The buckling was prevented by implementing a lateral support system constraining the consumable rod during the initial transient plunging period. The test specimens for extensive mechanical test and metallurgical analysis were extracted from tubes produced with 80 mm in length. These longer tubes were manufactured using the support system to prevent buckling. The results on tensile test, flattening test and flare test of F2T tube reveal that the mechanical properties of produced tubes are as good as tube of similar material produced by another manufacturing technique. The temperature during the application of the F2T process was monitored with thermo-couples. The mechanical properties of produced tubes were evaluated by hardness meas-urement of cross and longitudinal sections. Tensile test were applied to sub-sections of wall of the tubes, and flare and flattening test to the whole tubes. The metallurgical analysis encompasses optical microscopic analysis, and SEM/EBSD with grain size evaluation. The research work demonstrate the feasibility of producing seamless tubes by F2T in structural steel. A correct design of a dedicated system to prevent the buckling, enables to produce long tubes

Microstructural characterization of alloy 926 and 2205 duplex stainless steels coating layers deposited over high strength steel by friction surfacing

Friction surfacing (FS) is a solid state processing technique used to deposit thick coating layers over a substrate enabling several metallic combinations. FS can be used in the production of new components or as a repair technique, typically to recover base metals from corrosion or wear loading. In this paper, UNS N08926 and UNS S32205 duplex stainless steels were investigated as consumable rods for coating a substrate made of S700MC grade high strength steel. The FS was applied under controlled plunging speed. Rods with dimensions of 180 mm long and 12 mm in diameter were used to deposit a coating with approximately 100 mm in length. The rods were made of duplex stainless steels. An L-9 Taguchi experimental design was used to optimize and study the effect of the rod rotational speed, the rod feed speed (Vz), and the travel speed (Vx) on the coating geometrical characteristics and its microstructure. The rotational speed was changed from 1500 to 2500 rpm, the plunging speed from 2.0 to 3.0 mm/s and the traverse speed from 350 to 550 mm/min. The FS was done without inert gas protection. The geometry of the resulting coating was characterized by a stereoscopic equipment. The microstructures were analyzed by optical microscopy and by EBSD technique, at center, advancing side and retreating side. The thermal profiles were assessed by IR camera and thermocouples. Results show a difference in the austenite volumetric fraction using both characterization techniques, but the trend of the results were similar. Both duplex stainless steels presented an austenite percentage lower than the original rods percentages. In the case of UNS N08926, the minimum volumetric fraction was 1.0% and the maximum austenite content was 18.0%, compared to 26.0% in the original rod. For UNS S32205, the minimum value was 20.0% and the maximum austenite volumetric fraction was 40.0%, compared to 48.0% in the original rod. Also, the amount of austenite at the advancing side was larger than at the retreating side, and both larger than at the middle of the coating. These results can be explained by the thermal cycles and by the plastic deformation. The optimal welding parameter maximized the volumetric fraction of austenite in the three regions of the FS coating. Keywords: Friction surfacing; Duplex stainles