Journal of Welding Science

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A517 is a low alloy high-strength steels that due to its high strength, toughness and weldability is used in ship building and submarine hulks. The welded areas of this steel often require repairs. In this study, the effect of number of welding repair on microstructure and mechanical properties of A517 steel is studied. Four samples (samples without repair, once repaired, twice repaired, and three times repaired) were welded by SMAW welding. Microstructural studies were carried out by using optical and scanning electron (SEM) microscopes. The effect of the number of repairs on mechanical properties of samples were investigated by using tensile, bending, impact and hardness The profile of hardness illustrated that the hardness in the heat affected zone near the base metal increased by repeated repairs while the hardness of this zone reduced in the third repaired sample. By repeating the welding repair, tensile and yield strengths of the welding areas were reduced and fracture impact toughness of heat affected zone at -51^○C was increased. Generally, the results of tensile tests of second and third repaired indicated that the strength of these samples were not meet the ASME IX standard requirements, so welding steel A517 in the second and third repairs is not acceptable.
 

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The surface of continuous casting moulds with high number of castings may be worn or destructed. As result, an approach for increasing these moulds life is necessary. In this project, the goal is the restoration of the DHP copper sample. In this project, the destruction of the copper sample is done by creation of groove using a CNC machine. The restoration of the sample is done using OAW and filler to fill groove area. In this project, the effect of preheating temperature, filler type and heat treatment of welding area on hardness, microstructure, chemical analyses of welding area and thermal conductivity of the weld are investigated. The preheating temperature range of 300 to 450^oC was selected. The Cu-P and Cu-Ag-P fillers were chosen to fill the groove of the weld area. The scanning electron microscope (SEM), energy dispersive x-ray spectroscopy (EDS), micro hardness tester, optical microscope and thermal conductivity meter were employed for evaluation of the results in this project. The results showed that the increase of preheating temperature creates oxide layers and the decrease of preheating temperature causes the incomplete filling of the welding area. Finally, the preheating temperature of 400 ^oC was a proper choice considering the above mentioned factors. The stress relieving operation to decrease stress and preserve the mechanical properties in the temperature of 250 to 400 ^oC and duration two hours was carried out. The result demonstrated that the selected temperature causes no unwanted decrease on the hardness. It was also found that increasing the annealing duration, decreases the hardness of weld for Cu-P filler for Cu-Ag-P filler increasing the annealing duration, first decreases the weld hardness and then increases the weld hardness. The Cu-P filler was compared with Cu-Ag-P filler. The results showed that the Cu-Ag-P filler has less hardness (around 10 percent) than the filler without silver. On the other hand, the thermal conductivity of the Cu-Ag-P filler was around 10 percent more than the thermal conductivity of the Cu-P. It is obvious that the selection of the filler type depends on the type of base metal and its geometry. The results showed that the segregation in the Cu-P filler with 7.2 percent phosphorous, because of the proximity of the weld structure to the eutectic point, has slightly happened; while, the selection of the Cu-Ag-P filler with 6 percent silver caused severe segregation of silver to 90 percent silver at the center of weld at the non-dendrite area

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In this study, in order to modify the weld structure obtained from repair welding of AZ91C magnesium alloy and improvement of tensile strength, input parameters such as current intensity and preheating temperature were optimized for this alloy. T6 heat treatment was separately done befor and after the welding to homogenize the microstructure and improvement of the mentioned properties. Using variance analysis, the accuracy of the models was checked and analyzed. Optical microscopy, scanning electron microscopy (SEM), Energy dispersive X-ray spectroscopy (EDS) and tensile tests were used to characterize the microstructure and mechanical properties of the repaired parts. The results of microstructural studies showed that the samples 2 (samples that were subjected to T6 heat treatment before and after welding) had continuous precipitates which these precipitates affected the strength due to the interruption of more slip planes and creating stronger barriers in the path of dislocations, resulting the better mechanical properties as compared with samples 1 (samples that were subjected to heat treatment only after welding). Also, by plotting response surface graphs and level diagrams, the highest tensile strength for samples 1 was observed at preheating temperatures of 493 to 513 K and current intensities of 80 to 90 A, and for samples 2 at temperatures of 513 to 553 K and current intensities of 100 to 110 A.

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