چكيده به لاتين
In this research, the effect of friction stirring process on the microstructure and mechanical properties of tempered martensitic steel has been investigated. The main purpose of this study is to increase the hardness of the steel sheet. From a steel sheet with a thickness of 9 mm and dimensions of length and width of 10×10 cm, which is kept for 30 minutes at a temperature of 900 °C and then cooled in water, then for tempering kept for 30 minutes at 550 °C and cooled in air (25 °C). The friction stir processing is applied to the sheet using a tungsten carbide tool with a rotation speed of 600, 950 and 1180 rpm, and a welding speed of 40, 60 and 180 mm/min. Performing the process at a rotational speed of 600 rpm and a welding speed of 180 mm/min the tool were failed, and at other conditions, tools wear and corrosion was observed. To solve this problem, the tool was cooled by spraying a solution of water and soap.
Microstructure study showed that fine grains were formed in the stirring zone due to continuous dynamic recrystallization. The microstructure of the base metal includes martensite and residual austenite, and the microstructure of the stirring zone includes martensite and ferrite bonds. The results of the X-ray diffraction pattern from the stirring zone showed that the peaks were flattened and the surface of the samples were oxidized. Optical microscopy images from the stirring zone of the specimens showed that the onion-rings were formed. In the tensile test results, the failure of the specimens occurred from the stiring zone and at lower than the base metal yield stress, which is due to the formation of onion-rings due to high rotational speeds, and is due to the formation of carbide tungsten, which has been inserted into the stirring zone due to wearing of the tool. The results of the microhardness diagram on the surface of the samples showed an increase in hardness, so that in the Advance side of the specimen with a rotational speed of 950 rpm and a welding speed of 40 mm/min, the hardness increased from 400 HV in base metal to 763 HV. The results of microhardness on the cross section of the thickness of the samples in the stirring zone also showed an increase in hardness by an average of 45-50% of the base metal, but in some places the amount of hardness was significantly different from other neighboring parts, which is due to the structure of onion-shaped rings and the presence of soft bands in the microstructure.
