Experiment and research on high strength bolts in specific environments

The crane beam is located on the 205 ton loader between the open furnace span and the ingot span. Steel production is very tight. There are no conditions for full suspension of reinforcement. Therefore, it brings great difficulty to the reinforcement of the crane beam. The reinforcement program was studied several times with the production plant and the design and construction unit. Finally, use the jacking scheme - replace the upper and lower strings and the contact plate with the column joist. The connections are made of torsion-cut high-strength bolts. However, whether this torsion-shear type high-strength bolt can be used in the reinforcement and maintenance project can be applied to the connection between the old and new steel structures, especially whether it can work at high temperature. What is the difference between the connection performance and the normal temperature? These are the problems that must be solved. .

The bolts of the specimens are all 26 mm in diameter. They are made of torsion-cut high-strength bolts made by Shanghai Pioneer Screw Factory and have a diameter of 2 mm. Considering that the rivet holes of the original crane beam gusset plate are large, the bolt holes and bolt diameters differ by 4 mm. The bolts are fastened by the Japanese electric wrench. In order to accurately obtain the pre-tension of each bolt, two resistive pieces are symmetrically attached to the polished rod portion of each screw. To prevent the wires from being squeezed. The temperature is controlled by a thermocouple, an adjustable temperature measurement millivoltmeter (E-type l) and a contactor. The loading is stopped when the secondary main slip load is obtained. At this point, the test machine draws a load-deformation curve. To determine the slip load, several lines were drawn on the side of the test piece to observe the misalignment of the cover and core. Loading situation. The tensile test of the hot state test piece was carried out on an East German ZDML-400T horizontal tensile test machine.

350°C cycle 20 times test piece load-deformation curve 5250°C load-deformation curve weight 18 tons 45 test piece 12 weight 204-56350°C load-deformation curve 2 under normal temperature weight 18, heavy Sichuan test piece load-deformation curve ton 45 test piece 1 less C45 °C load-deformation curve load-deformation curve at normal temperature ton 45 test piece 4420 °C load-deformation curve When the normal temperature test piece slips, it produces a loud sound, and the specimen slip after heating or hot state is slowly carried out. Only intermittently weak sliding sounds occur.

In order to understand the decline of the friction coefficient at different heating temperatures, it is defined as the coefficient of reduction of the friction coefficient, and N is defined as the reduction factor of the slip load. among them. f and N are the friction coefficient and the slip load at each temperature. f and N are the friction coefficient and slip load at room temperature; this can eliminate the difference in the pre-tension of the friction surface treatment. The pre-tension of the bolts of the two batches of the test is different. The average pre-tension of the specimen after heating is 23.4t. In the hot state, the top tensile force of the test piece is 2.7t. If the slip load reduction coefficient is used to compare the error, it is more appropriate to use the coefficient of reduction of the friction coefficient. This test data and the data of Japan’s Tanaka and others The test data (l) is plotted on it, and it can be found that the variation law at 3 ° C and 450 ° C is similar to a straight line. Tanaka’s data is on the right side of the line. Because the test is carried out under normal conditions, rather than over large holes, the reduction factor is slightly higher than this test, but the downward trend is consistent. Therefore, 350 ° C can be used as a sudden change in the high-strength bolt slip load and friction coefficient, which is the same as the foreign test results. Relationship between slip load, friction coefficient and number of thermal cycles Table item cycle 1 cycle 10 cycles 20 times slip 5,636.1 Slip load, friction coefficient and number of thermal cycles can be seen from 1 and with thermal cycle When the number of times increases, the slip load and the friction coefficient decrease, and decrease by about 3 compared with the cycle, but it does not decrease indefinitely as the number of thermal cycles increases.

The reasons for the gradual decrease of the sliding load and the friction coefficient of the friction type joint at high temperature can be considered to be caused by the following factors: the thread portion of the bolt and nut subjected to high pressure is significantly deformed due to annealing, and the axial force of the bolt is relaxed. The gasket is plastically deformed at high temperatures, and the bolt axial force is also relaxed. After heating, the friction surface is oxidized to reduce the friction coefficient. The elastic modulus of the bolt and nut material is reduced. Therefore, it can be considered that the main reason for the high temperature downshift load and friction coefficient reduction is the stress relaxation of high-strength bolts. 2 Conclusion According to the experimental results, the friction coefficient of the high-strength bolt friction joint is 0°C when it is heated to 0°C.