3.1 .4 special electrode method
The method is to develop a new type of welding rod, the liquid metal and the liquid slag have high solubility, can improve the transition radius of the weld, reduce the toe angle, and reduce the stress concentration at the weld toe. Thereby increasing the fatigue strength of the welded joint. Similar to the shortcomings of TIG welding, it has strong selectivity to the welding position, especially suitable for flat welding position and flat fillet welding, but its advantages are significantly reduced for vertical welding, horizontal welding and overhead welding. .
3.2 Method for adjusting residual stress field to generate compressive stress
3.2.1 Pre-overload method
If a tensile load is applied to a specimen containing stress concentration until yielding occurs at the notch, accompanied by a certain tensile plastic deformation, after unloading, compressive stress will occur at the tensile plastic deformation of the loading gap and its vicinity. On the other cross-sections of the sample, there will be tensile stresses below the yield point that are in equilibrium with it. The sample subjected to this treatment, in its subsequent fatigue test, will have a different stress range than the original sample to which the pre-overload is not applied, i.e., significantly smaller, so that it can improve the fatigue strength of the welded joint. The research results show that large-scale welded structures (such as bridges, pressure vessels, etc.) need to undergo a certain pre-overload test before putting into operation, which is beneficial to improve fatigue performance.
3.2.2 Local heating
Localized heating can be used to adjust the welding residual stress field, that is, to generate compressive residual stress at the stress concentration, which is advantageous for improving joint fatigue strength. This method is currently limited to longitudinal non-continuous welds or joints with longitudinal stiffened plates.
For a single-sided corner joint, the heating position is generally about 1/3 of the width of the weld from the weld, and the heating position is the center of the plate for the double-sided angled joint. This ensures that compressive stresses are generated in the weld and the fatigue strength of the joint can be increased. Different researchers have different effects obtained by applying this method. For single-sided corner joints, the fatigue strength is increased by 145%-150%, and for double-sided angle joints, the fatigue strength is increased by 70%-187%.
The local heating position has an important influence on the fatigue strength of the joint. When the point heating is performed at the end of the weld, the compressive residual stress is caused at the notch of the end of the weld, and the fatigue strength is increased by 53%. However, when the spot heating is performed at the center of the end of the weld, the distance from the end of the weld is the same, although the same metallographic structure is produced, but since the residual stress is tensile residual stress, The measured joint fatigue strength was the same as that of the non-treated sample.
3.2.3 Extrusion method
The local extrusion mechanism is the same as the point heating method, that is, the fatigue strength of the joint is improved by compressing the residual stress. However, the point of action is different, and the pressing position should be at a position where residual compressive stress is required. High-strength steel samples are more effective than low-carbon steel by extrusion.
3.2.4 Gurnnerts method
Ohta used this method to successfully prevent fatigue cracks inside the butt pipe. The specific method is that the outside of the pipe is heated by induction, and the inside is cooled by circulating water. Therefore, a compressive stress is generated inside the pipe, thereby effectively preventing fatigue cracks from occurring inside the pipe. After the treatment, the fatigue crack growth rate of the butt weld pipe is greatly reduced, and the same crack growth rate as that of the base metal is achieved.
3.3 Ways to reduce stress concentration and generate compressive stress
3.3 .1 Hammering
The hammering method is a cold working method which acts to cause compressive stress on the surface of the joint weld toe. Therefore, the effectiveness of the method is related to the plastic deformation generated on the surface of the weld toe; at the same time, the hammering can also reduce the existing sharpness of the notch, thereby reducing the stress concentration, which is also the reason for greatly improving the fatigue strength of the joint. The hammer pressure recommended by the International Welding Society should be 5~6Pa. The top of the hammer should be 8~12mm diameter solid material. It is recommended to use 4 impacts to ensure the hammer depth is 0.6mm. Recent work by the International Welding Society has shown that for non-loaded T-joints, the joint fatigue strength increases by 54% at 2 x 106 cycles after hammering.
Since it is sometimes difficult to accurately determine the heating position and heating temperature of the local heating method, in order to obtain a satisfactory effect, Gunnert proposed a method in which the main point of the method is to directly heat the notch portion instead of the nearby portion to be plastically deformed but lower than the phase. Change the temperature to 55 ° C or 550 ° C, then spray it to cool. Since the underlying metal and the unsprayed metal around it are cooled later, shrinkage will cause compressive stress on the cooled surface when it is cooled. The compressive stress can thereby increase the fatigue strength of the member. It should be noted that in order to achieve the heating effect of the bottom layer, the heating process is slower, Gunnert recommends a heating time of 3 min, and Harrison recommends a heating time of 5 min.
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