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конструкционная прочность 05.09.12..docx
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8.2 Mechanical properties

Mechanical properties are directly related to the material structure of weld and base metal. Weld metal is comprised of the metals of electrode and molten edges of base parts. High temperature affects the structure of base metal. Grain size enlarges at boundaries of the weld joint - in the heat affected zone (HAZ). Large grains have relatively poor mechanical properties. Outside the HAZ grain size is the same as in the base metal. In molten weld metal the grains grow from colder parts of base metal. Lower amounts of molten metal correspond to lower heat and smaller HAZ. HAZ is smaller for electron beam welding: A. Arc welding, butt-joint B. Electron beam welding, butt-joint C. Multi-layer arc welding, butt-joint D. Gas welding, build-up weld For steel welds mechanical properties are usually highest at the weld. The hardness is lower in the weld. Mechanical properties of a high-strength steel weld depend on carbon content in the material. Increase in strength corresponds to decrease in ductility (elongation is a measure of ductility). Fast cooling, similar to quenching, could result in strength increase. Yield strength and ultimate tensile strength increase with higher cooling rate. There are special tests for weld joints. The welded specimen is tested until a crack first starts. The weld is stronger if it lies for a long time before a test. A large angle a characterizes the ductility of the weld joint. There are welded joints for which the angle could reach 180o.

8.3 Stress concentration

Passing through welds, inner forces meet obstacles on their path. They concentrate at the ends of weld. The force lines bend smoothly as they pass through the welds, lines cannot bend sharply. The figure shows a stress profile in the butt-weld. There is stress concentration in corners. Stress in the wider central section of weld doesn't exceed the nominal value. This stress pattern is typical for a welded joint. Sum of the area (force) under the curve must be equal to the sum of the area under the line corresponding to nominal value. Nominal shear stress is twice as large as the shorter welds. Stress concentration is higher if rigidities of connected parts are different. Stress is higher in the beginning of the short weld. Stress concentration depends on the surface shape, not inside geometry of the weld. The larger the angle q, the smaller the stress concentration factor. In order to fulfill these requirements, a special cutting operation is made. Fatigue strength of a machined joint is higher than the first one. A large fraction of inner force goes through the end nugget in the row. The numbers indicate approximate values of the parameter. Stress concentration can be evaluated by stress concentration factor as which is equal to the ratio of maximum and nominal stresses.

8.4 Defects

The quality of weld depends on many factors: A. undercutting is caused by high amperage B. porosity is caused by fast travel or dirty material surfaces C. slag included in bead is caused by low amperage and short arc D. lack of fusion is caused by low amperage and improper edge preparation E. overlap is caused by electrode shaking Surface defects perpendicular to tensile force are usually more dangerous than an inner defect of the same size. Lack of fusion, D is the sharpest and the most dangerous defect. The quality of manual welding is usually less than that for other methods. Some imperfections that are not dangerous: A. Electroslag welding: 0.56 defects / 10 meters. B. Automated welding under flux: 2.5 defects / 10 meters. C. Electric arc manual welding: 35 defects / 10 meters. A. Incomplete penetration B. Excess metal handing C. Curved weld D. Narrow weld at underside Incomplete penetration means that tensile force lines meet obstacles on their path, causing high stress concentration. Other defects from the list do not cause high stress concentration. Better melting takes place if there is a gap between the parts of Tee-weld. Residual stresses and cracking are smaller in this situation. A. Cracking: toe, longitudinal, transverse, and underbead cracks B. Incomplete fusion C. Undercutting and underfilling D. Surface damage: small droplets and arc strike (electrode touch) Cleaning the weld area prior ro welding improves the fusion of weld and base parts. The operation can guard against incomplete fusion, B. A surface defect E in the heat-affected zone can be considered the most dangerous defect in the Tee-joint. Residual tensile stresses in the vertical plate is less than in the first instance. The second instance is poor for heavy welded construction, showing lack of ductility through the thickness of the material. Friction welding of steel bars. The bars are rotated relative to each other and squeezed together. A. Uniform weld is preferential B. High pressure or low speed C. Low pressure or high speed Two defects can be considered as one if the distance between them is on the order of it's own size. The depth of the defect is more important than the length.

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